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Solid State Amplifiers => Amplifier Discussion => Topic started by: sewage666 on July 03, 2012, 01:26:40 PM

Title: Fried Ampeg SVT-200T
Post by: sewage666 on July 03, 2012, 01:26:40 PM
First post after finding this forum...

I'm looking for some help. I've done just a tiny bit of tube amp repair for me and friends, but now I've got an old humdinger of a fix-it project back on the bench.

My old Ampeg SVT-200T was fried some time ago at a Bay Area club with bad power. Two other bass rigs were burned up that night (I guess the power wiring there is crap, or three people in a row don't know how to hook up their amps). It was fixed by a tech who just installed an entirely new power amp board from SLM. As I recall, it fried again in the same club and I just put the thing in storage until I could suss it out. Installing a new power amp board is no longer an option, Ampeg having been bought by LOUD.

On opening, the T-90 relay was burnt and fused and R28 and R29 were toast. I replaced all of these. Not one fuse was burnt, though, so I guess those didn't really work to save anything.

Now the amp will power on, but no sound. The pre-amp works when I put a lead from the pre-out to another amp. The contact from the pre to the power amp in the loop section seems good. I've removed and tested the power transistors and they all seemed fine.

I tested all the points listed on the schematic... being a bit of a n00b inside an solid state amp, I'm not sure entirely what's going on, but TP10 is definitely way off, reading .2mVDC when it should have 16 volts. I'm going to replace the diode and electrolytic there.

I've attached the schematic here with my test notes on it (hopefully it's big enough to view). Is there anything else I should be looking for? And what the eff could've happened?

I've searched for anyone else working on this amp, but didn't find much online. It seems like when a '90s Ampeg solid state breaks, it gets tossed. These PCBs are a pain, but I'm looking to this as an educational experience and maybe get a back-up amp for my tube monster. Any help is appreciated.
Title: Re: Fried Ampeg SVT-200T
Post by: Enzo on July 03, 2012, 07:10:03 PM
The fuses are not there to protect the amp.  They are there to protect your house FROM the amp.   They are there to help prevent the thing catching fire in event of failure.

TP10 is your +16v rail, it needs to be there.  Measure the diode, is it shorted?  If not, I suspect an open R24 rather than the zener and cap.   Is there +60v on the other end of R24?

SImple test.  Leave the amp on a couple minutes.  I suspect you have +60, but start with that, is there a +60 supply present?  (Can't make +16 from +60 if there ain;t no +60)  So assuming +60 on one end of R24, and essentially zero on the other end, either the resistor is open, which would leave it cold to the touch.  Or the zener/cap is shorted, which would make R24 REAL hot.
Title: Re: Fried Ampeg SVT-200T
Post by: sewage666 on July 03, 2012, 08:06:53 PM
Enzo... you are king.

I did as you advised. The resistor got hot, and then I realized I'd hit the wrong part of the test point I'd marked on the PCB while tracing the circuit. Oops.

It is indeed getting 16V there.

Now I'm really stumped. I'm attaching the revised schematic with notes. Any idea of where to look next would be appreciated. The only real idea I have now is to do an audio probe.
Title: Re: Fried Ampeg SVT-200T
Post by: sewage666 on July 03, 2012, 08:29:27 PM
I was able to follow the audio up to R1 that leads back into IC1 on pin 6. Then it disappears. R1 tests fine, as does the cap and resistor between pins 6 and 7 on IC1.

My limited knowledge would assume I could have a problem with the IC and / or with the optocoupler OC1. I don't even know what an optocoupler does here, though, much less how to test it.
Title: Re: Fried Ampeg SVT-200T
Post by: sewage666 on July 03, 2012, 08:36:58 PM
I found this to test an octocoupler...
http://www.antimath.info/electro/how-to-test-an-optocoupler/
And by my reckoning, I guess my octocoupler is working.
Title: Re: Fried Ampeg SVT-200T
Post by: J M Fahey on July 03, 2012, 10:00:48 PM
That Led/LDR optocoupler is being used as part of a compressor/limiter.
I distrust more IC1.
Check that it's fed +/-15V to pins 8/4 respectively.
Also that there's no DC (beyond a couple millivolts) on pins 1/7 .
You will *not* have audio on pin 6 but yes on pin 7 .
I mean, *if* you inject some signal on the Line in jack.
Worst case, carefully replace IC1.
You may temporarily unsolder and lift the LDR end of the Opto, just to put it out of the way.
In that case, you must have 2X what you inject.
Title: Re: Fried Ampeg SVT-200T
Post by: Roly on July 03, 2012, 11:15:40 PM
First up, thanks for being your own best friend and posting the circuit/schem complete with your measurements - that's a huge flying start.

I'm curious about how the club power situation could cause an amp to fry, but we'll leave that for the moment.

IC1 - you won't see any signal on pin 6 because pin 5 is connected to ground and that makes pin 6 a "virtual earth" point, but you should see signal on the output pin 7.

The readings at TP4 and TP5 say the +/-16V supplies are okay, despite the apparent reading at TP10, and I wouldn't expect any AC at TP4 or 5 (see table).

Like JM, if I had to guess between the opto and the op-amp I'd go for the op-amp every time.  These LDR optos hardly ever fail, and it would be very unlikely for it to fail shorted across the LDR without some external help in the form of a loose solder dag or such.  Op-amps however are a bit more fragile and not unknown to simply give up.

I feel quite strongly that you have two different, and quite possibly unrelated, situations here; the left hand side around the low level stages and op-amps, and the right hand side around the main power output stages, and I don't think they are actually related.

I think the reason the amp has stopped is around IC1, and that the damage around the output stage is an accumulation over time that had not yet got so bad as to do any serious damage - that TP13 in the speaker protector goes low and pulls in the relay T90 strongly suggests that there is nothing wrong with the power amp portion.

Again the voltages at TP8 and TP9 tell us that the main supplies are okay.

I find R28, R29 and the relay T90 being toasted particularly interesting.

TP13 should only be low and relay T90 pulled in if the output of the main amp ("half rail") is very close to ground, which it should be (the relay etc being there to protect the speakers from being burned up by DC).  So it's possible that R28 & 29 had only got very hot, and not actually failed, otherwise I would have expected to see some other damage (dead transistors?) or off voltages around the output stage.

That is, the primary problem in the output stage may have been the protection relay itself, or at least its contacts.

If you could provide a voltage reading between point (E) - the main output - and ground, and across each of R31 through R38, the 3.3 and 0.33 ohm emitter resistors (~1V, and ~10-20mV), that would help a lot in getting a picture of the condition of the output stage.

I would nominate IC1 as your most likely suspect simply because it is fairly exposed to the outside world via the 'Line In' connector, and dodgy club power could potentially have put a high voltage in via this connector if it happened to be being used at the time.

HTH
Title: Re: Fried Ampeg SVT-200T
Post by: sewage666 on July 04, 2012, 02:16:05 AM
Thanks so much all for the help! I'm gonna try to address this all logically and in order...

J M Fahey:
Check that it's fed +/-15V to pins 8/4 respectively.
Also that there's no DC (beyond a couple millivolts) on pins 1/7 .
You will *not* have audio on pin 6 but yes on pin 7 .


Pin 8 is getting 16.5V and pin 4 is getting -16.5
I could hear but the faintest of audio with the gain and volume cranked all the way on pin 7. I mean REALLY faint.

J M Fahey:
Worst case, carefully replace IC1.


The local Fry's Electronics store has the NTE equivalent in stock, so I'm going to do just that, after I install an IC socket... doing myself a favor for the future.

Roly:
First up, thanks for being your own best friend and posting the circuit/schem complete with your measurements - that's a huge flying start.


I've done enough lurking around forums to know better.  :)

Roly:
The readings at TP4 and TP5 say the +/-16V supplies are okay, despite the apparent reading at TP10, and I wouldn't expect any AC at TP4 or 5 (see table).


I'd misread TP10, it's good at 16V. I don't know what's going on at TP3, however... my new reading is .47VDC and -1.2mVAC. That's all sorts of not matching the table.

Roly:
TP13 should only be low and relay T90 pulled in if the output of the main amp ("half rail") is very close to ground, which it should be (the relay etc being there to protect the speakers from being burned up by DC).  So it's possible that R28 & 29 had only got very hot, and not actually failed, otherwise I would have expected to see some other damage (dead transistors?) or off voltages around the output stage.


It's been so long since I reopened this can of worms... I should've checked if those resistors were toasted or just toasty.

Roly:
If you could provide a voltage reading between point (E) - the main output - and ground, and across each of R31 through R38, the 3.3 and 0.33 ohm emitter resistors (~1V, and ~10-20mV), that would help a lot in getting a picture of the condition of the output stage.


I'm not getting any voltage at E... and I'm wondering about R39. I checked the Ohms in circuit, and it reads 0... I know that's not the proper way to check, but R40 is the same type and it reads 10 Ohm. I'll unsolder R39 tomorrow and double check. I'm not sure what you mean by read across the emitter resistors though... read each side of each resistor? Or from emitter to emitter?

Tomorrow, I'll celebrate Independence Day by shopping for some parts. Yes!
Title: Re: Fried Ampeg SVT-200T
Post by: Roly on July 04, 2012, 11:06:10 AM
Quote from: sewage666
I'm not getting any voltage at E... and I'm wondering about R39. I checked the Ohms in circuit, and it reads 0... I know that's not the proper way to check, but R40 is the same type and it reads 10 Ohm. I'll unsolder R39 tomorrow and double check. I'm not sure what you mean by read across the emitter resistors though... read each side of each resistor? Or from emitter to emitter?

Zero volts to ground on point (E) is good, it tells us that the whole DC-coupled output stage seems to be balancing itself correctly, which in turn means it is very unlikely to have any dead or dodgy devices.

I mean measure the voltage across each of R31 to R38, probe at each end of each resistor in turn to measure the voltage between the ends of each resistor.  R31 and 32 should have about a volt across each, while the rest should have about 10 to 20mV across each.  Basically this is to check that the two drivers, Q11 and 12 are passing about the right current through R31 and 32, and that each of the output transistors Q13 to Q18 are all passing about the expected current, and therefore can be presumed healthy.

The reason this is important is that the toasty resistor R28 and R29, and the relay contacts on T90 all suggest excessive current in the output stage at some time.  This may have been due to a fault since repaired, or simply that the resistors and relay contacts were a bit under-specified and are showing some age.  I strongly suspect the latter, that there is no serious problem around the output stage (which is a damn good thing 'coz these stages can often develop terrible cases of galloping silicon cancer).

R39 is generally only used as a former for the coil L1 but may also be a damper; shorted by the coil it will only read zero ohms.

Until you can see about a volts AC of signal at TP2 there isn't much else you can do, and I suspect that when IC1 is replaced you will restore the signal level and overall operation.

Just be certain you get the new op-amp in the right way around.
Title: Re: Fried Ampeg SVT-200T
Post by: J M Fahey on July 04, 2012, 02:06:18 PM
Agree and add: please do not use an NTE equivalent if at all possible.
NE5532 are very common, order a couple from a known supplier , keep tem as spares.
That said, they are not even needed in IC1, *very* light duty there, you can use a common TL072 .
Of course, as Roly said, make sure you replace it pointing the same way.
Good luck.
Title: Re: Fried Ampeg SVT-200T
Post by: sewage666 on July 04, 2012, 03:23:56 PM
Agree and add: please do not use an NTE equivalent if at all possible.
NE5532 are very common, order a couple from a known supplier , keep tem as spares.
That said, they are not even needed in IC1, *very* light duty there, you can use a common TL072

I replaced IC1 with a TL072. No result.

I mean measure the voltage across each of R31 to R38, probe at each end of each resistor in turn to measure the voltage between the ends of each resistor.  R31 and 32 should have about a volt across each, while the rest should have about 10 to 20mV across each.  Basically this is to check that the two drivers, Q11 and 12 are passing about the right current through R31 and 32, and that each of the output transistors Q13 to Q18 are all passing about the expected current, and therefore can be presumed healthy.

I've measured 0V passing through R31 or R32. That can't be good. Same for the R33-R38. I assume since these are ceramic resistors, they should also be running at least a little hot, which they aren't. I just pulled out Q7, Q9, Q11 and tested them according to first test here:
http://www.kpsec.freeuk.com/components/tran.htm#testing
I don't know if there's a better, more assured way to see if a transistor is failing?

I'm baffled what to try next. I'm beginning to understand why, years ago, one tech I asked about working on this amp said it was on a short list of amps he refuses. Oi vey.
Title: Re: Fried Ampeg SVT-200T
Post by: J M Fahey on July 04, 2012, 04:03:30 PM
There's something that does not add up, so please do the following:
1) inject, say, a 200 mV sinewave into the line in jack.
Use an actual plug there, not simply soldering some wire to its pads.
2) measure that you actually have those 200mV there.
Any half decent multimeter will have a 200mV AC scale.
If you do not have an audio generator, download a 400Hz to 1 KHz MP3 wave and play it with a portable MP3 player into the line in.
Most of them can provide around 200mV at the earphone jack.
3) follow those 200mV along their path, you may have a cracked track or bad solder anywhere.
So check: Line in jack "hot" terminal> C1 (both ends)> R1 (first end)> IC1 pin 7 (you should have 300mV AC there).
4) if AC reaches R1 and can't be found after that, turn the amp off, wait for all PSU voltages to disappear and measure the resistance between IC1 pins 6 and 7 (I expect around 33K) ; and between 6 and 5 (I expect "open" or at least over 100/200K).
Use the 2K and higher resistance scale.
Title: Re: Fried Ampeg SVT-200T
Post by: sewage666 on July 04, 2012, 09:16:51 PM
Big update... I was trying JM's test, and I started getting the faintest hint of a sound coming through my speaker. A little 500Hz sinewave I was playing through my iPod into the line in. Weird, I thought. I switched ICs into the sockets I'd installed, and it didn't make a difference. So I think the ICs were fine.

While I was probing to do the voltage test, I accidentally made contact on the tip end of the line-in jack/switch. Noise. Big noise. I switched my iPod onto The Fog soundtrack (yay) and there it was, nice and loud, coming through the power amp section. Then I accidentally bumped my probe into the thermal switch. POP! I blew one of the NE5532s. So I swapped a TL072 in.

I'd had this same kinda problem with the effects loop of a VH-140C (it was cutting the signal in and out), so I had some of the weird L leg Cliff jacks that Ampeg used in the 90s lying around. This is all despite the original jack continuity testing fine, keep in mind... same thing on the VH-140C. I can't explain it.

So, I swapped the jacks for new ones, and behold! My problem was mechanical all along (and I feel really dumb)! Many thanks in helping me trudge along, or I wouldn't have kept testing. And I hate Cliff jacks, now. BUT...

Now that the amp is amplifying, it's buzzing and distorting REALLY bad, and the signal cuts abruptly while the instrument's strings are still decaying. It also only makes noise if the volume and gain are at least half way up. Could this be because of the TL072? I guess I can't really answer that until I order some new NE5532s and get this thing back to spec. Could it be the octocoupler? Could be from the original damage inflicted the first time it fried, or damage I just did shorting the thermal switch? Time to stop for now, order some ICs, and then watch fireworks. Happy fourth!
Title: Re: Fried Ampeg SVT-200T
Post by: Roly on July 05, 2012, 12:00:34 AM
Quote from: sewage666
I'm beginning to understand why, years ago, one tech I asked about working on this amp said it was on a short list of amps he refuses. Oi vey.

If they can build it and get it going, you can repair it.  The tech's attitude may have had more to do with the commercial aspect, being time consuming and unprofitable; but as it's your gear, and your time is your own, the economics are quite different, so don't get discouraged - okay?  You have already made very serious progress.

Quote from: sewage666
I accidentally made contact on the tip end of the line-in jack/switch. Noise. Big noise.

(AAARRRGGHH!  :grr )  YAY!  :dbtu: I'm sure JM and I would both have mentioned the jack switch contact as a very common cause of this sort of problem, were it not for the fact that it was marked on the circuit "ckd continuity"!

Quote from: sewage666
So, I swapped the jacks for new ones, and behold! My problem was mechanical all along (and I feel really dumb)!

Most of the time it is, and some of the time you do, but with time you learn to double check the stupid stuff first.

Now;

Quote from: sewage666
Now that the amp is amplifying, it's buzzing and distorting REALLY bad, and the signal cuts abruptly while the instrument's strings are still decaying.

Okay, now you are WAY ahead, but earlier you said...

Quote from: sewage666
I've measured 0V passing through R31 or R32.

...and what you are now observing is perfectly consistent with what you previously observed, the output stage is operating without any bias; there is no voltage across them because there is no current flowing through them (and stony cold) - and this most likely goes back to the burnt resistors R29 and R29.  We are very hot on the trail here, but now we (you) must be very careful because one wrong move around the output stage could produce quite a lot of damage.

I think that this is almost certainly a pre-existing problem, not something you have just caused.

The table says you should have +40 volts on TP6 and -40V on TP7.  My guess is that you won't have, that these voltages will be quite a bit lower.

At this point I normally revert to cold checking, and in this case I'd be pulling first Q6, the bias transistor, and Q7 and Q8 the pre-drivers to make sure they are good.

It could well be that somebody has simply fiddled with the bias setting pot (looks like "AR1" on my cct, in series with R23), but I'd want to be sure before I went changing it.  I'd also want to know the two protection transistors Q2 and Q3 are healthy.

In all these cases you must be extra careful to restore the right transistor to the right position, the right way around.

I doubt the op-amp has anything to do with this problem, but by all means restore IC1 to a 5532 if you feel happier about it.

While testing around the output stage it is best to disconnect your loudspeakers and check for near-zero volts between (K) and ground before you reconnect them each time. (Yeah, I know it has a speaker protector, but the amp also has faults and an unknown tech work history; better to be cautious...).  I would also be using a limiting lamp in the mains feed while I was working on the output stage.

You have made considerable progress and I think you are not far off finding and clearing the remaining fault(s)
Title: Re: Fried Ampeg SVT-200T
Post by: J M Fahey on July 05, 2012, 12:32:06 AM
Agree and add:
BUILD A LAMP BULB CURRENT LIMITER AND USE IT
Search this Forum for it.
And   disconnect speakers for now, we'll suggest some DC voltage measurements, no signal applied.
Title: Re: Fried Ampeg SVT-200T
Post by: sewage666 on July 06, 2012, 01:23:28 AM
Ha! I SWEAR I checked the continuity of that switch jack... in fact, checking it again out of circuit, the old jacks test fine. They just didn't work... can the metal corrode or something where it starts to act like a resistor? All I know is this is the second time I had this same mysterious problem with old Cliff jacks... test fine, but don't work in circuit.

Moving on...

Fired up the amp again today to update my test points, and +16v rail went down to around 6? +61 rail is okay (tested around +71)... but right after the 1.5kohm resistor (which tests okay), voltage drops with 66 volts measuring across resistor. If the resistor isn't bad, then where is that voltage going?
Measured D20: measured 1.7 one way, .3 the other. Bad diode?! Could I have shorted that out?

Tested
Q6: good
Q7: good
Q8: good
Q2: good
Q3: good

Moving on and waiting for some parts in the mail...
Title: Re: Fried Ampeg SVT-200T
Post by: sewage666 on July 06, 2012, 01:40:19 AM
Also just noticed... Q6 (bias transistor) is listed as 2N3402 on schematic, but the actual part in my amp is a 2N3403. Could be equivalent?
Title: Re: Fried Ampeg SVT-200T
Post by: Roly on July 06, 2012, 03:28:46 AM
Sorry, I mis-read the TP table; the DC voltage at TP6 and TP7 should be +1.45V and -1.45V, not +/-40 as I previously said (that's the fully driven AC voltage at these points)

Quote from:
Ha! I SWEAR I checked the continuity of that switch jack... in fact, checking it again out of circuit, the old jacks test fine. They just didn't work... can the metal corrode or something where it starts to act like a resistor? All I know is this is the second time I had this same mysterious problem with old Cliff jacks... test fine, but don't work in circuit.

Yes, all jack contacts seem to be prone to this.  One trick is to give them a good scrub with the edge of a bit of newspaper to clean any crud off, then they work fine again, for a while...

+16V rail needs to be 16 volts or very close to it.  6V implies it is being heavily loaded by something. The output stage depends on the +/-16V rails being about right, so this won't help at all and you need to find out why it's so low.

D20:  presuming you didn't get your fingers into the act, and you measured it with one end disconnected, this is very wrong - should measure about 600mV one way and open circuit the other.  You can't get a reliable reading in circuit (because of C10).  But these measurements won't give you 6 volts on the supply either.

I'm inclined to think there is actually nothing wrong with D20 or R24.

Now previously you had about the correct voltages on TP4 and TP5, and more importantly TP10 and TP11.  If the +16V is now only +6V then something on the +16V rail is drawing too much current, and the most likely suspect is one of the op-amps powered off that rail.

Pull the one you have socketed (IC1?) and see if that restores the 16V supplies.  IC1 is not needed in place to measure the DC conditions of the output stage.

The only difference between the 2N3402 and 2N3403 is the gain range, 75 to 225 against 180 to 540, so they overlap.

I think we need a fresh set of DC voltage readings for all the Test Points.
Title: Re: Fried Ampeg SVT-200T
Post by: sewage666 on July 06, 2012, 11:44:57 AM
Quote
D20:  presuming you didn't get your fingers into the act, and you measured it with one end disconnected, this is very wrong - should measure about 600mV one way and open circuit the other.  You can't get a reliable reading in circuit (because of C10).  But these measurements won't give you 6 volts on the supply either.

I'm inclined to think there is actually nothing wrong with D20 or R24.

I did all the measurements out of circuit. R24 is fine. D20 tested funny, so I compared it D21, which tested good. I've got a batch of 1N4745As on their way to me now.

Quote
Now previously you had about the correct voltages on TP4 and TP5, and more importantly TP10 and TP11.  If the +16V is now only +6V then something on the +16V rail is drawing too much current, and the most likely suspect is one of the op-amps powered off that rail.

Pull the one you have socketed (IC1?) and see if that restores the 16V supplies.  IC1 is not needed in place to measure the DC conditions of the output stage.

Luckily I socketed both ICs, because I had the same thought... but unluckily, it didn't make a difference. I traded around ICs in both. My TL072s were recycled from some random circuit boards I found in a salvage store, so their reliability is suspect. I've ordered NE5532s.

When I get these parts replaced that I know are variables, I'll be taking new test point notes. Or, maybe I'll be playing loud bass through my now working amp. I'd prefer the latter!
Title: Re: Fried Ampeg SVT-200T
Post by: Roly on July 07, 2012, 09:26:54 AM
I'd be checking that you have good +/-16V rails before you plug the op-amps in.
Title: Re: Fried Ampeg SVT-200T
Post by: sewage666 on July 10, 2012, 02:30:35 AM
And, I'm back.

I got my parts in the mail... I replaced both diodes on the 16V rail, D20 and D21. Both, just because I thought I should. I got my +16V back, I assume the diode was bleeding to ground.

I plugged in my two new NE5532 ICs... and nothing. Well, not nothing. I ran the sine wave back into the power amp line-in, and it definitely was coming through. It's coming through about the level before I replaced the shorted jack. Very quiet. I tried the same trick to short the jack connection, in case that was the problem, but no effect. I also tried to short the speaker jack, no result. Messed with the biasing resistor (and put it back to where it was), no result.

I measured across the .33Ohm 5W resistors and didn't get a reading. They're stone cold, too.

I re-measured all my test points, and there is definitely still some discrepancies from the table. Namely 3 (giving me a negative DC voltage reading) and 14 (way off from the table)

Again, I'm stumped. I'm gonna approach this again with daytime eyes, but maybe someone else has a thought? Thanks. New schematic and test point results attached!
Title: Re: Fried Ampeg SVT-200T
Post by: Roly on July 10, 2012, 11:42:03 AM
{this drawing seems to double in size every time you upload it. (???)}

You need to be specific about what voltages you are seeing where - you are our eyes here.

You appear to have added two voltages at the ends of R30 which are most interesting; +5.2V and +5.4V.

Now if we start at the half-rail (point (E)), and we apparently have no current flowing in any of the output emitter resistors R33-R38, then we have no voltage drop across them.

The typical voltage drop across the base-emitter junction of a transistor is around 600mV or 0.6V, so this is what the bases of Q13-Q15 should be (similarly for the negative side).

Q11 adds another 0.6V so the voltage at the base of Q11 should be;

2*0.6=1.2V.
3*0.6=1.8V

TP6 and TP7 are given as 1.45V+/-20mV

1.45/3=0.48333333

So let's compromise and estimate around 0.5V across each of the three EB junctions in series.

If your voltages are right then there has to be an open Base-Emitter junction in both the upper and lower path, e.g. Q11 and Q12.

We have two choices; mistaken measurements, or some dead silicon between Q11 and Q18, and I'll bet on Q11 and Q12.

What is a serious concern here is that the TP table says you should have only +/-1.45V at TP6 and TP7 (i.e. across Q6), and until you determine why there is so much voltage across Q6, almost 12 volts when it should be closer to 3V, simply replacing dead drivers Q11 and Q12 will only fry the new ones.

Also, note that the voltage at TP3 cannot be corrected if the loop is open, and if there are transistors with open BE junctions in the loop then TP3 could be just about anything, so don't worry about that for the moment.

Important point; can the voltage between TP6 and TP7 be brought down to +/-1.45V by adjusting AR1, and if not we need to find out why and fix that before anything else.


BTW, you said "I also tried to short the speaker jack" - I hope I misunderstand because that is something absolutely NOT to do at any time.

So,
* post the table of DC voltages for all your Test Points.
* Does adjusting AV1 bring TP6 and TP7 to about 3V apart, or a bit less?
Title: Re: Fried Ampeg SVT-200T
Post by: sewage666 on July 10, 2012, 12:23:41 PM
Oh boy, that's my bad!! Those voltages aren't on either side of r30, those are TP6 and TP7. I realize now I wrote them at a different spot before... Sorry! I wiped clear my old readings and just re-added each point.
Title: Re: Fried Ampeg SVT-200T
Post by: Roly on July 11, 2012, 02:31:18 AM
Well okay, but that doesn't really change anything much; you still have +/-5 -something volts where you should only have +/-3V (2x 1.45 = 2.90), and it just means that another couple of transistors are suspect.

You still need to establish if altering the bias control will get these TP's to a difference of 3 volts or a bit less, then you have to move down each line of B-E junctions getting lower voltages as you go.  The emitter of Q9 (base of Q11) should be about half a volt lower than TP6; the emitter of Q11 (bases of Q13/14/15) should be about half a volt lower again.

Similarly working along from TP7 on the negative side through Q10, Q12 and Q16/17/18.

At one of these transistors (on both the positive and negative sides) you will find a base-to-emitter voltage much larger than half a volt, and those devices are dead.

But as I said before, there is no point in just replacing them until you find out why TP6 and TP7 are so far apart, and getting the voltage between them back in spec - +/-1.45V.
Title: Re: Fried Ampeg SVT-200T
Post by: sewage666 on July 11, 2012, 01:22:19 PM
I hadn't seen your response until I'd gone mad and decided I needed to test every diode!!! Well, I tested them in place, and any that didn't read normal I tested out of circuit as well. Of course, that was just about every diode. Fruitless search?

Not necessarily... I found D10 and D13 were passing no voltage... these are connected to the bases of Q7 and Q8. Also suspiciously near the original resistor burn out zone. D12 also tested funny (was reading as passing 2x as much voltage as other 1N914s) so I'm just going to replace that whole array of diodes there.

My parts are already on there way via snail mail... When I get them put it in, we'll again 1. see what happens and then 2. I'll start tests down the rails as you've suggested.

Thanks a lot for your help and patience, Roly!
Title: Re: Fried Ampeg SVT-200T
Post by: Roly on July 12, 2012, 06:47:45 AM
I'm confused by your "D10 and D13 were passing no voltage".  Components pass current and have voltage across them, or between points.

Specifically did you find D10 and D13 open or shorted?

Shorted I hope because open wouldn't explain the symptoms.  The diode test range on a DMM displays millivolts, thus a good diode will look like an open circuit one way, over-range in reverse; and somewhere between 500 and 700mV the other, forward, way.  A shorted diode will generally read something low or quite low, less than 100mV, both ways, while an open diode will read over-range both ways.

Patience is something you must have if you are going to solve these sorts of problems - they simply don't yield to macho gust and bust but to thorough and methodical.
Title: Re: Fried Ampeg SVT-200T
Post by: sewage666 on July 12, 2012, 10:38:08 AM
I'm confused by your "D10 and D13 were passing no voltage".  Components pass current and have voltage across them, or between points.

Specifically did you find D10 and D13 open or shorted?

Sorry, nomenclature fail... There is no voltage across them either direction, so they're passing no current. They're shorted.

Patience is something you must have if you are going to solve these sorts of problems - they simply don't yield to macho gust and bust but to thorough and methodical.

Do I get points for being pissed at every diode, but methodically testing them all in order? :)
Title: Re: Fried Ampeg SVT-200T
Post by: Roly on July 12, 2012, 12:34:16 PM
Quote from: sewage666
There is no voltage across them either direction, so they're passing no current. They're shorted.

That's good because that makes sense with the voltage readings.  :tu:

Now you may think I'm being picky here, but unless you think about the properties of these components the right way in terms of resistance, voltage and current, you are never going to make sense of what is going on.

In this case they are shorted, like a length of wire, they have no voltage across them - but does a length of wire, a short, pass current?


D10 and D11 aren't in this particular part of the circuit to prevent current from flowing backwards, but rather because they have a fairly constant voltage drop independent of current; in other words they are acting here like very low voltage zeners to provide a fixed drop of 2x 0.6V (roughly).

Because one on each side is shorted the effect is to turn Q7 and Q8 on too much, giving too much current though Q6 and therefore excessive voltage between TP6 and TP7.

Hopefully when you get good diodes in those positions TP6 and TP7 will only be about 3 volts apart.

Yes, you do get +1 for methodically testing them all in order.  :dbtu:

This is one hell of an amplifier you are servicing, a bit like learning to drive in a tank, and you are gaining ground, but with an amp like this I would "hasten slowly" myself, so you are doing fine.  ;)
Title: Re: Fried Ampeg SVT-200T
Post by: J M Fahey on July 12, 2012, 06:14:01 PM
Agree and add:  those diodes *are* passing current.
They show no voltage drop because being shorted they "have no resistance".
Please be careful with the use of words because we won't understand what you mean and you will not understand our instructions.
Good luck.
Title: Re: Fried Ampeg SVT-200T
Post by: Enzo on July 12, 2012, 07:14:37 PM
I think part of the word problem is that for many people the words voltage and current both mean the same thing:  electricity. 
Title: Re: Fried Ampeg SVT-200T
Post by: Roly on July 13, 2012, 09:14:57 AM
Yeah, what happened to the good old hydraulic analogy; the hose on the garden tap?

Voltage is pressure.

Current is flow.

Resistance is friction.
Title: Re: Fried Ampeg SVT-200T
Post by: J M Fahey on July 13, 2012, 11:42:03 AM
Dear Roly: I used it to explain things long ago, until I found it creates more problems than it solves, so now I avoid it as a pest.
What problems?
Well, non electronic guys take it *too* literally and jump to conclusions such as:
1) say a +B wires is shown reaching 2 caps in parallel; obviously it will reach one "first" and then the other, at least it's what the drawing shows.
Guys think the first cap gets more charge or voltage or whatever, because it's , well, "first".
2) say that current can flow along an unterminated, open end wire "because that's what water does along a hose".
They don't understand the need for a return path.
3) the hydraulic analogy would imply that a puncture on a wire insulation "might leak some electricity through it" which obviously is not the case
4) imagine trying to explain a transformer !!!
5) imagine your own along these lines.
If I have to add lots of sub-explanations such as "electricity is "water" but has no mass (or weight)" ; "electricity needs a return path", "electrons move instantly, "all at once" (or at least at the speed of light)" I end up finding that explaining things as they really are, no analogies, ends up being the shortest path.
jm2c
Title: Re: Fried Ampeg SVT-200T
Post by: Enzo on July 13, 2012, 06:25:57 PM
Well, it isn't all or nothing.  I'd be hard pressed to get many concepts over without using analogies.  But analogies inevitably break down.  SO we have to temper them with caveats.  I like the water analogy, but I also find my self even more often turning to automobile analogies for many things.

If someone thinks a punctured insulator will leak electricity, then they don;t even get electricity, never mind the concept of "circuit."   The water analogy is used to demonstrate the concept of circuit more than the concept of electricity.  At least in my mind.

One very important distinction to make early on is that the schematic shows one thing and a wiring diagram shows something else.  SChematics show electrical relationships and wiring diagrams show physical/mechanical relationships.   Always bums me out when I ask someone to post his schematic for troubleshooting, and he puts up his layout wiring diagram instead.
Title: Re: Fried Ampeg SVT-200T
Post by: Roly on July 14, 2012, 05:18:10 AM
The water analogy certainly isn't going to help much with something like conjugate impedance, but there seems to be a general need here to get the basic concept of Ohms Law across, the very basic relationship of a voltage driving a current through a resistance.

While I don't seem to encounter the sort of thing JM mentions, it comes up in a different way with things like the current capacity of plug packs, and the fear that a 12 volt 600mA pack will somehow force more current through a circuit (a device like a stomp) than one rated at 12 volts at 300mA. It's a bit of a worry when you find that this is the sort of wrong advice people have got from sales staff, who should know better, in electronics retailers.

I got razzed on another forum for posting a picto-graphic "circuit", and while I'm not keen on them it's sometimes hard to remember the initial struggle you had relating a circuit to the reality when you've been doing it for a long time.  It's a bit like reading music vs using guitar tabs.
Title: Re: Fried Ampeg SVT-200T
Post by: J M Fahey on July 14, 2012, 11:25:57 AM
Yes, it's hard to explain things when to begin with the asker does not only have the (legitimate) doubt but does not handle the language or concepts necessary to explain it.
I also get razed often because I explain something trying to adapt to what the asker seems to understand, and some tech minded guy corrects me: "it ain't xxxx but yyyy , didn't you know? "
Yes, brother, I agree, but I'm trying to explain it to the OP who had *one* specific problem and just wants it solved, he's definitely *not* taking "Electronics 101" classes.
Oh well.
Title: Re: Fried Ampeg SVT-200T
Post by: Roly on July 14, 2012, 08:07:19 PM
{JM; You will notice how often I use the words "generally" and "typically"  ;) }


CAUTION: @sewage666 - I want to hear specifically that you now have a limiting lamp in series with the mains feed to the amp.  With an amp this size it is not optional.

http://www.ssguitar.com/index.php?topic=2093.0 (http://www.ssguitar.com/index.php?topic=2093.0)

This is SOP for bringing up a faulty or (we hope) repaired solid-state amp.

The reason is that the output stage is connected directly across the power supply.  If, for any reason, both the top and bottom rank of output transistors should turn on together they would short the supply.  This normally results in the rapid demise of the output transistors, sometime the drivers and pre-drivers, followed a short time later by the power supply rectifier, and the power transformer will burn up next.

So it's really important to avoid this happening.  If there is such a problem then the input power will appear in the lamp, lighting it brightly, not the output stage (and lighting it brightly  :-[ ).

In your case it is almost certain that there are dead transistors (open circuit base-emitter) somewhere following the diodes you are about to replace; most likely the pre-drivers Q9 and Q10, or the drivers Q11 and Q12.

At this point I would remove the drivers Q11 and Q12, test them, and if okay put them to one side for the moment.  Note carefully that these are a NPN/PNP pair and that they must go back in the right places (all the MJ15022's are along the top, all the MJ15023's are along the bottom).  With these transistors removed there is no way that the output set can be driven - a safety factor while we get the voltages in the pre-driver right.

Once the voltage between TP6 and TP7 is something more reasonable (like about 3 volts), AND we have located and replaced the dead transistors that are lurking downstream somewhere at the moment, then we can try a power up with limiting lamp and no speaker, checking for reasonable voltage (and currents) before we let it fully loose.

I can't stress enough how important it is to have located and replaced ALL the dead silicon (and any other dead components) in a solid-state output stage before you let it rip on unrestrained power.

Even then you need to be certain that it is balancing the output to zero volts before you connect your speakers (this one does have a speaker protector, but most don't, and we can't be certain that even this one is working properly yet).

HTH
Title: Re: Fried Ampeg SVT-200T
Post by: sewage666 on July 17, 2012, 01:32:29 PM
Update:

I built a limiting lamp. Replaced the four 1n914A diodes D10-13...

(http://i27.photobucket.com/albums/c160/sewage666/photo-2.jpg)

In my impatience, after testing Q11-18 multiple times now, I just decided to fire it up. The light bulb went bright, then dim as it should.

I checked the test points... 6 and 7 are reading 57 and -57 DC. What the F? TP 3 is still in negative mV. On the happy side, 4 and 5 are right where they're supposed to be.

Out of curiosity, I plug into a speaker and guitar cord anyway, and pow... buzz. Loud buzz. Plug in a guitar, and FINALLY this amp is breathing again. It's loud. It's the first time I've heard this thing make a peep in years and years, and it sounds awesome.

Okay, I probably broke a lot of rules here. I have a degree in painting, so, I'm an idiot. Also, I meant the diodes were OPEN, not shorted... another failure of my lexicon in electronics. This project, however, has schooled me immensely.

I'm gonna do some more testing, because really, I'm trying to figure out what I did wrong on the test points. In the meantime, my testing will be more fun, because this amp works again! Thanks for all the help!

P.S. I kinda hate the water analogy for current. I get it, I've thought about it. I think just about every analogy fails eventually, though, some quicker than others. It's like saying the Internet is a series of tubes. It's actually a decent metaphor, if you think about data flow, but it just doesn't translate.
Title: Re: Fried Ampeg SVT-200T
Post by: Enzo on July 17, 2012, 07:27:20 PM
The point of any analogy is to describe a relationship in the IMMEDIATE discussion.  They are not intended to be used as definitions.   That they all break down is a given.  After all, if it didn;t break down, that would mean the analogy exactly described the whatever, which instead of analogy, then becomes just a definition.

The water analogy is meant to suggest the idea of flow in a circuit.  It is not intended to generate inferences, it is not meant to suggest, "Oh then that must mean that..."  No, there is no "that must mean" for an analogy.
Title: Re: Fried Ampeg SVT-200T
Post by: sewage666 on July 17, 2012, 07:44:17 PM
@Enzo, of course this is true... I guess what I hate is that I'm one of those n00bs who has a hard time not taking the analogy too seriously, thinking of electricity "flowing."

I definitely could use an "Electronics 101" course, but since I'm fixing all my own stuff because of a lack of cash flowing, it'll be some time.
Title: Re: Fried Ampeg SVT-200T
Post by: sewage666 on July 17, 2012, 10:01:38 PM
Speaking of n00b... I hate to admit this, but in the interest of total transparency for any future readers this could help, I shall.

I had test points 6 and 7 in marked wrong. Oi vey. I was thinking about it on the way home from work, traced the circuit again, and found I'd been testing in the wrong place. I adjusted the variable resistor, and they're well within their test range now.
Title: Re: Fried Ampeg SVT-200T
Post by: Roly on July 18, 2012, 05:16:41 AM
Quote from: sewage666
I probably broke a lot of rules here.

You sure did,  :trouble  and you are one lucky fella that you aren't surrounded by curling silicon smoke.

Quote from: Enzo
After all, if it didn't break down, that would mean the analogy exactly described the whatever, which instead of analogy, then becomes just a definition.

 ;D


Quote from: sewage666
I had test points 6 and 7 in marked wrong. Oi vey. I was thinking about it on the way home from work, traced the circuit again, and found I'd been testing in the wrong place. I adjusted the variable resistor, and they're well within their test range now.

Phew! 

You should check;

a) that you are getting the same small voltage across each of the 0.33 ohm resistors R31 to R38.  Apply Ohms Law; you are looking for an idle current of around 50mA for each; somewhere between 10 and 20mV.  This will confirm all the main output transistors are hauling their weight.

b) that the voltage between point (E), the main output, and ground (i.e. across the speaker(s)) is no more than about 100mV.

If I were doing this repair I would also go to the trouble of putting a low resistance load across the output and gently lifting the drive until the protection (Q2 and Q3) started to work, to confirm that it is working, but in this case...

What I would like you to do however is to go over all the Test Points and at least confirm that the DC voltages are correct for the table.

Normally there would be a few other health checks we would do, but if it's now making a very loud noise I think we can be satisfied that it's going okay.

On one hand you deserve a gold star and Win! button for carrying off the repair of a pretty major amplifier.  When confronted with something like this I consider it to be a fairly serious piece of work, and I've been doing it for a long time now.

But it must be said that you got very lucky just plunging ahead, and if you think you can get away with this on a regular basis ... well ... sooner or later you are going to be spending a fair bit of money and quite a bit of time muttering under your breath that you will be more circumspect in future, as you replace every transistors in the output stage and all the diodes in the power supply.

I'm sure we've all done it at some stage, I know I have, and it tends to be a salutary lesson that makes you a lot more careful in future.

Anyway, very well done  :dbtu: ; I had no doubt that you would get there eventually.  It is very pleasing to hear an amp you have been working on come to life, so much so it can be a bit addictive and cause you to actually seek out more "lost causes".   ;)
Title: Re: Fried Ampeg SVT-200T
Post by: theonewhohowls on September 10, 2013, 10:37:03 PM
Do any of you guys have the schematic for the amp?
Title: Re: Fried Ampeg SVT-200T
Post by: Enzo on September 11, 2013, 01:00:21 AM
What part of the amp are you working on?  The power amp half schematics are already posted here in the first couple posts of this thread.
Title: Re: Fried Ampeg SVT-200T
Post by: g1 on September 11, 2013, 04:02:40 PM
Here is the complete schematic
Title: Re: Fried Ampeg SVT-200T
Post by: Used.Benz.Parts on October 31, 2018, 10:44:57 PM
Hi guys,

Hope you’re still out there in the ether, and have a chance to reply to this novice’s posting...

I’ll try to get you guys up to speed with the basics, the quick and dirty issue I’ve got, and maybe we can get to the root of the problem if/when I hear back [fingers crossed].

Simple set-up:
SVT-200T
15” Bag End speaker
Custom cabinet I fabbed so the amp / cab combo would look nice collecting dust.
Squier P-Bass
Dean Edge 4
Patch cord from bass to amp
Patch cord from amp to cab
That’s it (I’m an enthusiast, barely an amateur)
[Of note: I’m a veteran diagnostic tech from the car industry, but it’s been more than a decade since I had to test electronics -from a schematic- at the component level. And I’m well aware of the “you could get fried working on this amp” warning.
So please be patient.]

Anyway...

The SVT-200T I’ve had for almost 3 years has been finicky. Worked ok-ish when I first plugged it in (see list below), took it apart to clean the attachment hardware and re-Tolex the case, and tried to renew this beast to annoy the neighbors with my loud, terrible playing.

Jan 2016:
New cab operational and I’m proud of what I’ve built.
Put ‘em both in the living room, plugged it all together, tried it out.
The 0dB input crackled.
The 15dB input was clean.
(Or vice-versa; I can’t remember ...sorry.)
Also can’t remember if there was any crackling / popping when adjusting the knobs.

Fast-forward to Summer 2018
Tolex the cab.
Build a grille cover.
Paint the corner hardware.
Assemble the cab.
Set in living room and admire my hard work.
Try out the set-up.
Can’t remember if it worked, but probably not too well.
Take PCB for front panel / pots out and inspect solder joints.
Input jacks (both), found solder joints are cracked.
Re-solder joints.
Notice transistors (3-pin) have either:
a) missing screws to fasten them to the PCB or
b) aren’t supposed to have screws to fasten them to the PCB.
I think that’s on the “heat-sink” end (?) - it’s the metal tab with the hole, opposite the BCE prongs.
Project side-tracked due to noise from truck rear axle; commence rear axle rebuild.


Oct 31, 2018
Plug in the amp (for sh*#s and giggles).
Hook up the Dean.
Nothing - no sound.
Dead battery in the Dean?
Hook up the Squier - doesn’t need a battery.
Nothing - no sound.

Then...
Crackles and a flickering Gain light.
Limit light (under stage volume knob) flickering.
Unsure if the two flicker at the same time, or which is flickering while the crackles come through the speaker.
Narrow down a starting point:
Gain knob causes crackling as adjusted from lowest setting to 1/4-turn up from lowest: too obnoxious to handle.
Turned everything off and tried a different wall outlet: slightly better, but essentially the same result.

Try to find schematic and pics of the internals, maybe some troubleshooting help, had very little luck (except DoktorSewage.com and this thread - with, I suspect, DoktorSewage on it).

From the pics on DoktorSewage’s page, I can tell there are a few differences between my amp’s guts and his, e.g. the power cable comes in from underneath on mine; his, in the back. Mine has a blue trim stripe at the bottom, where the ‘input,’ ‘gain,’ ‘effects loop,’ etc... labels are. I’ve tried to narrow down the year it was made, but the best info I’ve got is from the (?) serial number (?) on the back [20H-1073], and the assumption that part of the pot part numbers includes the year they were made [“8720,” “8719,” maybe 20th week of 1987...???...]. But I’m really not sure about a lot of this amp, except I know it was great once, played well for me for a while, and may be able to be brought back to life.

Thanks for reading through this - it was more than I expected to write. I hope we can get crackin’ and stop that cracklin’. I’ll post more info, and some pics if/when I hear back.

Thanks,
Dave
Title: Re: Fried Ampeg SVT-200T
Post by: phatt on November 01, 2018, 07:53:18 AM
Try plugging the guitar into the *Effects Return* if the crackle stops and the signal sounds clear then the Gain pot is likely stuffed.
You will have a bit less volume but the rest of the front panel controls should all function normally.

A common issue is FX loop switches.
The switch is built into the socket and with older gear that contact corrodes and fails to pass signal.

The schematic that *g1* kindly posted shows 2 fail points, the FX loop and there is also the *Line in/Line out* which is the same setup.

If you suspect fx loop problems then use 2 short patch cables and bridge those 2 loops if the amp then works fine you likely have an easy fix.
Phil.
Title: Re: Fried Ampeg SVT-200T
Post by: Enzo on November 01, 2018, 09:44:45 AM
And really, it would be so much better to start a new thread for your amp instead of adding on the end of a thread that has been dead for five years.
Title: Re: Fried Ampeg SVT-200T
Post by: Used.Benz.Parts on November 02, 2018, 07:56:50 AM
Wow! Thanks for the quick replies!

Phil, I’ll try your suggestion when I get a chance in the next few days.

Enzo, you’re right, a new thread would have been a better idea (given how old this one is). But I’ve found so little info on this amp, I was trying to write to people well versed in it and get their attention.

I’m any case, I’ll post results once I get it tested and we’ll go from there.

Thanks,
Dave
Title: Re: Fried Ampeg SVT-200T
Post by: Used.Benz.Parts on November 08, 2018, 03:25:44 PM
Well, plugging into the FX loop had the same effect: crackling and popping until it just stopped. Nothing from the speaker after that. Except for a quiet hum - but you’ve gotta put your ear about 2-3 inches from the grille.

I’ve got the case open, amp sitting on a plastic crate. A closer look showed that there are globs of solder at 3 different resistors and 2 pair of the resistors are in parallel ...pretty sure they aren’t supposed to be that way because of the equivalent resistance and the solder is on the wrong side of the PCB [see pics, if they attach]. Also, are the transistors supposed to have screws through them? [Phillips heads, last pic.] I installed the one with the larger washer on it because it was either missing, or shouldn’t be there.

If I understand correctly, the corrosion failure in the FX loop switches MAY be contributing to this issue. If so, cleaning the corrosion off should allow a signal to pass through them. Yes?

I’m assuming that they were right on the verge of not passing a signal and by inserting the cable, the contact area may now be fouled completely.

—> I’ll get that board out and try to clean them as best I can. Granted I’m assuming a few things, and we all know what happens when assumptions are made, but it’s one place to start.

More details as it evolves.

Thanks!
Title: Re: Fried Ampeg SVT-200T
Post by: phatt on November 09, 2018, 06:38:22 AM

YES, If the contacts in those switching sockets are dirty it WILL cause crackling. 8|

You need to bridge *BOTH*,, cable from *FX loop out to FX loop in*,,,
as well as another cable from *Line out to line in*.
Both could be faulty.

The switch is on the *return* and *line in* sockets only so get some electrical cleaner spray and give them a good blast, Also you may want to slide the plug in and out a few times to get contact again. if it's crackling try gently wiggling and reinserting the plug,, see if you can get the signal to play clean.

If you have access to those sockets you can bridge the terminals right on the sockets.

Regards those extra resistors soldered on the top side;
Someone has modified the circuit it's often easier to add a new value to the top side,, saves pulling the whole amp apart you just add the mod on top,, no big deal and I doubt it would cause the problem you describe.
You need to establish that those 2 loop switches are working or not FIRST,,, then you move on to other things.
Those 2 TO220 transistors don't need the screws.
Phil.
Title: Re: Fried Ampeg SVT-200T
Post by: Used.Benz.Parts on December 07, 2018, 02:57:06 PM
Hi again,
Had some free time this morning, so I got the PCB (with the pots and inputs on it) out of the case and ohmed out the 0dB, -15dB, Effects Send, and Return jacks ... and they were all high resistance due to corrosion on the contacts.
Got ‘em cleaned up with a small file and contact cleaner, checked the tension on ‘em with the patch cable (making sure they opened and closed - several times), and rechecked the resistance: 0.40 ohms on ‘em all.
Put it all back together...

It works. Still a little crackle from the Squier due to patch cord contact at the output of that bass, but the Dean sound comes through nice and clear.

Guessing the years of use before I got my hands on it was the culprit.

Thank you guys so much for your help. I really didn’t know where to start, but now she’s up n running and I’m happy.

Thanks!
Title: Re: Fried Ampeg SVT-200T
Post by: 66cccfff on February 08, 2019, 05:27:46 AM
toast resistors, fried amps, fused relays......Why're geeks always speaking so FuNnY?
In my opinion the problem might be caused by neutral wire breakdown in the club , which could lead to a 380-volt high voltage on mains.
It's tragic but, even switching power supply modules can't handle such a CRAPPY voltage so always use a surge protecting extension cord