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Author Topic: Fried Ampeg SVT-200T  (Read 15653 times)

Roly

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Re: Fried Ampeg SVT-200T
« Reply #30 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.
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: Fried Ampeg SVT-200T
« Reply #31 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

Enzo

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Re: Fried Ampeg SVT-200T
« Reply #32 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.

Roly

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Re: Fried Ampeg SVT-200T
« Reply #33 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.
If you say theory and practice don't agree you haven't applied enough theory.

J M Fahey

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Re: Fried Ampeg SVT-200T
« Reply #34 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.

Roly

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Re: Fried Ampeg SVT-200T
« Reply #35 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

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
If you say theory and practice don't agree you haven't applied enough theory.

sewage666

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Re: Fried Ampeg SVT-200T
« Reply #36 on: July 17, 2012, 01:32:29 PM »
Update:

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



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.

Enzo

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Re: Fried Ampeg SVT-200T
« Reply #37 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.

sewage666

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Re: Fried Ampeg SVT-200T
« Reply #38 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.

sewage666

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Re: Fried Ampeg SVT-200T
« Reply #39 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.

Roly

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Re: Fried Ampeg SVT-200T
« Reply #40 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".   ;)
If you say theory and practice don't agree you haven't applied enough theory.

theonewhohowls

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Re: Fried Ampeg SVT-200T
« Reply #41 on: September 10, 2013, 10:37:03 PM »
Do any of you guys have the schematic for the amp?

Enzo

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Re: Fried Ampeg SVT-200T
« Reply #42 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.

g1

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Re: Fried Ampeg SVT-200T
« Reply #43 on: September 11, 2013, 04:02:40 PM »
Here is the complete schematic

Used.Benz.Parts

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Re: Fried Ampeg SVT-200T
« Reply #44 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

 

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