Hi all,
Thanks for stopping by this post, I hope you find it interesting and can offer me some advice to get this bad boy working.
It was working a year ago and it's not been played since, turning it on this week and it produces a loud hum in the 4x12 cab or headphones.
The schematics are attached and I have worked my way through them to try and track down the problem, without the aid of signal tracers etc..
The guitar plugs in and in clean channel 1 you can hear nothing but the hum. in channel 2 (Warp) the only time you hear guitar is when maxed out on the Gain and Master.
If phones are in FX Send socket it sounds like the preamp and all pots and channel switching works, you can hear the effects of the pots etc so I think it must be post-preamp and in the poweramp section somewhere.
I've checked power supply for all voltages as per page 3, all good. page 2 bottom, right to left, all voltages are spot on too.
I've removed lots of the resistors and capacitors too to check them
The issue appears to be in the poweramp stage so I changed the two large power transistors T5 (TIP142) and T6 (TIP147), checking the removed ones with a DMM on diode setting shows there are no issues. I changed them for some off the web, but they were smaller in size and when I powered up with these and then R66 released some magic smoke and then the transistors no longer checked out correctly.
I put the original TIP's in and replaced the 39ohm R66 and I'm still having the same issue.
Can anyone see anything obvious that I'm missing? I've checked for dry joints and have tested all resistors, some caps, some diodes but I think I need advice now on where to narrow my focus into from people such as yourselves who might recognise the symptoms and immediately have a suspect component.
Any help will be massively appreciated and I thank you in advance for your time and support.
Looking at the Power transistors a little more closely, I think I substituted the TIP147 and TIP142 for a TIP147T and TIP142T, which look smaller, have a different hfe (gain?. I wonder then, if that's why R66 popped. Should I get some transistors without the T suffix?
The originals did check out with the diode tester on my multimeter though. Weird.
Don't use the T suffix, they are lower power spec. and not suitable. Not sure why you changed them, but now there may be an added fault. This is why it's best to isolate the problem first rather than just replacing parts.
Is R66 still burning? If not, try your guitar into the FX return.
G1, thanks for the fast reply.
In truth, and ashamed to admit it but I didn't realise they were different (i.e. TIP147T) until after R66 went. After replacing R66 and refitting the original TIP142 and TIP 147 it's gone back to the original state.
I put the guitar into the FX Return and heard nothing but humming. If I put my iphone on max volume into FX Return using a cable and 3.5mm to 1/4 jack I can just about hear the music but it doesn't alter with Master Vol ( i didn't expect it to either)
I've attached a sound file recorded from my headphones by my iphone so it's not great quality but it should give you an idea of the sounds it's making.
It starts with Chan 2, max gain and volume producing humming and crackling interference, with guitar being played on. then second half is clean channel with high e string and how it cuts off, then back to chan 2 again at the end.
It sounds something like there's not enough signal to drive the poweramp stage unless you turn guitar all way up and hit that string as hard as you can, then when the string slows the sound cuts off.
Check T8 BF245 & D24 the slow start circuit. Sounds like T8 is on all the time.
Thanks Tony, I'll give that a go this evening and report back.
From my understanding T8 is a JFET Transistor so I'll google how to test one of those with my DMM.
Thanks again.
Ok, so I had the jfet out, T8 (BF245B) and I did the 2 multimeter test on it.
one multimeter across Source and Drain on Resistance showing 4-6 Mohm
other on Diode setting across the Gate and Source to turn on the channel, it dropped to a few ohms, not 0 ohms but a few.
with the second dmm reverse polarity and it didn't turn on the channel, just stayed as it was, 4-6Mohm
I also had D24 IN4007 Diode out and tested that with diode checker, it checked out fine..
Then I checked C37 as I hadn't tested that, that checked out too. 4.7 uf
Only thing I haven't changed, but I do have some are the BC5468's T1 and T2, worth a try?
Thanks
I'm a little iffy about that T8 testing or your identification of the terminals?
T8 drain to source resistance should be low (on) until it gets it's turn off voltage applied to the gate.
Maybe just try the amp with T8 removed. If you get sound you know the problem is either T8 or it's not getting it's gate voltage.
Cool, Thanks g1. I've removed the jfet T8 and now the hum has stopped but the plucking a string gives a loud sound and is then cut off almost immediately. It's the same on both channels, if I do it on channel 2 and give the low E a pluck it is deafening and then instantly cuts off to a lower level of sound.
Should I get another JFET to go in there or is it something like R45, C37, R44, D24 or R43 which I have already checked and all appear to be ok?
Thanks again for all of your assistance.
Bob
The T8 fet is just a mute for the turn on thump. Leave it out until the rest of the amp is fixed, then you can go back to the mute circuit.
What happens now when you go into the FX return jack?
g1, you're a star for helping me through this. Thanks.
With the guitar into the FX Return, I can hear it, but it's very faint.
When I plug into the input and use clean channel I can hear the guitar build up volume as I turn up volume but it's still quieter than I imagine it should be.
Once the volume pot goes so far, the drive of the string picks jumps in volume to something really loud and distorted and clippy. When I come back down the volume it goes from noisy, clippy and distorted to clean and quiet.
I've added a sound clip of a mic in my headphones, I can't do it from the 4x12 cab as the sudden burst of power makes the whole family jump, even if they're in the other rooms!!
I'm not sure what else it can be, almost every other component in the Poweramp section has been taken out and tested or tested in place. I'e also attached a photo of the circuit that I've been using to identify stuff and everything highlighted has been tested to the best of my ability.
Thanks for your help.
Bob
Well, there are only 6 transistors in the actual power amp section, so how about some static DC voltage checks (no signal applied).
Base, collector, emitter, for T1 thru T6.
g1, static DC voltage checks, not sure if it's what you meant but I switched on the amp with nothing in the input and measured DC voltage at each base, collector and emitter to ground.
The results are attached in the PDF with the voltages written next to each leg of the transistor.
I've stuck the img in here too to save opening up the pdf.
(http://www.arkham.uk/onlineimgs/warp7voltagesontransistors.png)
Thanks again, Bob
There is something seriously amiss with T3's voltages.
You have 44Vdc on one side of R53 (1K) and 43Vdc on the other.
That is to be expected.
But 43Vdc through R57 (100) seems amiss at 130Mvs.
I would check R57 and T3.
Thanks Jazz. I'll double check it after work to make sure I've not got my pin outs wrong. I was working on the underside of the board and getting confused as to which was e,c or b. I think I got it right but will double check.
The resistor I've had out of circuit and it read ok but I'll chexk it again too.
I'll update later today. Thanks again.
Bob
Hey,
That voltage was indeed a bit weird. I've not got confused with the BCE pinout but the voltages are different than I measured the other day.
T3 Base is 44v, Collector at 44v and Emitter at 2.5v
Not sure why that happened unless I hadn't grounded the meter lead properly or the probe was not touching the solder.
Doesn't help though really does it I guess?
This is how it stands now.
(http://www.arkham.uk/onlineimgs/warp7t3voltagecheck.JPG)
Did you put T8 back in?
You were supposed to leave it out.
No I left it out and took voltage readings on the connections where it should be.
Not putting it back until the main issue is resolved.
Don't know what the next move is though.
Thanks all, for your advice and guidance.
I'm not the expert but from my experience;
The base of T3 would not be at Zero as there is always some slight DC imbalance there, maybe 50mV or so. Just a thought?
It may have already been mentioned but have you checked the Speaker output for DC volts?
Phil.
Quote from: batbob on March 07, 2018, 02:34:53 AM
No I left it out and took voltage readings on the connections where it should be.
Not putting it back until the main issue is resolved.
Excellent, we can use those numbers later.
For now, most of those DC voltages look good except T3. Can you measure from E to B (one probe on each leg). We are looking for about 0.6V difference between the two.
Please also recheck T5 and T6 voltages, I think B & E are mixed up, and T6 should be -1.1V, not +.
Phil,
I rechecked the voltages around T3 and there must have been an issue with me not connecting correctly. The Collector was 2.5v with Base and Emitter both at 44 v
g1, I measure the voltages on T3 Emitter-Base whilst the amp was on and it was about right, at 0.530v.
I did the diode check on the DMM on T3 too to see if that identified anything, there was an oddity in the Collector to Emitter of 1.75v (whilst amp was switched OFF)
The rest were (- means black lead and + means red lead on my Fluke 79III)
B- > C+ =0.53v
B- > E+ = 0.53v
B+ > C- = OL
B+ > E- =OL
C+ > E- = 0.53v
C- > E+ = 1.75v
I also rechecked T5 and T6, traced the circuit to make sure I was right on the E,C and B.
The measurements with the amp on are consistent with the above post apart from the emitter on T6 should have been -1.1v (you're spot on), with the amp off E>B drop of each is 1.1v
There's 30mv DC on the speaker output.
This has me foxed....
Quote from: batbob on March 07, 2018, 06:33:19 PM
g1, I measure the voltages on T3 Emitter-Base whilst the amp was on and it was about right, at 0.530v.
I did the diode check on the DMM on T3 too to see if that identified anything, there was an oddity in the Collector to Emitter of 1.75v (whilst amp was switched OFF)
The rest were (- means black lead and + means red lead on my Fluke 79III)
B- > C+ =0.53v
B- > E+ = 0.53v
B+ > C- = OL
B+ > E- =OL
C+ > E- = 0.53v
C- > E+ = 1.75v
I'm sorry but I'm getting confused as to what readings are amp on, which are amp off, and whether the diode checks are with transistor in or out of circuit.
The leads of T3 are E,C,B (left to right) looking at the printed face.
If there is .53V from B to E running, then one should be around 44V and the other around 43.5V
Please check again the exact DC voltage for the three terminals of T3 with amp running.
Quote from: phatt on March 07, 2018, 08:01:08 AM
I'm not the expert but from my experience;
The base of T3 would not be at Zero as there is always some slight DC imbalance there, maybe 50mV or so. Just a thought?
It may have already been mentioned but have you checked the Speaker output for DC volts?
Phil.
Opp's sorry meant to say
base of T2 :duh:-[
@ G1, Q, with only 30mV's on the output,,,,is it possible the amp might be working but with a cracked trace on the output? Or even a frayed speaker wire/cold solder joint.
Reason I say,, *Batbob* mentioned it breaks through on big signal.
I've had that happen and it sends you round the twist. :grr
Phil.
Cool. I'll get on those further tests when I get home from work.
With regards to whether readings were taken in circuit and amps on/off
The amp on was t3 e-b .53
The other voltages in that post are t3's readings in circuit and amp off.
I'll also inspect the tracks from output backwards to see if I can spot a crack or dry joint
Thanks all for your help.
Whereabouts are you guys based?
Quote from: g1 on March 07, 2018, 01:33:24 PM
Quote from: batbob on March 07, 2018, 02:34:53 AM
No I left it out and took voltage readings on the connections where it should be.
Not putting it back until the main issue is resolved.
Excellent, we can use those numbers later.
For now, most of those DC voltages look good except T3. Can you measure from E to B (one probe on each leg). We are looking for about 0.6V difference between the two.
I agree, adding the amp would have to be ON otherwise readings won't be valid.
Only saying that because *batbob* mentions power off readings as well as on readings which makes no sense to me. Or have I missed something? xP
Maybe probes are not fully contacting giving false readings?
@ *batbob* if that test fails them T3 has an issue.
If T3 checks out ok then Amp might be working but you likely have an intermittent connection.
One common issue is the speaker braids, the 2 flexible wires that go from speaker terminals across to the speaker cone. They often fail by fraying right at the solder point.
It is often fixable.
Phil.
Thanks Phil I'll take a look later.
In terms of output the sound is the same through the headphones or the speaker cab which is a seperate 4x12.
I'll take a look when I finish work and get home. Thanks for the help.
Well if the Headphones cut the speakers off then that Hphone socket is a prime suspect for intermittent operation. :trouble
If it's one of those plastic encapsulated types that are pcb mounted then look for hairline solder cracks around the pins.
Phil.
Batbob, I'm in Canada. (rural southern Manitoba)
Sorry if we missed trying the headphone output, good catch by Phil.
Phil: Agree that it is probably some bad connection bursting through when sufficient signal is applied. But, first I wanted to be very sure it was not that T3 is turned off and is only turning on when big signal applied to it. Seeing 44V listed for both base and emitter, I'd like to know the exact voltages there. The .53V 'across' was listed for both power on and off conditions, so I was confused about which it was and would still like to see the exact numbers with power on.
Cool stuff, Canada eh? Thanks for taking the time you guys to help me out.
I've rechecked voltages on T3 with amp on and no input, double and triple checked pinouts.
T3 Emitter > Base was 511mv (.511v)
E>C was 41.3v
C>B was 40.9v
This sounds like the T3 was behaving normally.
T2 Base = 0.006v, Collector 44v, Emitter -600mv
With the headphones in it breaks through at higher signals (higher gain & volume)
With the 4x12 Cabinet plugged in it also does the same thing.
With the talk of dry joints and other broken joints I'm just going to heat up every single connection on the board to ensure there are none left.
The pinouts I've been working from have been scavenged from the web and are as follows:
(http://www.arkham.uk/onlineimgs/warp7tranisistorpinouts.JPG)
Thanks again.
Hmm, I've just re-soldered every single connection on the board, there doesn't look to be any cracked tracks that I can see anywhere across the board.
The only components I've not changed in the poweramp stage are T1 and T2 but they don't appear to have too much wrong with them looking at the voltages.
I've plugged a battery powered radio into the input of the amp and I can hear it faintly through the phones. When I turn it up and lift gain and master over halfway it crackles so the louder bass notes trigger the amplification but the quieter notes stay quiet. It feels like something isn't getting enough signal to start working but I don't know what or where...
If I max out the volume on the radio and then lift the gain and master up on channel two it amplifies everything but it's obviously mega distorted and way too much for the headphones. I can't plug it into the 4x12 as it's midnight here and my wife would be rearranging my genitalia if I started making loud noises like that. It's the same as the other day though when the same effects were witnessed on the head into the headphones and then head into the 4x12.
The next step will be applying a constant tone to the FX return and tracing the signal to see where it is getting lost.
Do you have something like a tone generator? Like a phone app or computer app will work, or something like a portable keyboard with a key taped down.
And a signal tracer can be a simple practice amp with a capacitor probe to block DC (.1uF 200V or so should work) Or your meter set to AC volts with same capacitor probe.
But before that there is one more quirky reading that needs checking. T8 that is out of circuit right now, you show that there is -15V (or -7V) at the terminal that is supposed to be ground? That should not be. Is there a ground missing at the board?
Well well well, thanks for the great idea of constant tone tracing. I did look into this last week but figured I wouldn't have the kit to do it. I made a probe out of an old screwdriver, a 10uf 400v capacitor (nearest I had) and an old guitar cable. I downloaded a tone generator app for my iphone and plugged it in. I connected the "probe" to my old Park 10w practice amp, not my 800w Line 6 L2T and started playing.
I found that the signal seemed quiet at the speaker out and phones out without too much bother but not amplified at all. I guessed this meant that it was further back in the circuit. Everywhere I probed sounded the same, on the Base and Emitters of T5 and T6 then back further into T1 & T2, I checked at R51 which is the first component after the "To The Poweramp" section change.
I knew that the signal was good until the FX Send as it sounded good at that point so I looked between the FX Return and the "To Poweramp" C16 still sounded clear and the same at Pin 3 of IC5 (op amp chip MC33078P). Changing the voltage on Clean and Warp channels to this point sounded nice.
Probing Pin 1 of IC5 showed up a quieter signal, weird. I turned up the volume and found that it was quiet and then crackling and noisy as per the output on the phones and speaker out.
(http://www.arkham.uk/onlineimgs/ic5issue.jpg)
Checking pins 8 and 4 for voltage gives me a 16.8v on 8 and -15.5 on 4 which is in the operating voltages for this IC. (+- 18v)
Checked Resistors R35 and R36 both read 10k so it might not be a grounding issue but actually an issue with the IC itself.
I went and checked the same IC5 which has pins 5,6 &7 connected for the op amp after Channel 2's Treble and Presence Pots, signal sounded good on all three pins so I think that it's definitely the IC internals that are used for pins 2, 3 and 1
Now I've ordered a replacement for this IC (and two spares), I can't help wonder if it'll blow again once I put it back.
Can you see a reason why T8 out stops Hum if the op amp in IC5 is knackered?
Does the problem in IC5 mean that it's the issue and not related to T8, remember taking out T8 stopped a loud hum but didn't alter main issue of signal breaking through at high gain.
(http://www.arkham.uk/onlineimgs/IMG_2582.JPG)
Thanks again for all of your help with this, I think we've nearly got this cracked.
So, forgive my thinking on this, I might be 100% incorrect. I was worried about the fact that if I replace IC5 it might blow again.
I checked the voltages around IC5 inputs.
for earlier stage for Channel 1 treble/presence I found that the + input was .203v and the - input was .226v, output was .226v. All seems legit.
I checked the input voltages for the problematic op amp and the + input was -14.23v and the -input was -7.75v, output was -15.6v, looks dodgy to me, not sure why though.
I measured voltages back from the +input and it was -14.4 that side of C16 (a 220nf 63v cap), the other side where it went off to fx loop and R46 was 0v. I can't find why it would ground to 0v so now I suspect that C16 is knackered, when I capacitor test it with my DMM it shows as 230nf so it looks ok.
Just as a double check, I think that this is normal behaviour to block the DC but I wanted to check with you guys and girls that this looks normal.
If you think that the op amp IC replacement will work then I'm happy. Just worried about these weird voltages......
(http://www.arkham.uk/onlineimgs/IC5voltages.jpg)
To block DC and or to separate 2 different DC circuits.
jason
Pin #3 is an input pin.
It can't possibly have +14 Vdc on it as shown.
I vote that the ic is toast.
To prove pin #3, remove the ic and see that the pad on the pcb reads 0 volts.
Agree with Jazz, remove the bad IC and check the pads.
Please also see the end of reply #31 above and double check the empty pads for T8.
g1, regarding post #31, I've just been testing that and it appears that I had my drain and source mixed up. G=-6.9v, D=-15.6v and S=0v.
For some reason I'd watched a video on JFETS and they had the source as the top and drain as bottom on the diagram. I see where I went wrong now.. Sorry
Jazz and g1, 14v sounded wrong to me, I'll get unsoldering and let you know.
Thanks,
Bob
IC5 removed and pad voltages are all 0v apart from 4 (-15.8) and 8 (16.3v)
I guess that this means it was goosed and the weird voltages were a symptom of this.
To move forward in the circuit I went back to the drain of T8 pad, that was -15.6v before. It reads 0v now so I guess that the IC was sticking some strange voltages across T8, causing the hum too.
If it is the IC as we suspect, what explains the sudden gain achieved when the signals were large? Is there any logic in that effect happening like that? I suppose that it could be the inside of the op amp starting to function for some reason...
Thanks,
Bob
Thanks to you all from r your help so far. I'll be receiving my new IC on Wednesday hopefully and I'll let you know how I get on.
I'll put the ic in and try it before I put TC8 back in the circuit too.
Thanks again
Well, it's a big thanks to you all, g1, Jason, Jazz,Phil and Tony for all of your help.
After the IC arrived today I soldered it back into the board and then tried it, and wouldn't you know it, all worked!
I banged in the T8 jfet too and it was all still working superbly. :dbtu:
It's all back together and plugged into the H&K 4x12 and sounds good, but I can't test it at the high end yet as the family were watching tv earlier and now I've just come back in from shooting its 11:30 at night!
Many many thanks again and I'll be hanging around on here from now on to see what other stuff people get up to, and look at trying some of those cool electronics/home brew pedals projects.
I've added a couple of chip points to you all as we've been going through this but it's a very little gesture compared to my gratitude.
May you all have a superb 2018 and I'll hopefully see you all on here again.
Great to hear you got it working,,, enjoy :dbtu:
As to why / how the chip failed but still half worked,,, well same way a bad battery terminal connection will refuse to start a car,, try a couple more times and it suddenly turns the starter motor.
If you frig around with electronics long enough you will come to understand that up to 90% of most electrical breakdowns are due to bad connections.
whether it be a rusty RCA plug or a failing connection on a silicon wafer inside a chip the result is the same.
I remember a motorcycle rider asking why his self cancelling indicators were not working?
I replied saying; It's electrical mate, like all electrical circuits they eventually self cancel. ;)
Resolved by cleaning the contacts. 8)
ramble over,,
Phil.
Cool, thanks Phil. You're a gent. I'll bear that in mind in future with any other projects I take a look at.
Regards,
Bob
Bollox.
Just got it up to half vol for a bit of Muse Psycho and it stopped working, 500ma fuse in rear has gone. I'll check out why it's gone and get back..
Bob
OK don't panic,,,Q What channel was engaged when it blew?
Phil.
It was on WARP when it went. It was working earlier when I had the volume a bit lower. Turned it up to halfway had a play on clean and then switched to warp then got about a minute into A muse song then it went off.
Maybe pop in a new fuse and plug into the *Line in* to check if the power stage is still working.
I'd be inserting a radio signal or similar into Line input to check the power stage is stable and taking voltage readings as you turn up the volume.
I just use the headphone output of and old radio cassette for that stuff.
Now although the preamp is not in use while using the line input the *Warp select* does engage the CS resistor R75. (CS stands for Current Sense) which increases the gain of the power amp.
Power amp gain is changed via *slc2* (little box next to R37)
There is the possibility that *IF* R75 was shorted the amp would run fine on clean but pull too much current in warp mode.
Although R75 is only .33 Ohm you might think it would make little difference but if the speaker negative terminal is accidentally grounded to case or similar situation then it would quite likely blow the fuse because the warp channel also switches on R75 turning up the power stage gain. (I'm guessing by quite a bit)
Regards checking voltages, these are shown at bottom of sheet 2.
That 16 Volts node between R50 and R67 can't be right,, should read more like 18~19 volts. If it was 16 Volts then D20 and D21 would not be able to regulate the 15 volt rails. Zener diodes need 2~3 volts above the zener voltage to work properly.
Other point that confused me for a while, It seems that IC4 is marked wrong way round.
Those 3 *slc hi/lo boxes* the pins are likely correct but looks like all 3 are marked wrong way round.
IF the CS resistor is shorted then it will be at ground in both clean and warp mode.
Checking low value resistors can be a trick but I just self test first.
Switch to low Ohms,, short the leads scrape them together till you get a stable reading,, jot that down ,,NOW hard press either side of R75 and deduct the shorted lead reading will get you close. If it reads .1 Ohm then go hunting for the short.
Check the Speaker neg wire that runs from pin 6 of J5 (HP socket) does it ground back to the case anywhere?
There may well be other problems but that would be a possible reason for what is happening.
I once had a CS resistor shorted out via a badly replaced speaker socket which I accidentally grounded and your problem reminds me of that stuff up.
Amp worked ok until you crank it up then poof,, no sound.
Phil.
Quote from: batbob on March 15, 2018, 04:13:17 PM
500ma fuse in rear has gone. I'll check out why it's gone and get back..
Bob
You are on 230V mains?
Did you at any point adjust the bias trimmer when you were working on the output transistors?
Should probably check the idle current anyway, what is voltage (DCmV, no signal) across the 5W emitter resistor, R69? R70?
Thanks g1 and Phatt, We are on 230v, I didn't adjust bias trimmer.
I can't test anything really as the fuse went in the rear of the unit, 500mA. I changed it and then the 2 inside 2000mA went too. I've rigged up a current limiting lamp in the middle of the power lead and it's at it's brightest.
When I put an ordinary cable in, it blows the 500mA fuse in the rear. Even when the leads from the Transformer are disconnected from the board.
I think that the transformer is a dead 'un for some reason.
I've measured resistance across the primary at 22 ohms, as it's a 0.44A primary at 230v, I reckon it should be 522ohms.
The secondaries are 0.4 ohms and 0.5ohms (0.9 across both). Sounds like the transformer is now a deceased transformer.
I'll have to take a look around for a 230v 0.44 primary, 28-0-28 1.5A Secondaries transformer.
I'll let you know what happens from there. This might be the point I stop spending on it to be brutally honest.....
Thanks for your help too.
Regards,
Bob
Ok, I unsoldered the connections from the transformer and I'm getting the right voltages out, so it's not knacked.
I removed R47, R48 and R43 to try and narrow it down and then remembered that the +40 and -40 rails come off before that so then I started looking at what could short out in the poweramp stage again and tracked it down to transistor T6, a TIP 147 that I thought was buggered originally but seemed to be working ok once the IC had been changed.
I've checked it in circuit and saw a short between emitter and collector, thinking it might be something in the circuit other than T6 I unsoldered it to check it, it's a dead short on the desk so I'll need another TIP147 by the look of it.
I don't know if this is the last fault with it but I may try to find a decent TIP147 and bang it in. That will deffo be the last attempt. I'm reluctant to spend any more on it...
Anyone have any good contacts for TIP147's?
Oh dear,, I'd just replace T3,T5 and T6 as they are likely stressed and could cause another failure.
Fire it up again (with limiter) check all is well then remove limiter and check bias as per *G1* posted above.
Phil.
Thanks Phil, you may be right.
What's everyones thoughts on replacing the TIP 147/142's with TIP 147T/142T's?
I know early on someone said I shouldn't do that, but looking into the spec sheets, they're the same, except for Package Type.
I ask this as I can't find any TIP147's and TIP142's that are the same size as these two in the amp without paying through the nose and waiting for too long..
Cheers,
Bob
The data sheet I'm reading tells me that they are different.
The TIP147T is only 90Watt while TIP147 is 125Watt. :-\
Try searching for a power Darlington with the same specs or higher should work.
100V, 10A, 100W Darlington, but check pin out.
T3 is a BD238
which is PNP 100V, 2A, 25W unit, again anything with those ratings or higher should work.
The main specs to look for;
Max Collector Voltage, Volts
Max Collector Current, Amps
Max Collector Power dissipation, Watts
I notice that Onsemi.com data sheet for TIP142/TIP147 states that after 2012 they are changing package to TO-247 which have same pinout which would fit. :tu:
Phil.
Yes, use the TO-247 like phatt suggests.
Cheap and readily available at Farnell or RS components, or other UK suppliers.
Not sure if you will need different mica insulators to fit, but they will be cheap as well.
The smaller package "T" are lower power and not suitable (they will burn up).
Parts ordered from CPC, now awaiting the TIP147,TIP142 and BD238.
I'll let you know how it goes.
I was looking at the TIP147 and 142 with the new package but thought that they'd be a similar problem as the TIP147T, now I've been through and checked the ratings I see why they're ok.
On Farnell the 147 had an hfe of 1000 and the 142 had an hfe of 500. The knacked transistor has a 500 so I bought two the same, I figured that it's not good to have two different gains through the circuit...
Cheers, Bob
hfe spec. is dependent on test conditions.
Farnell was a bit sloppy there using the 2 different specs.
You can see from page 3 of the attached spec sheet that both specs are valid for both parts, but under different test conditions.
Tip142 & 147 are a complimentary pair, so you should be fine if both have the same suffix (package type).
Hi all, got the two TIP's (142 and 147) all soldered in and working on the headphones like a good 'un.
Just as a final check to make sure I don't blow it again tomorrow after work when I thrash it a bit, is there anything about the connection to the Cab that could cause an issue?
I've measured resistance at the in and out of the cab, both 7.1 ohms, the Amp is an 4-8amp output so I don't see there being an issue.
Just wondered if there was anything I should look out for, someone mentioned DC on the output. I havent checked it yet but is there an acceptable level or should it be 0v dc?
Thanks for all of your guidance and support!
Bob
YES always check the DC after heart surgery or expect,,:trouble :trouble.
The basic concept is that it's a DC Amp (Directly coupled, No decoupling caps) which means each section passes it's DC offset onto the next.
Because it's split supply then the output should be very close to Zero volts.
But these things are never perfect so there is always some slight DC offset at the output. +/- 50mV of DC is a respectable number for such a basic amp.
If the whole circuit is balanced well then you should see ~ 50mv DC at T1 Base, T2 Base, and speaker output. don't expect all 3 to be exact,, that's just a ball park number for you.
My concern for the unit was WHY did it burn out in the first place but now that I've re read page one I see it was you who inserted the wrong transistor which burnt out R66. ;)
So I assume there was no fault in the power amp stage only the dead IC5.
But anytime you replace components (especially on high current power stages) you always check the DC conditions are still stable.
That is how it works; You set the DC conditions so that the AC signal can pass and be amplified to the maximum ability of the DC potentials that are available from the components used.
If any DC part is not right then something will burn out.
Phil.
Thanks Phil, that's an ace answer and easily understandable.
I'll check the DC on the output tonight when I get home.
If it is out, I'm guessing its the trimmer pot that would affect the Bias?
Thanks and I'll keep you informed as to whether it blows again!.
Cheers,
Bob
Quote from: g1 on March 16, 2018, 01:56:28 PM
Quote from: batbob on March 15, 2018, 04:13:17 PM
500ma fuse in rear has gone. I'll check out why it's gone and get back..
Bob
You are on 230V mains?
Did you at any point adjust the bias trimmer when you were working on the output transistors?
Should probably check the idle current anyway, what is voltage (DCmV, no signal) across the 5W emitter resistor, R69? R70?
Hey *batbob* don't forget this post from *G1* 8|
Phil.
I measured the DC on the speaker out and it was under 10mV so I'm happy that it's all behaving nicely now.
It sounds awesome turned up loud again, instead of the humming and crackling I was getting.
Many many thanks for your help and support.
I've added some chip points for each of you that have helped as I appreciate your assistance a great deal.
Cheers, from a slightly deafened, Bob
Great to hear,
Hey you got it fixed and learned a lot in the process. :dbtu:
Phil.