Amp keeps blowing reverb driver chips

[wizard333]

This is a Koch Twintone 2 that I modded for someone about 6 years ago. Came back this week with complaint of a few things, including "verb not working". He took it to some local nut who destroyed some circuit traces and switching transistors through slop and a too big/hot iron. After I fixed all that, including hand wiring parts where he killed the traces, it worked fine, for about an hour.

It keeps blowing the verb driver chips bd135/136. I checked voltages, circuit traces, etc, all seems fine. The chips get REALLY hot before blowing (emitter/collecter short). There isn't much room given where they are mounted to add heat sinks. Luckily I socketed them when fixing all the other BS because they were blown when the amp showed up and I didn't want to keep having to pull the board if it kept happening while I tracked it down (and it has kept happening).

Other than the heat, I can't see a reason they keep dying. Electrolytics that feed them are fine, pos/neg rails are showing the right voltages (nominally). Not sure why this is an issue on this amp either; I own 2 of these and have modded a lot of others. None had this issue. When I say they get hot, I mean almost too hot to touch, and after only a few minutes. I could try to mount a heat sink (or part of one as a full one won't fit), but it seems an odd issue as I don't see a reason they should be committing thermal suicide.

Any ideas? Page 2 and 5 show the reverb and power supply schematics, respectively.

Koch Twintone-II schematics 1434-3070.pdf

[nickb]

This is a Koch Twintone 2 that I modded for someone about 6 years ago. Came back this week with complaint of a few things, including "verb not working". He took it to some local nut who destroyed some circuit traces and switching transistors through slop and a too big/hot iron. After I fixed all that, including hand wiring parts where he killed the traces, it worked fine, for about an hour.

It keeps blowing the verb driver chips bd135/136. I checked voltages, circuit traces, etc, all seems fine. The chips get REALLY hot before blowing (emitter/collecter short). There isn't much room given where they are mounted to add heat sinks. Luckily I socketed them when fixing all the other BS because they were blown when the amp showed up and I didn't want to keep having to pull the board if it kept happening while I tracked it down (and it has kept happening).

Other than the heat, I can't see a reason they keep dying. Electrolytics that feed them are fine, pos/neg rails are showing the right voltages (nominally). Not sure why this is an issue on this amp either; I own 2 of these and have modded a lot of others. None had this issue. When I say they get hot, I mean almost too hot to touch, and after only a few minutes. I could try to mount a heat sink (or part of one as a full one won't fit), but it seems an odd issue as I don't see a reason they should be committing thermal suicide.

Any ideas? Page 2 and 5 show the reverb and power supply schematics, respectively.

[ATTACH]42506[/ATTACH]

1) If the driver chip IC3 is drawing too much current it will in turn cause both output transistors to turn on too hard and overheat. Has that chip been changed?
2) An error in the re-wiring will cause havoc.
3) Disconnect the reverb tank and see if they still heat up. If OK then check R52 is the right value and wired correctly.
4) It's possible that the specific transistors you are used just so happen to have a low Vbe. In that case the solution will be to reduce R49 & R51 to lower the bias current through the two transistors. My guess is the intent of the original design was to have very low bias current and deal with the crossover distortion with the massive negative feedback. It would be helpful to measure the bias current either by putting a meter in the emitter circuit or by measuring the voltage drop across a small (say 1 ohm) resistor added into the emitter. It would also me helpful to know the voltage drop across the 6.8K resistors without the transistors fitted.

[wizard333]

1) If the driver chip IC3 is drawing too much current it will in turn cause both output transistors to turn on too hard and overheat. Has that chip been changed? .

Well yes, but what would cause that on a functional, non-shorted op amp? Yes, it has been changed.

2) An error in the re-wiring will cause havoc.

No error.

3) Disconnect the reverb tank and see if they still heat up. If OK then check R52 is the right value and wired correctly.

4) It's possible that the specific transistors you are used just so happen to have a low Vbe. In that case the solution will be to reduce R49 & R51 to lower the bias current through the two transistors. My guess is the intent of the original design was to have very low bias current and deal with the crossover distortion with the massive negative feedback. It would be helpful to measure the bias current either by putting a meter in the emitter circuit or by measuring the voltage drop across a small (say 1 ohm) resistor added into the emitter. It would also me helpful to know the voltage drop across the 6.8K resistors without the transistors fitted.

R52 is correct and is of course wired correct, else the verb wouldn't work. I'll try it with tank disconnected, but during diagnosis it killed a few with the tank out. If the transistors I'm using have a low Vbe, then a whole lot of them have and the originals that died did as well, unlikely. I could lower 49 and 51, but that again begs the question, why? All the parts are wired correctly, measure w/in tolerance, and work in every other version of this amp I've seen. Should be a good reason why it's happening here and not elsewhere. I'll try a few things and report back.

[Enzo]

Your local nut may have replaced the IC. WHat IC did you find in there and what did you replace it with? And more important, the factory specs TL061. Note they used a TL071 for the recovery. The TL061 is a low power version of the op amp, it is often used in battery preamps inside acoustic guitars for better battery life. So if you have a TL071 or other single op amp in that position it could draw excess current through those transistors.

With no reverb pan, is ther any DC offset on the output?


yes there should be a good reason, and we like to figure thatout to point to the repair, but sometimes it only becomes clear in retrospect.

[nickb]

Well yes, but what would cause that on a functional, non-shorted op amp? Yes, it has been changed.

If it has been changed then it could be have a higher current draw. That will cause excess drop across the 6.8K resistors and blow up your transistors.

No error.


R52 is correct and is of course wired correct, else the verb wouldn't work.

The point was that if R52 were shorted due to a wiring error or was too low a value then you could get DC flowing through the tank and cause the problem and the verb would still work.

I'll try it with tank disconnected, but during diagnosis it killed a few with the tank out. If the transistors I'm using have a low Vbe, then a whole lot of them have and the originals that died did as well, unlikely. I could lower 49 and 51, but that again begs the question, why? All the parts are wired correctly, measure w/in tolerance, and work in every other version of this amp I've seen. Should be a good reason why it's happening here and not elsewhere. I'll try a few things and report back.

Disconnecting the tank will not cause them to blow. If it is a low Vbe issue then I would not be at all surprised that they all do it as that is a manufacturing process dependant so will come in batches, but I think it unlikely. I wanted to list as many reasons as possible to cover all the bases for you.

I think your likely culprit is IC3. What is fitted? Remove the transistors and measure the drop across the 6.8k resistors.

[J M Fahey]

1) Pull T1 and T2 from the board.

2) re re recheck IC3 is TL061 and nothing else.

3) measure voltage drop across R49 and R51

4) post results

[R.G.]

All of the advice is correct. Something is causing both T1 and T2 to be turned on with too much idle current. They both conduct and try to pull down the power supply. The power supply always wins, of course.

The advice is to track down what is causing the larger idle current. This particular circuit has been around for quite a while. I remember it from the 1970s, when IC opamps were new. The way it works is to use the two transistors as both current and voltage boosters after the opamp. Adding voltage gain to an IC opamp is a recipe for disaster, as most of them are only marginally stable anyway. Any additional gain pushes them over the edge and they oscillate.

This circuit works by forcing the internal current use of the opamp to turn on the external transistors. This adds more current to the load. Current from the opamp to pull the load positive comes through the V+ pin and goes out to the load. This current is sucked through R49 and the base of T1, letting T1 dump beta-times the current into the load, and reducing the current needed through the opamp. Feedback lets the opamp back down on current until both it and T1 are doing their share. We hope. Same situation for V-, excepting that load current pulled from the load goes through R51 and T2 base, sucking beta-times the base current from the load, with T2 helping the IC. So far, so good.

When there is no load current, the idle current for the IC goes through R49/T1 base, through the IC, and out R51/T2 base. Beta-times as much goes through T1 and T2. If everything is just right, this current is just enough to cancel any tendency to crossover distortion. "Just right" is obtained by sizing R49 and R51 to just tickle the correct Vbe so T1 and T2 are on the edge of conduction, but not much. That means that the opamp idle current must be (1) known and (2) of a small enough variation that you can the idle current times the value of R49 (and R51) be just about 0.45 to 0.5V, to suit the turn on of T1 and T2.

What could possibly go wrong here?

1. If you change the opamp, and what you put in pulls too much idle current, the transistors fight trying to pull down the power supply. The power supply wins.
2. If R49 and/or R51 are too high a value, the transistors fight the power supply, and we know how that goes.
3. If you change the opamp and the new opamp doesn't have as much gain-phase margin, the whole mess oscillates at a frequency that could well interfere with aircraft VHF radio and coincidentally be way beyond what a 20MHz scope can see, and the excess heat kills T1 and T2. This circuit was usually festooned with pf caps to try and prevent this. Sometimes it worked.

[wizard333]

Your local nut may have replaced the IC. WHat IC did you find in there and what did you replace it with? And more important, the factory specs TL061. Note they used a TL071 for the recovery. The TL061 is a low power version of the op amp, it is often used in battery preamps inside acoustic guitars for better battery life. So if you have a TL071 or other single op amp in that position it could draw excess current through those transistors.

With no reverb pan, is ther any DC offset on the output?


yes there should be a good reason, and we like to figure thatout to point to the repair, but sometimes it only becomes clear in retrospect.

I used a TL061. In fact, I went through a couple of them because there WERE shorts and such caused by knuckle head and the popping driver transistors were taking some op amps with them.

[wizard333]

All of the advice is correct. Something is causing both T1 and T2 to be turned on with too much idle current. They both conduct and try to pull down the power supply. The power supply always wins, of course.

The advice is to track down what is causing the larger idle current. This particular circuit has been around for quite a while. I remember it from the 1970s, when IC opamps were new. The way it works is to use the two transistors as both current and voltage boosters after the opamp. Adding voltage gain to an IC opamp is a recipe for disaster, as most of them are only marginally stable anyway. Any additional gain pushes them over the edge and they oscillate.

This circuit works by forcing the internal current use of the opamp to turn on the external transistors. This adds more current to the load. Current from the opamp to pull the load positive comes through the V+ pin and goes out to the load. This current is sucked through R49 and the base of T1, letting T1 dump beta-times the current into the load, and reducing the current needed through the opamp. Feedback lets the opamp back down on current until both it and T1 are doing their share. We hope. Same situation for V-, excepting that load current pulled from the load goes through R51 and T2 base, sucking beta-times the base current from the load, with T2 helping the IC. So far, so good.

When there is no load current, the idle current for the IC goes through R49/T1 base, through the IC, and out R51/T2 base. Beta-times as much goes through T1 and T2. If everything is just right, this current is just enough to cancel any tendency to crossover distortion. "Just right" is obtained by sizing R49 and R51 to just tickle the correct Vbe so T1 and T2 are on the edge of conduction, but not much. That means that the opamp idle current must be (1) known and (2) of a small enough variation that you can the idle current times the value of R49 (and R51) be just about 0.45 to 0.5V, to suit the turn on of T1 and T2.

What could possibly go wrong here?

1. If you change the opamp, and what you put in pulls too much idle current, the transistors fight trying to pull down the power supply. The power supply wins.
2. If R49 and/or R51 are too high a value, the transistors fight the power supply, and we know how that goes.
3. If you change the opamp and the new opamp doesn't have as much gain-phase margin, the whole mess oscillates at a frequency that could well interfere with aircraft VHF radio and coincidentally be way beyond what a 20MHz scope can see, and the excess heat kills T1 and T2. This circuit was usually festooned with pf caps to try and prevent this. Sometimes it worked.

Very interesting explanation of the circuit! Yes there is a lot that can go wrong with this, I'm surprised that I haven't seen it before given how many of these amps I've worked on. If everything else looks right; any suggestions on where the PF caps are typically placed to prevent that oscillation?

[J M Fahey]

Start by getting DC voltage and current right, then you can worry about stability and possible oscillation..
Please answer post #6.
Thanks.

[wizard333]

Ok here's the numbers:

With op amp TL061 in place: (Makes no difference whether verb pan is attached or not)

R52 = 32.8ohms, 0 VDC present

R49 = 6.7K, voltage drop across R49 = ~.4vdc

R51 = 6.73K, voltage drop across R51 = ~.4vdc

F - = -15.8vdc

F+ = 15.3vdc

Without TL061 installed, there is no voltage drop across R49 or R51, and no heat build up in the drivers BD135/136.

60ua per side doesn't seem like it should be heating up those drivers...

[nickb]

Ok here's the numbers:

With op amp TL061 in place: (Makes no difference whether verb pan is attached or not)

R52 = 32.8ohms, 0 VDC present

R49 = 6.7K, voltage drop across R49 = ~.4vdc

R51 = 6.73K, voltage drop across R51 = ~.4vdc

F - = -15.8vdc

F+ = 15.3vdc

Without TL061 installed, there is no voltage drop across R49 or R51, and no heat build up in the drivers BD135/136.

60ua per side doesn't seem like it should be heating up those drivers...

Agreed. With those numbers, the transistors should not be conducting at all. Therefore, it's either wired up wrong and/or one or both transistors are bad. Could the transistors be transposed?

[wizard333]

Also, using a 1ohm resistor to measure emitter current on the drivers, I get ~40 to 50ma (some Hiesenberg effect going on there).

[wizard333]

Agreed. With those numbers, the transistors should not be conducting at all. Therefore, it's either wired up wrong and/or one or both transistors are bad. Could the transistors be transposed?

No, they are not transposed, and no, there is no wiring issue (unless it was wired wrong at the factory, in which case it wouldn't have lasted as long as it did).

The complaint of "reverb stopped working" is a few years old. Customer didn't want to ship it to me, so he took it to some yahoo who said he couldn't figure it out, but didn't do anything to it (ha). Load of BS. Then other complaints emerged (all due to the 'nothing' yahoo did to the amp). When I finally got it back, it was clear that yahoo couldn't figure out the issue, but poked around extensively with a large, overly hot iron, burning various wires, parts, and traces right off the board. So I fixed all that, traced everything more than once to be sure it was right, etc.

I suspect that one of these driver chips blew to start with (given the description of "amp kinda hums now") I got back when the verb issue first surfaced.

SO what do I glean from this? At some point a few years ago, when the amp was several years out of my shop and probably 8 years or so old, one of those verb drivers went tits up. Then a whole lot of damage was caused by some boob poking around in it. The latter is fixed, but I'm left with, something caused that driver chip to crap out way back when, and that something is STILL apparently causing the same issue after x time, but what that something is, I can't seem to put my finger on.

Here's what it's NOT: 1) wiring 2) wrong parts 3) "transposed" parts. It's also not any particular "bad" part , at least not an op amp or driver, because multiple parts have been in those places.

[wizard333]

Also the transistors DO conduct, as noted by R.G. above, the voltage drop across R49/51 is right in the range it ought to be in, and the verb works perfectly, until one of the drivers dies thermal death. Then the amp hums and the verb stops, until the other driver also dies, then the hum dies and, of course, verb remains non functional.