Sunn Amplifier Power Amp Puzzle

[crutschow]

The incandescent light bulb is basically a current dependent resistor and it's really based on the resistance temperature curve of Tungsten. Tungsten has a resistance about 12x lower at room temperature.

So that characteristic obviously makes it a very good limiter for testing purposes since it has a low resistance to drop only a small voltage at low currents but rapidly goes up in resistance at higher currents to limit the current at a much lower level then if just a fixed resistor were used.
It has the added handy feature of the glow giving a rough indication of the current the unit is taking.
(What will we do when they no longer make incandescent bulbs. )

 

[KeepItSimpleStupid]

Well, some of this riddle may be solved. I've been assuming I'm supposed to see 120VAC at the AC side of the rectifier on the Sunn because my brain was thinking this was the line, but it's not. It's the secondary of the transformer. Looking at the schematic, I'm only *supposed* to be seeing 66VAC. The GOOD news is that I *have* been running the amp at pretty much full voltage! Which means it only draws 220mA at the outlet.

That's all good news.

That out of the way, here are my findings with a 1KHz 240mVp-p sine wave injected directly into the power amp (bypassing the preamp board for now, which has its own problems), and the master volume turned up to about 3 or 4 (hard to tell as it's not mounted) we get a 5Vp-p sine wave at the output at full power. Actually, we get that even at around half power, where it starts to drop off as I turn the variac down.

Line levels are about 1.4 V pp for consumer products. See:
https://en.wikipedia.org/wiki/Line_level

1 kHz is good.

Increase the gain until the amplifier clips. Then reduce it slightly and look at the +-symmetry.

All we really care about at this point is amplification and symmetry and no excess power draw.

At full power, there is detectable crossover distortion, but pretty tiny--might not see it if you're not looking for it. Around half power the output wave begins to lose amplitude and the crossover distortion grows more severe as we get down to 1/4 power, 20% power, etc.

Good.

At 10% power the amp starts singing Elvis Presley tunes. Not sure what that's about.

Not sure what this means.

Other finding: Sometimes I had trouble getting the signal to appear at the output. I had to reset the function generator a couple times and/or disconnect the signal wire and reconnect. I remembered you had said to the inject the signal as I was turning up the variac, so maybe this is normal? Next time I run tests I'll have a 2nd probe at the input to see where the signal might be getting clamped down, if it is.

250 mV is tiny anyway.

No other distortion of the sine wave as I was turning it up.

Q15/Q6 etc is actually the VI limiter.

Thoughts?

Initial thoughts:

* Bias regulator is messed up based on earlier tests.
* Get it just below clipping. It should almost reach the power rails. i.e. A few volts less.
* Take a peak at the -15 V supply with a signal applied.
* Take a peak at the DC output voltage (speaker terminals) with a signal applied.

Symmetry. One you have a nice sine wave at close to the rails, use GND reference and check symmetry.

Comments:
Not sure what oscilloscope probes your using, if any.
Crash course in SCOPE PROBES
Standard oscilloscope input is around 1 M || 22 pf. There are 50 ohm input scopes and some that are switchable.
There are other capacitance values.

Why capacitance?

The scope probe (e.g. (1x/10x/gnd) has a variable capacitor in it. in 1x, the likely is covered or non-adjustable. The probe has 1 M input Z at this point.

In 10x the variable capacitor comes into play. It tries to make the probe cable purely resistive and compensates for the cable capacitance.

Compensation:
Is using the square wave calibrator output on the scope and adjusting the capacitor, until you get a "square wave". Fourier theory says that the square wave can be constructed of the sum of an infinite number of odd harmonic + fundamental sine wave. The probe must be >1X to work.

Mentally, you generally have to take the 0.1 V/div and multiply it by 10 and realize it's 1 V/div in 10X

The input impedance of the scope with probe using the 10x setting (variable or fixed) is 10 M. I think it's 10 m for all other attenuations like 100x etc.

So, when selecting a scope probe bandwidth is most of the cost. A 400 MHz probe on a 10 MHz scope is pointless and MAY NOT work. The amount of capacitance that the scope can compensate for is also required.

The compensation adjustments can be at the input connector or probe. You can get 10x, 100x and 500x and other fixed attenuations. Tektronix and I believe HP had a tab on their scope probes that would change the scale.

On a 3-prong plug scope, the ground clip is not your friend. It's connected to earth ground.

 

[Bud_J]

Q15/Q6 etc is actually the VI limiter.

Just read up on VI Limiters. Interesting. This might explain why Q19 blew when I pulled IC1. I read in another thread where someone pulled IC1 from a Sunn Beta Lead to take measurements. Some of his output transistors got very hot, but did not blow. He was able to pull the IC several times to take various measurements. I just read this yesterday and thought maybe my Q19 fried because it was a cheap sacrificial transistor in the first place.

Now your info sheds new light on that.

As noted about 100 posts ago now, after frying Q19, I found Q15's socket had two cracked legs when I was looking things over. I've since soldered Q15 directly to the board. So maybe it was the combination of my removing IC1 and the lack of functioning VI Limiter that caused my Q19 to blow, where this other guy was able to get away with pulling IC1.

I think I'll pull IC1 right now to test that theory . . . . NOT!

Still, good theory?

 

[KeepItSimpleStupid]

I think I'll pull IC1 right now to test that theory . . . . NOT!

Still, good theory?

It may not be a bad idea. But postpone it for a bit. Again, checking the -15V supply with it removed.

You will need to consider replacing Q13 and moving R42 to minimum in the future.

 

[Bud_J]

* Get it just below clipping. It should almost reach the power rails. i.e. A few volts less.
* Take a peak at the -15 V supply with a signal applied.
* Take a peak at the DC output voltage (speaker terminals) with a signal applied.

Brought the amp up to full power again, this time monitoring the input and output sine waves. Started the input at 1Vp-p.

I cranked the master volume up to 11. Could not get the power amp to clip. It may very well be designed that way, leaving the "drive" for the preamp. So then I cranked the input to 2Vp-p just to see if I could clip the output. No. Stopped there because I don't know how much is too much.

At 2Vp-p input, the output was close to 60Vp-p
Rails were about +/-41 on my probably-not-true-rms multimeter. If I cranked the input some more I might have seen clipping, but again I wasn't sure how much was too much. Anyway, the output was pretty much following the rails w/o distortion, which I think is the info you're after.
-13 and +15 again. Seems like nothing can shake that.

I did lose the output a couple times again. And only the output. The input signal was fine, so it wasn't clamping there. I switched probes and it still happened. Tried 1X and 10X and it still happened, but if I touched the probe it would come back sometimes so might have been the connection. I connected it in a new spot and that seemed to be better. I'll keep an eye on it.

Symmetry. One you have a nice sine wave at close to the rails, use GND reference and check symmetry.

Wasn't quite sure what this meant, but the top and bottom of the sine wave looked symmetrical in amplitude and shape.

Alright, going to see what I might have for a Q13 replacement . . .

Thanks for keeping me moving along.

 

[KeepItSimpleStupid]

Thanks for keeping me moving along.

Your Welcome. So, basically it looks good except the -13 V thing?

Has that OP amp been replaced?

It must be my second full-time job. Physically, I'm whacked and mentally it just depends. Migraines when it rains and some foods as well as full-time caregiver for an elderly parent in a wheelchair.

 

[Bud_J]

Has that OP amp been replaced?

It may have been. It's in a socket that looks like it was cut from a 16-pin socket, which is something I would do. Some info: This amp fried something like 25 years ago. About 20 years ago, I was playing around with it and got the Power Amp board "kind of" working. It worked, but parts would get hot. I lost my notes from that era. They're probably still around, but I can't find them just now. I'll probably find them the day after I get this amp working again.

Anyway, I moved a couple of times and since my first repair attempt, the amp has been moving from attic to attic in an old suitcase. Earlier this year, I started getting interested in electronics again. I was also thinking of getting an old tube amp and restoring it. So I went up in the attic, got the suitcase and brought the Sunn down. My thought was I could cut my teeth on this amp, if I'm going to get serious about working on amps. I know solid state is more difficult to work on, but I understand it's also safer. Anyway, I needed to re-familiarize myself with the basic principles. Plus, I used to really like this amp!

I was using it as a bass amp, playing bass in a blues band. The initial symptom was a popping sound while playing. After my earlier insight today, I wonder if it was that Q15 socket with cracked legs that had been the original problem. I can't remember if the popping sound was in time with the bass or independent of it. Later on, I was working on the amp and all of a sudden it started smoking. Maybe the VI Limiter failed and that's why? Way back then, I replaced a LOT of parts on the power amp board -- mostly in the vicinity of Q11, and some of the output transistors. I re-ran several burnt-out traces. One or two on the preamp board, too. The damage was that extensive. Back then I thought Q1 and Q11 were "drivers." Only this time around, digging into things more, did I finally learn their true function.

I needed the internet to be invented and for the Chinese to start selling every chip there ever was before I could work on the amp again. For a long time, I believed the fried 14506 on the preamp board was my entire problem. I had that part number in my eBay search for YEARS. Finally it turned up and I got 3 of them earlier this year. Well, that chip solved some problems, but not all.

Anyway, that's the gory history that may offer some insight.

And yep--you read it right--the amp has been waiting 25 years to be fixed! I've played thru it some this year -- just a few minutes at a time. And it was kinda cool to have it kinda working again. There are still issues on the preamp board, but I got one channel working and the other halfway traced. That got put on hold when I blew up the power amp board again ;-)

I feel I can get it going this time. Maybe not without this forum. But as long as you guys are willing to keep bumping me in the right direction, I'm willing to leave this thing sprawled all over my living room until it's finally fixed.

I really appreciate the opportunity you guys are giving me here. I might also get enough education to keep from zapping myself with 500 volts when I do finally get around to restoring a vintage tube amp.

END OF NOVEL

Bud

 

[KeepItSimpleStupid]

Valve/Tube amps MUST have a load.

 

[Bud_J]

Valve/Tube amps MUST have a load.

That's one thing I've already made note of. You and/or Crutschow had mentioned it above. That's why it's good I'm cutting my teeth on this amp. And don't worry . . . you guys will be the first to know if I have a valve amp on the way ;-) I'm kind of a "one thing at a time" guy, so I won't actually buy a valve amp till the Sunn is done. But I've been doing a lot of research . . .

 

[Bud_J]

You will need to consider replacing Q13 and moving R42 to minimum in the future.

Do you think I should remove the jumper across C13 then retest the transistor? For this application, do you think a regular ol' 2N3906 could be dropped in there? Or do you see some characteristic of the 2n4250 that might be required for this part of the circuit to function properly? The two devices seem fairly close when I look at the datasheets.

I remember testing the pot for resistance, but not for varying resistance. Might be time to peel back the RTV, yes?

You had mentioned a reading of > 90mV across any emitter resistor would not be a good thing. Any other precautions before I attempt this?

Thanks for the morale support. I always thought peeling off RTV and tweaking scary-looking esoteric adjustment pots was only for the big league techs

 

[KeepItSimpleStupid]

Do you think I should remove the jumper across C13 then retest the transistor? For this application, do you think a regular ol' 2N3906 could be dropped in there? Or do you see some characteristic of the 2n4250 that might be required for this part of the circuit to function properly? The two devices seem fairly close when I look at the datasheets.

No time to think right now. Think it's shorted. test the B-E junction with diode mode.

I remember testing the pot for resistance, but not for varying resistance. Might be time to peel back the RTV, yes?

I do prefer 10 turn pots here, but 99% of the amps don't have them. Take off the RTV. Multi-turn pot is an upgrade. Make sure the POT isn't touchy.

You had mentioned a reading of > 90mV across any emitter resistor would not be a good thing. Any other precautions before I attempt this?

Monitor voltage across C13. It's going to start increasing the voltage across the emitter resistor at >~2.4 or >~3.2V. It needs to be set at idle, but power the amp up with a signal first and least see if you can vary the voltage across C13. For now, stay below 90 mV on the emitter resistors. I have to do some stuff.

Thanks for the morale support. I always thought peeling off RTV and tweaking scary-looking esoteric adjustment pots was only for the big league techs

Your learning to be a big league tech. You afraid of a little bit of "Magic Smoke"?

 

[KeepItSimpleStupid]

@crutschow I think R39 is of by at at least a factor of 10. I think 27 ohms is way way too low. 270 maybe, but 27 doesn't make any sense to me.

 

[crutschow]

@crutschow I think R39 is of by at at least a factor of 10. I think 27 ohms is way way too low. 270 maybe, but 27 doesn't make any sense to me.

Yes, I was wondering about that myself.
First, what is its purpose, and second, why such a low value?

 

[KeepItSimpleStupid]

.What I can think of:

R22 (10K) +R39 (27) basically set the input Z and discharges C1, C4 and C12.
R2, C4 is the RF filter.
The 4558 is short circuit protected indefinitely. The datasheet hides the output current max.
At 0V out, it's (27 ohms) should not be an issue.
The whole mess around U1 may also do some pop suppression as well.
I think 27 ohms puts more EFFORT into transistor matching.

Q9, Q10 and Q17, Q18 matched as pairs are the ones I care about.

 

[Bud_J]

It was listed on the schematic, but nit where to measure it, right? You have to set it in volts, so why not use volts. It really is the "Idle current ADJUSTMENT". e.g. Adjust to 90 mV

I had wondered about that. And also wondered how you knew to take the measurement across an emitter resistor. I reckon this is just a "known thing" among amp techs and the manufacturers expect anybody working on the amp to know this, which is understandable.

Likewise, with the some of the AC test points. I had gone thru and verified voltages at the DC test points. But when I got to the AC ones I held off because nowhere on the schematic does it say what the input signal should be. So my measured output signal would be meaningless. I've watched a bunch of amp repair vids online and I see the techs typically input a 1KHz sine wave. But at what voltage? This, too, seems to be an unspoken and understood thing.

I'd like to go through and check these AC test points. Is the expected input signal 1Vp-p? (Unless otherwise noted on a schematic?)

For what it's worth, before the whole IC1 fiasco, all of the DC test points were pretty much right on (except for the -15vdc).

Removed the jumper from C13 and re-tested. Seems to have no crossover distortion at full power. But when I turn the variac way down I do see it.

Haven't done the bias pot test yet. I may pull the bias transistor again and send you guys the measurements. I'd had it out before and it gave me an in-range Hfe in my transistor checker, so I put it back in. But I'll get those diode mode readings you suggested and post them soon.

 

[KeepItSimpleStupid]

ikewise, with the some of the AC test points. I had gone thru and verified voltages at the DC test points. But when I got to the AC ones I held off because nowhere on the schematic does it say what the input signal should be. So my measured output signal would be meaningless. I've watched a bunch of amp repair vids online and I see the techs typically input a 1KHz sine wave. But at what voltage? This, too, seems to be an unspoken and understood thing.

No hand-holding. You set TP-1 to 2.2 V p-p and check the others. Think!

1 kHz, you can hear and it's not annoying. 600/1kHz is common too.

The amp has to work with the sensitivity it was designed for and they are all over the map "somewhat".