Sunn Amplifier Power Amp Puzzle

[KeepItSimpleStupid]

With 20vac across the input of the rectifier, measuring the voltage across the C-E of Q13, I get 99.5 mVdc after it settles down (kind of jumpy at first--goes high then low then climbs back up). R42 has RTV holding it in place, so I was hesitant to mess with it.

OK for now, but not in the long run.

A 4.0 GPA is hard to do. I did it with my 2-yr AAS electronics technology degree. I also have a general 4 year BAS degree in general engineering.

 

[KeepItSimpleStupid]

Rule #1 is usually inspect. It usually includes the "smell test".
Rule #2 is usually check the power supplies - magnitude and ripple

 

[Bud_J]

Check the ground connection at C8, C7 before you do anything. It's hard to see how the negative voltage is higher than the supply? The + voltage looks right.

Hmmmm. Good point. The ground is fine, but in checking the area, I realize it's a spot that I'd done a lot of rework to in the past because of blown PCB traces. Recently, trying to make it better, I re-heated things again. I wonder if the TIP30A I had there was already failing, and if in re-heating things I pushed it over the edge? Same with one of the caps. I don't have an ESR meter, but one of the caps "reads" differently than the other when I do the the charge test with my multi-meter leads across it. In fact, it doesn't seem to charge at all.

I might as well replace all those parts. I don't have any more TIP30 series but I have a TIP42. I think it should work. Here are the datasheets:

TIP30A (what's in there now):
https://pdf1.alldatasheet.com/datasheet-pdf/view/2779/MOSPEC/TIP30A.html

TIP42:
https://www.alldatasheet.com/view.jsp?Searchword=TIP42

Think I should change both caps and TIP30A? Or just try changing the caps first?

Thank you for all your help!
Bud

 

[Bud_J]

OK for now, but not in the long run.

A 4.0 GPA is hard to do.

I had a feeling you'd eventually ask me to tweak the POT if wanted to earn a 4.0 ;-)

 

[crutschow]

I was wondering if I should look there next. IC1, pin7 yes?

Yes.

 

[KeepItSimpleStupid]

Caps. measuring the ripple for the +-15V supply and comparing it should tell you something, but you really need the +-40 V supplies to up to |16-20} V.
They (+-15V) should be regulating then.

 

[Bud_J]

What's the output of the bottom op amp doing at that time?

With 20vac at the rectifier input, pin7 of IC1 reads 335mV. Takes about 5-10 seconds to rise up to it. Checking TP6 at this time reads 3.8mV.

 

[Bud_J]

Check the ground connection at C8, C7 before you do anything. It's hard to see how the negative voltage is higher than the supply?

I just realized . . . it's not higher than the supply. Supply is +12.8 and -12.8. So perhaps the disparity is just due to the two regulator circuits behaving differently until the supply is high enough to get each Zener to regulate. Like you say in your other email, maybe I should push it till I have +/- 20vdc. Then if there's still a disparity in the +/-15vdc supply then I'll know there's still a problem.

I'm still a bit hesitant to push it till I know IC1 is working as it should. I tend to think it is. We'll see what Crutschow thinks, now that I've posted the voltage at pin7 ;-)

 

[KeepItSimpleStupid]

TP6 is good. The 4558 won;t work until the supply is at least +-5V which you have it this point. I'd still go for pushing the +-40 V supplies
to +-20 V and measuring the +-15 V supplies and ripple.

 

[crutschow]

With 20vac at the rectifier input, pin7 of IC1 reads 335mV. Takes about 5-10 seconds to rise up to it. Checking TP6 at this time reads 3.8mV.

Sounds like the op amp is working okay.
It has a integrator time-constant of 0.8s so it can take awhile to reach a steady-state output level.

Seems like you are ready to up the voltage to the values KISS suggested.

 

[Bud_J]

Sounds like the op amp is working okay.
It has a integrator time-constant of 0.8s so it can take awhile to reach a steady-state output level.
Seems like you are ready to up the voltage to the values KISS suggested.

Thank you! I was waiting for your advice with the op-amp, so it's good to know I can move forward with upping the voltages. Should be able to get to that soon and post the results.

 

[Bud_J]

Curiosity... Do you have a way to inject a sine wave signal? It's another thing that helps.

Kept meaning to get back to this question. I recently bid on a function generator on eBay from a surplus/salvage seller. Says it powers up and appears new. Will be a good deal if it works as I only paid $40. Should be here any day. I'll check it when it gets here and post the results.

 

[Nigel Goodwin]

You can get very useful function generators for little money, such as this one:

https://www.banggood.com/FG-100-DDS...or-Frequency-Counter-1Hz-500KHz-p-942123.html

Or build a kit, even cheaper:

https://www.banggood.com/DDS-Function-Signal-Generator-Module-DIY-Kit-Pulse-Sine-Wave-p-958215.html

And you have the fun of building it.

 

[Bud_J]

You can get very useful function generators for little money, such as this one:

https://www.banggood.com/FG-100-DDS...or-Frequency-Counter-1Hz-500KHz-p-942123.html

Or build a kit, even cheaper:

https://www.banggood.com/DDS-Function-Signal-Generator-Module-DIY-Kit-Pulse-Sine-Wave-p-958215.html

And you have the fun of building it.

Thanks, Nigel. I did consider those two models initially, or ones very similar I saw on Amazon. Then I saw this one on eBay--a desktop model with a nice interface. It appealed and seemed to fit with my other gear:
https://www.testequipmentdepot.com/instek/signalgenerators/gfg8215.htm

Just checked the ETA and it's due in tomorrow. I'll post back as to whether it was a bargain or a bust. I got a good feeling about it, though. And seller offers 14-day returns.

 

[Nigel Goodwin]

Just checked the ETA and it's due in tomorrow. I'll post back as to whether it was a bargain or a bust. I got a good feeling about it, though. And seller offers 14-day returns.

Good luck, have fun

Even if there's something wrong with it, it's probably a simple fix.

 

[Bud_J]

TP6 is good. The 4558 won;t work until the supply is at least +-5V which you have it this point. I'd still go for pushing the +-40 V supplies
to +-20 V and measuring the +-15 V supplies and ripple.

Well, here are my findings.

Cranking the rails to +/- 20vdc, I was still unable to get the +/- 15vdc supplies up to full value, so I cranked to +/- 25vdc and got the -15 supply to read -12.9, which was what it used to read (the original problem I was tracing). At this point the +15 supply read +14.1.

I was getting a brief glow on the 40W bulb limiter at power-on and I wasn't sure if this was kosher or not, so didn't push the voltage further. I'd say the 40W bulb flares up to about 1/10th full brightness for about 1/2 second at power-on. Maybe a full second before it fades out.

Anyway, I did take ripple readings at that time. I suspect they're probably not good. With the rails at +/-25vdc, here's what I got:

The +40 supply at C16 had a 1.5vpp sawtooth ripple.
The -40 supply at C16 had a 2vpp sawtooth ripple.

The +15 supply had a 520mVpp nearly sinusoidal waveform at it. Clean looking wave.
The -15 supply had about 36mV of noise. The Q11 collector had a 1.8vpp sawtooth ripple, but very noisy looking. Whereas the other side of R25 (which is the -40 supply) had pretty much the same size ripple, but cleaner looking.

Thoughts?

 

[KeepItSimpleStupid]

Thoughts:

Replace C2, C3. C7, C8 and C13
Replace C16 and C17 and upgrade voltages to at least 80.

Buy a TIP30 but don't replace yet.

How old is the amp?

 

[crutschow]

What is TP6 doing at this point?

 

[Bud_J]

Thoughts:
Replace C2, C3. C7, C8 and C13 Replace C16 and C17 and upgrade voltages to at least 80.
Buy a TIP30 but don't replace yet.
How old is the amp?

All the caps have been replaced (even on the preamp board) except for C16 and C17. However, the ones on the +15 supply I did a long time ago. No miles on 'em, but maybe I'll replace 'em anyway. Someone had said the big caps usually don't go bad, and I didn't have any those size, but in looking at them now they are only rated 50V. Not much headroom on a +/- 45 supply. Plus I'm assuming you guys agree that 2vp-p ripple when the supply's not even all the way up is too much, yes?

How 'bout these puppies:
https://www.digikey.com/product-detail/en/nichicon/TVX1J332MDD/493-14824-ND/2539922

The next jump up is 250v and they're not even in stock.

Buy a TIP30 but don't replace yet.

I worked on that part of the circuit yesterday, looking for a bad ground. I ended up putting in the TIP42. When I measured after, everything was exactly the same. The TIP30A read fine, too, so I've put it aside. Meant to post that.

How old is the amp?

33ish years. I've done just about all of the electrolytics. Just have to do the big cans now--if you guys agree that those Nichicons are good enough.

Thanks for all the help! It's going to be a great amp once I'm done

 

[Bud_J]

What is TP6 doing at this point?

I kept going back to that, partly out of fear, partly out of curiosity. It seems to like that 3.2mV range, and seems to be less jumpy as I crank the voltage up. I don't get why Q19 blew out a 2nd time after I popped IC1 back in. Perhaps in my frazzled state I improperly recorded the order of events. Or maybe the open legs of Q15 threw something off (though it doesn't seem to be part of the circuit to me).

Or maybe it's because I"m using cheap, sacrificial power transistors till I feel confident enough to start popping in good ones. May be a forever-mystery.

Thanks for all the help and moral support!