Problem with Alto MAC 2.3 stereo amplifier

[spy]

Ok I uploaded the service manual to mediafire. So if anyone would like to have a look he can download the service manual!

[J M Fahey]

Hi spy.
I downloaded the service manual but it's on some higher PDF version, my reader opens the PCBs and block diagrams but not the schematics themselves.
Anyway, seeing the thumbnail you posted , I find it very complex, yet the bias, for example, seems to use an obsolete thermistor, the short protection is very strange, it *seems* yo have a transconductance op amp compressor, in all a very strange beast.
It might be marginally stable, maybe enough to pass a few quick tests and be sold massively in the West.
Steve's

This is a mass produced amp, not some boutique hi-fi thing as I thought, so the design should be OK

applies 99% of the time, considering serious manufacturers, but *anything* you make in China will sell a lot here, and a "test run" might mean a few hundred units or more.

[spy]

Hey, thanks for your help. Here are some files in jpeg. Hope you can see them. You can download them for better quality...

MAC_series_service_manual_Page_08.jpg
MAC_series_service_manual_Page_09.jpg

By the way, can I check the output stage with only one pair of power transistors at the output? Or I have to place all the pairs? I'm more into tubes you know?!

[J M Fahey]

Hi spy.
those jpegs are good and very clean, thanks.
The amp is a salad of technologies from different ages, I guess the designer took a little bit from everywhere.
Trimmers: VR1 is indeed for bias.
VR2/3 in principle should not be touched now, they are part of a very old fashioned protection circuit.
They clamp the base drive of Q4 and Q5 and must certainly be adjusted by driving the amp to just clipping with a suitable load resistor, and adjusting them until the amp starts clipping a little more.
The signal limiter based on an LM13700 *can* still be made, but it's a "don't use for future designs" part.
That thermistor compensation is 60's technology.
The amp topology is that one where you ground the output rail and drive the speaker from the midpoint junction of the power supply capacitors.
Apparently you only blew some output transistors (lucky guy)
The amp is unstable.
I would:
1)reinforce the zobel network as suggested by Steve, by parallelling another equal value capacitor with the one originally there, and do the same with the resistors, without pulling the originals.
If we "overdamp" the output somewhat, it shouldn't hurt.
2) get a 100r or 220r , 25W resistor, to load the amp with *something* , yet not with the full 8 or 4 ohms.
That load may be driven with a single output pair.
Put back a pair of original, salvaged ones, that you know worked before.
If it works properly, add pair by pair until you fill the board.
If any new pair causes instability, there is your culprit.
Try to get those recommended by Enzo, those transistors are *heavily* counterfeited.
As of a possible future stability tweak: those output transistors are driven by Q4 and Q5 (poor abused guys) which are driven by U2b (1/2 NE5534).
The only frequency ( and phase) determining elements I see there are C8 and C9, which are probably there to better the 100KHz square wave response or something like that, I dislike them and would pull them out (at least lift one pin each and see what happens) and U2b's compensation: C22, which is more complicated to set.
Well, now you have something to keep you busy a couple evenings.
Work slowly and carefully.

[spy]

Still trying to repair this amp!
Not enough time the last weeks so I didn't do any progress. But today I did some checkings with my oscilloscope and found out that I was totally wrong the last time or half wrong.
Anyway, without the output transistors and with a signal at the input, I traced with my scope the signal and I realized that I have nothing at the output of the NE5532 ( U2-B ). I replaced it with another one but still no output! So the problem is somewhere else...

Here are the updated links for anyone who would like to help!
http://www.mediafire.com/file/bmotmj...service_manual 8.jpg
http://www.mediafire.com/file/buyidy...service_manual 9.jpg

[spy]

Hey, please help!

Just a question... is it possible to be the LM13700? It's the only part I haven't replaced because it's an SMD type. But if I'll do it carefully I think I'll replace it with no problems.

[spy]

I think I'll through this amp to the garbage!