Problem with Alto MAC 2.3 stereo amplifier

[spy]

Hi,
I'm trying to repair a transistor stereo amplifier. It's an Alto MAC 2.3 and I'm using it in my studio. A few days ago I rented it to some friends but they blew it in their live!

Anyway, I changed all the output transistors 8 pairs of 2SC5200/2SA1943 in both channels, but...
One channel is working great but at the other channel I have noise. When I was having the amp on the bench and trying to find where is this noise from using an o'scope I found out that it's on the supply rails. Then one more pair of power transistors blew out.
I took off all of them from this channel and measured the existing ones to be sure that I can use them again.
I powered up the amp without the outup transistors of this channel and everything works fine! Then I soldered a pair to keep safe the other ones... they're a little bit expensive... and carefully I scoped the noise. It's an oscillation around 6-8Vp-p @ 1000KHz! I have no idea what is this!? And, you can scope it at the output too.
Checked all the components and everything looks fine. Checked all the small transistors, diodes, resistors etc

Unfortunately, I don't have a schematic. Asked from Alto to send me one and they send me from the newer version MAC 2.3 MKII that it's different.

Any help? Without the power transistors I have a perfect sinewave at the drivers. But with a pair of powers I have that noise at the output. It's loud when I plug a speaker. Hope someone has something in mind....

[Steve Conner]

First check the Zobel network: a series resistor and capacitor from the speaker to ground. If either the R or the C has gone open, the amp will go unstable. Maybe the design was marginal with capacitive loads, and your friends blew it out with some long speaker cables: that would probably take out the Zobel along with the output transistors.

Next, are you sure you got real 5200s and 1943s? They're quite commonly faked. The fakes might be slower, and that could make the amp unstable. You've already blown some of your replacements, so break them open and compare the die size with the original blown ones.

Third, the 5200/1943 are quite old. Maybe you have the opposite problem: the ones they're making now could have more gain than the ones Alto tested the design with. You might need to beef up the Zobel network, or change the compensation capacitor, increase the stopper resistors, or something.

Hi-fi amp circuits vary so much that it's hard to give any more advice than this. The classic "Lin" circuit is plenty good enough, but every hi-fi designer thinks he can improve on it with some crazy topology. I once had one of my homebuilt amps go unstable in a similar way, one channel sounded rough, and on scoping it, it was oscillating at several MHz. Turned out that I'd put a tantalum cap in backwards in the bias circuit, it had shorted after several years, and the MOSFETs were now getting no bias at all. When they both turned off at zero crossings, the feedback loop didn't know what to do, so it started to oscillate. So there's the last thing to check: Is your bias generator doodad working?

[spy]

Thanks Steve,

Just measured the zobel network. It isn't open.
The other channel is fixed so I don't think it's due to new or old 5200/1943. I changed all of them. Both channels.

I'll try to find the feedback loop...

EDIT: Don't know if this plays any role but the collectors are all going to the ground....

[Steve Conner]

The other channel might seem "fixed" but it could be only marginally stable, so I wouldn't rule out the transistors right now.

[spy]

Just checked the old transistors with the new ones. They look like exactly the same on size and they both are Toshiba.

I feel that I have a serious problem!

What do you say to beef up the Zobel network?

[spy]

By the way, which is the compensation capacitor?

And one more question, the amp has 3 trimmers in each channel. Is it possible to be an adjustment the problem?

[Steve Conner]

I have absolutely no idea. I don't have the schematic here. You'll have to figure it out yourself.

One of the trimmers will be bias, and that will need adjusting after changing the transistors. The other two are Lord knows what... DC offset and something else?

[spy]

Steve, what do you say to beef up the Zobel network?

[Steve Conner]

I meant to make the resistance smaller, if it was 10 ohms or 8, make it 4.7. And the capacitance bigger, if it was 0.1uf you could up it to 0.22.

I didn't realise this was a PA amp, though, it should be 4.7 already. And as our uber repair guy Enzo says, we're trying to fix a problem, not re-engineer it. This is a mass produced amp, not some boutique hi-fi thing as I thought, so the design should be OK and it's probably more likely to be a blown component that you missed.

[spy]

Hey, yes it's a PA amp!

The Zobel circuit has 2x 12R resistors in parallel and .068u cap in series...
what do you say?

I can't find that blown component!

[spy]

Don't know if this is any helpfull... but the amp has a switch that you can connect the amp bridged or stereo. When the switch is set on the bridged position then I don't have an oscillation at the output....

[Jazz P Bass]

Bridged

Some amplifiers when they say Bridged, actually bridge the inputs.
Other amplifiers mean Bridged Output.
Seeing that the sparks did not fly (Bridged output is usually the two red posts for the speaker connection), I will assume that we are talking about bridged input.
I seem to recall a phase inversion in the preamp when you select bridge input.
Without a schematic that is all that I have.

[spy]

Good news! I have the schematic!



If anyone would like to help I can e-mail it for better view. I don't know how to post it bigger.

It's a pretty weird output stage...

[Enzo]

Don't know where you have been buying them, but 2SC5200 are from $2 to $2.50 at both MOuser and Digikey. I expect the PNP to be similar.

COunterfeit parts often LOOK just like the real ones, so it matters where you buy.

[spy]

Thank you Enzo for that! I bought them higher in a local store!

Any help with my problem? Do you believe it's due to counterfeit transistors? Would you like to send you a clearer schematic to have a look?