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Teac AH380 DC offset problem

J

juice667

New Member
Hello,

I've got a Teac AH380 amplifier (made in 2008) which has a problem with DC offset. On startup it is over 1 volt in both channels, but some sort of feedback corrects it to a few tens of millivolts after a minute or so. I think that this affects the sound of the amplifier, and it surely causes a "thump" in the speakers when the output relay is energized by the "thump" suppression timer. The same thing happens with both channels.

I've changed all the dubious caps to reputable Panasonic and Nichicon ones, and I've also changed the diff amp transistors to a well-matched pair. Moreover, I've checked that the supply rails for the amplifiers rise up quickly. Do you have any ideas where I should look next? I've attached the schematic to this message. Any help is greatly appreciated.

- Jussi
 

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Hello Jussi,

I am not any kind of expert but, fwik, when you have a positiv voltage on the output then you have some kind of problem in the negative rail supply. So you have to check the -15v rail (c918,c920, ic94). Ic94 is a 7915, a very common 3 pin regulator.
Also i sugest changing the c286, c287 10uf/35v capacitor on the input of the power amp and c260, c261 10uf/50v capacitor on the pre out .

Also i can see a cn15 and cn16 conectors for the input in the power amp. Can you disconect them? If so, you can check the power section only and see if you have the same simptoms. If not, you have to check before it, in the preamp section (ic23).

Dimitrios.
 
dimanag said:
Hello Jussi,

I am not any kind of expert but, fwik, when you have a positiv voltage on the output then you have some kind of problem in the negative rail supply. So you have to check the -15v rail (c918,c920, ic94). Ic94 is a 7915, a very common 3 pin regulator.
Also i sugest changing the c286, c287 10uf/35v capacitor on the input of the power amp and c260, c261 10uf/50v capacitor on the pre out .

Also i can see a cn15 and cn16 conectors for the input in the power amp. Can you disconect them? If so, you can check the power section only and see if you have the same simptoms. If not, you have to check before it, in the preamp section (ic23).

Dimitrios.
Click to expand...
Many thanks for your input Dimitrios.

I have measured the +-15 and +-VDD rails and they are ok (correct voltage levels, no significant ripple). I've also replaced all the electrolytics in the power amp section with the exception of C341-C344. I tried disconnecting the CN15 and CN16, and shorting the power amp input to ground. The problem persisted, so the fault must be in the power amp section.

Still looking for the fault and waiting for your ideas.

- Jussi
 
Okay, you're not going to believe this.

It turns out that the amplifier had been fitted with capacitor C304 & 305 of value 470 µF instead of 47 µF. Generally I'm not afraid to take the blame, but this time it certainly wasn't me, as the problem was the whole reason I decided to pop the amplifier open and recap it. While recapping, I did notice that the capacitance does not match the schematic, but I thought it's not unusual for design documents to not be up to date because of some improvements or other design changes.

So, I fitted the amplifier with 47 µF capacitors and the "thump" is gone. The physics behind the problem are quite logical now when I think of the schematic, but this is the first time I encounter a problem of this nature. Has anyone else seen assembly mistakes like this?
 
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