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Accuphase E-204 - help needed for restoration

Machineghost

Machineghost

Ghost in the machine
Good evening folks! Its been a while since I have posted here.

Very recently I acquired an Accuphase E-204 Integrated Amplifier. Cosmetically its in a very good shape but electronically its a bit of a mess.

The unit switches on and plays audio but with certain conditions. Only when the TONE control function is engaged will the unit switch on normally. The reason for this is that I found that there is severe DC offset on the pre-amp output (11vdc reading). This makes the power amplifier section trip.

My question here is - where do I start looking for the issue?

Also - does anyone have a photo of the pre-amplifier section power regulator PCB that sits vertical in front of the transformer? In my unit it has been replaced by a shoddy veroboard unit. I will post pictures tomorrow. My plan is to make a replica PCB of the original if I can only see a photo. I have the schematic.

And finally - the power amplifier has a mixed assortment of output devices, yet miraculously it functions! Probably not to spec but ...

My plan is to restore this beaut and keep it in my vintage stack.

Thank you in advance!
 
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Are both channels of pre-amp putting out 11v dc? If so, then its likely the power supply at fault. Which, from your description, sounds likely.
 
Good day - yes, both channels are puttiing out 11 vdc. The home made PSU tests fine in regards the voltage being output.
 
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That actually looks pretty good. Double check (assuming you haven't) to make sure the +/- voltage is getting to the preamp section where it should be. You might then double check preamp section for shorts. I know the E-203 preamp section was op-amp based. Not sure about the E-204.

Looking at the schematic, it uses a TA7322p preamp IC that is shared between the channels. After that is a TA75558 for each channel driving a pair of 2SC2236/2SA966. If the output voltage is roughly the same in each channel I'd guess the TA7322p is shot or one of the power supply rails isn't getting to one or more of the ICs. Check R57 and R58.
 
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Good day Foot - I have checked the voltages from the make-shift PSU and it checks out albeit having some 50 Hz hum on them. The pre-amp uses a combination of op-amps and discreet transistor/fet devices for the audio.
 
Is the preamp section in a place where you can safely measure voltages - ie easy to get to without risk of shorting something with a probe? If so, check the voltages to the power supply pins of the opamps.
 
Does it sound fine with the tone controls enabled? It would seem strange that a solid state device had issues and the output wouldn't be distorted.

Enabling the tone controls puts a capacitor into the signaling path after the preamp. However, the amp section has an input capacitor as well. You measured the preout voltage with the switch in the back set to separate right?
 
It clips early with the tone controls enabled - also very loud hum.

When the pre-power section is coupled it makes the power amplifier trip repeatedly. So the DC gets through to the power amp input.

I did measure the DC offset with the switch in both positions - same measurement.
 
Ok, then its probably one of those solid state devices. Maybe they went out with the original supply. Double check the 2SC2236/A966 for shorts. The TA75558s could probably be subbed with a 4558 type opamp. Normally I'd say its more likely the TA7322p given its a shared part. Unfortunately, given the PS board was replaced, anything is possible.
 
I have measured all the transistors in the pre-amp. They all checked good. But the DC offset was still there. Then I found a DIP8 op-amp hidden beneath the selector switches. That seem to be connected to some DC offset correction circuit which is not listed on the schematic. Replaced it with a TL072 and voila! DC offset gone. Unit plays beautiful now.

I also made a new PSU layout. Just need to get the board made and populate with the correct parts as per schematic.

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