So, I had a bit of a session today with an intermittent fault that was putting the amplifier in protection mode briefly, and then recovering as if nothing had happened. A combination of noisy MV103's, and troublesome 2SA1145//2SC2705 pairs was to blame.
Also I've been tackling the Control Amp, here is a couple of views of the track side where I found these apparently late additions. Not too worried about the resistors and the film capacitor, but those huge electrolytics need something done with them.
Another view
I find that the four capacitors in the lower picture form 2 bipolar capacitors, and I can't see that they need to be 50V rating each - a combined rating of 100V, so I replaced them with quality bipolar capacitors, with the board now looking like this. (I'll put some self adhesive foam on the new bipolars to cushion them against the underside of the PCB.
). The original topside positions for these capacitors are marked, but don't now (if they ever did?), have the correct track pattern//connections to allow possible mounting on the topside of the PCB.
The two capacitors that were mounted in the centre foreground (on the plastic pad) have been replaced with same value/voltage capacitors, but now located on the component side of the board in their provided locations. The originally fitted capacitors were too big to be accommodated in their component side positions - presumably the result of component shortages all those years ago. This is also borne out by the original use of 220µF 50V capacitors on the Main Amplifier board where 220µF 10V (or 16V), was the correct component - because it is placed across a 6.2V Zener diode.
You might be able to see a rounded blob of something underneath the remaining plastic capacitor mounting pad in the last picture - this is some kind of hot glue, almost clear but with a milky appearance. I needed to remove the upper 2 blobs & pads, because there were capacitors on the topside that had to be changed, but this is really tough stuff and wouldn't come off. So I after some chin scratching I decided to try 'silicone shower sealant remover' - it worked! - I decided to leave the last bit as an indicator of what had been there for a future restorer.
Component change details for F-5643 Control Amp PCB.
C13, C13, C23, C24 (x2) 4pF Ceramic, replaced with 4pF Murata MLCC C0G/NPO.
C16, C17 (x2), 220µF 50V, (bipolar pairs measured at ~105µF each) - each pair replaced with single 100µF 50V Nichicon ES Bipolar.
C36, C37 (x2) 10µF 25V, each pair replaced with 4.7µF 63V WIMA MKS2. (vacant position jumpered).
C41, C42, 220µF 35V - replaced with 220µF 35V Nichicon PM.
I have just been having a listen to the amplifier
before I deal with the last two items - The EQ Amp board, left until now because of
so many connections, and the remote mounted metal tape driven switches, and the PSU because of the heavy duty soldering to the main PSU capacitors, also it is buried in a 'sea' of interlocking plates - awkward items to restore.
Despite all my efforts to avoid it, I ended up having to de-solder the main PSU capacitors from the PSU board. I just couldn't get the access I needed to properly clean the glue off the PSU board. It all went well and I think I made a neater job of re-soldering the lugs of the capacitors. One thing that struck a chord with me, was that for example, the original two 1000uF capacitors on the PSU board appeared to be in excellent condition, and had me thinking I wonder why I am changing them. This echoes the sentiment expressed by AK member Bumpalump regarding some original capacitors which are still in good condition. The difference is I can't afford/justify an ~£800 (~$1120) piece of test gear to prove it.
That's how it stands at the moment, only changed in one channel. But I'll be trying a set of different substitutes for these as the output capacitance of the current subs (TTA004B//TTC004B) is a bit higher than the originals at 17pF and 20pF as opposed to 2.5 pF and 1.8pF for the originals.
, thanks for the reply 
