I've just bought a 1801 in dire(?) state and set to restoring it so I've read your posts here and in OCAU with great interest.
Seems you're doing a great job!
Funnily enough my item suffers from the same power cord fraying as yours!
Upon opening it up to attempt to replace the power cord found a number of things.
- it appears one channel completely failed at some point as there are signs of extreme heat that caused the paint to disappear from one of the metal panels and black suit deposited on the top cover. Not good...
- Also the molex(?) connector is blackened too
- The damaged channel board has been replaced with one marked Rev. 5 on it.
- One of the main PSU caps. has been replaced at some point (can't tell which one but both are Sangamo, same size one with white jacket, one with clear jacket).
- 4 ceramic resistors half soldered across(?) the delay relay (possibly one end came detached at some point)
- mains earth disconnected from the chassis!!!
Thankfully, all cabling is intact.
The other channel board has the following changes that I can see:
- 3 output transistors replaced with ones marked 2n3442
- all sorts of component changes
- a resistor soldered on the back side
- about a quarter of the copper tracks heavily and really sloppily tinned
- blackened molex(?) connector (I guess the board had been swapped to the other side at some point? as the mating end of the connector is more blackened on the other channel)
- only 3 original screws left - the rest are all sorts and have probably damaged the threading on the heatsinks / case
- phono socket and plug missing - phono cable directly soldered to board
Now this board has no rev. marking on it (so does that make it the original version?), no component markings / drawings on the parts side and generally inferior parts quality (although I can't tell if this is due to them having been replaced at some point).
So my conclusion is, at some point one of the channels failed, possibly taking one of the psu 'lyitcs with it and was replaced.
The other one had a number of repairs done by someone who couldn't care less (I think the amp was used in a PA system).
One of the boards is probably original the other is probably revision 5.
One of the mods carried out includes adding power resistors on one side of the mains transformer (still to identify their purpose & function).
My aim is to restore this with quality components but keeping as close to the original as possible.
I'll probably start by locating transistors, carbon resistors etc. that match the originals.
Later on, I'll attempt reforming the psu caps.
I think I'll also go with replacing the ceramics with silver mica or polystyrenes though. They just look naff - original or not.
I would like to clean the boards too and remove the sloppy tinning carefully (hopefully it's not there to repair track damage underneath)
I'd be keen to hear of any advice and find out how your restoration is coming along. If finished, are you pleased with the sound? Made any additional changes ?
Best regards
Stelios