Very often these http://www.ebay.co.uk/itm/Eagle-LT700-LT44-Radio-Output-Inter-stage-Tapped-Audio-Matching-Transformer-/120788551407 are used as replacements but not actually having tried them I cannot comment.  It maybe worth a try

Schematic heaven mirror here - http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/index_HTML.html

Look here < http://www.hawestv.com/amp_projects/fet_preamp/fetpreamp1.htm > some information about equivalents.

So who's going to buy one to pull to pieces

Thats a 2SD526 transistor many mfg leave out the 2S part. Data sheet here http://www.datasheetcatalog.org/datasheet/toshiba/3449.pdf
The T is the manufacturer's logo - Toshiba.

Probably a Class D chip with linear preamp. Horrendous price though about $/£190 :-(

Have you looked here - http://www.beavisaudio.com/projects/NoisyCricket/

People usually recommend Deoxit for this task. Beware of some Servisol products they contain solvents which attack plastics etc including the track on pots (potentiometers, volume controls etc.) Guess how I found out

calx see here http://sound.westhost.com/project10.htm . check out the rest of the site as well. A wealth of info!

The pics you posted do not appear to be the same as the diagram.  Can you please state which model Coliseum you have as there seem to be a number of different types?  The details should be on a plate on the back of the amp.  Also is it a head or a combo?

A circuit schematic would help with your problem.  Have you got one you can show us?

The red button just looks like a lamp with the fixing bezel broken off.  Thats why it can be pushed in?  Can you show the back of it in closeup?

What you need to look for is SENO tin powder.  A chemical when dissolved in water will tin plate PCBs in a few minutes.  Made by a company in Germany I believe.

These older SS amps had a Capacitor in series with the speaker. The pop is caused by this charging up.  It is quite normal for this to happen. Do not remove it as you will probably then have DC on your speaker.

3.4v on the output indicates that the output amplifier is faulty, possibly caused by the brown spots?  There should only be a few mV on the output - TP17 on the diagram.
Are there any spots near the output amp chip U6 a TDA1514A integrated circuit. (mounted on the Heatsink) Can you read a circuit diagram? Its available from here http://support.fender.com/schematics/guitar_amplifiers/Champion_30_schematic.pdf . This should help you.