The service manuals usually go by serial number, what is it?

Follow the trace from the bridge + lead to filter cap.  Is it going to + or - end of cap?

Quote from: markorock37 on April 13, 2024, 02:57:58 PMI don't see 2SA872 on Mouser, is KSA992 a good sub?

Yes, those should work.

Quote from: saturated on April 12, 2024, 07:53:18 PMSo I'm going out on a limb here and proposing that the 8k ohm 10w resistor is shorted to ground.  It measures 170v on one end and zero on the other.
A quick look at the schematic (a forensically enhanced version kindly provided by the Mighty Mick) to me places cross hairs directly on the zener diode.

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So I'm guessing that the zener diode is shorted...idk.

That's not going out on a limb, that is good solid logic. (assuming you verified the resistor is ok) 

Quote from: RookieRecurve on April 11, 2024, 10:09:19 AMMy main concern is with the ground.  I am unsure if sharing a ground with an external amp and an internal amp would cause any issues? 

It can cause issues with ground loops or worse.  Some amps require their output (-) terminal to be isolated from ground for things like current feedback schemes.
If you want it to be safe as possible, use a Cliff S2 style jack, which will be insulated from chassis, and also will allow switching of both the positive and negative wires.

Yes it would seem T1 is bad as you verified R5 and D1.  But you also need to check their solder connections.
As T1 and T2 form a differential amplifier, it is best to replace them as a pair.

I think there may be some confusion now about the 'distorted sound'.
You are getting the proper sound of the Boss pedal using the FX return?

Quote from: Bandito on April 06, 2024, 04:15:56 PMWould this one work? https://www.hotroxuk.com/accutronics-blue-reverb-amc2bf3.html

That is too low input impedance.  If you want to go with that miniature type, use the AMC2EF2.  In my experience, the miniature type do not sound very good.
If there is not enough room for a 17" tank you can get a 9" type.  For MOD or Accutronics brand, 4EB2C1B is 17", 8EB2C1B or 9EB2c1B are 9" long types.
You may find them for less money at other UK sellers, like Watford valves, etc.

Sometimes the removal and re-install of tubes will clean up (deoxidize) the sockets.  Maybe that got it working.

Depending on the droning sound, it could be a microphonic tube.  Try tapping them gently one at a time, or holding them with a rag or something to damp the vibration but keep from getting burned.  Generally, microphonic preamp tubes will be higher pitched whistling or squealing, where power tubes will more likely rattle or make low pitched rumbling sounds.

The E to B voltage of T4 (0.3V) seems to not be enough to turn it on.  Maybe checking directly with probes at E and B will give a more accurate reading than checking voltage to ground.
The voltage to turn it on should be developed across R5.  That is created because of current through T1.  What are E,B,C voltages for T1 and T2 ?

Do you have those reversed or is it a typo?  T4 should be the TIP29.

That's not the right schematic for the G.  It's actually for a 1000S (printed on sheet).
100G schematic attached.  Shows 600 ohm impedance for the tank so 4EB2C1B tank should work.

Can you post E,B,C voltages for TR4 and TR6 ?

It is very common for the FM212 (and 65) to develop problems with the differential stage of the power amp (Q9 and Q10), which needs to be well balanced.  That could very well be the cause of your distortion issue, but even if not, it still needs to be dealt with.
Check R83,84,85,86 and replace any that are out of spec.
Replace Q9 and Q10.  This may be tricky as they are no longer made and there will be a lot of fakes available.  Try to get them from a reputable electronic parts supplier, not from Amazon or China.  * edit:  looks like Audiolabs of Georgia has them and they are a legit supplier.
It may be better to find a modern production sub for the 2SC2362K.

Yes, either will be close enough.  Looks like 8EB2C1A is available.