Avalon 737's that is my language and has been for some time now.
There are a few things to look at that could cause such an issue.
For starters the valves/tubes used in the units are 6922. I believe the euro version is an E88CC.
From there the PSU outputs 5 X different voltages B+, heaters, 12V for relays, +/-34VDC for the class A circuits. There is further onboard regulation on the main board to give around a +/-18VDC for the IC's that do the meter drivers and DC offset. Further more, phantom power voltage is taken from the B+ feed.
It's pretty neat how that is done.
Anyway, 2 ways to check the PSU without a load and with a load. Because of the design, we can measure the PSU unloaded and see the expected voltages when powered on but not connected.
the color code for the 8 pin molex is as follows
green and blue wire is ground
Red = +34VDC(class A circuits V+)
Black = -34VDC(Class A circuits v-)
Orange = +12VDC(Relays, leds, vu buls)
Yellow = +6.X, usually around 6.2 - 6.3(heaters)
White = B+(usually around 180VDC)
As always measure in reference to ground.
Once connected back into the main board we can measure at the following points when turned on and see the following
Solder point T1 on the main board. This is where the B+ line runs into the tube card.
solder point T2 on the main board. This is where the heaters(DC) run into the tube card.
On card 5600-7373 measure the top of R34. It's the +34VDC for the class A circuits.
On card 5600-7373 measure the bottom of R43 as that is the -34VDC for the class A circuits.
There are acceptable variances on the class A stuff usually with in a volt or two. the B+ has an acceptable swing of around 5V. so while it is expected to be around 185V, anything with in 5V is considered acceptable, so like 180VDC is is often found.
From there the 12V for the relays can be measured at the lamps as it also feeds the lamps and the leds. If they are working then your 12V is o.k.
If the voltages are o.k. but the level is low, you either have a tube issue, or you have an issue with card 5600-7372. That is the Class A circuit I/O card. Aside from the tubes, In and out happen on that card. So we should confirm the card is working. Usually if there is a fault with that card it is visible(burned resistor, blown capacitor, that kind of action).
Lastly We should also confirm what is working and not working. I have seen where a faulty tube in the V1 stops the mic preamp from working but the unit works with line level.
The tubes themselves layout from left to right, chassis wall to center V1, V2, V3, V4. V1 is the preamp tube and should be low noise, V3 is the output tube, V2 is needed for the eq path and V4 is needed for the compressor. V2 also plays a hand in the line input and if pulled the line input will not function.
best to test the unit as follows assuming there PSU is o.k.
start with the line I/O, no compressor, no eq and see if the level is healthy. With preamp and output at center detent the level at the line input should be the level at the output. It may very slightly due to calibration but it should be very close with a .10 of a dB or so but expected to be less then a dB. anything beyond that means there is another issue at fault.
Assuming the line check outs, test the mic pre side. If the line works but the mic does not, you most likely have a tube related issue.
If both of those are o.k. then add the eq, if there is a significant drop in level, you either have a fault on the 5600-7373 card or a tube issue(V2)
Then lastly add the compressor if it does not function either it's a V4 problem or a fault on card 5600-7374....
