Randall RH150 mystery

[JKK]

I recently got this amp for free because it had low and farting output. So far i haven't found the problem. Fuses on psu are fine, the output transistors seem fine and the .1R/5W resistors are fine. But there is something weird going on.
The tube should be 12at7 but it was a tad 7025 and there is one resistor and two diodes soldered on the backside of the pcb that i don't find in the schematic. This is still a work in progress, but if anyone has any thoughts on this i would appreciate it.

The resistor is 1M and it's soldered across c12 to d7. The cathode sides of the diodes are soldered to legs 1 and 3 of tr7 and then soldered together.

[glebert]

This amp has an effect loop, right? I would plug a guitar into the return jack and see if the output is still farty. This should be bypassing most or all of the preamp, including the tube. If you have another amp or mixing board or something you can also send the preamp signal to something else and check the sound quality.

[JKK]

Yeah. Probably should have checked that before dissembling. I just assumed the problem to be in the power amp or psu based on an earlier topic here and the sound it was giving out.

[glebert]

Yeah. Probably should have checked that before dissembling. I just assumed the problem to be in the power amp or psu based on an earlier topic here and the sound it was giving out.

I think 3/4 of amp repair is assembly/disassembly, kind of like gardening is mostly weeding. MIght as well get used to it and get efficient at it.

Most Randalls have separate preamp and power amp boards, I'm assuming yours is that way too? Kinda curious that the schematic shows the tube as part of the power amp instead of early in the preamp.

Also one FYI, I have found Randall amps to often have some changes or differences and you can't find a schematic that lines up exactly with your amp.

[JKK]

The 12at7 is a driver for the power amp on this amp.

[glebert]

Curious to anyone who knows, what is with the op amps on the bottome of the page with the tube? Looks like a graphic equalizer without the pots, is that a spectrum shaping thing?

[Enzo]

Looks like a graphic equalizer without the pots,

Well, if you already knew...

[g1]

The diodes across TR7 are possibly zeners for protection of the Fet.
U1A is part of another feedback network so the 1meg across C12 was probably just a tweak to correct something.

[teemuk]

Curious to anyone who knows, what is with the op amps on the bottome of the page with the tube? Looks like a graphic equalizer without the pots, is that a spectrum shaping thing?

Yes. Mimicks the frequency response of a low-damping factor power amp (i.e. a tube amp) driving reactive load presented by loudspeaker.

[JKK]

I was just putting the boards back in the chassis when i noticed the psu was making a rattling sound when i would shake it. Probably the relay. That doesn't seem good...

[JKK]

So turns out the relay works and so does the amp. I switched the 12ax7 to 12at7 and cleaned the power amp and psu pcbs that were covered in dirt or mold. Also cleaned all the pots. Thanks for the replies anyway

[g1]

So the low output is fixed? Did you find the source of the power supply board rattle?

[JKK]

Sounds clean and proper, not farty anymore. Still need to test it at higher volume when i have time to take it to rehearsal though. The rattle must have been the relay cause there was nothing loose on the board, but i can hear it clicking when switching on.
Anyone know the second hand price for these? Don't plan on keeping it.