Randall V2 - preamp not working

[johnnyreece]

Good afternoon!

I have a friend who brought in a Randall V2 (tube pre, SS power) for me to look at. It seems one day, the preamp stopped working: No sound, and the lights for each channel don't light up. Best of all? He's taken it to other techs. I see a number of what appear to be optoisolators have been replaced. I'm posting on the off chance that maybe someone has a schematic, or has seen this before. If it matters, the PCB version is 1.2. If someone has seen this before and could kindly point me in the right direction, I'd be grateful. I was hoping for something obvious, but alas; no luck. I've checked all the fuses (and boy, are there a lot!), and they all seem to check out. I'm hoping to not have to pull the boards to see what's going on (all the traces are on the back side, so it's a WAG what's going on under there). Anyhow, thanks for stopping in to take a look! If I can ever get done doing projects for other people, I might actually have something new here soon. So many parts, so little time...

[xtian]

This amp will have a low voltage supply for switching. Did you measure it voltages?

[johnnyreece]

Not having a valid schematic makes this interesting. I finally located the low voltage that supplies the optoisolators. It appears to be a 12v supply that is reduced to 5.75. However, the schematic I have (for the similar T2) shows that it should be 7V. Judging from the appearance of this circuit, though, 5.75 seems to be the design. I attached the T2 schematic. On page 3, you can see the 12V supply go to a 7805 voltage regulator. With the three diodes on ground, around 7V would be expected. However, in the V2 I'm looking at, there is only one diode, and as such, I only get 5.75 (which seems close, the way this circuit is).

[Stevem]

Part of checking the basics is also testing the preamp tubes Cathodes to see that they're idling and drawing at least 1 volt and to also confirm that no plate load resistors have gone open.

It should be pretty easy to signal trace thru the amp even with an AC volt meter, just remember that as you go down stream from the input jack that you will only read signal voltage ( besides on the grids ) with a meter after the plate coupling cap, not on the tubes plate pins itself.

If signal is coming into a opto and not coming out you will find it in this way without a O scope!

[johnnyreece]

Thanks, I'll check that out, too. I don't do much with channel switching, so I don't fully know what to expect/test on this. The thing I'm curious about is, the channel indicator lights aren't lighting up. I can't wait to get back to single channel amps built on turret boards...