B-52 AT-100. No signal on over drive channels.

[goldtop5]

I have a b-52 at-212 that doesn't pass signal on the over drive channels. The clean channel works fine. I tried swapping preamp tubes, still no progress. There is voltage on pin 1 on each preamp tube and they all light up. Anyone have any suggestions? Thanks!

[nevetslab]

No image is appearing from your attachment. Preview the attachment so you know it pops up.

[g1]

Try this one:

[glebert]

If you crank the master volume up when drive channel is engaged, do you hear any little sounds if you twiddle the Contour or EQ knobs for the drive channel? I find this helpful to know if there are parts of the channel that are working.

Lots of relay switching in this amp. Maybe look at those?

[nevetslab]

And, do you have a scope available? As there are so many stages in the Drive Channel, you may be able to quickly find where you loose signal. And, what glebert said in fiddling with the Contour/EQ knobs in the drive Ch. If you get system noise thru there and hear the character change, then you're no doubt good from that stage on, with the problem being ahead of that.

[goldtop5]

No signal. When you pull the preamp tubes out, it doesn't make a popping sound coming out of the speaker. No hissing noise either. I don't have a scope. I have a signal tracer that I'm going to use, but it's still on order at the moment. Fiddling with the contour/eq knobs in the over drive channel does nothing. It's like it's muted somehow.

[glebert]

I'm still going with relay problem. Wish the schematic gave the expected control voltages.

[goldtop5]

It seems the relays are clicking in and out on both overdrive stages. There's got to be something that's controlling both of them. Maybe RL5?

[nevetslab]

RL5's DPDT switch contacts selects the LED for CLEAN & OVERDRIVE, as well as Muting the signal thru both the Clean Ch @ V2-2 input grid & the Overdrive Ch @ V3-2 input grid. The schematic of RL5's circuit seems backwards, as it shows when Overdrive is selected, it's grounding the Overdrive circuit, and the same if the Clean Ch is selected. The 'default' relay switching shows CLT1 selected, enabling the Clean Channel signal to flow all the way from the input jack thru the master volume and to the power amp stage.

RLY 1 both routes the signal from V1-1 to either the tone stack of the Clean Ch while grounding the feed to V1-2 of the Overdrive Ch, or Routes that signal to V1-2 and grounds the input to the tone stack of the Clean Ch.

All of the switching logic is shown in the lower left-hand corner of the schematic. I assume it's logic is momentary 'locking', which toggles the relays accordingly to their Clean Ch path or the Overdrive Path. I haven't studied the logic to see if it does this or not. So, it could be a relay that's sticking. Since all of the switching is via relays, you should be able to verify each has the coil voltage it should have for the circuit path to be enabled or not.