MstrVol Super Reverb w Pull Boost Channel Bleed - Stumped

[captntasty]

Hey All,
I'm pulling my hair out on this one and I figure it's time to ask for help... without mentioning everything I've thrown at this one (just chasing my tail) here's the symptoms:

Normal Channel works perfectly
Vibrato Channel is feeding the Normal Channel but at much reduced volume... I can hear everything (reverb, vibrato, EQ, pull Boost, etc) from the Vibrato Channel circuit (barely) just behind the much more pronounced bleed from the Normal Channel. If I pull V1 all is quiet when plugged into Vibrato.

The only voltage that seems off is the shared cathode on V4 - schematic shows 2.3VDC - I only read 1.3VDC. My thinking tells me a bad decoupling cap somewhere... I've tried some subs with no luck. Definitely could use a fresh pair of eyes on this...

http://schems.com/manu/fender/cbs_45...-bmstr_rev.pdf

Any help is, as always, much appreciated.

[Enzo]

Decoupling caps wouldn;t affect the cathode voltage at V4, or is that not what you meant?


First question is this: is this an actual problem or is it just something you didn't expect, but noticed the amp doing? In other words, you may have some channel crosstalk, but when you are playing can it be heard then?

How do you even hear bleed into the normal channel when you are playing the Vib channel? There is no post control on the vib channel. I would expect the sound through teh vib channel to be 10-20 times louder than any crosstalk from the other channel.

V1b and V2b have their cathodes wired together, and that is certainly a place for interaction, especially if the shared cathode bypass cap is dried up.

If you suspect a decoupling cap, then don't guess, find out. The cap in question would be the sideways one beside V4b. CLip another one in parallel with it and see if it does the trick.

Clearly if you pull V1, then the normal channel can;t amplify anything, V1 is the whole channel.

Crosstalk can come from many things. Decoupling is just one of them. SIgnals radiate around inside the chassis. Regardless of problem or no problem, isolate the symptom. SO the normal channel picks up the other channel. I will just assume you have a way to listen to just that channel. So listen to the crosstalk sound. Does the normal channel volume control affect its loudness? DO the normal channel tone controls change the tone of it? Does the normal channel bright switch make the crosstalk brighter? And is the grounding contact on BOTH inpout jacks of the normal channel working? Are both jack tip contacts grounded? Those tests will tell you if V1a or V1b is picking it up. Poor wording, while it is possible I don't mean the tube itself picking it up, I mean the tube STAGE picking it up, V1a or V1b. Using a clip wire, what happens if you ground the grid of V1a? And the grid of V1b?

[captntasty]

Thanks for replying Enzo... there isn't really "crosstalk" between the channels. My explanation wasn't very good - let me try again in light of your questions. When plugged into Normal everything is as it should be - Vibrato Channel controls have no effect - everything sounds great. When plugged into Vibrato, Normal controls have no effect. The signal is much weaker than on Normal but the EQ controls on Vibrato all do what they are supposed to - but the predominant signal is clean/normal. The reverb, vibrato and boost are only audible as distorted background noise behind the clean/normal signal. It seems the Vibrato inputs are feeding the Normal somewhere after the EQ/Volume (somewhere in the mixing network? or at V1b?) The Master Volume (more precisiely the Boost switch) is definitely throwing me off - it's kinda goofy tapping off the reverb. I will try grounding V1 grids tomorrow and bypass caps and report back. Thnx.

[Enzo]

OK, I THINK you might be using terms for more than one meaning. Normal is a channel name, let us not use it as a description, it may confuse no one else, but it confuses me. The two channels are identical, the only difference is the Normal channel has a larger value output cap - .047uf instead of .022uf. They should sound the same, the one is not more overdriven than the other or anything. The output of the vib channel then goes through an extra tube stage to mix the reverb in. SO if I read right, your complaint is that the vib channel sounds weak. And the effects are not right on it.

You are not hearing the vib channel through the normal channel, you are hearing the vib channel as weak and sounding LIKE the normal channel. Is that closer?

My stage grounding and stuf was in support of what I understood your trouble to be. It has no particular value to what you have now described, other than as examples of techniques.

You reported that the cathode voltage of U4 was low, roughly half what it should be. Since the two cathodes share one resistor, that means half the current is flowing. I tend to think that it is more likely one of the two triodes is not conducting rather than that they both are going halfsies. (Halvsies?)

So what voltages are at the plates of V4? The schematic wants 225v on each. Makes sense they'd be th same, both operate off the same B+ with the same value plate load resistor and a shared cathode. SO you either have both at something remotely close to 225v, or they are different. I'd be looking at V4b, pin 6. If its plate is at zero volts, then the plate resistor may be open - it is the 100k up just above the pull switch by V1b. If pin 6 has about 350v, then that resistor is OK, and the tube is not conducting. You mentioned making subs, but not what the subs were. If it were tubes, then hopefully you tried a new V4. If not, then do. If the tube is not conducting and it isn't the tube itself, look at it, are BOTH heaters glowing? Monitor the plate voltage and wiggle the tube. Does the plate voltage try to drop back to 225? Looking for socket not making contact that way.

My spider sense is tingling, it says V4b is dead and you really are hearing crosstalk, just not the kind I originally had in mind. I suspect, and may be wrong about, the signal path is stopping at V4b, and the other channel is picking up radiated signal from the vib channel circuits prior to V4b. That is why pulling V1 kills the vib channel entirely, it is the only way ANY sound from the vib channel gets out past the dead stage.


How did I do?



Forget the boost switch until the amp is working. The boost switch may not do much now, but the channel isn;t working. Get the channel working, and I bet the boost falls right into place.

The reverb sucks because it can;t get through V4b, and the trem sucks because where it goes in, at that little B in a square up between V1b and V6, there ain't no signal coming through to tremulate.

The reverb drive is nothing more than a small power amp, driving the reverb pan instead of a speaker. In fact, you can connect it to a small speaker instead of the pan and hear the music. The signal coming out of the reverb drive stage is good and strong, so all that boost switch does is take that strong drive signal, and send it on over to V4b to mix in with everything else. The signal still goes to the reverb too, of course. Think of it as the same thing as taking a tap off the speaker out of any amp, running it through a couple resistors and using it as a line out. Only here, that "line out" was sent to the mixer stage.

[captntasty]

Phew! Took a while to get this back on the bench... thnx Enzo for pointing me in the right direction. I did replace all plate resistors as the original carbon comps were all over the place as far as value. One of the those plate resistors was installed improperly so V4b had no voltage applied - doh! I guess the lesson here is... take your voltage readings from the tube pin in question rather than the component on the board that is supposed to be connected to said tube!

[Enzo]

Or do BOTH.