Fender Blues Deville Issues

[Enzo]

R60 is fine. If you really must measure it, lift one end from the circuit board, so it is out of the circuot. Now it will read 10k. If R60 was bad, none of the switching functions would work.

In normal operation, Q1 is only on for a brief pulse right when channels change. It is not a steady on/off thing.

Fix one problem at a time, the reverb is not the reason you have low output, so forget it. Chances are when you fix the main problem, the reverb will fix itself.

My schematic shows TP33 as greater than -0.6v for ON and less than -0.6v for OFF. So -0.6v is where it changes. 5v is greater than -0.6, so is 15v, and -12 is less than, etc. The actual voltage is not specified, at least not on my print.

The idea of pulling Q1 is that it instantly shows whether the whole mute thing is involved or not. If pulling it cures the amp, we are right. if it has no effect, we can ignore all the switching/muting stuff.

You are right though, the reverb should default on, so swap out U4 and find out. This isn't a time to be cerebral, we just swap out U4, lift Q1, and chances are all the questions are answered.

[g1]

U4 is also controlling the Q1 fet, so if it is bad, chances are that Q1 is ok and just not getting it's turn-off voltage.
I had not realized there was another problem with the switching circuit, so just figured snipping off Q1 would be the quickest test. Normally we would check the voltage at the gate of Q1 to see if it was getting it's proper control signal.

[greenmeanh1]

A little progress. I put in the 470 ohm resistors and got the amp to fire up but still have issues.
I have good volts on X,Y,Z, B+ C- and also the 16v feed. The power section seems to work as i can get sound going into the preamp in jack although its a bit farty. The pre out is not working and if i plug into either of the channel inputs i get very little volume. Less than a watt for sure. I can hear the eq's working, the channel switch seems to be functioning but its not getting signal into the power section of the amp. A bit stumped at this point. I tried numerous 12ax7's with the same result.
There is a TL072 (U1) that appears to buss the preamp line signal to the pre in and power amp out jacks. Is this part ever known to be an issue or blow? Not sure how to test other than to see incoming voltages on pin 4 and 8 which both U1 and U2 have.
Thoughts if any??

R60 is fine. If you really must measure it, lift one end from the circuit board, so it is out of the circuot. Now it will read 10k. If R60 was bad, none of the switching functions would work.

In normal operation, Q1 is only on for a brief pulse right when channels change. It is not a steady on/off thing.

Fix one problem at a time, the reverb is not the reason you have low output, so forget it. Chances are when you fix the main problem, the reverb will fix itself.

My schematic shows TP33 as greater than -0.6v for ON and less than -0.6v for OFF. So -0.6v is where it changes. 5v is greater than -0.6, so is 15v, and -12 is less than, etc. The actual voltage is not specified, at least not on my print.

The idea of pulling Q1 is that it instantly shows whether the whole mute thing is involved or not. If pulling it cures the amp, we are right. if it has no effect, we can ignore all the switching/muting stuff.

You are right though, the reverb should default on, so swap out U4 and find out. This isn't a time to be cerebral, we just swap out U4, lift Q1, and chances are all the questions are answered.

OK Cool info!! U3 had -.2v for drive channel; and 2v for clean channel at TP30 so i based my assumption for U4 TP33 measurements on that working circuit.. I will pull Q1 now and report back. I don't have a 4560 in stock so i will have to wait for that.
I do have a working but old Tek 465 scope but know little about using it effectively so out of fear i don't use it on things i don't know enough about. My biggest worry is poking around on those test points with some 300 or so DC volts on the same pins i would be testing for VAC. I have been using a cheap ass VOM for most of the measurements i have been taking.
Anyway off to the bench!

[Enzo]

A 4580 would work, in fact an old 4558 would work.

[greenmeanh1]

Ok Pulled Q1 and the amp is working as was suggested it might. Quite happy with that and thanks again for all the great tips!
Put J111 in my multi function tester and it comes up as two resistors in series. A 39 and 51 ohm.

[greenmeanh1]

As expected no reverb operation at this point but the rest of the amp is great. You can hear the channel switching noise but its not terrible. I ran it on the bench for a couple hours. So, i gotta wait for a new J111 a new 4560 a one meg master volume pot and a couple other minor details. In the meantime i put the amp back together so i can give it a few test runs over the next week until i get the new parts.
I put some Marshall Knobs on and added a few pics. As you can see my face plate has been modded slightly along with the amp color.
So far decent rescue.
For some reason this post got separated from my thread.

[Jazz P Bass]

Flat face towards you.
Legs from the left to the right: D S G

The J111 should read 30 ohms from D to S unpowered.
D or S to G should be in the mega ohms.

[greenmeanh1]

U4 is also controlling the Q1 fet, so if it is bad, chances are that Q1 is ok and just not getting it's turn-off voltage.
I had not realized there was another problem with the switching circuit, so just figured snipping off Q1 would be the quickest test. Normally we would check the voltage at the gate of Q1 to see if it was getting it's proper control signal.

I didn't snip it off i de soldered. Seems the part is toast anyway. I also left the U4 4560 in for now. More parts ordered.

[greenmeanh1]

Well its been about a week or so since i last worked on this amp. Still waiting for a J111 and a 4560 op amp.
In the meantime i have been using it for several hours a day and done a few sessions with it. Amp sounds great even without the reverb. Out of the six Fender amps i have, this one may now be my favorite.