Pro Reverb(pro tube) fender hum...

[pontiacpete]

I was wondering if anyone is familiar with these amps. I've worked on several but not with this problem. On the back where the send and return jacks are there are 2 pots. Is this amp suppose to work when in bypass mode? The amp here is at a very low volume when in bypass, but when it is engaged and the pots are turned all the way up there is a normal level of volume but a fairly loud AC hum.
Does anyone here know about this amp and could explain it to me?
thanks
pete

[Enzo]

Of course the amp should work in bypass mode, otherwise it would be called mute mode.

With the loop off, K3b selects the cathode follower output of V3b to feed to the power amp. This completely bypasses the loop level controls and the loop tube V8. If sound ever comes out of the amp, everything after K3b must be working, and things up to V3b as well. SO if there is no sound, then either K3 is faulty, or the cathode connection from V3b to the relay.

CLick on the loop, and all that bypassed stuff is in the circuit. Now the loop controls work. Don;t plug anything into the loop. Does it still hum? Or only with something plugged into it? Neither loop control should be able to go to zero sound.

Make sure little R56 10 ohm resistor in the ground return for the SEND jack has not gone open. That would leave the send jack hummy.

With the loop on, isolate the problem. Plug a guitar into the loop[ return jack. SOund clear? ANy hum? Now plug into the regular input and connect the FX send jack to some other amp. Is the send signal hummy coming out over there?

[pontiacpete]

"CLick on the loop, and all that bypassed stuff is in the circuit. Now the loop controls work. Don;t plug anything into the loop. Does it still hum? Or only with something plugged into it? Neither loop control should be able to go to zero sound."

yes it hums with loop on with nothing plugged in. when turning either the return or send pot the hum goes away.

"Make sure little R56 10 ohm resistor in the ground return for the SEND jack has not gone open. That would leave the send jack hummy".

R56 is not open.

"With the loop on, isolate the problem. Plug a guitar into the loop[ return jack. SOund clear? ANy hum? Now plug into the regular input and connect the FX send jack to some other amp. Is the send signal hummy coming out over there?"

when plugging into the return jack the sound is clear, no hum.
plugging into the reg input and connecting to another amp from the send there is a loud hum.

[Enzo]

OK, you have isolated the problem to the loop drive.

V3b is the split point. If the bypassed loop doesn't hum, then chances are that V3b is OK, because any hum on its cathode (used in byopass) would also be on its plate.( used for the drive signal)

From the plate of V3b, the signal to the drive goes out C24, then through shielded wires - "coax" - to the rear panel for the control and back, winding up at the grid of V8a. V8a itself is a cathode follower driving the loop jack.

Does the send level control affect the level of hum?

At this point I would check the two sections of coax wire. Note on the schematic the shield connections at WJ9 and WJ11. An open shield could allow hum And there could be a problem with V8, swap it with something.

[pontiacpete]

"V3b is the split point. If the bypassed loop doesn't hum, then chances are that V3b is OK, because any hum on its cathode (used in byopass) would also be on its plate.( used for the drive signal)"

In bypass mode there is a little less than half the volume but no hum. (should I go ahead change K3? would that solve the bypass problem?)

"Does the send level control affect the level of hum?"

yes, it lowers the hum.

"At this point I would check the two sections of coax wire. Note on the schematic the shield connections at WJ9 and WJ11. An open shield could allow hum And there could be a problem with V8, swap it with something."

If the ground shield of coax JW9 goes through C39 but when measuring across C39 I get .2 ohms, dead short. It looks like in the schematic that the shield of JW11 on the opposite side of V8 is not suppose to be grounded, right? But in this amp it is. Am I reading that wrong?

Swapped out V8 and still there is hum

[Enzo]

I wouldn;t swap K3 unless I knew it was bad. Apply a signal to the amp. Scope the signal on the two lines coverging on K3. One from the bypass, and the other from the loop return. If one is much larger than the other, then the relay is not doing anything. If they are the same level, but one comes through the relay diminished, then the relay may be at fault. Get out the ohm meter, for the normally closed K3 contacts, measure resistance across them. SHould be about zero, less than half an ohm certianly. The amp will have to be running for this, but so what, click the relay on so the contacts transfer, now measure resistance across the other set of contacts now selected. Same low resistance expected. Hell of a lot easier to measure than to replace the relay and hope that was it.

WJ9 and WJ11 both show their shields connected to circuit ground. The schematic shows the grounds on the main board ends, but I have to assume they carry circuit ground to that rear jack board. C39 connects to that circuit ground at the top end, and the bottom end goes to CHASSIS ground - note the different ground symbol. If the circuit ground is somewhere in the amp connected to the chassis - highly likely - then that cap will measure shorted - both ends go to grounds that are connected together. I see the input jack of the amp is connected to both grounds for example.


If the send control controls hum level, then I have to think the hum is from before the control. That narrows it down to V3b over to the control - unless we are missing something. Scope that plate, then scope after each component until you get to the pot. See any hum? Not sure what it looks like? It ought to be on the top leg of the pot. View that. Hint, hum signal does not have to be large to be obviously audible.

Try this, tack a short across WJ8 and WJ9. Basically shorting out the signal fed to the send pot. Don;t short it to chasssis, short between the two. DO it on the main board end of that coax, then also at the jack board end. Either one kill the hum?

Oh, and is the jack board mounted, with all jack and control nuts in place and snug? Those bits of harware can be involved in providing ground connections to things.

[pontiacpete]

here are some pictures. I put a 1Khz, 500mVAC signal into the input. The lower waves are on the output at the speaker jack. The upper is, in order, JW10, JW6, and JW8.

All the nuts and washers for the back panel pcb are fastened to the chassis.

When playing thru the return jack there is no hum at all, nice and clean.
When playing thru the send jack there is the hum. The hum is over the guitar signal but the guitar signal is stronge. Hum sounds like a jack that left unconnected to an instrument with amp cranked.

There isn't any difference to the upper wave when switching from loop to bypass.

As for the resistance test with amp on, I'm not sure where to clip to, but I tried clipping across JW10 to JW6 and it reads open when switching loop.

[Enzo]

WJ6 and WJ10 are not related, I am not surprised they check as open.

You were concerned the relay was killing the signal. So I was proposing to measure the contact resistance. So when the relay is off, the normally closed contacts on K3b are pins 13 to 11. SO we measure from WJ6 to C32 or R76 near the reverb circuit. That will measure the resistance of those contacts. That would be the bypass condition. That you can do even without power. With the loop on, K3b transfers and closes the normally open contacts between K3b pin 13 and pin 9. To measure that, power must be on so we can energize the relay. Convenient points for the measurement would be the same C32 or E76 on the pin 13 side, and R63 way over to the right.

SInce bypass was what you report as being weak, it is the first condition we should measure.

Try that shorting WJ8 to ground thing I mentioned.

[pontiacpete]

OK, the resistance between WJ6 and C32 is .2ohms with the amp off and in bypass mode.

With amp on, resistance between C32 and R63 with loop on is .3 ohms.

The shorting to ground of WJ8 to WJ9 on both main board and rear board kills the hum

[Enzo]

SO that pushes us back against the first tube, I don;t have it open now, V3 was it?

[pontiacpete]

Are you saying the relay is good, right? At least that is what i was thinking. The first tube is V3 before the loop send. Is that what you are talking about?

[pontiacpete]

I made a mistake,,,oops, when I short WJ8 to WJ9 it not only kills the hum but the all sound.

[Enzo]

The hum is there whether you play guitar or not, right? SO killing the hum is all we care about now. By killing all the sound - including the hum - we have shown that the hum is coming into the system before the point we grounded. Follow?

That jumoper we grounded to kill hum was along the signal path, so yes, we grounded the signal path, so any signal - music, hum or whatever - would be killed. If the hum had remained, then obviously it would have been coming into the system after that grounded point.

[pontiacpete]

Thanks Enzo, I follow now.
Here are some voltages on V3B

pin 6 - 206vdc
pin 7 - 14.8vdc is this normal ?? where is it getting voltage?
pin 8 - 36 vdc

in loop mode when I touch the probe on pin 7 there is a loud fast motor sound, like high speed motorboating louder than the hum but the hum is still there.

Also, when probing pin 8 the hum disappears

[pontiacpete]

pin 6 - 206vdc
pin 7 - 14.8vdc is this normal ?? where is it getting voltage?
pin 8 - 36 vdc

That's easy enough to answer, the voltage is coming from V3B cathode 36vdc, dropped across the 1.5k and the 1M resistor.
I'm still surprised about the reaction to the probes on pin 6 - 8. Anyway still moving back.

What should be on the FET Q1, because there is nothing there even when switching the relay K2?
Using the previous resistance checks on K2 show that it is working.

Also, now looking a the K1A and B relay and was wondering what makes that switch? Is it the turning of R17, the gain pot?