Hum in 1978 Vibrolux Reverb

[Gene]

Hi,

I have a 78 Vibrolux Reverb.  It has 120 hz hum (not 60hz).  The hum is not too loud with the volume boost off.  However, it does become quite loud when the boost is on.  I have replaced all the electrolytic capacitors.  The heater wire is grounded with a pair of 100ohm resistors.

I believe this hum is in the reverb circuit area.  By pulling out V3, the hum from the volume boost can be stopped.  Of course, with V4 out, all hum stops but no guitar sound from channel 2.  

If you follow the wire from Pin 7 V4, it connects to a 3.3meg resistor.  I grounded that junction and can stop the hum.  Anyone have experience with these amps?  Are loud hums when the boost is on common?  I see that it uses the reverb circuit for the boost.

[Tiny_Daddy]

Lots of things can cause that. Have you tried a new tube for V4?

[Gene]

Lots of things can cause that. Have you tried a new tube for V4?

Yes, the tubes have been changed and tested.  It is not the tubes. 

What are the "lot of things" you are thinking of?  The hum is 120hz.  So, I've eliminated such things as bias and filaments.  The power filter caps have been replaced with good F&T 16uf/475v.

The ground shows good with 0.3ohms pretty much throughout the circuit.  Can bad resistors cause a loud hum?  Visually, the resistors look fine but I suspect the 3.3meg resistor that connects to pin 7 on V4.

[Tiny_Daddy]

Some or all of the old Fenders have a separate ground wire for the first filter cap that runs to ground at the power transformer. This is important for reducing hum. I don't have a Vibrolux wiring layout. Other causes: heater-cathode leakage but you already changed the tubes. And improperly grounded reverb tank. But usually it's an improper ground.

[Gene]

Some or all of the old Fenders have a separate ground wire for the first filter cap that runs to ground at the power transformer. This is important for reducing hum. I don't have a Vibrolux wiring layout. Other causes: heater-cathode leakage but you already changed the tubes. And improperly grounded reverb tank. But usually it's an improper ground.

I've checked the grounds quite carefully.  Both visually and with a continuity check.  It all seems good.  The amp hums louder without the reverb pan connected.  I've also tried several different pans.  No change.

One thing I noticed.  The reverb transformer on the primary shows open.  Should it not show continuity between the red and blue wires?  the secondary shows continuity with the green and black wires.

[Tiny_Daddy]

That's another thing: a reverb transformer that is shorted, primary to chassis can draw excessive current from the B+ which causes excessive power supply ripple, therefore hum. Try disconnecting the transformer.

[Shrapnel]

Some or all of the old Fenders have a separate ground wire for the first filter cap that runs to ground at the power transformer. This is important for reducing hum. I don't have a Vibrolux wiring layout. Other causes: heater-cathode leakage but you already changed the tubes. And improperly grounded reverb tank. But usually it's an improper ground.

I've checked the grounds quite carefully.  Both visually and with a continuity check.  It all seems good.  The amp hums louder without the reverb pan connected.  I've also tried several different pans.  No change.

One thing I noticed.  The reverb transformer on the primary shows open.  Should it not show continuity between the red and blue wires?  the secondary shows continuity with the green and black wires.

IF what you say is true, with power to the chassis, and in run mode, carefully check the voltage on the plate pf your driver reverb driver tube. IF it is as open as you measured, you should have 0v on it (0v on one side, full node voltage on the other.) IF shorted you should have full node voltage on it (measures the same no matter what side of the primary you measure from.) IF it is indeed open, replace it. (Warranty it if under warranty.) IF it was working properly, there should be a slight, but measurable, difference in voltage.


The fact that it hums worse when you disconnect the reverb tank says the problem may be in that area though, so concentrate there after looking at that driver transformer more closely. When the tank is connected, is the shell of the tank grounded? Is your return line shielded and the shield grounded? IIRC, both line in and line out require a ground connection to be present. Also if you reverse connections (in and out) you can wind up with hum. Try shorting the return to ground and see if the hum disappears. IF it does, definitely double check how you are hooking up the tank, otherwise look to the return stage itself. (pull the return tube and see if the hum goes away.)

[edit] fixed the close tag for the bold.

[Gene]

The blue wire from the reverb transformer goes to the 2 plates of the 12AT7 reverb driver.  This measures 388v.  The red wire of the primary shows 406 volts and this goes to a power filter cap and other areas.

If you short the return from the reverb pan to ground, the hum stops but so does the reverb and the pull volume boost makes no change.  This 120hz hum becomes very loud only when the pull boost is used.  With it off, the hum is very quiet and not a show stopper.

[Shrapnel]

The blue wire from the reverb transformer goes to the 2 plates of the 12AT7 reverb driver.  This measures 388v.  The red wire of the primary shows 406 volts and this goes to a power filter cap and other areas.

If you short the return from the reverb pan to ground, the hum stops but so does the reverb and the pull volume boost makes no change.  This 120hz hum becomes very loud only when the pull boost is used.  With it off, the hum is very quiet and not a show stopper.

"pull volume boost" isn't something I've seen on any of the Vibrolux Reverb schematics or layouts. What chassis is in that? (aa270, aa964, ab568, ???) So, when you short the tank (like switching off the reverb) that boost becomes completely ineffective (as in permanently off or on, and which?)

[Gene]

With the reverb tank shorted at the input, the boost becomes off whether the switch is on or off.  Ofcourse, the reverb is off as well.

Here is the schematic.  This amp is a 1979.  This is the closest schematic I can find.  The schematic shows a boost pedal jack as well but my amp does not have the jack.  Just the pull switch.  No master volume.



You might have to save the image onto your computer and zoom it up. 

[Geezer]

Although not labeled as such, I believe this schematic might apply>>>

http://www.schematicheaven.com/fenderamps/cbs_45w_mstrvol_pullsw_super-pro-bmstr_rev.pdf

[Gene]

Thanks for the link.  I looked at it and there is significant differences to my amp.  The one I posted is pretty much exact except the volume boost pedal jack.

[Gene]

It looks like your version was shrunk to fit the forum so it's hard to resize properly.

PM me your email.  I'll send you a pdf of the schematic.

Gene

[RicharD]

Have you tried a different set of RCA cables?  Those thangs can be pesky.

[Shrapnel]

LOL, erased the one that was replied to, but content is quoted.

Anyway, until I get that one, closest I might have is the Princton Reverb  with boost pedal, and the deluxe reverb with volume boost (don't appear to be the same schematic part # as yours though.)

Based off those though, shutting off the reverb by shorting out the return would kill the boost too. Have you checked lead dress in that area? with the components and wires available, AND powered up with a speaker load, chopstick (non-conductive probe, wood chopstick, etc.) the wires related to the boost and its components. I might have a better Idea when I see the specific schematic in better detail though.

[Gene]

Have you tried a different set of RCA cables?  Those thangs can be pesky.

Yes.  And different reverb pans as well.  It hums louder with the reverb unplugged.  Only with the boost pulled on.

[Gene]

LOL, erased the one that was replied to, but content is quoted.

Anyway, until I get that one, closest I might have is the Princton Reverb  with boost pedal, and the deluxe reverb with volume boost (don't appear to be the same schematic part # as yours though.)

Based off those though, shutting off the reverb by shorting out the return would kill the boost too. Have you checked lead dress in that area? with the components and wires available, AND powered up with a speaker load, chopstick (non-conductive probe, wood chopstick, etc.) the wires related to the boost and its components. I might have a better Idea when I see the specific schematic in better detail though.

Yes, I've done the chopstick with the wires connected to the boost switch and the circuit.  No change.  I can ground out the junction with the 1k and 220k resistors coming from pin 7 of V4.  Also the junction where the 3.3m and 10pf cap ties together.  This eliminates hum.

[Shrapnel]

Yes.  And different reverb pans as well.  It hums louder with the reverb unplugged.  Only with the boost pulled on.

Ok, reviewing here, as to be honest, I haven't dealt with reverb before, but see this as a learning experience. 

Pulling V3 (the reverb driver tube) the hum disappears. Shorting the input to V4b turns off both reverb and boost (as expected) and kills hum.

Pulling V4 would not only cut off V4b, but V4a, required for the Vibrato channel to work and also kills the hum. (Normal channel should work fine.)

Transformer winding appear to be OK. (Higher resistance on the primary, adjust scale to see continuity. Voltages look OK and say primary is good. Secondary will be low resistance, as it will be for a low impedance.)

Removing the tank causes extra hum. Did you disconnect the cables as well? (should kill hum, but then again no boost.)

The boost seems to take the channel's output and after the 470K resistor, runs IN PARALLEL, a 220K and 10K voltage divider and feeds it back into the tank through a 1K resistor along with the dry "in" signal to the reverb pan. (Switch closed. Switch Open, and the signal doesn't get to the reverb circuit.) IF the jack  existed, it would short out the 10K resistor in the schematic when the foot-switch was used, (adding a 1K load in parallel with the reverb tank when the PP switch is "on.") removing the "boost" from the signal.

Circuit Examination:
One Leg of that 10K resistor should be grounded. the other connects to both the 220K and the 1K resistors (as well as the foot-switch if it had a jack for it.) the other leg of the 220K resistor should  connect to pin 7 of V4, along with that 3m3 resistor and the 470k resistor and the 10pF cap. The side of the 1k resistor NOT connected to the 10k and 220k resistors should go to the switch with the other end of the switch connecting to the GREEN wire of the reverb transformer.

Questions:
When the boost is pulled on, what's the status of the switch? open or closed?

What are the conditions of the 1K, 10K and 220K resistors that are part of this boost circuit? I know you said physically they look good, but what do they test? (crank the reverb level to full while testing the resistor values to minimize interaction.)

Reverb functions fine in either state of the boost? (Other than the hum.)

Did you check the component connections in addition to the grounds you already tested? (especially those I've mentioned here?)

[Gene]

Pulling V3 stops the hum as well as reverb.
Pulling V4 stops the hum.  Normal channel works fine.
Transformer reads 1k ohm for primary, 0.7 ohm for secondary.
Yes, cable disconnected to reverb tank = louder hum only with boost pulled.

In my amp, there is no 10k resistor.  It is a junction of 220k and 1k.  I suppose because there is no foot switch jack.  I sent you a schematic of a deluxe reverb with boost switch/no jack and it shows how my amp is wired.

When the boost is pulled, the switch is closed.  I'll recheck the resistors but I believe they were in the range.  I did do a visual check on connections.  Maybe I'll remelt the solder on this circuit area.

[Gene]

As a long shot, I replaced the 220k and 1k resistor that connects to pin 7 of V4.  No change in hum   

[Shrapnel]

Ok, I think I understand the boost circuit now. It takes part of the signal from the reverb driver, runs it into the 1K load (in parallel with the reverb tank) and then uses the 220K resistor to mix the signal back in.

Possibilities left include the reverb transformer's orientation with the choke (based on proximity) and shielding between switch and reverb transformer (or wiring proximity to choke between switch and reverb transformer.) After that, I'm not sure what else it could be.

You could try unbolting the reverb transformer and turning it 90 degrees if there's enough wire, and bolting it back down and see if it fixes it. Is it the original reverb transformer? If not, the internal wiring may be different enough for inductive coupling.

[Gene]

I now know it is not the reverb transformer.  It is also not the .1uf capacitor from pin 6 of V4.  The choke is quite a ways away and is between the power and output transformer.

The reverb transformer is original as well as the choke and the others.  This amp is not giving it up easily.

[Gene]

I checked some voltages and the B+ shows 400v.  It seems a bit low.  I think 450v at the standby switch is normal.  The power transformer shows 345-0-345 which is correct.  I tried couple of different 5U4s.  Same 400v.  I then tried a 5AR4 and it shows 430 volts.  I don't think it has anything to do with the hum though.

Still hunting.  Any ideas welcome.