Revox model 40 phono hum

Winters

Active Member
I am currently repairing a Revox Model 40 amp, made in Switzerland in the early 1960s. I have figured out most of the problems with the power amp section (see thread: http://www.audiokarma.org/forums/showthread.php?t=620580 ) and I got it to work nicely using line inputs - good sound, absolutely no hum whatsoever.

On the other hand, I have a problem with the phono section. It works and produces sound in both channels but there is a slight AC hum present that varies with the volume. It does not overpower the music but there is too much of it for a hi-fi amp.

I checked basic ground connections (phono rca inputs/mode switch/first preamp tube) and I see no faults there. Tried different turntables with different cartridges - nothing really changes.

I would be grateful if someone had an idea concerning this hum.
 
examine the filament DC, grounding, the electrolytic cap etc. See what happens if
you replace the DC with batteries.
 
Phono preamp filament is supplied by an independent rectifier (new, silicon) through a 2200uF (originally 1800uF) capacitor (new, good quality - F&T brand).

Are those "mustard" capacitors subject to failure due to wear? There is quite a few of them in the amp:

DSC04652.JPG
 
Hard to tell but the pc board on the standoffs looks cracked on the left side by the two yellow wires. Maybe a problem there.
 
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No, it can't be that because:

a) the board is not cracked
b) it has nothing to do with the phono section specifically - those PCBs are responsible for the EQ controls and that works splendidly.

Has anyone any experience with mustard caps like those in my amp?
 
I do see a electrolytic buried underneath one of the switches that doesn't look to healthy.... :scratch2:
 
Again,
the initial 12ax7 is most sensitive for hum. By feeding it's filament from an external
battery you can exclude hum from the DC heater AND the tube itself. If hum disappears
you have the problem in in DC PSU or the tube itself( some 12ax7 might experience
less then wanted isolation between filament and cathode).
If hum remains you can start lookung elsewhere.
This method, dividing the problem in half, is very powerful and save time since you
only have to spend time in the half that causes ( hum,hiss distorsion or whatever)
 
It won't affect bias to disconnect the V1 and V2, they are connected in paralell with the
bias .
V1 and V2 however are series connected, so unless you have 24V of batteries you will
have to rewire bothV1 and V2 filaments.
 
Looks like the phono tubes power from the output tube bias supply. If the 600uf capacitor in the bias supply is good, then the power supply should be healthy. Checking the ground from the bias supply and on the phono tubes would be worth doing though. In theory, removing those tubes should not affect output tube bias, but I wouldn't count on it. It may cause bias voltage to decrease (become more negative) and cause the output tubes to conduct less. It would not harm anything, but it may cause increased distortion. Thats an un-regulated power supply and reducing the load on it will probably affect it's output voltage.

I would also be checking wire routing. If any AC wires are near the phono circuit or its wiring, see if you can move them further away. Check where the grounds are for the phono inputs too. It looks like it may be a shielded wire from phono input jack to the phono tube. Make sure that the input jack is not shorted to the chassis if it should have it's ground wire run to the phono tube cathode resistor instead.

It looks like this is a pretty compact amplifier. Some designs like that were prone to noise just because everything was packed too close together to get good quiet signal.

schematic from the other thread in case anyone wants it:
http://elektrotanya.com/studer_revox_model_40_ser2_sch_1964_sch.pdf/download.html
 
Thus far, I'm trying to eliminate the simplest possible causes. I have replaced the wires from the phono input to the first preamp tube because one of them was visibly oxidized.
No change whatsoever after that.

Also replaced AC wiring between the on/off switch and the voltage selector with two solid core wires twisted together. This is how it should be according to photos of the amp I've found on the Internet.

What I didn't mention before but may be significant is that the AC hum appears only with a phono cartridge connected to the phono input. With nothing connected, there is a constant swooshing/hiss in one channel when the phono mode is selected. I've never seen anything like that in other amps. Something is wrong and I don't know what.
 
None of my turntables have external grounding, I'm running several 1960s record changers and manual record players. Everything is OK with the turntables, there is no hum etc on any other amplifier.
 
If it's only got hiss with the phono source disconnected, and it hums with the phono connected, then the problem is IN THE PHONO. Meter the ground's and connections from the cartridge to the phono output cable far end. It's possible you have an open at the cartridge or other connection.
 
It's not just a regular amp hiss, it's loud and ONLY in one channel.
No way I've got a bad cart or a loose connection somewhere. Every turntable/cart I have tried with the Revox works fine with other amps - no hiss, no hum, only good, clean sound. I know my TTs well, as most of them required repairs when I had first got them...

With the Revox, I get good sound using the phono inputs, BUT with pronounced hum.
 
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