Fisher 500b Phono Hum

[Lohengrin03]

I have a restored Fisher 500b. All electrolytic and coupling capacitors have been replaced. This is a bizarre problem so bear with me…

Everything is dead silent with no inputs connected. When in phono mode, there is hum in the right channel when something is connected to the AUX input. The cable connected to AUX picks up hum and radio stations. If I move the cable away from AC sources, the hum goes away. This happens only in phono mode and only happens in the right channel. When in AUX mode, it is nice and quiet no matter what. If I disconnect the cables from the AUX input my phono mode is nice and quiet, even with a record player connected. The only time it hums is when in phono mode and i something is connected to the AUX input. I have tried different cables and tried switching the phono tubes: no change. Any ideas here?

 

[dcgillespie]

Two things --

1. A pic or two would help. Some caps used today are larger than the original caps, so a pic would help to see how the installation was performed to see if that has any bearing on the issue.

2. When did the right channel start humming a tune? Can we assume it was quiet before the recap was performed, or do you know for certain? If it was quiet before the recap, at what point during that work did it start to hum -- or do you know?

Detail information like this can help pinpoint the problem --

Dave

 

[Lohengrin03]

I cleaned the phono level pots and it got better. Still not great. But better.

 

[Lohengrin03]

Somehow, anything touching the ground connection for the AUX jacks, acts as an antenna that picks up radio stations and 60hz hum. I can move the cables away from AC cords and the hum gets less. Also, different radio stations come in and out as I move the cables around. What's weird is it only does this when something touches the ground of the AUX in jacks…could be a bare wire, rca cable, etc. and it only happens in phono mode. So, the cable is picking up AC hum and radio signals and they are making it into the phono stage through the AUX jack ground. Please note it is leaking in via the GROUND of the AUX jack, not the actual AUX in. While looking at the way this is wired, the only way I can see this interference making it from the AUX ground to the phono input is through the phono level pots. The hum/signal interference does change as the phono level pots are moved. Not just the volume of the hum either, the sensitivity changes when the phono level pots are moved. What is going on here?

 

[dcgillespie]

It sounds like you live in an area of strong Radio Frequency Interference (RFI). In such a location, the early stage amplification tubes can act like detectors, allowing the radio station to be head. Quite often, RFI will mask itself as a hum, or other socially unacceptable behavior, because of the overload it causes in the stages it affects.

You can try connecting a 330 pf cap from the Aux ground terminal to the chassis, using the shortest possible leads to make the connections.

Another thing you can try is installing some ferrite beads around the Aux jack ground wiring, and even on the signal leads from the Aux jacks as well.

RFI is quite insidious to eliminate, often affecting one channel, but not the other, and changing it's impact at different times of the day. All you can do is to keep working with it, using good lead dress, and making sure any and all shields are in place as well, in addition to the remedies mentioned.

I hope this helps. Good luck with it!

Dave

 

[Lohengrin03]

Update:

This happens also if something is connected to the tape monitor in jack as well. It seems that any wire connected to any ground on the aux or tape mon rya jacks acts as an antenna picking up ac hum and radio stations. BUT, only the right channel does this and it only happens in phono mode. I do live in an urban area with strong FM signals, however I used to have a 400 and did not have this problem.

 

[dcgillespie]

Fisher became more aggressive with their anti-RFI efforts in their later designs. You might check for the use of such caps at the output and input connections on the 400 schematic, versus what is installed in your unit. RFI can be insidious to resolve. Even using shielded cable for your speakers can help. I'm not saying it is absolutely your issue, but it sure sounds like it is. I used to live in an area of strong RFI, so I had to battle it regularly -- but it can be overcome.

Alternatively, I just worked on a fisher X-202B that -- from the factory -- left one whole bank of input jacks ungrounded in one channel. It simply got missed. You might also check that each jack has a good continuity reading to the chassis as well.

Dave

 

[Lohengrin03]

Each jack that is supposed to be grounded is. All have good continuity accept for the phono jacks which are connected to ground through a 10 ohm resistor and 100 mfd capacitor. I don't know that my location is the problem. Given that it only happens when a cable is connected, it only happens in the right channel and my 400 didn't have this issue, I'd say it's a problem in this receiver. I just haven't a clue as to what would cause this. What would cause leakage into the right phono section via a cable connected to ground?

 

[dcgillespie]

OK, if you've ruled out RFI as your problem, then the next thing I'd check is the integrity of each ground lug that is mechanically attached to the chassis by a rivet. Sometimes they can develop an ohm or two of resistance, which is plenty enough to cause hum problems in the channel with the affected ground connections.

What happens if you touch just the ground sleeve of a cable to the ground sleeve of the affected jacks in the right channel -- that is, not actually connecting the input into the jack? Does it hum them? How about if the ground sleeve is simply touched to the chassis around the jack area?

Dave

 

[Lohengrin03]

OK, if you've ruled out RFI as your problem, then the next thing I'd check is the integrity of each ground lug that is mechanically attached to the chassis by a rivet. Sometimes they can develop an ohm or two of resistance, which is plenty enough to cause hum problems in the channel with the affected ground connections.

What happens if you touch just the ground sleeve of a cable to the ground sleeve of the affected jacks in the right channel -- that is, not actually connecting the input into the jack? Does it hum them? How about if the ground sleeve is simply touched to the chassis around the jack area?

Dave

I will check all the rivet connections, however both aux jacks, A & B share the same riveted ground lug, so its weird that both channels would be effected. With just the ground sleeve (not actually connected to the input) it still hums. If ANY wire is connected to the ground sleeve of an RCA jack for aux and tape monitor in, it hums and picks up radio signals while in phono mode. It does not hum when touched to any part on the chassis.

 

[Lohengrin03]

So the AUX jacks ground sleeve (the outer portion of the RCA jack) is physically isolated from the chassis like all the other jacks. The AUX RCA jack grounds are jumpered to the phono level pots and then each are connected to ground via two separate wires running through two separate shielded metal tubes that runs to a single tab that is riveted to the chassis up near the preamp tubes at the front. If I touch a wire to this tab, no hum just like there is no hum if I touch the chassis anywhere else. I can't for the life of me figure out why one channel has a problem and the other is fine. What I can say is that I don't think it has anything to do with the AUX jacks themselves because it also happens when a cable is connected to the tape monitor jacks. I am really at a loss here.

 

[Lohengrin03]

In the phono section, I did replace the .01 ceramic caps with vitamin q's and the .022 with Russian poi's. I'm pretty sure it did this before I replaced the caps, but I'm not 100% positive. I kept the leads as short as possible and placed the new caps in the same position as the old ones, but I guess it's possible I got a bad cap…could a bad coupling cap cause this problem? I would think I would experience other issues if a coupling cap was bad, but I could be wrong.

 

[Lohengrin03]

I have touched a wire to every rivet on this chassis and no hum. The only points where hum is introduced is when a wire is touched to the outer ground on the AUX or any of the tape monitor and reverb expander jacks…and not matter what jack you touch the wire to, it is only the right channel the hum shows up.

 

[Lohengrin03]

What is also extremely weird is that if I have a cable plugged into an aux jack and the right channel is humming like it normal does, the hum does not go away if I touch the sleeve of that cable to the chassis.

 

[dcgillespie]

Is the shielded tubing grounded at both ends, or at multiple points along the way? Does it follow the same route as the shielded tubing does for the other channel? I have diagnosed instances where shielded tubing was carrying magnetic current through it, that then induced this current as noise into the wires running through the tubing. Again, some pics wold help tremendously.

Dave

 

[Lohengrin03]

The shielded tubing is grounded at multiple points along the way. Both channels follow the same path. I have ruled out the coupling caps in the phono section as a possibility.

 

[Lohengrin03]

more pics...

 

[Lohengrin03]

If the problem were in both channels, I'd say it was just environmental with my location being urban and close to FM towers, but the other channel is just fine...

 

[Lohengrin03]

Something interesting…If I use my DMM to check voltages, when I touch my probe to pin 1 on each tube for the phono section…On pin one of V7 I get a normal pop, but when I touch my probe to pin 1 of V8, I get amplified FM interference.

 

[dcgillespie]

If you will, please identify which preamp tubes serves which channel in your pics. One other thought -- is the integrity of the top-of-chassis flexible ground wire that grounds the frame of the phono preamp tube sockets to the chassis still in tact? If one is broken, it will definitely cause a hum in the channel with the broken lead.

Dave