Need help getting rid of hum

With the controls as you requested, Dave, it's not just a soft hiss. "Hummmm" goes up in both channels with raising the volume control. But this is interesting... For kicks I took a solder sucker plastic tip and at half of full volume, jiggled the hole in both PHONO LOW jacks. Result = One of the channels amplifies the probing with the plastic. The other channel does not do that. I'm thinking I must have a problem at the jacks. I know Larry suggested tightening them up which I will do next time I open it up. I didn't know the jacks could be tightened. Usually I tighten the RCA cable tips when I feel they're loose.

You're right that with a TT connected that it adds very little hiss and hum.

EDIT: It's the plastic ring (insulator) inside one of the PHONO LOW jacks that appears to be loose, probably a piece or two that broke off. None are broken completely through to the metal though. Investigating further. It looks like several of those plastic insulator rings have deteriorated over time. Some of them I can move easily in a full circle. Also I noticed that each jack inside the chassis has the hole for the RCA plug pin and then a flat plate underneath it (on top of it?). I'm not sure, but some of the wiring to the flat plates look like they might have been touching the pin part. The insulation around those wires is not very good. They're just pieces of cloth insulation. It's easy to move the insulation back and forth on the wire. I created more space between the 2 pack parts to ensure the wiring cannot touch the pin part. I was able to tighten the RCA jacks.

EDIT: Another thing I noticed... The wires that are white with colored stripes that go from the input jacks to the various front panel switches... a few of them have bare wires inside the insulation in addition to inner-insulated wires. Some of these bare wires are connected only at one end i.e. connected at the jack but not connected to anything at the switches end where the inner-insulated wire connects to a switch. Is that normal? At that end you see a piece of wire sticking out of the insulation.

An example is the white wire with a blue stripe that goes to the volume switch. the inner wire connects to the middle terminal on the volume pot. But the bare wire inside the white/blue insulation just sticks out of the insulation. Same thing with 2 wires going to the TAPE MONITOR switch on the front panel.

Thorne

EDIT2: When I have only dummy loads attached, with selector on PHONO STEREO, if I touch the chassis, the 60cps hum seems to disappear and leaves it as a 120cps hum. Also, if I move my hand near the selector switch or the PHONO LOW jack (I'm doing the tests in this paragraph with only headphones), the hum gets louder. I've poked at just about everything inside the chassis with a plastic tool and this hum doesn't seem to be caused by anything that's loose.
 
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I can't believe how silent the Fishers can be most things if I put my ears up to the speaker I can hear something but not Fisher! I realize they cheated somewhat with the whacky volume control circuit but even with it up it is pretty silent.
 
So I think it's fair to conclude that the hum in my 500-C precedes the volume control since the volume control makes the hum louder. So I should start troubleshooting in the preamp section, correct? I'll start reading, but can someone please point out some good chassis locations on which I can probe with my scope? I have an x10 probe and my scope is a Rigol DS1104z:
https://www.tequipment.net/Rigol/DS1104Z-Plus/Digital-Oscilloscopes/
(it's actually a DS1054z, but I upgraded the firmware).
 
if it only does this in phono, the problem is somewhere in that area. Are the phono tube shields installed? Is the bottom on?
 
Bottom was off when I did the testing in post 42. Tube shields are on V5, V6, V16 and V17. I didn't try to reproduce the same symptoms as in post 42 on any other setting, only PHONO STEREO. BUt so far the hum has been also present in PHONO MONO and TAPE HEAD.
 
I searched for info on your scope, but can't determine what the max DC voltage is on it. I'd hate for you to damage it trying to measure the ripple on the B+.

In any case doing a quick search there are some hits that talk about setting it to AC for looking for the source of the hummmmmm. I'd start with the lower voltage circuits such as the heater and bias on the preamp until you can determine how to use it safely for HVDC. Maybe Dave will pop in and correct me.
 
Thanks, Tim. Here ya go...

Maximum Input Voltage
(1 MΩ)
Analog channel:
CAT I 300 Vrms, CAT II 100 Vrms, transient overvoltage 1000 Vpk
With RP2200 10:1 probe: CAT II 300 Vrms
Digital channel:
CAT I 40 Vrms, transient overvoltage 800 Vpk

My scope has no digital channels. So I think 300 Vrms is the max? Guess I have to stay away from plate voltages.
 
Maximum Input Voltage
(1 MΩ)

Got me confused. 1Mega Ohm? How's that voltage?

In any case I don't think you have the voltage handling you need for HVDC unless you buy a special probe. That looks like it's suited to low voltage circuitry work with the stock probes. I could be wrong.

Anyone else got an idea?
 
There are high voltage probes I can purchase. Guess I'll get one that handles up to 1500V. For now I guess I'm limited to 300V.

EDIT: Yup, it says 300V max right on the probe in 10x mode. 150V in 1x mode. Ha! It also says that on the scope over the channel inputs. Duh. "All Inputs 1M ohm // 13pF 300V RMS CAT1"
 
OK, so I have to read the voltages in the schematic (hope they are correct) and stay away from those circuits over 300V.
 
thornev said:
OK, so I have to read the voltages in the schematic (hope they are correct) and stay away from those circuits over 300V.
Click to expand...

Thorne, yes, but I was hoping someone would chime in with an alternative such as a voltage divider or some such.
 
you can usually couple into these things after a cap so there isn't any voltage. Or just stick a junkbox film cap between the HVDC and the probe. Value won't be critical, anything in the 0.01 to 0.47 range will work. Basically whatever you've got laying about thats rated for at least 400v.
 
Thanks, gadg. Sounds like a plan. I went ahead and poked around last night, but couldn't get anything meaningful from the scope. Went back to good ol' multimeter to check voltages, but that's not gonna find hum. Gotta do more reading about scopes. All I got was noise on the scope. Seems like I forgot everything I learned on the scope. (rolling eyes into head)
 
Thanks, Rap. Per POST #1, TAPE MON does not make it go away. So far we've solved the hum in only AUX. I believe that was fixed by rerouting wires around a t-strip.

EDIT: I was wrong. TAPE MON does make the hum go away. But then I get no sound out of the speakers!
 
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Well, the scope probing didn't turn up anything useful. I probed all these tube pins with a 25mV 1kHz signal into PHONO LOW, volume turned up about half, with these results:

preamp:
V14 pins 1 & 6 - nice clean sine wave; same for V15.
phono:
V16 pins 1 & 6 - couldn't get the signal
V17 pin 1 - noise
V17 pin 6 - nice sine wave that wouldn't stand still no matter what scope controls I turned.
phase inverters:
V12 pins 1 & 3 - nice sine wave; same for V13.
output:
V8 pin 6 - nice sine wave (that at times floated like a wave)

I didn't get the 60cps hum anywhere except maybe V17 pin 1 (but I don't know what the noise is). Maybe I drowned out the hum with the 25mV? I purposely chose a quiet signal, maybe not quiet enough. My assertion was that since the hum gets louder with the volume control that the hum must originate before the stage with the volume control.

I tried scoping with no signal, but all I got was noise everywhere, so that wasn't useful.

Any suggestions? Thorne
 
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If I were to disconnect temporarily the IBAM board power supply, it wouldn't change the current to the output tubes or hurt anything else, correct? I want to see if my IBAM board is causing the hum.
 
NO! What you'd be doing is disconnecting the BIAS Voltage on the tubes. VERY BAD JU-JU! You could however with the power OFF, make sure the ground wire is secure to the closest point available, and possibly reroute you're supply wire to the IBAM away from the AC lines.
 
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