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Need help getting rid of hum

Rats. I knew the 200k IBAM resistors were going to be suspect eventually. Does it matter that the IBAM wiring does not go anywhere near the power section of the chassis? I have the wires threaded through the holes in the chassis next to can cap C92. That side of the chassis is on the other side of the long metal shield that separates the power section and INPUT jacks from the output section.

I followed that thread about the resistors' best location. I grimaced because I interpreted the original Drew schematic as if they are to be on the board, not left in the chassis. Oh well. Thanks for the input, Dave.

EDIT: I reread your post, Dave. With the volume all the way low, no, there is no hum. The hum volume follows the volume of the volume control.
 
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So far I found that 2 wires emanating from the SDS board were held on to chassis wires by wire that had melted and broken off. I opened those heat-shrink tubings, stripped off insulation exposing more wire, resoldered and replaced the heat-shrink tubing. Then I removed the 2 blue wires from the t-strip terminal, soldered them together going around that t-strip and insulated them with heat-shrink tubing. I'll report back later.
 
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Good news and bad news.

Hum is completely gone when AUX is selected. I mean even at full volume...not any hint of hum. Fabulous!

However hum is still present with PHONO (stereo and mono) selected. I didn't notice this before, but the hum gets louder when I select TAPE HEAD. More investigation needed. Both cases are with no devices attached to the INPUT jacks.

Suggestion that remains is moving 200k IBAM resistors into the chassis. Also try shielding the long wires that go from the external IBAM board to the various connections inside the chassis. Those wires enter the chassis through the holes in the chassis around can cap C92. The B+ and ground wires for the IBAM board are wrapped around the metal tube in which the INPUT device wires travel to the selection switch. Could that be a bad thing?

EDIT: Also I might try shielding the RIAA mods to see if they are picking up any EMF.

Thank you everyone. Thorne
 
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are you testing with no turntable connected? Hum with nothing on the phono jacks isn't unusual. Try a shorting plug if you have nothing hooked up. Its just an RCA plug with the center pin wired to the outer part.
 
Yes I was testing with no turntable attached. When I attach a turntable, I get the same hum at the same volume, and it still gets louder as volume control is turned up.
 
Thorne,

Are you sure that the TT isn't introducing it's own hum? I.e. The TT doesn't hum on other amps? The reason I ask is that I'm constantly refurb'ing TT's that I pickup and dealing with hum issues with them is a regular part of my refurb activities.
 
Tim - I'm sure because I have the same hum with no TT connected. I tested with a TT connected after gadget's question above, but did not turn on the TT. What I do get with a TT connected and powered up is a lot of vinyl surface noise. But that's because I have a very expensive cart in my Pro-ject TT. It picks up just about everything in those grooves. I have the cart horizontally and vertically aligned to get the best tracking possible. My turntable is always grounded to the Fisher chassis.

EDIT: To be honest, if I turn the volume up all the way on the headphone amp. I do hear the 60cps hum. But I would never listen in headphones at that volume. My ears would be ringing for decades !
 
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Thorne,

As some indicated it is common to have hum on TT inputs with nothing connected. That by itself isn't necessarily an indication of a problem.

A reasonable test is to hook up a proven TT that is shown to not hum with other amps. The reason I say that is that TT's can hum - A Lot. I regularly have to mess with headshell contacts, muting switches, interconnects, etc to get TT's to not hum. Some cartridges are more prone to hum too. There is the famous (infamous?) Grado hum. I'm not saying this is your problem, but TT's can be hum bugs. Now if your Pro-ject TT is relatively new then you may be ok.

Also, you might try not using the ground wire on your TT. I found I had hum problems with a Dual TT /w Shure M91 and my Fisher KX-100 when also using the ground wire. I left the ground wire off and I'm getting no hum that way, but get hum with the ground connected. (And yes, I know to use the 4ohm tap for ground on the KX-100. It's a weirdness on the KX-100 and some other Fishers that have that Center Channel feature. I don't think your 500C has that weirdness though.)

Again, I'm not saying this is your problem, but my experience is to be suspicious of TT's when hum is present. Trust but verify.
 
Tim - I tried disconnecting the ground wire and the hum gets slightly louder.

I understand what you're saying about TT hum issues. I've certainly had my share as a vinyl junkie through the years. It wouldn't bother me except for when I wear headphones and the hum is loud enough to be annoying, especially during quiet music passages. That's really why I'm trying to get rid of it. I know it is possible because when I hook up my TT to just a headphone amp, no hum.
 
The reason for the PHONO and TAPEHEAD Hum with nothing connected is that they are HIGH GAIN STAGES (more so than tuner, Aux, Tape Monitor, etc. (I added Tuner in case some people wonder about integrateds here.)). So putting a shorting plug in the inputs is a good test of the section. As both Tape HEAD and PHONO use the same gain stage (with different EQ) Hum with a source NOT connected is pretty normal.

Does your turntable have a muting switch (arm in rest position, muting on, and when in play position muting off). If so move your tonearm to the playing position (anywhere on the LP) and listen to see if hum drops off. Also check the grounds internal to the Thorens. And continuity from the plus side cartridge wire to the center pin of the output's RCA, with the Negative side cartridge wires to the corresponding outer rim of the RCA Jack.
 
My TT does not have a mute switch. It's a Pro-Ject Debut Carbon DC Esprit SB (https://www.project-audio.com/en/product/debut-carbon-esprit-sb-dc/). The cart is an Ortofon 2M Black, very expensive. I get no hum when I play it through only a headphone amp. I believe that indicates the turntable is not the problem.

Is PHONO LOW also a high-gain stage? That's the input I use for my TT. The hum follows the volume control. In other words, no hum when volume is at zero.
 
+1 on what Larry said. Much better than what I said.

Thorne, so you said there is no hum on AUX. Did you try to run an input into AUX to see if it sounds ok? If it sounds good, then your problem is probably not in the power amp section where you IBAM lives. I would think the problem would be limited to the phone/tape preamp (and not the line/aux preamp) and possibly even external in the TT.
 
Yes, I ran a CD player thru AUX and it was a pleasure without the hum. I agree the problem is in the "phono/tape preamp" stage.
 
Others are better equipped to deal with issues in the phono section, but if it truly is a 60 hz hum then there are only a few ways I can think of that can bleed in 60Hz.

- Heater circuit? DC filtering not working right?
- B+? <Same reason as previous>
- Induced from something nearby? Any AC anywhere near this stage?
- Bad ground or a ground loop in the circuit?

So some of these would affect both channels. You could check the DC on the heater circuit for that stage with a scope to see if there is any 60Hz ripple. (Did you say you had a scope?)

I've exhausted my ideas for the moment. I really hope you figure this out. Those issues are maddening.
 
Tim, thanks so much for your replies. Yes, I have a scope. Is it OK to probe at anywhere in any circuit? I know there are test points in the schematic but not many.

I'm not sure where the heater circuit is because I have that SDS board which serves as the power and filtering stages. But I know now where all its wires go.

I have considered that the problem may be EMF from the Fisher's environment. It is next to a 32" LCD monitor and there are LED lights all around the small 9x9 room. I have to turn all that stuff off to test. Thanks for reminding me. I could also try plugging in the Fisher to a different AC receptacle. I have a special 115vac power strip on the floor for the Fisher.

EDIT: It's not the monitor nor the LED lights. And I tried a different AC receptacle and that's also not the problem.
 
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Phono LOW and Phono High are differentiated by whether your MM cart is below 5mv or above 5mv. They both tie into the RIAA circuit at the same point. TAPE HEAD also goes into the RIAA circuit on the other side (NARTB EQ). Try the TT on Phono High, just for shits and giggles. More than likely it'll hum also. But 1st try shorting plugs on both tape head, and both of the phono inputs. See the Owners manual for description of the Low and High PHONO JACKS.
 
Now this is interesting... I tried shorting the PHONO LOW jacks as requested. When I short each channel individually, the hum goes away in that channel ! (the hum remains in the other channel) Are we on to something? Same thing shorting the TAPE HEAD channels. But the TAPE HEAD inputs need work as the hum gets a lot worse when I jiggle the cable around. The PHONO LOW jacks behave similarly but not nearly as bad.

I tried PHONO HIGH with my TT a long time ago and the audio was distorted IIRC.
 
Killling the Hum with a shorting plug is the Expected and Normal result of a good section. Your jacks sound like they are loose tho. Tighten them up (center pin hole) and recheck with the turntable. One other thing with the turntable, if the cable have plugs on both ends, try another set of cables.
 
thornev said:
Tim, thanks so much for your replies. Yes, I have a scope. Is it OK to probe at anywhere in any circuit? I know there are test points in the schematic but not many.

I'm not sure where the heater circuit is because I have that SDS board which serves as the power and filtering stages. But I know now where all its wires go.

I have considered that the problem may be EMF from the Fisher's environment. It is next to a 32" LCD monitor and there are LED lights all around the small 9x9 room. I have to turn all that stuff off to test. Thanks for reminding me. I could also try plugging in the Fisher to a different AC receptacle. I have a special 115vac power strip on the floor for the Fisher.

EDIT: It's not the monitor nor the LED lights. And I tried a different AC receptacle and that's also not the problem.
Click to expand...

Regarding using your scope indiscriminately, you need to be careful about using it on the HVDC on B+ circuits. Check the owners manual for the capabilities of your scope to be sure you won't overload it. You could certainly use it for the heater circuits.
 
Hummmm......(pun intended) -- A reality check if I may: Under the conditions of the bottom cover installed, Stereo Phono selected, tone controls set for flat and filter switches turned off, and no inputs (or shorting plugs) connected to the phono jacks, then advancing the volume control to full should result in nothing but (basically) a soft hiss from each channel. Connecting a TT to the most sensitive phono input will reduce the hiss somewhat -- and -- introduce any hum baggage that the TT has with it (which should be minimal), but it should not take connecting a TT or installing a shorting plug into the phono jacks to eliminate any residual hum when operating properly.

Dave
 
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