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Need help getting rid of hum

Tim... I changed the RIAA network to be connected without the PEC's, bridging terminals 2 and 3 (per Gadget's diagram which makes TAPEHEAD into a 2nd PHONO input) to where the PEC leads were connected. Doing so made the hum even less. And when I tested with the shorting plugs, the hum was the least it's been. There still is some hum, but to really hear it I have to turn the volume control way past 12 o'clock. I can turn the volume to maximum and the hum is not obtrusive and would certainly be drowned out by most music (turning up the volume that much would surely cause me to lose what hearing I have left!). Combining all this with shortening the leads on the RIAA networks has reduced the hum significantly. I'm pleased.

Since I was on a roll I decided to try replacing C35 and C36 (1000pF) with 2200pF which in theory is supposed to boost deep bass frequencies. It does although it seems that higher frequencies have been affected or maybe it's the phasing angles that are affected. The result is not displeasing though and I plan to leave it until some day when I become adept at using a scope to test the amplitude across the frequency range to see how the 2200pF's have affected the frequency range.
 
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EDIT: Gadg - you already answered my question earlier: "At the input jacks, you'll need to add a 100k resistor from the center of each tape head jack to the outer ring in order to get the proper phono cartridge loading."

Gadget... In my RIAA network I bridged terminals 2 and 3 so that terminal 2 is a 2nd PHONO input jack (like in your diagram, stock PEC removed), do I need to add any resistance at the TAPEHEAD input jacks? The schematic shows in series a 150k, 68k and 220k at PHONO LOW and PHONO HIGH before terminating at the SELECTOR switch. The TAPEHEAD input jack has no resistors and is wired directly to the SELECTOR switch. It would make sense to wire the TAPEHEAD jack into the same series resistors as the PHONO input jacks since I would always use only PHONO LOW with my Ortofon 2M Black cart which puts out 5mV at a 47k load resistance.

Or is bridging PHONO LOW and TAPEHEAD input jacks good enough?

What about adding a different resistance at TAPEHEAD input jacks? I wonder what effect different resistances might have. Alters the resistance the PHONO preamp sees I imagine. Maybe a more "ideal" resistance for my Ortofon cart? Ortofon says 5mV @ 47k so would a 47k be a good choice? See the attached schematic.
 

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  • RIAA pec schematic 47k.jpg
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with a 100k in there, it ends up loading at 50k. If you want it bang-on 47k, an 89k resistor is the better choice. It has a 100k grid to ground resistor after the phono stage.

Stock it actually works out to 68k on phono low or 40k on phono hi.
 
Gadg - It seems that bridging terminals 2 and 3 where the PEC leads used to go is enough to be able to use TAPEHEAD without a resistor. I'm playing a record and tried the TAPEHEAD input jack and it sounds fine as is. Strange.
 
Thorne,

You might want to read-up on phono cartridge loading. That's what all this talk about the resistors is all about. It's a topic all its' own. I think industry standard is 47K ohm shunt between terminals on phono (others can correct me). Audiophiles play with that loading because different carts prefer different shunt loading resistances. To be stock though you would want the phono pre-amp inputs (both of them now) to be 47K loading.
 
The resistor needs to go at the input jack to load the cartridge correctly. It has nothing to do with the PEC. In stock form there is a 100k grid to ground resistor at the first phono tube, just after the selector. Thats in parallel with whatever is across the input jack in the back. It will work without it, but the loading isn't right and its likely to be on the bright side since the cart is seeing a 100k load rather than the usual 47k or whatever the mfg specifies.
 
It looks like the ground connection of the TAPEHEAD pin already has a resistor connected to it (see arrow in image) for the PHONO LOW input jack. Is it OK to add the 89k resistor to the same terminal so I can make the TAPEHEAD jack a 2nd PHONO input jack?

upload_2020-6-29_19-51-44.png
 
yes, the two phono and the tape head jacks have their outer ring tied together. Running the new resistor to the same point will work fine. Prob installed how it is just because it physically fits well.
 
Just finished reading this whole thread...Wow man it was like watching a movie. I never knew what was going to happen next ! Interesting how things went. I too have a 500C that has a low level hum issue. Not a volume dependent one though. Thought I would read the thread anyhow. Found a few things I am going to try in the thread. Thanks, looks like you did a lot of work here and glad you sorted most of it.
 
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