Revised Guide: Conversion of 500C Tape Head Input to AUX Input

[daylite]

TAPE HEAD input to AUX input (Part 1 of 2)
Dave Gillespie developed a mod to convert the TAPE HEAD input to an AUX input. Dave wrote it requires some ‘fancy rewiring of the selector switch’.* I made the plan below for myself to help me finish the project and understand what was being done in this mod and why. *[Thanks to Drew Bolce for a description of the mod he posted in 2013. His post got me started on making these illustrations to accompany his words.]

[EDIT 01-14-2022: I have revised this guide to simplify the instructions and show how to apply Drew Bolce’s 2013 suggestion to eliminate signal crossover from the new AUX input to the PHONO stage. Dave explained below why the crossover happens. This revision incorporates another idea that’s found in Dave’s reply below – the direct wiring of the phono inputs to the phono preamp.]

EDIT 11/12/2023: Thanks to Dave451 for pointing out the need to show the correct 'after' wiring. The full schematic for this 500C project has been corrected accordingly.

IDENTIFYING SELECTOR SWITCH TERMINALS:
Selector switch terminals are named and located as shown in the illustration below.

STEP-BY-STEP INSTRUCTIONS:
TAPE HEAD to AUX - Before and After Schematic.png

Illustrations adapted from this source.

The result of the above steps is that the TAPE HEAD jacks can handle volt-level input from a CD player or iPhone or similar AUX device and there is no audible crossover affecting the PHONO or other functions. -d

 

[daylite]

TAPE HEAD input to AUX input (Part 2 of 2)
Pictures of the rewiring.

(A) This shows the stock wires (blue/gold) that traveled in the helical shield detached and insulated with heat shrink tubing and new leads (GRY/YEL) laid in with the striped volt-level wiring.
(B) Everything hooked up to jacks and selector switch.
(C) Closeup – the white arrow shows a jumper between ‘blobs’ that must be cut to allow the TAPE HEAD GND connection to be detached from ‘PHONO GND’ and reattached to chassis GND.

Thanks in advance for all corrections, additions, comments, and advice – have a great 2022! – d.

 

[JamesML]

I'm not sure I'd be up to implementing this MOD myself. But, I am appreciative that people, such as yourself, take the time and trouble to do this stuff.
Diagrams are excellent too. Many Thanks.
Jim

 

[dcgillespie]

Day -- The reason you hear some of the new Aux 2 input bleeding into the phono preamps is that with the approach used, the new Aux 2 input is not shorted out to ground when not selected.

With the final approach I used, I remedied that problem by permanently connecting the input of the phono preamps to the phono preamp input jacks, and then used the low level input wafer to either select the new Aux 2 input (old Tape Head) jacks, or the output of the phono preamps (after the 82K mixing resistors, which are disconnected from their Section 1 and 3 rear wafer terminals ). The output of the low level input wafer would then go to the terminals that the 82K resistors originally connected to.

With this approach, the signal from the output of the phono preamps is shorted to ground when the new Aux 2 input jacks are selected, or the output of the new Aux 2 input jacks is shorted to ground when the phono functions are selected. That way, the new Aux 2 input is created from the old Tape Head input jacks, but the shorting feature is retained so that no source bleed-through occurs.

Remember when performing this modification (mine, or the one you show), to disconnect the ground terminals of the Tape Head input jacks, and connect the ground terminals of those jacks to the grounds for the High Level input jacks. The high level input jack ground terminals go directly to ground (which the old Tape Head input jacks now need to do), where as when used for their original purpose, they were grounded to the phono preamp ground circuit, which is partially isolated from circuit ground in each channel.

Dave

 

[gadget73]

I've usually converted the tape head inputs to a second phono input, but when a guy has more turntables than anything else, thats what you do. Any time work involves poking at the selector switch, its not exactly a joyous experience though. Seems like the connection you need is always on the opposite side of where you can actually reach easily.