Fisher 500C restoration and modification project

[daylite]

Tone Control Bypass Modification
The bypass removes the Baxandall BASS and TREBLE tone controls from both left and right signal paths. These notes and part numbers are for the Fisher 500C LEFT channel. The RIGHT channel will involve the same changes.

STEP-BY-STEP (left channel)
1. The LOW FILTER (LF) switch components are disconnected and then reconnected ‘off-switch’ with C39 (20 nF) wired parallel to C38 (2200 pF). This renders the LF function permanently ‘OFF’. The LF switch is left empty.

2. One side of the LF switch is bridged end-to-end with an 8pF cap and a 1.2M resistor, in parallel. The black wire that connected to the center terminal of the LF switch (and to C38/C39) is moved to an end position on the LF switch and the lead from PC3 Pin 7 is moved from the center of the TAPE MONITOR switch to that same end of the LF switch. The other end of the LF switch is connected to the ON end of the HIGH FILTER (HF) switch. [The LF switch still moves but now has no function. Its terminals are being used only as mounting posts.] The lead running from PC3 Pin 4 to R72/C47 (220K/100pF) is disconnected. PC3 Pin 4 is then connected to the ‘OFF’ side of the HF switch and the lead from R72/C47 is connected to the center of the HF switch.

This is another very desirable circuit modification. It allows the user to immediately hear output with and then without the Baxandall tone control circuits in play. Many thanks and all respect to Dave Gillespie and those who have written here about their experience with this mod. -d

 

[thornev]

When you ask "Is it OK to alter Fisher’s ‘art’?" you are speaking for yourself. There are members on this forum who prefer to restore to Fisher's original design but perhaps with higher quality components. There may even be a contingent that believe it is sacrilege to alter the stock design. And as you point out, there are many who prefer to modify the design to improve the efficiency and performance. I enjoy tinkering same as you.

 

[daylite]

as you point out, there are many who prefer to modify the design to improve the efficiency and performance. I enjoy tinkering same as you.

Hi Thorne – I’m certainly happy to be on the same page with you and many others here.
Tinkering? I can’t help it! Glad to have found this great forum. - d

 

[thornev]

What does the 500C sound like with no tone controls? Is it like putting the controls at their center position? Without some kind of EQ I can't imagine not being able to compensate for room acoustics or crappy song mastering. What advantage does that mod produce?

 

[larryderouin]

Would that make these receivers "Tinker Toys"?????

 

[daylite]

thanks, Larry!

Thorne - I liked the idea of being able to hear a baseline output wo/ any added tone shaping. As you know from experience, rebuilding the tone control PECs is sometimes needed. I was also considering whether to rebuild the tone control PECs and I wanted to hear what the sound was like without them. The bypass switch makes it easy to do an immediate A/ B listening test. And while square wave testing the unit, I didn't have confidence that centered controls had no effect. The mod eliminates that worry. (Dave and grindfix discuss square waves with and without tone bypass here.)

To answer your other questions: with the mod done I found I could not hear a difference between centered controls and tone bypass. So I guess my "pots and PECs" are not in bad shape. However, I could easily see a difference in the 10 kHz square wave. Dave reported he often finds the closeness in sound. He wrote - “On the pieces I've installed this modification on, I've always been impressed with just how close the tone control flat setting, and the bypassed setting (forced flat) setting sound. Assuming the tone controls and their PCs are in good shape, the original design is actually pretty darn good.”

Different strokes for all......for myself I find "less is more" with equalization and tone controls. But that’s me. Everybody gets to have their own opinion on what sounds 'good'. Almost all my listening now is with the controls bypassed.

 

[daylite]

At the end of December 2021, I started a separate thread to share a guide for doing a TAPE HEAD input to AUX input conversion and got a reply from Dave. In early January I made a revised guide that greatly simplifies the instructions and shows how Drew Bolce’s 2013 suggestion to eliminate signal crossover from the new AUX input to the PHONO stage is completed.

Because details and history of this work are covered in that other thread, here for the project record I’ll just post the before and after schematic for the revised conversion:

 

[thornev]

daytlite - Just want to say thanks for your detailed journaling and great imaging of the mods. Very helpful and educational.

 

[daylite]

hi thorne! your posts were a favorite read for me before and after I joined the forum, and you were very welcoming when I started this thread. thanks for the encouragement then and now. -d

 

[thornev]

Dave G in post 36 wrote this:
" At the ratio detector, adjusting the DC balance via the top slug with the temporary installation of balance resistors has never failed to deliver a very low distortion result at the output in my experience. No modulation is needed when adjusting the IF strip in this manner."
Can someone please explain what the "balance resistors" are? And I interpret "no modulation is needed" to mean that the 10.7mc signal injection with the "jig" is not required, true?

In the 500-C FM alignment instructions, step 2, it makes reference to "ground to junction of resistors (47K) connected in series from TSP3 to GND." Does this mean two 47K resistors connected in series and connected to Test Point 3 and ground? If so then where is the multimeter "+" probe connected so one can adjust the Z5 Ratio Detector for zero VDC?

I've got Z1 - Z4 adjusted nicely, but adjusting Z5 has been problematic. The only way I could adjust it was with injecting a modulated signal at antenna IN that is the same frequency as the 500-C FM frequency dialed in and then measuring for highest voltage at Test Point 3.

 

[daylite]

You got it. I adjusted the bottom of Z5 for maximum voltage at TP3 (measure TP3 to GND). Then attach two 47K resistors in series from TP3 to GND. Attach one side of your multimeter to the junction of the two resistors and the other side to TP4. Look at the schematic and you'll see TP4 is at one end of L9. You'll see L9 on a 500C chassis is very close to TP3. As I remember there is a terminal strip that the hot end of C85 connects to (that's TP3) and the coil L9 is attached to another lug of that terminal strip. TP4 is the end of L9 furthest from TP3 on my 500C. Adjust the top of Z5 to get zero volts (null) on the meter. For both adjustments inject your 10.7MHz signal (steady, not modulated, and as low power as possible while maintaining steady readability) at TP1 - the little stub on the side of the front end shield box using the jig Dave described. Good luck! -d

 

[thornev]

That's another way that my physical circuit conflicts with your schematic - my "-" end of C85 is connected to ground and "+" end is connected to Test Point 3. I vaguely remember that reversing that lytic was for some other reason.

NOTE - Dave G on Feb 8 2018 gave me the 2-resistor instructions for adjusting Z5. Apparently I have too many handwritten notes to scan.

UPDATE - OH! 10.7mc UNmodulated ! I was injecting modulated. No wonder my scope image was garbage.

 

[daylite]

Your C85 is the wrong way 'round. I have no idea what reason there could be for that orientation.

 

[thornev]

And yet the receiver works fine with it's being turned around. Hmmm. I'm afraid to turn it around. What could happen if "+" to ground turns out to be wrong in my case?

EDIT: Sho' nuff'...every other 500-C chassis into which I've looked has that lytic with "+" to ground. I wonder why its being reversed works???

EDIT2: AHA! I replaced C85 because it was suggested in the Metalbone restoration notebook. I must have accidently replaced it backwards. It was among the 1st restoration work I did back in Jan/Feb 2018. I was very green behind the ears as they say. So I will reverse it and see what happens. It's part of the FM amplifier signal path according to the Metalbone book. I wonder how the receiver will be affected.

 

[daylite]

At the low voltage on C85 the damage may not be apparent. Power caps the wrong way 'round can heat up, swell up, and spew hot vapors and/or brown goop. The following came from this reference - lots more out there.

 

[thornev]

Reversing C85 seems to have fixed my issue where the dial pointer was slightly off from the actual center of the frequency. Now the strength meter is right on. And the UNmodulated 10.7mc + jig + two 47K and with daylite's (and Dave G's) help, I've got all the IF transformers (Z1-Z5) tuned nicely with FM strength noticeably higher than it ever was and that's with using my finger and an FM antenna. YAHOO ! (Sorry to derail the thread - now back to our regularly-scheduled program).

EDIT: Apparently reversing C85 so it's correct per the schematic made my output tube currents jump from 34mA to 40mA. I adjusted them back down to 34mA.

 

[daylite]

Here is v27 of the project schematic showing all mods and corrections. One correction was revealed when Thorne wrote about R106 being used to label two different resistors. Dave G mentioned early 500Cs used 180 ohms for R103/R106 but later Fisher moved to 220 ohms. My map originated with the early model schematic found in the stickies area. This schematic is meant to be for S/N 30001-49999 versions so I made that update. This also has the correction to selector switch wiring Dave G told me about last month.
The highlights in the first schematic identify the modifications/additions made to the original hardware. The second schematic is shown without highlights.
EDIT: As of 11/12/2023 this includes final versions of all mods. Thanks to Dave451 for pointing out the need to update the areas from TAPE HEAD to the selector switch and PHONO inputs to the preamp to show the correct final wiring for TAPE HEAD to AUX2 conversion.

Thanks to the many forum members who helped with this project! -d

*P.S. - The receiver is back in the box and off the bench!

 

[thornev]

Back in the box ?! All that great work and it has to live its life in a box?! Sacrilege ! (just kidding)

EDIT: What R106 correction? There are still 2 of R106 in the latest schematic. What I did was make R106 = R102 at the 180 ohm position (follow pin 8 south from V13).

Also there's a 1000uF cap in the FM tuner box next to coil L1 that is not labeled. It is C6.

And speaking of tuner box, as another piece of my 500-C PHOTOFACT project, I have finally labeled all the components inside it in case anyone is interested. I have attached it so feedback is welcome.

 

[daylite]

I only corrected the resistor values - the schematic is correct in that it matches what I see in my receiver. The duplicate numbers and error in the parts list didn't bother me. But now that you have provided a substitute number to use, I may get to it and change it as you have done. Thanks!

 

[thornev]

Regarding the made-up R102... It may confuse people since it's not in the parts list. I picked 102 because it's in the same neighborhood as surrounding labels. I had to say something because that's just my big mouth nature.