Fisher 500C restoration and modification project

[daylite]

PCBs: IBBA......EFB......Buffer......Phase Inverter
TAPE MON to AUX input......Tone Control Bypass
TAPE HEAD input to AUX input
500C selector switch function animation
Loudness contour modeling and component selection
This project is close to an end. A signal strength meter problem came up but is now fixed.

Summary of Thread Contents
Introduction...................................................................................... Post #1
List of mods completed 8/29/2021 to 10/12/2021.............................Post #9
Dave Gillespie’s advice on normal signal meter levels.................... Post #12-#18
Troubleshooting and alignment advice from Dave........................... Post #23-#38
Advice on PCB clearances/update to mods completed................... Post #67
Schematic full map of this 500C project showing mods................... Post #69
Photos of 500C under chassis Before&After as of week 11............. Post #71
Parts purchased list for this 500C mod project as of week 11.......... Post #72
Images of make/install of IBBA printed circuit board......................... Post #73
Images of make/install of EFB printed circuit board.......................... Post #75
Images of make/install of Phase Inverter Circuit Mod PCB............... Post #76
Phase inverter adjustment using Wheatstone bridge........................ Post #77
Images of make/install of Preamp Buffer PCB.................................. Post #78
TAPE MON jacks to AUX input circuit mod....................................... Post #80
Tone Control Bypass modification..................................................... Post #81
TAPE HEAD input to AUX input conversion...................................... Post #87
'Final' schematic for this 500C project............................................. Post #97

Hi, all.
I have worked on only a few radios (RCA K130, Telefunken Rondo). Last year I received a Fisher K-100 chassis. I bought the K-100 chassis because of all I had learned here reading about Fisher products. Many of you will know the K-100 is very much like the Fisher 500 Mono. I recapped, modified the bias voltage supply, added a transistor buffered zener regulated individual bias control, added a tuning meter (the magic eye is faded), and built a front panel of wood and aluminum with backing plate for the knobs (the K-100 was used on consoles - I only had the chassis. Happily all brights and all knobs except one :-( were there.) Finished K-100 picture below. The sound was so great I dug out a turntable and speakers I bought in 1970 and I’m enjoying vinyl and great FM as I sit at the bench!

Last week USPS dropped off a Fisher 500C. It was great working on my first Fisher and I expect this will be a great adventure, too. I had a little panic when I opened the box and found one knob bright to be missing. Gave up looking through the packing, but the seller had shown a pic with all brights. Turned out it had gotten caught behind the dial glass and I heard it when I was moving the chassis up on end. Picture 2-4 below show some of the receiver. Picture 5 shows one of four PTs. Are these General Electric?

Seller said ‘all original’ but I doubt he took off the bottom panel. Picture 6 shows what I found. Someone had done a recap. (I guess it is all original, and then some.) First job for me is to reverse that. Picture 7 shows labels on the caps scattered around that replace C91 and C92. He/she kept C98. Looking at pic 6, does anyone recognize the replacement caps used? I’m just curious about how long ago the recap was done.

Other than the added caps a new R137 was added. It had to be way over on the right to bridge the recaps C91A/C91B. The original R137 is still under C91. More closeups are available if needed, but I want to wait until after removing the recaps before posting a quadrant photo series. There will be fewer pics per post in the future!

I’m not retired (just old) so progress may be mostly on weekends.

Thanks for reading. And thanks so much for all the knowledge shared and explanations found on this forum. I’ve read a lot of 500C relevant posts, but there's a lot more to see. I know I will need help with this project! I have a list of mods including 10R cathode resistors, 100 ohm screen stability resistors, IBAM, new output coupling caps, bypass resistor change (I read this may not be needed if IBAM is put in, but somewhere I think Dave said it could be ‘insurance’ ), RIAA upgrade, safety caps (C102,C103), CL-80, rectifier, upgrade R143, R139, R132, phase inverter (C69,C70), de-emphasis mod (R222, R223, R98), tuner (C85, C59, C61, C52-53, C57, C72), MPX (C210, C214, C221-222), phono preamp caps (C16, C17). The list for Mouser is about 45 line items!

Next post will be about reversing the recap (using Hayseed cans). After that the chassis will look better and I’ll be seeing what I expected to see when I took the bottom cover off.

(My apologies in advance for all the mistakes I will make or have already made!)

 

[larryderouin]

From the Light Blue Nichicon Axials, I'd say mid to late 70's thru early 2000's. I've found similar NICHICON's in a lot of late 60's Magnavox's. All they did that I can see is redo the main filter caps. Consider Hayseed Hamfest Cans.

95% sure those 7591 coin base are GE!

Don't touch any of the Tuner caps unless you plan on doing an alignment! In otherwords, leave the ceramics alone unless it's physically BROKE! Fisher used TOP TIER
ceramics and they don't go bad. Watch yourself in /on the MPX. Don't mess with the Filters near C222 and C223. Do the math 1st on the De-Emphasis components 1st to see how close or far it is. As much compression and overloading that stations put out (100%+ modulation, 100% of the time), you might just want to leave the tuner alone.

 

[daylite]

Thanks - two questions answered and valuable advice received! In fact I ordered the Hayseed cap set about a week ago (didn't get the under-chassis can, will use two 1000 uFs). At the time they were saying production was scheduled for 9/5-9/11. I'm working on a before-and-after schematic to map what's there now and what I will want to have after the Hayseed cans are installed. I really appreciate that the person who put in the recaps simply snipped the stock wires and left them in place. The tuner will be on the back burner. For now I am thinking that first after Hayseed will be the rectifier, safety caps, output tube protection/caps, R143, R139, R132, and bias adjustment control.

 

[daylite]

I want to get the wiring and lead dress right when I remove the electrolytics used in a previous recap and install Hayseed cans. Based on Larry’s comment, the grey/blue caps aren’t ancient, but they take up a lot of space. Also, I just don’t like the look of them in there.

[Side note: Looking around, I found vivatubes https://www.vivatubes.com/vintage-bp-japan-axial-electrolytic-capacitor-200uf-300v/ calling one of these capacitors ‘vintage’ and charging $18 for the one 200uF/300V capacitor. Maybe they’ll buy mine! Hah.]

On the left below is my attempt to draw the existing layout of power supply caps and a few key resistors (See post #1, pic 7 – the black traces represent the skinny yellow recap wires shown in that pic.) On the right is how I think things were before the recap - and will be after this ‘retro-recap’. View is as seen from below. [Edit - image corrected.]

Photos by others and old wiring left when the recap was done helped with the wire position and wire color choices. Points X, Y, and Z are terminal points where (I think) C91B, C91A, and C92A connect. The arrows around X, Y, Z show what else connects to those points.

If you have time to check the drawing I’ll be grateful to hear of any errors or advice. If you have a 500C on the bench, I would also like to have confirmation of the placement of R138. No doubt many of you will know where R136/137/138/ are found simply from memory and experience.

It will be a while before the Hayseed Hamfest cans arrive. Until then I’ll be working on cleaning and on the Mouser list. Many thanks.

 

[thornev]

daylite - You can check stock part positions using my maps:

All these PHOTOFACT files are somewhere, but I forget where.

 

[daylite]

fantastic! thanks, thorne. nice to meet you.

 

[thornev]

Back at ya, daylite. Welcome to the AK world of Fisher enthusiasts. If you can't find a part on my image, let me know. I did omit a few parts which I couldn't locate and I have a map of where parts are located by quadrant (well by 16 areas). Caps in cyan; resistors in black; other part types have their own color.

 

[flyweed]

wow what a beautiful old Fisher. I am hoping to find something like that to add to my collection.

 

[daylite]

It’s been 6 weeks since I started this thread. Seems to be time to post a brief progress report.

Thorne’s lists of his many mods was an important example for me on how to do this quickly, and he made a great map of component locations that also influenced me.

I’ll break this into two posts. This part will have two lists - one for things done, and one for things yet to be done. [ EDIT 12/2/21: As I remember the second part was going to be the 'planning map' - a schematic showing what's been done and what is left to do. That finally shows up in Post #69! ]

1. Here’s a list of mods completed:

2. Here’s a list of upcoming mods:

Thanks again to thorn. And thanks to Dave and all of you that have contributed here and provided instruction and helpful discussion.

If you find a mod I completed but should have done differently, or not done, please offer your advice. Also, if you look at the upcoming mods list you are welcome to send advice on whether the mod is needed or how it is best done. I put some question marks next to some upcoming mods that seem to get mixed support.

I’m happy to answer questions, but all this is very familiar to regulars on this forum, and they can give better advice than I could. I’ll review my work, and in the very few cases where I might have something to say that is not well known and might be helpful, I’ll try to post. -d

 

[thornev]

Glad I could help. You sure did cover a lot. How do you like the results?

 

[daylite]

It's really great to hear. I haven't given it the best environment - it's still on my bench. But I have two speakers (Dynaco A25s) and a turntable hooked up. After an evening soldering session I usually lean back snd enjoy the sound for a while. In the future I'll be asking about recommendations for other speakers. The FM comes in well, although the meter never goes above 1.5. But then my bench is in the basement, and I'm using a folded dipole. I would do the test and move the receiver upstairs, but its HEAVY! It seems to be OK, so I''ll wait a while to haul it up. If it still reads low when it's upstairs I'll be asking for advice. The voltages seem to be what they should be. Thanks -d

 

[dcgillespie]

Day -- I live about 50 miles north of Atl, and my lab is in my basement as well. My standard antennas down here are folded dipoles stretched out and strapped to a PVC frame that's mounted about a foot from the ceiling. With this scenario, a properly operating 500C/800C will have most Atlanta stations produce a meter reading just above the 4, while the weaker stations (from TN) will still produce readings around 2 - 2.5 -- but those stations are over 100 miles away. I mention this as a point of reference for how you should expect your receiver to typically operate. It can vary of course if you're in a reception "hole", but there's little doubt your receiver should be producing greater signal strength readings than it currently is.

If you're only getting an indication of 1.5 on local PA stations of any strength, then something is clearly amiss. If the voltages are good, make sure the tuner related tubes are also good, and that the sockets for those tubes are clean as well. If that doesn't help, then either an alignment (if someone has "fiddled" with the adjustments), or further troubleshooting is required to determine why the sensitivity is so poor. But first eliminate the more obvious possibilities before moving on to more complicated measures of resolve.

Dave

 

[daylite]

It helps to have that calibration point - thanks Dave. In fact my K-100 next to the bench gets what I think is a strong signal no problem (-27V at detector). This morning the 500C seems to max out at 1.2, and show just below 0.2 when tuned to a noise area. DIdn't try removing the antenna. The voltage across the meter at the same time goes from just above 17 mV to 3 mV. Although the signal meter is down there, the sound quality is great and no problem with automatic mode. My hardest test is a WV NPR station, 5000 watts 65 miles away. I can understand the words, but there is a blizzard of near-white noise. All stations that lock in show the same 1.2 reading on the meter. That's all I can get to until this evening. I'll start a very careful cleanup on the tube pins and sockets, and switch in some alternate tubes - I have extras of just about all, including nuvistors. I will also measure any voltages in the tuner path/MPX that are shown on the schematic (Ser Nos 30001 to 49999). The low level reading was there from the first power up with DBT and Variac, so I'm not kicking myself for now.
If you have voltages you recommend I check and write down just let me know. Thanks again!

 

[thornev]

I'm not 100% clear on your issue, but if the stations come in OK and the meter just isn't registering correctly for strong signals, the problem may be in the MPX-65. One of my mods was to piggy-back a 47k ohm 1/4W resistor to R204 to lower the stereo threshold detection. After I made that change, the strength meter more accurately reflected the strength of stations.

 

[dcgillespie]

Thorn -- Your observation here would be purely coincidental. The signal strength meter appears in the tuner section proper, operating off the signal presented to the 4th IF stage (limiter), while the mod I devised that you refer to for adjusting the stereo/mono switching threshold appears in the noise detector circuit of the MPX section -- as you noted. Not saying your observation wasn't real, but that with there being zero correlation between the two circuits, it must be considered as coincidental in nature.

Dave

 

[Oerets]

On my 500C and on other Fisher tuners I own. A FM 100 200B and 200C the meters never show much better then 75 ~ 80% on a strong signal.
Never pegs the meter.

Have you tested the tubes associated with the FM?

 

[daylite]

No tester available. I do have extra tubes to swap in and out. No change in meter level.

 

[dcgillespie]

Day -- Here's an even better reference. This is a quick pic of the meter on the 500C currently on my bench that I took for my client -- after repairs have been made to the tuner section. I provided it for him to show what a difference in signal strength the repairs produced. Before, the meter could barely peak at 2, and now it easily surpasses 4 -- same station, all else equal. This is a very normal operating position for the meter when a 500C/800C receiver is operating properly. The station is a local Atlanta station, whose transmitting tower is about 45 miles away. Antenna and receiver location here at my home as previously described. Next up -- cleaning that filthy dial glass! It's driving me nuts..........

Dave

 

[daylite]

Dave - OK I'm kicking myself now. I was cleaning pins, but for the first time in a month dialed up beyond 94 MHz (my habit is three stations, from 91.3 to 88.5). At about 99 MHz all sound stopped. I thought I heard the relay click, too. Went back down and about 95 MHz - back to reception as usual. I did this several times, going up the cutoff was not consistent, sometimes later, seldom sooner. Going down the revival was also sometimes later, but never below 93 MHz. I wiggled nuvistors and nearby tube. I found finger tap on V3/6HR6 killed the sound in the same way. I was leaning over to look at the meter, and I smelled smoke. Turned all off immediately and looked underneath. After search I found it looked like R51 (1K) had been toasted. I lifted one end and it was 180 ohms. I thought perhaps V3 had shorted. I replaced R51 and the V3 position got a NOS 6AH6. Brought up on Variac - no smoke. The meter is behaving much the same, though maybe read 1.4 occasionally. The dial now had a different but similar problem. Stations at 94 and below play very well, good fidelity, good stereo, though meter 1.2-1.4. Going up now gives no dead sound, but reception is very poor, strong stations can be heard, but lots of static sound.
It's killing me to think something else might have been damaged, too. Something not visible. Praying Z2 is OK. I will need to order a new spare tube. I will measure voltages and hope to find help here and somehow get to a normal 500C. -d

Toasted R51 on left:

 

[Tim D]

Maybe dirt in the tuning cap that doesn't hit until it is rotated to a certain point? I'm not sure how to verify and how to clean if that is the case. You need to be careful messing with that variable cap.