Fisher 500C restoration - Printable Version
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Fisher 500C restoration - EdHolland - 02-03-2019
Hi folks,
As mentioned in one of the "Finds" threads, I was gifted a Fisher 500C receiver in need of some TLC. This will be the place where the work is documented. In a while, I'll add the as-found pics.
Yesterday, I popped off the bottom cover, and just started to take a look around in general to get familiar. The output transformers look to be in good shape, and nothing is obviously burned up, including the rectifiers. The original fuse is still intact. This is all is promising, as I was told by my benefactor that "At some point, it just stopped working" which leaves a lot of open questions.
There are a few electrolytics, to replace, the filter cans, of course, and some of the small axial types in the output tube circuits look leaky/suspect.
I'm looking forward to this one
Ed
RE: Fisher 500C restoration - morzh - 02-03-2019
Even if "at some point it just stopped working" , it might just start working when everything, caps and resistors out of spec are replaced and sll tubes are checked and replaced as needed.
RE: Fisher 500C restoration - Ron Ramirez - 02-03-2019
RE: Fisher 500C restoration - EdHolland - 02-03-2019
Mike - you're right about that. Most things that haven't seen the "golden screwdriver" can be saved by carefully returning things to factory spec. Thankfully, there seem to be no signs of previous work on this machine.
Anyway, there will be a bit of an interlude, as I have to try and learn how to align the FM on the 1947 Zenith in the other thread...
RE: Fisher 500C restoration - Ron Ramirez - 02-03-2019
You say you were gifted that receiver?
Some people have all the luck...Hmph!
But seriously...
My suggestions:
Get it up and running stock. Add a CL-80 inrush current limiter in one leg of the AC line.
Once you have it going, then you really need to add, at the minimum, the IBAM (Independent Bias Adjustment Modification) to help preserve the life of those expensive 7591A tubes. (I just bought a set of four new Russian Tung-Sol 7591As for my Fisher 500-B.) EFB™ (enhanced feedback) would also be a good idea. We'll get into all that when you're ready.
I have not built an EFB™ circuit yet, but I did put IBAM in my Fisher TA-600 to make those Soviet military surplus 6П14П-EB tubes a bit happier.
RE: Fisher 500C restoration - EdHolland - 02-04-2019
Hi Ron,
Yes indeed. A fellow at work passed it to me - I explained it likely would have some value (not really knowing the model I was being offered at the time). The offer held and I gladly accepted.
Thank you for the technical insight. I have glanced at a few sources that mention bias circuit changes - and this makes a lot of sense. It seems the original design is somewhat vulnerable to problems. Independent adjustment is a good idea, and I'll definitely add in the inrush current limiter - they are very cheap insurance.
As to caps, I can't decide yet whether to restuff the cans, or get the Hayseed kit. It is more money, but will save time and effort.
More as we go along, but I want to finish up the Zenith, and clear the workbench before starting on this project. Won't be too long now.
Ed
RE: Fisher 500C restoration - EdHolland - 02-20-2019
The workbench is now available, and thoughts are turning to this project, and how to approach it. The mods to the output circuit will be implemented, but to begin with, the set will be recapped with new electrolytics and made functional with the stock circuit, ahead of making changes. This way, there should be less chance for confusion/errors to creep in.
So first things - new caps, and replacement of rectifiers (at least the selenium bridge unit). I am still weighing up the cost vs. practical advantage of purchasing the Hayseed kit for the multi-section cans. There's a side of me that likes the idea of restuffing the original cans, and saving money. There is another side that whispers in my head to take the easier route. After all there are four of them, three on top of the chassis and one underneath... Beyond that, there are still a few electrolytics to buy that are not included in the kit.
Any thoughts?
RE: Fisher 500C restoration - KCMike - 02-20-2019
Good luck ED.
RE: Fisher 500C restoration - Ron Ramirez - 02-20-2019
A unit like a 500-C deserves the royal treatment. G'ahead, get the Hayseed Hamfest caps a'ready.
Agree 100% on getting the receiver going in its stock configuration first, then adding IBAM and probably EFB as well.
Ed, I hope you've read up on the various mods for Fisher 500-C receivers which not only include IBAM/EFB, but also changing some resistors and a few caps to help keep those expensive 7591As happy.
RE: Fisher 500C restoration - EdHolland - 02-21-2019
OK then. Hayseed order placed, and current limiters ordered. Next up (lunch break) I'll put together the remaining order for small electrolytics.
What have I started
RE: Fisher 500C restoration - EdHolland - 02-25-2019
Hi Ron - yes, I am doing more reading on the 500C. There is quite a lot of info, and many experts and project examples out there on the web, although it is a bit scattered.
What I am picking up is, aside from the usual capacitor suspects and mods to the bias circuit, the output stage grid resistors are not good values for the tubes that are sold today as 7591 replacements. I'll definitely look in to that area.
I'll do the "easy" jobs first, which will help me get familiar with the chassis. Getting "hands-on" with a piece of gear also provides the motivation to do the background reading, since it is possible to visualise the circuit and component layout. That in turn aids one's understanding of the repairs and mods done by others.
Regards,
Ed
RE: Fisher 500C restoration - EdHolland - 03-01-2019
The Haysed cap kit arrived yesterday, and so did a packet of surge protectors. I didn't get around to ordering the smaller electrolytics yet, perhaps a Mouser order over the weekend. Before getting started, I remembered another project that was waiting for the work bench. Perhaps each will get a little attention.
RE: Fisher 500C restoration - EdHolland - 03-15-2019
Operation Hayseed should commence this weekend. I do still need to track down the other electrolytics needed - too many distractions at the moment.
Operation Kitchen Sink may take precidence... it all depends on orders from High Command
RE: Fisher 500C restoration - TV MAN - 03-16-2019
RE: Fisher 500C restoration - EdHolland - 03-17-2019
I feel I ought to chip in for another tub of popcorn TV MAN, With all the other fix-it jobs and distractions around here, I managed the following, about half an hour ago ~5 pm Sunday evening:
1) Carefully lifted the receiver on to the bench and turned it over*. Caught the brass trim piece that fell from one of the control knobs, and placed this safely.
2) Unscrewed the lower cover to expose the under chassis wiring.
3) Tinkered with the on-off switch, noting that it does not seem all that certain of it's resistance in the on position. De-Oxit has been applied.
Oh well, I may get time in the week.
*If weight is anything to go by, these are very well-built units.