Tag sale 500-T

able.baker

Active Member
For some reason I decided to invest the princely sum of $10 in this unit, without knowing if it was desirable, rare, valuable or whatever. It came with a Ratshack branded BSR turntable and some small, cheesy no-name speakers. The turntable was jammed. The speakers work.

So I set up a dim bulb tester and fired up the 500-t and it actually works! Sort of. I only get sound from one channel. I opened it up to find a huge amount of dust. The seller assured me the unit worked when it was put in storage, but there were signs (missing screws on the heat sinks) that some one had been mucking with the unit. Haven't had any time to do troubleshooting yet, but will update this thread when I do. I already found the schematic/manual on this site so I should be able to track down the problem.
 

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you should post a photo of the cheesy no-name speakers, IMO. One is (ever) curious.

Investment-wise I'd say you did about right.
 
I have a Fisher 500T that I am using as my main unit now. I think you will be pleasantly surprised as to how good that thing sounds. I have the Fisher paired to some EPI-100Ms and I am telling you, it has a nice "tubey" sound. It has a nice, deep soundstage. I personally think it will be worth your time to resurrect it, assuming it isn't totally trashed. I believe it's 35WPC. Get the optional wood cabinet if you can find one once it the receiver is done.
 
I would call the 500T and its solid state Fisher kin 'barely competent' but as always de gustibus non est disputandum.

500t.jpg


I'll confess to having one lurking somewhere downstairs.
 
I've finally gotten around to doing some work on this. In the meantime I have sold the cartridge on fleabay and recouped 3/4 of my investment. I have also reorganized my electronics workbench and even purchased a functional Tek 564b oscilloscope! The scope has become a project in itself.

Based on the symptoms (both channels worked in MONO mode) and the schematic, I narrowed down the problem to the preamp board, which has a total of 4 transistors, 2 for each channel. I decided to try switching the transistors from one channel to the other, and after the 2nd switch the defect swapped channels. One of the BC109Bs is defective. One of the junctions measured open with my VOM. I found American Microsemiconductor still carries these. Ordered a couple and then we'll see what happens. Now for some real restoration!
 
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I have a Fisher 500T that I am using as my main unit now. I think you will be pleasantly surprised as to how good that thing sounds. I have the Fisher paired to some EPI-100Ms and I am telling you, it has a nice "tubey" sound. It has a nice, deep soundstage. I personally think it will be worth your time to resurrect it, assuming it isn't totally trashed. I believe it's 35WPC. Get the optional wood cabinet if you can find one once it the receiver is done.

Thanks for the encouragement. I'll probbly end up making a cabinet for it. I have a pretty complete woodshop and a nice, wide piece of mahogany I found in the trash a few years back.
 
Based on the symptoms (both channels worked in MONO mode) and the schematic, I narrowed down the problem to the preamp board, which has a total of 4 transistors, 2 for each channel. I decided to try switching the transistors from one channel to the other, and after the 2nd switch the defect swapped channels. One of the BC109Bs is defective. One of the junctions measured open with my VOM. I found American Microsemiconductor still carries these. Ordered a couple and then we'll see what happens. Now for some real restoration!

The tranisitors came in today. American Microsemiconductor is amazing! Two day turnaround and free shipping! :banana: Well the preamp is working now, but one channel is still losing the signal in the audio (tone) control board. Time to track that down...:confused:
 
On to the Pre Driver Board!

Traced the signal through the Tone Control circuits. A scope really is handy! Subbed in some transistors for the 2N2924s (one tested bad) and still no left channel signal through the board! Back to the drawing board... I mean the schematic... :scratch2:
 
Yay!!! Found a bad transistor in the pre-driver board. Subbed in a few I had that looked equivalent and still no left channel. Farted around forever tying to figure it out. It turns out I Had subbed in a PNP instead of an NPN (or vice versa - I can't remember). Got the correct type of transistor installed and success! It appears some of my collected electronic bits and pieces have wound up in the wrong drawers of my parts cabinet. At least there were no puffs of smoke, sparks or burning smells...
 
Recapping has started in earnest

Have done most of the Power supply, preamp board and predriver board. Got some 25 uf caps on back order so I won't be able to complete this for a while. Rather then desoldering all the connections to the boards I have found it easier to desolder all but one side and then just tilt up the board. Then there's enough room to scooch behind with the soldering pen.

In the attached picture I've secured the preamp board with a twist tie to hold it in its bent back position. I am happy that I am somewhat ambidextrous. A third hand or maybe some longer skinny nippers/pliers might help, too.
 

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You should have enough slack, after opening up all the band retainers to flip the boards without de-soldering wires from the boards. It's weird, but my 550-T, 500-TX, and 175-T all have the compression slip fittings on all wires that fit on flat pins on the boards.

I just got my 550-T running again in FM STEREO. NOT 100% but enough that I can listen for long periods. After caps and most of the Transistors, plus a couple of resistors, It definately needs an alignment on the MPX.

Larry
 
You should have enough slack, after opening up all the band retainers to flip the boards without de-soldering wires from the boards. It's weird, but my 550-T, 500-TX, and 175-T all have the compression slip fittings on all wires that fit on flat pins on the boards.
Larry

Mine was not built in such a repairable manner. The audio connections run through litle BX cables and the ground points on the boards are soldered, too. See the attached picture of the IF board. It only has one electrolytic cap, so I might just leave it, since it works now.

Cheers!
 

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Recap done but no stereo FM

I finished this up a while ago and it sounds fine - although I am still just using the cheesy speakers to test it out. Only problem is - no stereo light. Or stereo sound on FM. It worked (at least part of the time) before the recap. Guess I shouldn't have mucked with the multiplex board? It looked to me like the electrolytics wouldn't affect the oscillator circuits but maybe I was wrong. Anyone around here ever trouble-shoot anything like this? I have a multi-meter and a clunky oscilloscope - but no signal generator.

I'm thinking maybe I should just take it to Urban Radio - they are just up the road from here. Could just need an alignment?

schematic is here: http://akdatabase.org/AKview/albums/userpics/10004/Fisher 500T Service Manual.pdf MPX circuit is on page 8.
 
The stereo light being out is important. It's part of the circuit. IIRC there is 25V coming off both sides of the lamp (one from trnfrmr and one from board) when the board senses MPX it drops the voltage to the lamp to I think 14V on one side and it lights. If the light is blown the circuit is open and no MPX. You have to have that voltage feeding back to the mpx for it to work. I don't understand it but thats what I found out for my 550-T and 500-TX. The light bulb is a weird 10 or 12 volt bulb. When the voltage differential is there its enough to light up the lamp.
 
Thanks - but I sort of figured that out already

The bulb is OK. One of the tests in the Service Procedures is to ground out pin 4F on the MPX board. When I do that the Stereo lamp lights up.

Oh well...
 
Check the base, collector and emitter voltages on the transistors on the mpx board. Start at Q401 and work to Q406. You might have a bad transistor in the mpx. Have you cleaned all the switches and pots? Pay particular attn to the MONO/STEREO rotary switch. Multiple cleanings to Rule out a switch/pot problem is common in these old Fishers.


What's the input voltage on 4A with it in Stereo mode. Should be 24V.

Larry
 
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Checked all this stuff:

Pin 4a is at 25.1 V. Just a tad high.

Cleaned all the switches w. deoxit. Pots were already cleaned and never scratchy at all. Mono/Stereo switch is OK. It grounds pin 4D in mono mode.

Measured all the voltages:

Trans Ve Vb Vc

Q401 4.5 5.2 24.6
Q402 12.2 15.5 0.6
Q403 0.0 0.6 19.7
Q404 0.2 0.5 25.1
Q405 6.2 5.5 25.1
Q406 0.0 0.2 20.3

I pulled each transistor and checked them with my VOM and they all are OK.
I had previously replaced Q404 and Q406.

Also checked the diodes - all OK.

Any thoughts? I am stumped again! (almost typed stupid!)
 
If all the parts are Ok, then it needs to go to the shop and get aligned. This isn't something you can do, unless you have all the specialized equipment, and know how to use them. Even tech's today really don't understand them. You need to find a shop with an OLD (PHYSICALLY) Guy that knows tubes. I know its SS but FISHER used a weird system that harks back to the tube side.

Listen to it in Mono for a while. You might be surprised at the clarity of sound. Separation will suck, but the sound itself, without the hiss is so much better.

Larry
 
Thanks

That's what I was thinking. I also need to hook this unit up to some decent speakers. I have a spare pare of Advent Graduates I just refoamed. Gotta use them somewhere!
 
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