Fisher 500 Mono - no AM or FM troubleshoot

[rufleruf]

I bought a 500 on bartertown that had non functional AM and FM. I tested all the tubes - all good; replaced all the electrolytics (only 1 can section) and wax+paper caps; brought it up on variac and tested. I have reasonable voltage at all sections of the can capacitors. Plays great through AUX - sounds amazing. I get some bleed through on all other modes from aux - not sure this is normal.

So, neither the AM nor the FM work and there is no movement in the tuning meter. I am not sure what to look for - having very little tuner experience. I would guess the problem is where the two sections merge into the amplifier section, or have something to do with the 135V source for all the tuner section tubes. Looking for a schematic - not one on fisherconsoles.com

Smoke from my Oakland Deck - normally I can see about 15 miles to some distant hills

 

[larryderouin]

HIFIENGINE has the 500 SAMS. Which version 500. Some came with EL-37's and some came with 5881's I believe. (I've been wrong before, but won't admit it. LOL)

 

[gadget73]

I'd start by making sure all of the tubes in the tuner have voltage at the plates and screens. Having both totally dead makes me suspect a lack of voltage somewhere. Looks like the tuner runs off the 135v supply, but several of the stages have the AM and FM can wired in series, meaning if one of those coils went open you'd lose voltage on both tubes and both tuners would quit working. A quick volt check on the tubes would rule that out easy enough. I've also had bad resistors cause no screen voltage in the IF strip.

https://www.hifiengine.com/manual_library/fisher/500.shtml

 

[audmod01]

Matt;

Since you have no indication of signal at the tuning meter it means the fault lies back toward the front ends for AM and FM. Any IF stage could cause this. Check for B+ at plate & screen grids of each IF stage. If OK, then turn off set and check for low resistance in IF xfmr secondaries. Let us know.

Joe

 

[rufleruf]

Guys,

Back from Florida.

Two things...
1: I printed out the Sams and went through the resistance tables. V5, a 6BA6 reads open on pin 5 where it should have 1500 ohms. This goes through A14 in L12 then A2 in L12. What do I check next? V5 is the first FM-AM IF amp, so it could very well be the culprit, but I am not sure how to move forward.

2: How do these IF strip inductors work?

Thanks!

 

[gadget73]

I'd ohm check the two transformers. Should be under a few hundred ohms if all is well. If you get an open, try soldering a 100 ohm resistor across the open coil and see if the other band works. L12 is the FM side, L13 is the AM one. If you find an open, its possible that its just a lead broken inside the can. Opening it up might reveal a problem you can fix. Failing that, you can probably rob a can from a donor unit. I've got a TA-600 chassis here if you need an IF transformer.

They're transformers that are designed to pass only a narrow range of frequencies. Essentially its a filtering mechanism that only passes a signal at 10.7mc or 455kc, depending which band you're in. Anything outside of those two ranges is rejected.

 

[larryderouin]

If you have a spare, Change the tube 1st to rule it out. Also check R22, R80 and R81 for an open or high resistance. Sounds like you've got an open somewhere in the B+ for that section as you have an open btwn V5 pin 5 (6BA6) and V14 pin 7 (6X4) Provided you saw the footnote about meter between Pin 5V5 and Pin 7 V14. I'd power it up and check for spec'ed voltages from Pin 1 & 6 V1, Pin 6 V2, Pin 5 V3, V4, V5 V6 and V7 and back track to the rectifier, As the 135V section feeds the whole tuner section. Also make sure you have voltage across pins 1-2 and 12-1 on M3 Section2 Front (Selector Switch) when in AM or FM positions.

 

[rufleruf]

Pulled out coil L12. I get open across the A14 side. It appears to be burned on top and its very hard to get a reading off the little wires to the core. Looking at my picture I can see the problem (see below). I think I will take pictures and blow them up when looking for problems instead of squinting through a magnifying glass in the future.

How do I solder a little charred wire like this? Anyone have a spare ZZ-509-130? Is this equal to another coil produced later?



I wasn't able to see this in person, but here in a picture it is very clear what's wrong!

 

[audmod01]

Matt;

If the wire still has its external enamel or plastic coating on it you will likely have to use a solder iron to cook the end with some solder on it until you get it to a bare tinned wire again. then see if you can manage to extend down to the solder lug inside the transformer using a wire strand from a piece of stranded wire. I have done this a number of times by stripping the end of a stranded wire that has tinned strands, separating one strand then using it to make the connections across the gap. Slide a piece of tubing over the soldered wire strand and connection down to the lug if possible. If not, then make sure the wire is not going to touch the shield when reinstalled in the chassis.

It may be that the wire simply came apart due to corrosion, but it could also be that a bypass
capacitor is shorted. Make sure first before reinstalling the transformer that there are no shorts to chassis ground at the screen grid and the plate of the IF tube socket this transformer connects to. If the wire is truly darkened and burned at that spot it could also be due to an IF tube in the destination socket that shorted (even if only temporarily). Check to see if R22 is burned and if C37 may be shorted.

Joe

 

[gadget73]

yeah that would be a varnished wire. Might need to replace the bit coming from the base with a new longer strand and splice it up higher. Heat to cook the varnish off should do it. It'll be very fiddly, like soldering hair, but if you can make a connection and get continuity through the coil you should be good to go.

 

[rufleruf]

Houston - we have reception! Strong reception with my hand as antenna. Need to adjust the needle position, clean the dial glass, install 5 of the 110 replacement original 378 lamps I bought on the bay today for $15 when they show up and it's case closed.

Thanks everyone - hardest part was seeing the splice and hitting it with my crappy soldering iron. Splice is about an inch up the coil.

 

[rufleruf]

Quick post script.

I was listening to it while cleaning up the dial glass and doing some odds and ends and noticed one of the Sovtek 6L6GCs was starting to redplate. Took it about 5 minutes to start distorting. I let it cool down, swapped the two output tubes and turned it back on. Same tube began to redplate again in the new position. This is a basically new tube from a reputable seller. Oh well. Swapped in a tired Sylvania 6L6GC and it had no issue.

I'll post a picture when it's all buttoned up.

 

[audmod01]

Matt;

Is the negative bias of the grid well within spec. for the two tubes? If the one that red-plates begins to loose its negative bias, it may be an indication that the tube has grid leakage after it warms up. I have seen that happen. Since the problem moved with the tube it is pretty certain that the tube is the culprit. If the negative bias is marginal, it may not take much grid leakage for a tube to go into red-plate condition.

Glad that you managed to repair the IF Xfmr. It can be tedious, but a repair is worth the time and effort. This will build your confidence level for the future.

Joe