Recapped Fisher 400 -- All I can hear is the Ocean on FM

drspiff

drspiff

Fisher "T" Aficionado
I just finished recapping a Fisher 400 and fired it up. No smoke which I took as a positive sign. But...

The unit works with an AUX input, and may I say works nicely. However, when I switch to any of the 3 FM positions, all I hear is white noise, kinda like the ocean. Since I did not buy this receiver to lull me to sleep, I have to ask how to troubleshoot the lack of FM. Here are the existing conditions:

1. The preamp and amp work. I put a line level signal into the AUX inputs and get nice sound out of the speakers.
2. When I switch to any of the 3 FM positions, all I hear is white noise. Pleasant, but not appropriate.
3. The "magic eye" barely responds to a very strong local FM signal, and not all to any of the mega-power C&W stations around here.
4. I have a 39" piece od wire connected to one of the antenna posts.
5. The unit WAS picking up FM before I recapped, so I know it did work.

Any ideas on where to start looking for the problem?
 
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Perhaps while you were working on the amplifier section you loosened a wire connection.


You could try:

Checking your wiring to and from the tuner section.



Did you do any recapping of the tuner section?
 
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No transistors in a Fisher 400, Bub :-)

Exactly what all did you "recap"? Did you mess with any caps in the tuner (RF or IF)? If so, you've most likely detuned all of the tuned circuits. Probably not the end of the line, but total realignment will be needed.

If you didn't recap anything in the tuner... maybe you knocked a wire loose someplace (e.g., the FM 300-ohm input, which IIRC goes right over the bias power supply cap).
 
I put in some .047 caps that couple the 7868 final drive tubes to the 12AX7 driver/phase inverters, the 1k powersupply caps, new fast recovery diodes, 100uf heater powersupply cap, inserted 10ohm fuse resistors on the 7868's, and 2 .022uf caps in the tone controls.

I left everything in the tuner section alone. My Certificate of Wizardry (CW) does not include the black arts like tuner alignment. There is a completely separate add-on certificate for that. Besides goat entrails are hard to come by around here.

I'll check the 300 ohm lead.
 
drspiff said:
2. When I switch to any of the 3 FM positions, all I hear is white noise. Pleasant, but not appropriate.
3. The "magic eye" barely responds to a very strong local FM signal, and not all to any of the mega-power C&W stations around here.
4. I have a 39" piece od wire connected to one of the antenna posts.
5. The unit WAS picking up FM before I recapped, so I know it did work.

Any ideas on where to start looking for the problem?
Click to expand...

Try reseating all the tubes in the tuner section. If one or more pins are not making good contact the FM signal would get severely attenuated. Or a bad or dirty switch contact. Or a broken wire. If you didn't change any caps in the tuner section it's unlikely that you mis-aligned anything. The fact that you are getting lots of hiss implies that the loss of signal is near the front end, as several IF amplifier stages are working hard enough to bring up the background noise level. And that you are getting something from the strongest station in toen means that the FM local oscillator is working, and that I would guess that the fault is between the front end metal box (the part with the tuning mechanism inside (tuning cap or variable inductor) and the first IF amp tube. Often there is a coax cable between the two, see if a connector got loose.

Just be mindful of the supply voltages when pokeing around.
 
And yet people replace caps in tuners (recap them) all the time. The Fisher Doc kits had a lot of cap replacement.

In term of a general approach, How does one determine which are 'coupling caps' and 'audio path' caps if you are unable to read schematics? (Power supply boards and thier caps are fairly obvious.) This would be very valuable if someone could give a general idea of which are OK to change without realignment, and their general layout.

A fellow that does recap tuners frequently I spoke with indicated that in his experience only work on the "filter" sections required subsequent complete realignment. He suggested that the ability get quiet strongs signals with good stereo seperation could be taken as making a difficult alignment unecessary. (He had apparently recapped and resold many tuners with feedback from satisfied customers.) -sf
 
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sydsfloyd67 said:
In term of a general approach, How does one determine which are 'coupling caps' and 'audio path' caps if you are unable to read schematics? (Power supply boards and thier caps are fairly obvious.) This would be very valuable if someone could give a general idea of which are OK to change without realignment, and their general layout.
Click to expand...

The caps that one would replace in a recap are rarely involved in any LC tuned circuits, and thus you can do a recap and not hurt the alignment any. Wax paper caps of the sort you replace in the audio amp part of the receiver. Ceramic caps in the tuner section rarely require replacement, so don't touch those. None of them see any audio anyway.
 
Every tube tuner I've seen needed an alignment to work properly. They may work as is, but sensitivity and stereo separation are generally very poor. Those ipod RF modulators make a good signal source to align the stereo decoder without buying an expenisve MPX signal generator. A little adjustment in the MPX circuit can make a huge difference. IF alignment is more difficult.
 
andy said:
Every tube tuner I've seen needed an alignment to work properly. They may work as is, but sensitivity and stereo separation are generally very poor. ....... IF alignment is more difficult.
Click to expand...
The commnet he made was that if they were getting good quiet signal and stereo seperation, further alignment would be unecessary (and just a matter of altering necessary compromises). This was for solid state tuners. -sf
 
You mentioned that you connected a 39-inch wire to ONE antennae terminal. My 400 doesn't pick up much of anything unless i connect BOTH leads of a 300-ohm twinlead to the two terminals provided. The 400 has wonderful, silky, FM performance when working properly, but it is not a particularly sensitive tuner. Mine is the early version with 3 IF stages, later versions had four. Well worth getting into working condition if you have quality FM broadcasts in your area.
 
Make sure the tubes are not weak in the tuner section...we worked on one where the first RF amplifier (6HA5 if i remember correctly) was very weak and we could pick up almost no stations. Replacing this tube plus another weak tube made it work very well at picking up stations with good sensitvity.
 
OK, a little more information. The schematic shows 158v on pin 5 of the 6BA6. But I'm getting 172v on this pin.

Does this suggest that the problem is the 6BA6 or perhaps before it? In other words, do I need to look downstream or upstream from this datum?
 
I think you will find all the voltages to be high in these old Fishers simply because they were made in a day when 110 - 117V was the AC norm whereas today it is usually over 120V. Rather than high voltages I would be looking for low or no voltages. Unfortunately you can only get at the tuning head tubes by removing the bottom plate in the tuning head. Yet because of that you are unlikely to have done anything to that area.

Rob
 
There are 3 6AU6's in a row which are the 2nd, 3rd, and 4th IF amps.

What would I see if I were to put an o-scope on pin 5 of these tubes?
Would I audio frequencies?
Or would it radio freqs?
If I don't want pin 5, is pin 1 the appropriate choice?
 
I need to qualify my comments by clearly stating that I'm only an addicted hobbyist, and not even close to Craig or Terry or Rob or some of the other true techs in this group. You've been warned!

Based on the things you did, I'm guessing this is not a complicated issue, i.e. this isn't something that's going to need any serious troubleshooting. If it worked fine before, and you didn't touch the FM or mpx sections, then it's not likely you screwed up the alignment.

You changed coupling caps, power supply components, and tone control components. All of these would affect any input setting equally. That is, if you were using Aux, or Phono, or listening to FM, if you futzed something up in the areas you touched, the end affect would be the same no matter what you were listening to.

However, Aux works fine, but FM doesn't. If Aux works fine, then the power supply mods are ok, and the coupling caps are ok, and the tone controls are ok. The replacement of the ground connections with those 10 ohm resistors is not a no-brainer. It's possible to make errors in these connections. Again, however, if you made a mistake it would probably affect everything equally.

This leads me to think that you may have inadvertently bumped/broke/changed something that only affects the FM section. Since you're getting white noise, the connection from the mpx section through the amp section is probably ok. (And, the amp section itself is fine, since Aux works.) You're just not getting any mpx signal to the amp section, or, you're not getting any FM output from the tuner into the mpx section.

Have you done any signal tracing from the antenna inputs forward? Or, have you simply looked at solder connections, the function of the selector switch, etc.? I'm thinking you might have a bridged/shorted solder connection, or you might have snapped a wire somewhere. Or, you might have ended up with a piece of debris of some sort in a switch connection.
 
I was messing with the Fisher again this morning and I had the volume cranked way up. I was able to catch one station that did not override the white noise, but did sound like it was modulating the white noise a little bit.

I'm about to haul out the Tek scope. I can find no wires loose or obviously bad connections. All the voltage callouts on the schematic are measuring fine. When I get the scope out, what am I looking for?
Will I be seeing audio frequencies or radio frequencies?
 
I ordered some more tubes for the front end, and it seems that a 6AB4 gave up its life to screw with my mind. So it looks like Chad had the right of it. Good call!
 
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