HELP - Fisher 400 tuning problem

[TWJones]

I'm hoping someone can help this non-technical dilettante. I have a Fisher 400 receiver than I bought on Audiogon over a year ago and that until now has worked flawlessly. All essential mods were supposedly made before I bought it (e.g. rectifier). Today when I turned it on - no sound when set on FM stereo, mono or stereo filter.:tears: The amplifier section is working fine - I have no issues playing an iPod through AUX. The stereo beacon shows no reception anywhere along the FM band. Interestingly, if I crank up the volume beyond 12 o’clock, I can hear stations faintly.

I don’t know of any tube technicians here in northern NJ anymore, and the one I used to have (via Vacuum Tube Valley) was expensive and ineffective. I am hoping all I need to do is replace a tube or two, but not sure where to start. I have no testing equipment beyond a basic amp meter, but do have a soldering iron I can wield with some competency. Would be willing to get some additional gear if needed.

Any suggestions?

 

[FloridaSam]

I am no expert whatsoever but I have a Fisher 600 that had the same problem yesterday. Tuner was working perfectly and then nothing. Behind the FM tuning knob are two tubes. I took those out, cleaned the pins, re-inserted them and its working perfectly. The tubes in my unit are the ECC88 and ECC85, the ones in front of the FM tuning section. Try wiggling those tubes and see if it makes a difference.

 

[Dadbar]

You might just be a victim of a loose tube....I would suggest reseating the FM tuner tubes (don't bother with the multiplexer ones)

If that doesn't work I would suggest getting a new 6HA5 tube- the first tube in the FM RF chain. It's a cheap date (like $5) and easy to find.

 

[audiodon]

So it's only FM? You tried all switch positions, right?
I'm with Dadbar. With the volume low, wiggle the three tubes in the MPX. If that doesn't do it, turn it off.
Pull out one of the three tubes, keeping track of what tube went where. Substitute with one phono tube (the ones with the shields in front of the MPX. If that doesn't work try it again subbing in the other phono tube. If nothing, turn it off again, pull the third tube, and substitute it for one of the tube right behind the front panel.

This, of course, assumes you have no extra 12AX7 or 12AT7s hanging around. If you do, just sub three in for the three, or do them one at a time. Don't pull tubes or put in different ones with the unit running.

 

[sgmlaw]

No sound on MPX or Mono? That's usually the front end or the IF strip.

Start by pulling the front end and IFs and testing them. As stated, sometimes these need to be reseated to clean them after a time. If they all test good, that's no guarantee against a failing tube. Also clean up the tube pins before reseating.

If any of the IFs need replacing, replace them all, and send it out for a realignment.

If the tubes all test good, then you will need to start checking voltages to start diagnostic.

I presume ALL the small electrolytics have been replaced, including C55?

Also, "wiggle" or maneuver hot running tubes with great caution and care. It is very easy to screw up a tube that way. If you must do it, do it right after turn on, before the cage reaches full temperature. Although, it will wiggle in the socket just as well when turned off.

 

[TWJones]

Thanks for the suggestions - will try some of them out tomorrow when I have a free moment (wife and three boys...). I actually do have some 12AX7 and T7's around (from my phono preamp). Don't have a tube tester, though. Guess I ought to spring for one and bone up on how exactly to use it - shame I traded the BSE in EE I was going for way back when for a BA in Literature!

Thanks again.

Tom

 

[TWJones]

Quick update for all those who posted suggestions: Tried reseating all IF and 12AX7 tubes. Cleaned them all with deoxit. No luck. Have ordered 6AH5 and other IF tube replacements - for about $30 all in, cheaper than getting a technician. Fingers crossed to see if one or more of the tubes do the trick. Should know in a few days.

Thanks again.

 

[Bob91343]

Tubes are rarely the problem. It's possible there is an open resistor or some other small part gone south.

Get a diagram and make some voltage measurements.

 

[DaveWM]

antenna still on it right?

 

[TWJones]

Antenna is still on, and just cleaned with deoxit to be safe - but a good suggestion! You just never know. Figure if the tubes aren't right, I might get the ebay restoration kit mentioned in an adjacent thread and just replace all caps and resistors once and for all (minus the topside can caps). Kind of the brute force method, but what the heck? I have had a few old German cars (Audi and Porche) and have the same dumb-guy strategy: Dash light out? Open 'er up and replace them all just to avoid having to go back in. In the end, I hope this takes less time than wrapping up the receiver and sending it out for repair.

We shall see...

Cheers,

 

[sgmlaw]

Quick update for all those who posted suggestions: Tried reseating all IF and 12AX7 tubes. Cleaned them all with deoxit. No luck. Have ordered 6AH5 and other IF tube replacements - for about $30 all in, cheaper than getting a technician. Fingers crossed to see if one or more of the tubes do the trick. Should know in a few days.

Thanks again.

Don't just start throwing parts or tubes at it. That's a waste of money. If you don't have a tube tester, find someone who does, or who you can send your tubes to for testing.

You start with checking the tubes, because that's a simple first step (always start with the simple things first), and that's why they're socketed to begin with - occasionally one does act up. Most "troubles" are related to the tubes, or their sockets.

It sounds like all you've done so far is clean the sockets.

Again, if the tubes are testing OK, then you'll have to open it up and start circuit chasing to pinpoint the problem. With tuners, that may require a trip to a tech more experienced in RF matters. It's easy to mess up a tuner with haphazard repairs.

Some tube failures can be caused by malfunctions elsewhere, so analyzing how a tube has failed, and double checking that circuit's values, can reveal the underlying problem (otherwise it will just eat up more tubes).

You may find out that the restoration basics were ignored, such as replacing ALL the small electrolytics. Just about every restoration "beauty shot" I see nowadays beautifully displays all the original dried out little 'lytics (and all the equally shot black beauties) right next to the fancy new Auricaps and high-speed diodes some tech installed.

But confirm that the tubes you have in there are good FIRST. It normally takes all of 5 minutes with a B&K. THEN open it up.

 

[TWJones]

Don't just start throwing parts or tubes at it. That's a waste of money. If you don't have a tube tester, find someone who does, or who you can send your tubes to for testing.

You start with checking the tubes, because that's a simple first step (always start with the simple things first), and that's why they're socketed to begin with - occasionally one does act up. Most "troubles" are related to the tubes, or their sockets.

It sounds like all you've done so far is clean the sockets.

Again, if the tubes are testing OK, then you'll have to open it up and start circuit chasing to pinpoint the problem. With tuners, that may require a trip to a tech more experienced in RF matters. It's easy to mess up a tuner with haphazard repairs.

Some tube failures can be caused by malfunctions elsewhere, so analyzing how a tube has failed, and double checking that circuit's values, can reveal the underlying problem (otherwise it will just eat up more tubes).

You may find out that the restoration basics were ignored, such as replacing ALL the small electrolytics. Just about every restoration "beauty shot" I see nowadays beautifully displays all the original dried out little 'lytics (and all the equally shot black beauties) right next to the fancy new Auricaps and high-speed diodes some tech installed.

But confirm that the tubes you have in there are good FIRST. It normally takes all of 5 minutes with a B&K. THEN open it up.

Thanks. I was able to at least ascertain that the multiple 12AX7's were not the problem, by replacing them one by one with a tube that I know to be in good order (new EH tube that I had in my phono preamp). As I did that I cleaned the pins and sockets. Since the cost of replacing the 3 IF tubes is realtively small, went ahead and ordered them. Will switch them out one by one when I get them shortly.

If that does not work, I may try to replace the small electrolytics as you mention.

A question from real ignorance - how does one test a tube at home? Is there a relatively inexpensive tester (or series - I see there are a lot of old B&K models) I should consider purchasing (I have besides my Fisher 400 a Sonic Frontiers tubed preamp and a Project Tube-Box)? Keeping in mind I am NOT an electrical engineer

 

[Bob91343]

Used tube testers run $25 to $50. You can pay more but shouldn't. There is no simple way to test without one, although you could make a little project to build an emission tester. Then you'd have to calibrate it. Don't fall for the idea that the tubes are failure prone because they are in sockets; tubes fail but rather rarely in the big scheme of things.

As I say, check the voltages. A multimeter can be cheaper than a tube tester and will test many more things. Of course an oscilloscope is best but most people can't justify one.

 

[sgmlaw]

A question from real ignorance - how does one test a tube at home? Is there a relatively inexpensive tester (or series - I see there are a lot of old B&K models) I should consider purchasing (I have besides my Fisher 400 a Sonic Frontiers tubed preamp and a Project Tube-Box)? Keeping in mind I am NOT an electrical engineer

There's no real bargains when it comes to good testers. An old vintage B&K tester may be cheap initially, but it's vintage too, just like your 400. It may itself need to be reconditioned and/or recalibrated - many of them use tubes internally and are full of old worn out caps themselves. Finding an inexpensive and accurate older tester is the rare exception. A newer tester is not necessarily cheap, and the ones that may be priced low typically do not test the most meaningful parameters.

How many tubes you plan on having around should dictate the need. Many here have tube stocks in the hundreds, and plenty of caddy/shelf activity, so a tester makes sense. If that's where you're heading, then a tester can be a good investment. If an older model, try to find one from someone who has already reconditioned it, but be prepared to pay accordingly. But if you only have the tubes in your gear and perhaps a couple of spares, it may not make sense to potentially spend hundreds of dollars on a reliable and accurate tester. It may be better to find a friend nearby with a tester instead.

And always bear in mind that even the best tester is not dispositive. It sometimes will only cull out the obvious shorts and defects. We've all encountered bad tubes out there that defy the tester, but reveal themselves in service. Unfortunately, the equipment circuit is still sometimes the best tester.

There are tester vendors out there who can guide you to the best model for you based on your budget and the types of tubes you plan to work with. For instance, the old B&K 650/7xx series are nice, easy to use testers that test just about anything, are cheaper than a Hickok, and when calibrated are about as accurate. But if all you're testing are 12AX7s, there are better options.

 

[DaveWM]

A sweep gen is what would be next starting at the last IF and working forward.

even without a sweep gen a cheap unmodulated gen set to 10.7 mhz should at least find the problem stage, (not sure where the tuning eye take off is IIRC its not at the last stage), using a DMM on one side of the ratio detector.


That would isolate where the siganal is getting lost.

you could also check voltages on the plates of the IF/RF and hope you find a open resistor. That is the easiest thing with out proper test gear.

whatever you do, resist the temptation to start twiddling the tuning slugs in the RF/IF section.

My money is on a bad RF tube.

 

[sgmlaw]

My money is on a bad RF tube.

I wasn't going to say that, but it wouldn't surprise me.

I see a lot of these that after 45-50 years either have been used to death, and the emissions are now borderline, or have been bounced around so much (both hot and cold) that something gets damaged.

The worst are the "re-tubers", who yank out all the still-good TFKs, Mullards and Amperexes (for obvious reasons) and sell them "retubed" with unscreened new production, as if it's some bonus to the buyer.

At least with the RFs, the originals usually stay in the chassis as they change hands.

Of course, it also wouldn't surprise me if an original little capacitor that everyone ignored finally decided to go south.

 

[DaveWM]

I was thinking that too, but that little smoothing cap on the end of the ratio detector is more likely to open. Its very low voltage, besides I am pretty sure the tuning eye would still work even if that cap did short out.

if its not a tube I would look at the plate load resistors in the RF/IF.

 

[TWJones]

Success!

Many thanks to all for pitching in! Based on advice here and a schematic, I ordered the 6HA5 tubes as well as the other RF/IF tubes. Replaced them one by one and turns out that replacing the tube in the V5 position (3rd IF, a 6AU6 tube that cost me $9) did the trick. Reception is top notch now, and the eye tube is working again.
:

 

[dcgillespie]

Congrats on getting your 400 running again. One thing to remember in future evaluations of your receiver is that the tuning "eye" in the 400 is not so much a tuning indicator for station signal strength, but an indicator that the 19 kHz pilot signal necessary for stereo operation is present in the received signal. Yes, it will close a little more on stronger stereo stations and a little less on weaker ones, but it's primary function is to indicate the presence of a stereo signal being received. On reception of mono FM stations, the beam will not close at all.

Dave

 

[sgmlaw]

Congrats on getting your 400 running again. One thing to remember in future evaluations of your receiver is that the tuning "eye" in the 400 is not so much a tuning indicator for station signal strength, but an indicator that the 19 kHz pilot signal necessary for stereo operation is present in the received signal. Yes, it will close a little more on stronger stereo stations and a little less on weaker ones, but it's primary function is to indicate the presence of a stereo signal being received. On reception of mono FM stations, the beam will not close at all.

Dave

To clarify a little, the eye beam tube on a fully-aligned tuner should completely close with a tuned (on channel) MPX signal. It will modulate off-channel, which provides a fine-tuning aid. In mono operation, the eye beam will never fully close, but will nevertheless modulate when tuned. The amount of modulation is crudely consistent with signal strength variance. So it remains a helpful tuning aid, even in mono.

Non-closing eye-beam tubes in these units are a common occurrence. Why a particular eye beam doesn't completely close varies, but includes use of the wrong tube (EM84 v 84a), a weak signal, misalignment, or other circuit irregularity, or a combination of the same. FWIW, I have been able to get EM84 tubes to close on a perfectly aligned and operating Fisher, but the margin to accomplish this is much tighter.