Fisher 500-B FM and FM MPX Reception Issues - Looking for Some Guidance

[Pioneer Vir]

I have been restoring a 500-B for the past few months. I am having FM and FM MPX reception issues. I am receiving FM but the signal is not 100% clear and it has some noise present. FM MPX is receiving a signal but it is very noisy, almost to the point of not being able to hear the signal. I am only receiving two FM stations that are the closest to me and have the strongest broadcast signals. Stereo Beam is present but it is not showing any movement. I am using a basic t-shaped di-pole antenna connected to the Distant connection screws.

Volume, Balance, Bass, Treble, Low Filter, High Filter, Reverse are all working correctly. No audible noise on Tape Head, Phono, Aux and Tape. I have not hooked anything up try the other positions yet.

All electrolytic and mylar caps have been replaced (Hayseed Hamfest for cans). All power supply voltages are within 5% of shown schematic voltages. Line voltage is 119.5 volts and bucking transformer (being used) is 115.6 volts. "D" voltage for front end (6AQ8, 6AU6) is 173.2 volts (schematic lists 165 volts. I can provide all of the voltage readings if desired.

C97 bias supply voltages are +5.75 volts and -23.85 volts.

"Front End" tube voltages. All of these tubes are NOS and have been tested SAT and verified to work in another receiver.

V1 (6AQ8) RF Amp OSC-Mix

- Pin 1 - (schematic 158 volts) - reading 160.6 volts
- Pin 6 - (151 volts) - 151.5 volts

V2 (6AU6) 1st IF Amp

Pin 5 - (160 volts) - 161.5 volts
Pin 6 - (105 volts) - 79.4 volts

V3 (6AU6) 2nd IF Amp

Pin 5 - (159 volts) - 159.5 volts
Pin 6 - (108 volts) - 107.3 volts

V4 (6AU6) 1st Limiter

Pin 5 - (160 volts) - 112.6 volts
Pin 6 - (60 volts) - 46.8 volts

V5 (6AU6) 2nd Limiter

Pin 5 - (159 volts) - 112.1 volts
Pin 6 - (75 volts) - 54.2 volts

MPX tube voltages. 12AT7 tubes are NOS and 12AX7 tube is new production Tung-Sol. They have been tested SAT and verified to work in another receiver.

V100 (12AT7) Low Pass and 19KC Amp

Pin 1 - (95 volts) - 100.4 volts
Pin 3 - (7.5 volts) - 11.18 volts
Pin 6- (40 volts) - 28.98 volts

V101 (12AX7) 38KC Synch Osc

Pin 1 - (160 volts) - 117.9 volts
Pin 2 - (-0.,7 volts) - 0 volts
Pin 6 - (160 volts) - 118.3 volts
Pin 7 - (-6.0 volts) - -2.46 volts

V102 (12AT7) Audio Amp

Pin 1 - (94 volts) - 98.9 volts
Pin 3 - (19 volts) - 27.13 volts
Pin 8 - (19 volts) - 25.93 volts

"Front End" reception and multiplexing are not my strongest areas. I have more losses there than victories. Before I start troubleshooting I would like to have some guidance from the folks who know what they are talking about and doing. Where do I start at? It "appears" that V1 - V3 have good voltages with the exception of V2 pin 6. V4, V5 (Limiters) and MPX tubes have out-of-spec voltages based on schematic. I did not include the schematic as it's a Fisher 500-B and I suspect that you already have a copy of it. If not, I can certainly attach it. Same for pictures if desired.

Thank-you!

Don

 

[gadget73]

if all of the non-tuner inputs work properly with no distortion issues, we can probably ignore all of the audio stuff. If the output isn't properly clean in FM mono, the MPX decoder has no real hope of working correctly so I wouldn't worry about that for now either.

I don't suppose you have FM alignment gear? I wonder if you don't have a mistuned or bad IF transformer. A sweep response on the IF strip would show that pretty quickly. Silver mica disease might explain both the poor performance and the noises.

limiter voltages are off but I don't know that they are off enough to explain a serious problem. Those voltages also change depending on signal strength, I don't know what conditions they are taken under so it really may not be indicating anything.

If the tuning eye doesn't respond in stereo that means its got no 19kc pilot tone. If its not working in mono, there isn't enough signal strength getting through the IF strip for the limiter stage to actually do anything useful. Low signal strength will kill the pilot tone, and with no pilot tone you get no stereo so I'd focus on getting it properly functional in mono first.

 

[richiem46]

Probably stating the obvious but in several units I have worked on the 6AQ5 and the 6AU6 if tubes being weak caused poor FM reception. Once the weak tubes were identified and replaced the FM section performed much better and alignment of the FM/Stereo was possible.

 

[Pioneer Vir]

if all of the non-tuner inputs work properly with no distortion issues, we can probably ignore all of the audio stuff. If the output isn't properly clean in FM mono, the MPX decoder has no real hope of working correctly so I wouldn't worry about that for now either.

I don't suppose you have FM alignment gear? I wonder if you don't have a mistuned or bad IF transformer. A sweep response on the IF strip would show that pretty quickly. Silver mica disease might explain both the poor performance and the noises.

limiter voltages are off but I don't know that they are off enough to explain a serious problem. Those voltages also change depending on signal strength, I don't know what conditions they are taken under so it really may not be indicating anything.

If the tuning eye doesn't respond in stereo that means its got no 19kc pilot tone. If its not working in mono, there isn't enough signal strength getting through the IF strip for the limiter stage to actually do anything useful. Low signal strength will kill the pilot tone, and with no pilot tone you get no stereo so I'd focus on getting it properly functional in mono first.

I believe I have a signal generator around here somewhere. I'll need to find it, blow the dust off and check it out first. No tuning eye movement in mono or stereo. I'm sure I have 120 ohm resistors but they're probably not carbon.

 

[Pioneer Vir]

Probably stating the obvious but in several units I have worked on the 6AQ5 and the 6AU6 if tubes being weak caused poor FM reception. Once the weak tubes were identified and replaced the FM section performed much better and alignment of the FM/Stereo was possible.

They should be good but, I have more of them. I'll swap them out to make sure.

 

[dcgillespie]

Unless you have a generator of known 10.7 mHz accuracy (either crystal controlled or digitally synthesized with externally verified accuracy), you can easily do more harm than good if attempting an IF strip alignment on your Fisher with such equipment. Therefore, virtually all vintage RF generators of the kit variety using air tuned caps and manual adjustment knobs against a printed frequency face plate scale are nowhere near accurate or stable enough and need not apply for the job.

If your unit has (hopefully) not been touched by adjustment fiddlers, then the factory alignment setting should still be in place. Contrary to popular belief, vintage tuners and receivers don't just "go out of alignment". Rather, something fails to give the appearance it is out of alignment, when in reality, the alignment is fine, but some component has failed. As mentioned, an all too often case is that mica disease has developed in one or more of the IF transformers. If that is the case, and an adjustment fiddler has tried their hand on top of that, then the unit now really is out of alignment, so that the original problem still needs to be fixed, and then the unit aligned again. On the other hand, if the settings have not been moved and a transformer's defective resonant cap replaced, then alignment is usually either not required, or only a very slight touch up at best will be required of only the repaired transformer.

If any of the transformers have mica disease, one symptom is FM operation that will go in and out of working correctly -- possibly not so when first turned on, but then all of sudden springing into action after a little operating time -- or visa versa. One way of determining if mica disease is present fairly accurately without using test equipment is to carefully "rock" each IF transformer slightly all four ways from its normal mounted position. If doing so causes the FM signal to either snap into or out of normal operation, then the likelihood is high that one or both of the transformer's internal mica resonant caps are compromised, and the transformer will either need to be repaired or replaced.

I hope this helps!

Dave

 

[Pioneer Vir]

Dave,

Thank-you for the insightful reply!

No attempt at alignment has been made. I believe it's OK on that front. The FM has not been fading in and out. I will let it run longer and monitor it and see if there is any fading present. It's only receiving my two strongest local stations in FM mode only. It is a clear signal but it's weaker than normal. Volume and tone controls are working correctly. Switching to MPX results in the signal becoming very noisy and unlistenable.

I will do the IF transformer "rocking" carefully and report back.

Don

 

[larryderouin]

Just for S&G swap V2 with V3 and check all voltages again. If the Drop on pin 6 follows the tube, problem is the tube.

Whether the unit is tuned tighter than a slammed shut Tick's A** or off a little, you can't get the best from it with a mediocre Antenna. Most "T" antenna's are good for a metroplitan area but not so good in a suburban or rural area. The farther away from the transmitter the worse the signal. Hanging on a wall or on the floor hurts too. Get it up as high as you can. An Amazon "DIGITAL" Antenna on a rotor is a marked improvement. A Dedicated FM Band Antenna on a mast is better.

FM is "LINE OF SIGHT" to the Horizon. AM will bounce off the ionosphere (long wave). So conceivably you can with a decent AM tuner get Chicago from Atlanta(in my case Baltimore) or Memphis from New York with a simple 50ft wire out the window. FM doesn't work like AM. The Higher the antenna is the further the Range up to a point. I can pick up a college station in Philadelphia (85 miles up the road) on most of my tuners at home but it's somewhat fuzzy and in MONO ONLY. FM Stations of between 16KW to 25KW 15 to 25 miles are hitting 4by5 for the most part but need antenna rotation to get best results. I have a 100 mile channel Master VHF/UHF/FM monster on the roof with a Yaesu G450ADC rotator 12 ft above my roof. Washington DC stations are no problem for this setup. I'm in Northern Baltimore County (on Top of highest hill in B'more county) between york and B'more about 10 miles from state line. Very hilly here.

From your profile I gathered you are close to Norfolk (Dogs and Sailors keep off the grass) so it's fairly flat until near Richmond. With a "T" antenna spread perpendicular to the location (direction) of the signal you should get pretty good reception in FM at 20-30 miles. 40 miles is generally fringe area and depends on your height, the transmitter height and any intervening obstacles between (High trees, Tall bldgs and mountains.)

MPX is dependent on a correctly tuned FM circuit (ie: Tighter than a slammed shut ticks A**. Any deviation will Degrade the FM MULTIPLEX SIGNAL. The multiplex signal is an AM Carrier that is attached to the MONO FM signal and is pulled from it by the FM Multiplexer after the signal has been spindled, folded, and mutilated by the FM MONO Circuit. The FM STEREO signal will be slightly noisier because of this AM Carrier. Filters pull most of it out but not all of it. If possible extend a "T" about 10-15ft above the house and recheck. Trees and bldgs will cause ghosting of the signal as it's bent in different directions and all these get to your antenna at slightly different times.

Get the tuner IF Strip tuned up tight, and tune the MPX as Dave alluded to. Get a decent antenna (you won't need a monster for Norfolk) and mount it as high up as you can. HOA can't say anything if it's hung up in your Attic. If your close to NAVSTA Norfolk you might get some interference. Not sure about now, but back in the 70's-80's it was noticeable. Not so much NAS Oceana.

For Example WNOB is a 100KW station located in Chesapeake VA, but the tower is at the state line with NC. Even with an off tune tuner you should be getting a strong signal at 10-20 miles on a "T" antenna on the floor.




WUSH is a 11KW station out of Poquoson. I would expect a little degrading of signal strength but the tower is closer than WNOB Tower. I would call it a wash or extremely close depending on where you are in relation to the towers.


Go to Radio-Location.com type in Chesapeake va for location. Then click on your favorite station and click Area Coverage Map. Ideally you should be in the LOCAL AREA.

 

[Pioneer Vir]

Larry,

Guilty - Squadron Six bubblehead from the late 70s. Spent more time up North than in Norfolk though. Cold War... ah, the good ol' days.

I'm in Chesapeake and live 4 miles north of the NC state line. WNOB is the station that I am receiving the best. Roger on the di-pole antenna. It's being used for convenience. Normally, I get quite a few local stations using one. I have a metric butt-ton (good submarine technical term) of antennas. I'll hook up a digital one and see if reception is any better.

I'll swap V2 and V3 and see what the voltages are. I'll do the IF transformer rocking after that and report back. Appreciate the help!

Don

 

[Pioneer Vir]

Swapped installed 6AQ8 with original. No change in symptoms.
Swapped V2 and V3 6AU6. Low pin 6 reading (80.1V) stayed with V2 location. No change in symptoms.

Connected a Terk digital antenna to the receiver. Two additional FM signals were present with full amplification on the antenna. FM signals were weak but were present. Two existing FM signals were overdriven.

Two existing FM signals are stronger and clearer with the Terk antenna but overall, no change in symptoms. FM position working for these two signals and FM MPX is working but there is significant distortion present. Stereo Beam working with FM MPX position.

Don

 

[ramjet]

Swapped installed 6AQ8 with original. No change in symptoms.
Swapped V2 and V3 6AU6. Low pin 6 reading (80.1V) stayed with V2 location. No change in symptoms.

Connected a Terk digital antenna to the receiver. Two additional FM signals were present with full amplification on the antenna. FM signals were weak but were present. Two existing FM signals were overdriven.

Two existing FM signals are stronger and clearer with the Terk antenna but overall, no change in symptoms. FM position working for these two signals and FM MPX is working but there is significant distortion present. Stereo Beam working with FM MPX position.

Don

Is R48 still 47K as it should be?

 

[Pioneer Vir]

Yes. Original resistor is installed.

 

[ramjet]

Yes. Original resistor is installed.

And it is still 47K? I think something around V2 is not quite right hence the large difference in voltage.

 

[Pioneer Vir]

Yes. All original components. I follow your line of thinking though.

 

[gadget73]

I think what he's implying is you should measure the resistor to see if it has gone off value. Not a bad idea honestly, old carbon resistors tend to suck in that regard.

 

[gadget73]

If your unit has (hopefully) not been touched by adjustment fiddlers, then the factory alignment setting should still be in place. Contrary to popular belief, vintage tuners and receivers don't just "go out of alignment". Rather, something fails to give the appearance it is out of alignment, when in reality, the alignment is fine, but some component has failed

100% on this, I was thinking more just see if it looks correct or not. If the IF strip response looks normal, then it may be a problem in the RF amp section. if the IF response is whacked out then maybe someone has been in here or there is a bad part. Probably should have been a bit more clear that I was thinking this as a diag, not a "try to re-align it" step.


having screwed up more than one FM IF strip I'm sort of hesitant to mess with them unless there is a solid reason to, but I'm fine with checking them.

 

[Pioneer Vir]

R48 (47K) reads 50.0K. I lifted one lead for the reading.

After soldering the lifted lead back on during voltage checks (R48 read 80.4V), touching pin 5 causes increase in distortion. Touching pin 6 - no distortion. Touching pin 7 results in a significant increase of volume. I do not remember having this issue initially and I don't have anything written in my notes. Regardless, it's there now. Bad ground in the circuit?

Z2 leads 1, 3 and 4 all result in distortion when touched. No change when touching lead 2.

 

[ramjet]

R48 (47K) reads 50.0K. I lifted one lead for the reading.

After soldering the lifted lead back on during voltage checks (R48 read 80.4V), touching pin 5 causes increase in distortion. Touching pin 6 - no distortion. Touching pin 7 results in a significant increase of volume. I do not remember having this issue initially and I don't have anything written in my notes. Regardless, it's there now. Bad ground in the circuit?

Z2 leads 1, 3 and 4 all result in distortion when touched. No change when touching lead 2.

A ground issue is possible but, tested value of R44 please and pictures of the circuitry around V2 please.

 

[Pioneer Vir]

R44 reads 162.4 ohms.

Note: I went ahead and replaced R48.

 

[ramjet]

R44 reads 162.4 ohms.

Note: I went ahead and replaced R48.

If you have a good selection of 6AU6 tubes it would not hurt to plug and play a few in the V2 position to see if that changes the symptoms. Have you added/replaced wires in this area? Reason I ask is that wire routing in this area can affect performance.