Weird problem with Fisher 500 C radio

[robbarrow]

I am in the process of restoring a Fisher 500 C. Following what seemed to be "everyone's" advice, I didn't even plug the receiver in until I had recapped it. Along the way I had to replace one of the little switches on the front - I think it was the Low Filter and after I started listening to the unit - my phone through the aux, I noticed that the radio wasn't working. I finally discovered those tiny AC4s one of which was bad and replaced it and still no joy. Then, as I was testing, I accidentally left out the 6HA5 and the radio worked! Not stereo, but at least the right side was bringing in stations! I thoroughly cleaned the selector switch and ultimately had to replace the volume pot as it didn't respond to multiple(many) applications of Deoxit - not particularly fun with eleven connections. During the restoration, I replaced a couple of resistors in the multiplex section as recommended by my restoration kit. I have looked in that area to see if I created any shorts but it looks like I did an OK job - not elegant, but adequate (it is a particularly difficult area in which to work). I have replaced the 6HA5 with a new tube but this had no effect. It still works mono without the tube. I never did anything in the area where the 6HA5 actually resides. I love to tinker but I wouldn't describe myself as someone very knowledgeable in electronics. Does anyone have any ideas about how to proceed? I feel that the 6HA5 completes a short but don't know where it might be.

 

[thornev]

Rob - 6HA5 is V1. You might try looking at my 500-C PHOTOFACT thread to see if anything is missing around V1 (go to my last post that has an image of the wiring) or maybe a resistor or capacitor has weakened (if you have a multimeter to measure). I would bet someone will come along with better assistance. Thorne

 

[robbarrow]

Rob - 6HA5 is V1. You might try looking at my 500-C PHOTOFACT thread to see if anything is missing around V1 (go to my last post that has an image of the wiring) or maybe a resistor or capacitor has weakened (if you have a multimeter to measure). I would bet someone will come along with better assistance. Thorne

Thanks Thorne. I don't always trust my multimeter but It is always worth a try!

 

[dcgillespie]

If the receiver is producing a signal in both channels when set for mono operation, then V1 through V6 and V18 are all working correctly. These tubes must all be installed and operating for any signal to be produced from the receiver section -- mono or stereo. If the one channel is lost only in Stereo mode (either by automatic switching or from manual setting), then the problem is invariably in the MPX section, so your focus should be centered in that area. The usual cause for a dropped channel in the MPX section is that one of the output coupling caps has failed. The type Fisher used is not known to leak over time, but is notorious for becoming intermittent before finally just becoming open.

The recommendation to lower the stock value of the resistors used in the stereo de-emphasis and 19 kHz trap networks in the MPX section is provably dubious, and can potentially result in damage to the high frequency drivers in your speaker systems. Lower these at your own risk........

Good luck with your 500C!

Dave

 

[robbarrow]

If the receiver is producing a signal in both channels when set for mono operation, then V1 through V6 and V18 are all working correctly. These tubes must all be installed and operating for any signal to be produced from the receiver section -- mono or stereo. If the one channel is lost only in Stereo mode (either by automatic switching or from manual setting), then the problem is invariably in the MPX section, so your focus should be centered in that area. The usual cause for a dropped channel in the MPX section is that one of the output coupling caps has failed. The type Fisher used is not known to leak over time, but is notorious for becoming intermittent before finally just becoming open.

The recommendation to lower the stock value of the resistors used in the stereo de-emphasis and 19 kHz trap networks in the MPX section is provably dubious, and can potentially result in damage to the high frequency drivers in your speaker systems. Lower these at your own risk........

Good luck with your 500C!

Dave

I don't like to disagree with people who clearly know much more than I do but my original post was accurate as far as my 500 C is concerned. I did look at it a little harder after I got your post. As I said, no radio signal at all with 6HA5 installed. With it out, on mono FM and FM auto I do get radio in both channels but on checking it is much stronger in the left channel. With the balance to the right I do hear the signal in both channels. As I said, this seems to be a weird problem for the radio to function at all without what seems to be the main radio tube! And no function whatsoever with it installed.

 

[dcgillespie]

Rob -- Neither do I doubt what you're saying. My comment was made from a pure standpoint of theory -- not based on what you are actually witnessing.

So going on the observation that when the 6HA5 tube (V1) is installed -- the radio quits, then let's go at it this way:

1. Do you have another tube to try -- in case that one is seriously defective.

2. Is the tube actually lighting up so it can function?

3. Also, can you verify that there is plate voltage on this tube? That would be pin 5.

Either the tube being bad, no plate voltage, or no heater operation are about the only things that can kill the operation of the stage.

If the stage is not operating due to one of the factors mentioned above, it would be possible under the right conditions for the receiver to pick up some very strong FM signals with V1 removed -- but sensitivity would still be very poor. With the tube installed but not functioning, it would detune the resonant frequency of the tuning circuit that appears after V1, so that it would no longer operate within the FM band. In this way, it's possible for the receiver to operate without the tube, but not with, although as I said, performance would be very poor.

Unequal sound between the channels and the Balance control not fully cancelling out one channel are both completely irrelevant issues to the ones associated with V1.

Dave

 

[gadget73]

6HA5 is the RF amp tube. Possible that with it out you're getting some stray coupling from the antenna to the input of the mixer. With a bad tube installed its just not getting signal through. If another tube doesn't sort it, there has to be some failure right around that tube.

I'd expect a meter probe poked into pin 5 of the V1 socket to bring in more stations since you'd be essentially connecting an antenna direct to the input of the mixer, bypassing the missing RF amp stage and whatever coupling is happening with the tube out.

 

[robbarrow]

Rob -- Neither do I doubt what you're saying. My comment was made from a pure standpoint of theory -- not based on what you are actually witnessing.

So going on the observation that when the 6HA5 tube (V1) is installed -- the radio quits, then let's go at it this way:

1. Do you have another tube to try -- in case that one is seriously defective.

2. Is the tube actually lighting up so it can function?

3. Also, can you verify that there is plate voltage on this tube? That would be pin 5.

Either the tube being bad, no plate voltage, or no heater operation are about the only things that can kill the operation of the stage.

If the stage is not operating due to one of the factors mentioned above, it would be possible under the right conditions for the receiver to pick up some very strong FM signals with V1 removed -- but sensitivity would still be very poor. With the tube installed but not functioning, it would detune the resonant frequency of the tuning circuit that appears after V1, so that it would no longer operate within the FM band. In this way, it's possible for the receiver to operate without the tube, but not with, although as I said, performance would be very poor.

Unequal sound between the channels and the Balance control not fully cancelling out one channel are both completely irrelevant issues to the ones associated with V1.

Dave

I really appreciate your time in this Dave. So, to sum up, I have tried a total of four 6HA5 tubes in this spot. The last one I tried is brand new(new old stock Sylvania - out of the box). It is a taller tube than the first three one of which I have not actually tested. My recollection is that they have all shown a glowing filament but to be fair the silvering makes seeing the filament difficult on one of them. I will test the voltage between pin 5 and the chassis and let you know what I find. Thanks for your interest.

Rob -- Neither do I doubt what you're saying. My comment was made from a pure standpoint of theory -- not based on what you are actually witnessing.

So going on the observation that when the 6HA5 tube (V1) is installed -- the radio quits, then let's go at it this way:

1. Do you have another tube to try -- in case that one is seriously defective.

2. Is the tube actually lighting up so it can function?

3. Also, can you verify that there is plate voltage on this tube? That would be pin 5.

Either the tube being bad, no plate voltage, or no heater operation are about the only things that can kill the operation of the stage.

If the stage is not operating due to one of the factors mentioned above, it would be possible under the right conditions for the receiver to pick up some very strong FM signals with V1 removed -- but sensitivity would still be very poor. With the tube installed but not functioning, it would detune the resonant frequency of the tuning circuit that appears after V1, so that it would no longer operate within the FM band. In this way, it's possible for the receiver to operate without the tube, but not with, although as I said, performance would be very poor.

Unequal sound between the channels and the Balance control not fully cancelling out one channel are both completely irrelevant issues to the ones associated with V1.

Dave

I put a clip on the small capacitor lead and this time used the probe on the chassis - likely reversed the leads but I got a steady -191 vdc. All of the tubes I have tried test good. I did note an increased sensitivity with the test lead in place.

 

[dcgillespie]

Hi Rob -- OK, now we're getting down to the nuts and bolts of things real quick. That is way too much plate voltage, indicating that the tube is not drawing any current at all. The normal voltage at pin 5 will be under 100 volts. Check the secondary winding connections of the antenna coil L1. It is a tapped coil with one end connected to the cathode terminals of the tube (pins 2&7), the other end connected to the grid (pin 1), and the center tap is grounded.

If the secondary winding of this coil has opened (lightning strike?) on the cathode end, then there is no connection between the cathode and ground, so the tube cannot draw any current. Or, if the ground connection of the center tap is compromised, then the end result will be the same. If the voltage is present at pin 5, the heater is lit, and the tube is good, then the only reason that the tube won't conduct any current is because the cathode circuit is compromised. At the very least, you should show a very low resistance reading between the cathode terminals, and ground. If not, investigate why, which will likely be your problem.

The only other possibility is the tube socket itself being compromised on the pins discussed (2, 5, and 7).

Let us know!

Dave

 

[robbarrow]

Hi Rob -- OK, now we're getting down to the nuts and bolts of things real quick. That is way too much plate voltage, indicating that the tube is not drawing any current at all. The normal voltage at pin 5 will be under 100 volts. Check the secondary winding connections of the antenna coil L1. It is a tapped coil with one end connected to the cathode terminals of the tube (pins 2&7), the other end connected to the grid (pin 1), and the center tap is grounded.

If the secondary winding of this coil has opened (lightning strike?) on the cathode end, then there is no connection between the cathode and ground, so the tube cannot draw any current. Or, if the ground connection of the center tap is compromised, then the end result will be the same. If the voltage is present at pin 5, the heater is lit, and the tube is good, then the only reason that the tube won't conduct any current is because the cathode circuit is compromised. At the very least, you should show a very low resistance reading between the cathode terminals, and ground. If not, investigate why, which will likely be your problem.

The only other possibility is the tube socket itself being compromised on the pins discussed (2, 5, and 7).

Let us know!

Dave

The resistance from the antenna connection to the lead from the coil to pins 2&7 is 98K ohms. As you predicted there is little to no resistance between pins 2&7 and the ground connection at the center tap (lead coming out of the coil connecting to chassis)I do not have a brush small enough to clean the sockets of this or similar tubes. I suppose that I could dab some deoxit onto the pins themselves and repeatedly insert and remove the tube. There is no visible defect in the tube socket. I did this attempt at cleaning and it had no noticeable effect on the problem.

 

[dcgillespie]

OK. With the tube in place, measure the voltage drop across R37. This is a 3.3K 1 watt resistor located outside the RF can box. It is the B+ dropping resistor for V1. If V1 is drawing current, then there should be about a 45-50 volt drop across this resistor. The + side of your meter connects to the side of R37 that also connects with R36, the - side of your meter connects to the side of R37 that connects to a feed-thru cap in the side of the RF can box.

Dave

 

[robbarrow]

Dave, I am probably doing this wrong. when I measure across R-37 I get a voltage of only about a half volt dc. Measuring from the positive side to the ground (chassis) I measure 201.4 vdc. from the negative side to ground I actually measure an increase in voltage to 202.1 vdc.

 

[dcgillespie]

HI Rob -- Nope, you're doing it exactly correctly, and the result you're getting simply proves that V1 is not drawing any current -- assuming the tube was installed when you did the test. That resistor is in series with the RF choke inside the can that is connected between where R-37 connects to the feed-thru terminal on the RF can box, and the plate (pin 5) of the tube. We know the RF choke is good, because you were reading some 190 vdc between pin 5 and ground -- although it would be better if this could be confirmed with the tube actually in place -- but at this point we'll run with what we've got. The bottom line is, because the tube is drawing no current, you are getting virtually no voltage drop across the resistor.

So if pin 5 has plenty of voltage appearing at it, and if pins 2&7 have a very low resistance reading to ground, and the tube is still not conducting any current, then either:

1. The pins are somehow defective in the socket and not allowing the connection to be made to the actual tube pins that you think are being made, or

2. The tube is completely dead, as in incapable of passing any current at all, or

3. The heater is not lighting in the tube due to some bad connection somewhere in the wiring to pins 3&4, or something involving these terminals in the socket is bad, or of course, the heater itself is bad.

My bet is that the heater in the tube is not being lit, since items 1 and 2 have largely been eliminated.

Dave

 

[robbarrow]

Dave, I did some more checking. With the tube in, the voltage at pin 5 is 174 vdc, at pin 4 it is 7.4 vac, at pin 1 it is 7.1 vdc, at pins 7&2 it is 2 milivolts ac. Pins 6&3 are grounded. I can see the filament glowing in the tube. It helps to see it from the bottom.

 

[dcgillespie]

OK -- But now, things are different. With 174 volts at pin 5, that is the lowest voltage you've reported at that pin in all of your posts. And now the heater is confirmed to be glowing. This implies the tube is now conducting some current (measure the voltage drop across R37 again and see) -- but not much (maybe 7-8 mA, which is low for an RF amplifier tube) -- unless it is tuned to a station.

Check the pin 5 voltage reading with the tuning dial turned full down to the left of the scale, and disconnect any antenna. This is the condition for accurately checking any tuner voltages. Up until now, it hasn't mattered much, but maybe with V1 (possibly) coming to life a little bit (?), it just might now.

Dave

 

[robbarrow]

As you suspected the current across R-37 is only 7ma. To my shock and horror, when
I measure the voltage at pin 5 under the conditions you specified
(tuner parked at lowest frequency with antenna disconnected) I now find the highest voltage I have measured at 198 volts. I am beginning to not trust my multimeter and will at least change the batteries.

 

[larryderouin]

rob; If there are two voltages on any point, one not having a dashed or solid box around it, and one with a dashed or Solid box around it, it means that the voltage in the open is checked with the unit in AUX. Solid box means checked with selector in FM MONO, Dashed box means checked in FM MPX( Stereo). See schematic notes next to the selector switch position chart on the schematic.

 

[robbarrow]

rob; If there are two voltages on any point, one not having a dashed or solid box around it, and one with a dashed or Solid box around it, it means that the voltage in the open is checked with the unit in AUX. Solid box means checked with selector in FM MONO, Dashed box means checked in FM MPX( Stereo). See schematic notes next to the selector switch position chart on the schematic.

I will try measuring the voltages at these settings. Thanks

 

[robbarrow]

I will try measuring the voltages at these settings. Thanks

I replaced(recharged) the batteries in my multimeter and switched to clips instead of probes and consistently got much lower voltages at pin 5. When set on aux I got -1 volt. when on any radio setting I got between .8 and .9 volts. So this setting does make a discernable difference. There is no measurable voltage drop across R-37. The pin 4 voltage is steady 7.18 vac.

 

[dcgillespie]

Alright, this is entirely different now. You used to be getting at least 174 vdc at pin 5, and now you are indicating you're getting less than a volt. Batteries don't cause that big of a swing. You're sure you had the selector switch on an FM setting when you made your measurement? Maybe a loose connection somewhere? Even a dirty selector switch, as it switches the B+ supply voltage to V1 as appropriate depending on whether a FM setting is selected or not.

Dave