Fonovox 9070 Console No FM

[KMcCulloch]

I have acquired a Fonovox console AM/FM/Console radio that has AM working wonderfully but I cannot get the FM to work. I have obtaine the schematics off of radio museum dot org and have found that my resistance around the FM Tubes are correct. I have tried replacing all the tubes, especially the FM ones. I have replaced the M1 diode recifier, large electrolytic capacitors, and small caps in the FM circuit. When I measure voltages in the FM circuit, my multimeter test lead can cause a station to tune in no matter what the tuner dial is tuned to. The 285 volts gets pulled down to 220 in the FM circuit and the -4.5V is only around -0.45V. Again, the AM works great. Any ideas at what to look at next?
Thanks,
Kevin

 

[gadget73]

Absolutely any point in the tuner will pull in a station when the test lead is on it, or just stuff after the ECC85? Either way it sounds like at least the IF strip and detector are working correctly. Without any further diagnosis, I'll guess either the oscillator isn't running, or the RF amp isn't working. RF amp is reasonably easy to diagnose. Take a test lead and touch it to pin 1 on the ECC85. If the tuner works more or less normally, meaning you can change stations and such, the problem is in that area. Sensitivity won't be great but strong stations should work even without the amp.

If that doesn't do anything, grab a portable radio and put it next to this one. Set the dial on the Fonovox to 90 and see if you get some sort of signal on the portable around 100.7. If no dice, try 100.7 on the Fonovox and 90 on the portable. If nothing either way you have an inop oscillator. That will be the area around V1B.

The low voltage and low grid supply in the IF strip just means its not getting any signal. The more negative the grid is, the stronger the signal its getting, so closer to zero means no signal. The current flow in the tubes increases as the grid moves closer to 0v. I wouldn't worry too much about it, but at least its telling me the IF strip isn't oscillating and swamping any actual signal.

 

[KMcCulloch]

Hello gadget73. Thank you for responding to my situation. I was out of town on business and am now back hope trying to clear this chassis off my workbench. Powering it back up this morning, the volatges around ECC85 reamin the same as before. I tried a number of your suggestions to with no luck. But for some reason I am picking up a local radio station at 102 MHz all the time, no matter where the turning dial is from 87 - 107 MHz. I do not have much experience troubleshooting oscillators so your assistance is greatly appreciated. You mentioned that the area around V1B may be the culprit, how can I look into that further? Any other suggestions on how to troubleshoot?
Thanks - Kevin

 

[gadget73]

Does the portable radio trick turn up anything? That would tell us if the oscillator is running. Without the oscillator its not going to work correctly.

 

[KMcCulloch]

The only radio that I had handy this morning was a transistor radio and with the Fonovox receiving the 102MHz channel, I could not notice that the radio trick working. Shoudl I be using a portable tube radio?

 

[gadget73]

doesn't matter, any portable is fine.

will give an idea what it should sound like. 5:30 is what it will sound like on FM if things are working. Same idea on AM, just doesn't sound exactly the same.

 

[KMcCulloch]

Thank you for the video, it was very helpful in seeing a working setup. The ground lead of C19 (300mmf) had a cold solder joint from my earlier replacement and that is what was causing me to receive 102MHz all the time. Upon resoldering it, I am not able to receive any station again. I again tried the oscillator test at 90 and 100.7 MHz, and could not pick up anything like the video. When the non-working radio was at 90 and the working portable at 100.7, a weak local station came through. I took the portable down to 100.5 and may have had a little noice but by turning the dial on the non-working radio, no new noice appeared on the portable. Then I tried 90 on the portable and all around 100 on the non-working, and recevied no noice.
So it looks like my oscillator may not be working. Is that the area around pin 6 of ECC85? I have already replaced all the caps (C16, C18, C20, & C15) in the past. I have measured the resistance of all the coils and have not found any open or shorted to ground if they are not supposed to.R9 is good and the 100K resistor to ground below coil L9 has been replaced. I do have a signal generator, if that would help me where I have gotten to so far. Any other ideas would be greatly appreciated.
Kevin

 

[gadget73]

Was it working before the caps around the ECC85 were changed? usually those small value caps don't go bad but changing them does force a re-alignment. Should still be functional if the proper values were used, just the dial will be off, possibly way off. You did use picofarad value caps to replace the original mmf yes? If those got subbed with microfarad or nanofarad values its not going to work

 

[KMcCulloch]

Hello - THe FM has never worked since I have owned this console. You are correct, those small value non-electrolytic caps do not normally fail but I have been focusing my search around the ECC85 and there earlier as I was trying to raise the voltages on pins 1 & 6 and lower the negative voltage on pin 7, that was all that I could think of trying in addition to replace the tube numerous times. I did have to confirm that mmf was for picofarad and did use the correct values. Can we learn anything from the fact that when the ground leg of C19 was off, I could receive a local 102 MHz station throughout the whole dial and it was stronger towards the 100+ frequencies? The AM works perfectly and I would certainly like to find the issue with the FM for my son's first apartment. Thanks, Kevin

 

[gadget73]

probably just that the IF strip and detector are working. The RF amp is also probably working since you could hear it "peak" as the dial got towards 102. That would be where the RF amp is tuning to the station that leaks in.

Any chance there is something in between the fins on the tuning cap that cause it to short out? That would shut all this down but it would still read through the coils and such. Sometimes these are mounted on rubber grommets, not sure how this specific one is put together.

 

[KMcCulloch]

Hello gadget73 - I scrutinized the area around the FM tuner for any possible short or problem and could not find anything new. In the process, I accidently pulled off a wire from M1 and lost AM too before finding the culprit. It seems that no matter what I investigate around the ECC85 and the FM tuner area, I am not able to pick up an FM stations. Touching the test lead of the DVM does nothing as far as injecting a signal anymore. Would it be beneficial to try injecting a signal somewhere in this circuit with my signal generator? Thanks

 

[gadget73]

What do you have for a signal generator?

 

[KMcCulloch]

I have a HeathKit Model IG-102. I have attached a couple of pages from the manual.

 

[gadget73]

Set it for 10.7 mhz. Take a couple turns of wire around the ECC85 and connect the signal generator + lead to that. If the ECC85 has a shield, remove it or maybe use some tape to insulate it from ground and clip the + lead to that instead. Point is to couple the signal generator output to the ECC85 somehow. Sig gen ground to chassis. See if you can get a tone at the speaker. Looks like this has only amplitude modulation so it might come through fuzzy but you should be able to get something if the IF strip and detector is working as I suspect it is. The eye tube should also close even if the audio doesn't come through cleanly.

 

[KMcCulloch]

Assuming my signal generator is working correctly, I did not get any response around the ECC85 at 10.7 MHz. The tube did not have a full shield, just a short one around the chassis so I put tape around that and added a full shield over it and the tube. But had no response with the signal generator + Lead clipped to the shield and ground lead clipped to the chassis. Even moving the +Lead around that area of the chassis has no response on the speaker. I switched back to a wokring AM station and tried moving the 10.7 MHz lead around ECH81 and had no response. Either I am not working my generator correctly or somehting is not switching to FM correctly. I have tried and tried to use DEOXIT on the slide contacts for all the Band selection switches and everythign else works except FM. I have even tried measuring resistance across the contacts I can find and they good.

 

[gadget73]

10.7 mhz won't respond on AM. You may have to wind the signal level up a fair bit to get a response. ALso just make sure that any shield is not grounded otherwise it will just cancel out the signal

 

[KMcCulloch]

Limited Success! (At this point I will take any success that I can get). I was able to hear the 10.7MHz at the speaker when driving the shield around the ECC85 hard with the signal generator. With the volume max out, I could hear the freq. hum. I could not see any noticeable response on the eye tube, which works fine on AM. What can I try next?

 

[gadget73]

ok, since you have IF response, and we know the oscillator doesn't work, and you have a strong local around 102, try setting the generator to about 91.4 and see if you can pull that station in. Play with the dial on the generator and it should change the frequency that is recieved. Tuning dial on the radio should be somewhere in the 102 range to peak the RF amp.

 

[KMcCulloch]

Hello - I tried driving the ungrounded shield around ECC85 with a strong 91.4 megacycle signal from the generator and got no response no matter where the tuning dial was throughout the whole dial. Nothing new was noticed at the speaker. Thanks for the idea and I am still scrutinizing the power and FM circuits.

 

[KMcCulloch]

Hello All - I need to clear my bench and get this chassis installed before I do more harm than good. I am going to leave the FM alone now and have installed a Bluetooth receiver for my son to use on this console. Thanks for all your assistance and I have learned much that I will use in the future for troubleshooint. Kevin