JVC RC-770LB FM fault - UK Vintage Radio Repair and Restoration Discussion Forum

agardiner · 24th May 2023, 3:23 pm

Hi,

I am repairing this JVC radio cassette unit. All is now working except FM reception. The set is based around an FM front end which is working, and an AN7222 IF IC, which double up as the AM front end and IF; fairly common arrangement for the time.

So I attach a snap shot of the circuit around the IC, plus the IC's datasheet. When in FM mode, pin 1 (AM VCC) and pin 18 (AM oscillator) of the IC are sitting around 2V which is wrong. The wavechange switch is working correctly. If I ground either pin 1 or pin 18, then FM works just fine. Testing cold, pin 18 reads 4.7K to ground, even when completely disconnected from the circuit. When pin 18 is out of circuit, AM is dead as would be expected. It still floats around 2V on FM mode and pulling it to ground brings FM back to life.

My conclusion is that the IC has an internal leak and is the culprit, but before I order one I thought I would ask for other's opinions. Can anyone see anything I might have missed?

Thanks!

iceman303 · 28th May 2023, 9:42 am

Hello Adrian. I think you are correct. If pin 1 and 18 measure 2v when out of the circuit, I would think there is something wrong with the chip. Also I would check the voltage going into pin 1 ( am vcc ), and also if there is any voltage going to pin 18 ( maybe better with both pins out of the circuit ). The wavechange could also be faulty and passing some voltage to the am circuit when on fm, but you say you have checked it, so it should not be that. It looks like the am voltage is not being turned off when the radio is on fm mode. If everything is correct, I also think the chip is faulty.

Joao

agardiner · 29th May 2023, 5:15 pm

Fixed; although by a mod. For those of you who are following this thread....

New IC arrived and fitted, but of course no difference. I then removed the multi-band select switch and carefully dismantled it. All looked OK, but gave a nice good clean before re-assembly. Checked all contacts out with multi-meter, all OK. Put the switch back in the set and same problem. All very odd, as there is nothing else it really can be; anything leaky (except the switch) would pull pin 1 to ground and engage FM anyway.

To test further, I removed R9 which couples pin 1 of the IC to the switch. FM then worked and I could measure 1.9V on the switch contact proving that the switch itself was indeed the cause. I removed it again and disassembled. Careful examination showed no fault. Cleaned again, made sure totally dry and reassembled. Multimeter tests all OK, but the problem remained, so clearly a slight leakage from the actual paxillin base of the switch.

After carrying out some calculations and tests, I opted to add a 270R resistor between pin 1 of the IC and ground. This has resolved the problem. It pulls the pin to ground in FM mode, and still allows 4.8V in AM mode (originally 5.2V), with no excess current draw. Unit now working on all bands.

Mr 1936 · 29th May 2023, 7:47 pm

Hi

Well done for persevering on this one. It's rather unusual to get significant leakage on a paxolin based component in a low voltage application. I can't see there being a carbonisation/deterioration issue from arcing. Maybe the original material was not made to the best standards ?

A useful reminder to expect the unexpected !

28th May 2023, 9:42 am	#2
iceman303 Triode Join Date: Jun 2021 Location: Lisbon, Portugal. Posts: 36	Re: JVC RC-770LB FM fault Hello Adrian. I think you are correct. If pin 1 and 18 measure 2v when out of the circuit, I would think there is something wrong with the chip. Also I would check the voltage going into pin 1 ( am vcc ), and also if there is any voltage going to pin 18 ( maybe better with both pins out of the circuit ). The wavechange could also be faulty and passing some voltage to the am circuit when on fm, but you say you have checked it, so it should not be that. It looks like the am voltage is not being turned off when the radio is on fm mode. If everything is correct, I also think the chip is faulty. Joao

29th May 2023, 5:15 pm	#3
agardiner Octode Join Date: Sep 2020 Location: Thetford, Norfolk, UK. Posts: 1,741	Re: JVC RC-770LB FM fault Fixed; although by a mod. For those of you who are following this thread.... New IC arrived and fitted, but of course no difference. I then removed the multi-band select switch and carefully dismantled it. All looked OK, but gave a nice good clean before re-assembly. Checked all contacts out with multi-meter, all OK. Put the switch back in the set and same problem. All very odd, as there is nothing else it really can be; anything leaky (except the switch) would pull pin 1 to ground and engage FM anyway. To test further, I removed R9 which couples pin 1 of the IC to the switch. FM then worked and I could measure 1.9V on the switch contact proving that the switch itself was indeed the cause. I removed it again and disassembled. Careful examination showed no fault. Cleaned again, made sure totally dry and reassembled. Multimeter tests all OK, but the problem remained, so clearly a slight leakage from the actual paxillin base of the switch. After carrying out some calculations and tests, I opted to add a 270R resistor between pin 1 of the IC and ground. This has resolved the problem. It pulls the pin to ground in FM mode, and still allows 4.8V in AM mode (originally 5.2V), with no excess current draw. Unit now working on all bands.

29th May 2023, 7:47 pm	#4
Mr 1936 Heptode Join Date: Mar 2017 Location: Romsey, Hampshire, UK. Posts: 521	Re: JVC RC-770LB FM fault Hi Well done for persevering on this one. It's rather unusual to get significant leakage on a paxolin based component in a low voltage application. I can't see there being a carbonisation/deterioration issue from arcing. Maybe the original material was not made to the best standards ? A useful reminder to expect the unexpected !