Sanyo RP8880 AFC problem - UK Vintage Radio Repair and Restoration Discussion Forum

agardiner · 28th May 2023, 5:22 pm

Hi,

I have just repaired another one of these radios, carrying out everything as per my post about this set: https://www.vintage-radio.net/forum/...d.php?t=199510

All bands are working fine, but there is a fault with the FM AFC which I hope someone can assist with. If tuned to a station, when you engage AFC the set goes way off; if you tune up the band by approx 2.5MHz you will find the station again, although quite distorted. Now, my understanding was that AFC was based on the symmetrical IF waveform, and when engaged it shifts the tuning to the centre point of the waveform. Is that correct? I have carried out an FM alignment, and have a nice waveform and good S curve. I have tried 'tweaking' the IF so that it is off alignment to see the effect on AFC, but it doesn't really make any difference.

Any pointers for next step would be gratefully received.

Thanks all.

agardiner · 28th May 2023, 8:38 pm

Hmmm, might have been a bit stupid. Put the scope in DC mode, and I have noticed an offset voltage of around 100mV on the discriminator cap. I think I will replace it before carrying on.

Mr 1936 · 28th May 2023, 8:51 pm

Hi

Looking at the low resolution schematic online, this set appears to be all discrete. The output of the discriminator, suitably low pass filtered to leave only the DC component, will most likely be fed back to a dedicated AFC diode across the FM LO tuned circuit. This arrangement is pretty conventional for the period.

One wouldn't expect it to be tasked with correcting more than about a +/- 500 kHz error.

As you are doing already, check for any unwanted DC offsets. If the AFC line is switched, check the relevant contacts.

Sorry if any of this seems to be stating the obvious !

agardiner · 31st May 2023, 5:11 pm

OK, fixed. I replaced the discriminator capacitor and the problem is resolved. Should have checked this before!

Odd though, the old cap tests fine for ESR and has only drifted from 4.7uF to 5uF. Enough to cause the problem though.

Graham G3ZVT · 1st Jun 2023, 10:37 am

Quote:

Originally Posted by agardiner

OK, fixed. I replaced the discriminator capacitor and the problem is resolved. Should have checked this before!

Odd though, the old cap tests fine for ESR and has only drifted from 4.7uF to 5uF. Enough to cause the problem though.

The value of a capacitor in that position is not that critical.

It's more likely that it was (electrically) leaky

A leaky capacitor will often measure a higher capacitance than that printed on it. This increase is not real, it's a consequence of how the measuring instrument works.
__________________

28th May 2023, 5:22 pm	#1
agardiner Octode Join Date: Sep 2020 Location: Thetford, Norfolk, UK. Posts: 1,741	Sanyo RP8880 AFC problem Hi, I have just repaired another one of these radios, carrying out everything as per my post about this set: https://www.vintage-radio.net/forum/...d.php?t=199510 All bands are working fine, but there is a fault with the FM AFC which I hope someone can assist with. If tuned to a station, when you engage AFC the set goes way off; if you tune up the band by approx 2.5MHz you will find the station again, although quite distorted. Now, my understanding was that AFC was based on the symmetrical IF waveform, and when engaged it shifts the tuning to the centre point of the waveform. Is that correct? I have carried out an FM alignment, and have a nice waveform and good S curve. I have tried 'tweaking' the IF so that it is off alignment to see the effect on AFC, but it doesn't really make any difference. Any pointers for next step would be gratefully received. Thanks all.

28th May 2023, 8:38 pm	#2
agardiner Octode Join Date: Sep 2020 Location: Thetford, Norfolk, UK. Posts: 1,741	Re: Sanyo RP8880 AFC problem Hmmm, might have been a bit stupid. Put the scope in DC mode, and I have noticed an offset voltage of around 100mV on the discriminator cap. I think I will replace it before carrying on.

28th May 2023, 8:51 pm	#3
Mr 1936 Heptode Join Date: Mar 2017 Location: Romsey, Hampshire, UK. Posts: 521	Re: Sanyo RP8880 AFC problem Hi Looking at the low resolution schematic online, this set appears to be all discrete. The output of the discriminator, suitably low pass filtered to leave only the DC component, will most likely be fed back to a dedicated AFC diode across the FM LO tuned circuit. This arrangement is pretty conventional for the period. One wouldn't expect it to be tasked with correcting more than about a +/- 500 kHz error. As you are doing already, check for any unwanted DC offsets. If the AFC line is switched, check the relevant contacts. Sorry if any of this seems to be stating the obvious !

31st May 2023, 5:11 pm	#4
agardiner Octode Join Date: Sep 2020 Location: Thetford, Norfolk, UK. Posts: 1,741	Re: Sanyo RP8880 AFC problem OK, fixed. I replaced the discriminator capacitor and the problem is resolved. Should have checked this before! Odd though, the old cap tests fine for ESR and has only drifted from 4.7uF to 5uF. Enough to cause the problem though.