Bush VHF61 AGC problem - UK Vintage Radio Repair and Restoration Discussion Forum

unitelex · 14th Apr 2007, 8:26 pm

Although the set is receiving quite well, on all bands, and is quite sensitive,
There seems to be a problem with the AGC.

All the little Hunts have been replaced, and a new discriminator cap has been fitted.

The voltage on C56 the discriminator capacitor with a strong VHF station is around 35V - this seems too high to me, but I'm not sure what the normal range should be.

The tuning indicator is showing fully closed (all green) all the time
The trader sheet says there should be 170V on the target anode- this measures OK. The trade sheet says 24V on the anode, I get closer 160V

The discriminator has been checked for adjustment and I believe it is correctly nulled.

The voltage on the Anode of the freqency changer is also quite a bit out ( think it was too high ).

Not sure what might be causing this, seems to me that the AGC is not working correctly. I thought maybe i'd got a stability problem with the IF, but the audio is pretty good.

Any clues chaps ?

Steve_P · 14th Apr 2007, 8:55 pm

Try changing R28 first. Then head back down the AGC Line, S18 and S19 (Part of the wavechange switch - check all of this thoughrougly) and possibly R25. If the FM Reception's OK, then it's more likely to be the Tuning Indicator.

Cheers,

Steve P

ronbryan · 14th Apr 2007, 9:01 pm

Edit: My posting has crossed with Steve's - apologies

My understanding of the FM/AM switching is that on FM, the output is of the discriminator is only applied to the tuning indicator. No AGC voltage is applied to the stages preceding the discriminator.

You indicate that the discriminator voltage is -35V on a strong station, can you measure what voltage it is between stations?

Check the tuning indicator grid voltage (pin 1) to see it it varies in the same way as the discriminator voltage. If it does, and the eye remains fully closed, try substuting another EM81. If not, investigate the push button switching.

Ron

unitelex · 15th Apr 2007, 5:29 pm

Fixed it !

The tuning indicator worked fine on AM,

Found out by grounding the IF1 grid with a cap I could get a proper zero signal indication on the tuning indicator.
The FM IF was unstable and oscillating. Turns out when I had repaced the Hunts I had fitted the wrong value of heater decoupling caps on both IF valves

I had fitted 0.0022 in both positions C39 and C48, these should be 0.02 !.
Fitted the correct value 0.022 polyprop caps and problem gone!
Somewhat surprised it is so sensitive to the value of the decoupler.

Re-aligned the ratio detector and final IF and it now sounds better than ever.

Thanks gents for your help.

Paul Stenning · 15th Apr 2007, 5:44 pm

When I read your post I thought it was instability, which these sets do suffer from with the original caps. But when you said you'd replaced them all I ruled that out so didn't comment.

I'm pleased it's sorted, anyway!

14th Apr 2007, 8:26 pm	#1
unitelex Hexode Join Date: May 2005 Location: Greater Manchester, UK. Posts: 498	Bush VHF61 AGC problem Although the set is receiving quite well, on all bands, and is quite sensitive, There seems to be a problem with the AGC. All the little Hunts have been replaced, and a new discriminator cap has been fitted. The voltage on C56 the discriminator capacitor with a strong VHF station is around 35V - this seems too high to me, but I'm not sure what the normal range should be. The tuning indicator is showing fully closed (all green) all the time The trader sheet says there should be 170V on the target anode- this measures OK. The trade sheet says 24V on the anode, I get closer 160V The discriminator has been checked for adjustment and I believe it is correctly nulled. The voltage on the Anode of the freqency changer is also quite a bit out ( think it was too high ). Not sure what might be causing this, seems to me that the AGC is not working correctly. I thought maybe i'd got a stability problem with the IF, but the audio is pretty good. Any clues chaps ?

14th Apr 2007, 8:55 pm	#2
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: Bush VHF61 AGC problem Try changing R28 first. Then head back down the AGC Line, S18 and S19 (Part of the wavechange switch - check all of this thoughrougly) and possibly R25. If the FM Reception's OK, then it's more likely to be the Tuning Indicator. Cheers, Steve P __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

14th Apr 2007, 9:01 pm	#3
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Bush VHF61 AGC problem Edit: My posting has crossed with Steve's - apologies My understanding of the FM/AM switching is that on FM, the output is of the discriminator is only applied to the tuning indicator. No AGC voltage is applied to the stages preceding the discriminator. You indicate that the discriminator voltage is -35V on a strong station, can you measure what voltage it is between stations? Check the tuning indicator grid voltage (pin 1) to see it it varies in the same way as the discriminator voltage. If it does, and the eye remains fully closed, try substuting another EM81. If not, investigate the push button switching. Ron Last edited by ronbryan; 14th Apr 2007 at 9:20 pm.

15th Apr 2007, 5:29 pm	#4
unitelex Hexode Join Date: May 2005 Location: Greater Manchester, UK. Posts: 498	Re: Bush VHF61 AGC problem Fixed it ! The tuning indicator worked fine on AM, Found out by grounding the IF1 grid with a cap I could get a proper zero signal indication on the tuning indicator. The FM IF was unstable and oscillating. Turns out when I had repaced the Hunts I had fitted the wrong value of heater decoupling caps on both IF valves I had fitted 0.0022 in both positions C39 and C48, these should be 0.02 !. Fitted the correct value 0.022 polyprop caps and problem gone! Somewhat surprised it is so sensitive to the value of the decoupler. Re-aligned the ratio detector and final IF and it now sounds better than ever. Thanks gents for your help.

15th Apr 2007, 5:44 pm	#5
Paul Stenning Administrator Join Date: Dec 2002 Location: Cardiff Posts: 9,072	Re: Bush VHF61 AGC problem When I read your post I thought it was instability, which these sets do suffer from with the original caps. But when you said you'd replaced them all I ruled that out so didn't comment. I'm pleased it's sorted, anyway! __________________ Paul Stenning Forum Admin/Owner and BVWS Webmaster