Resistor query in Bush VHF90A - UK Vintage Radio Repair and Restoration Discussion Forum

high_vacuum_house · 22nd Jul 2017, 5:24 pm

Good afternoon,
I am currently restoring a Bush VHF90A radio and so far it works ok once the usual wax capacitors had been changed. There is a farting type oscillation when the set is not tuned into a strong station. Poking around the UL84 screen feed resistor (R23) with a multimeter can occasionally set it off into oscillation. On the manufacturers diagram it shows it as 33K though it actually measures 47K. Could this be a manufacturing change as it has not been disturbed since new. The screen voltage is not too far off. I was also thinking that C32 may have lost capacity and was going to try a 10uF capacitor across it.

Christopher Capener

Radio Tech · 22nd Jul 2017, 5:55 pm

What is the colour coding of the resistor in question? If the manufacture sheet says 33K then chances are the resistor has gone high in value, old style carbon ones do this.

Ken

high_vacuum_house · 22nd Jul 2017, 5:57 pm

This was the manufacturers service data from Bush though could this have been published before a modification was made. Drifting up in value from 33K to 47K seems possible though it is very close to 47K which makes me think it is a preferred standard value so was unsure if it is correct.

Christopher Capener

high_vacuum_house · 24th Jul 2017, 9:53 am

I have added a 10 uF capacitor across C32 (20uF) and this has effectively eliminated all of the oscillations and the sound quality is also much better. I am looking at replacing the UL84 as well as this is heavily stained on the inside of the glass.

Christopher Capener

Station X · 24th Jul 2017, 10:55 am

We never did find out what the colour code of the resistor was. 33k and 47k only share one colour.

The value will affect the screen grid voltage. How does that compare with the service sheet?

Herald1360 · 24th Jul 2017, 11:59 am

Quote:

Originally Posted by high_vacuum_house

I have added a 10 uF capacitor across C32 (20uF) and this has effectively eliminated all of the oscillations and the sound quality is also much better. I am looking at replacing the UL84 as well as this is heavily stained on the inside of the glass.

Christopher Capener

That rather suggests that C32 is past its use by date! A suitable 22uF replacement could be in order.

If the voltages around the UL84 are OK and the anode current isn't significantly down, there's no point in replacing it unless you plan to use the set a lot.

22nd Jul 2017, 5:24 pm	#1
high_vacuum_house Octode Join Date: May 2003 Location: Belper Derbyshire Posts: 1,935	Resistor query in Bush VHF90A Good afternoon, I am currently restoring a Bush VHF90A radio and so far it works ok once the usual wax capacitors had been changed. There is a farting type oscillation when the set is not tuned into a strong station. Poking around the UL84 screen feed resistor (R23) with a multimeter can occasionally set it off into oscillation. On the manufacturers diagram it shows it as 33K though it actually measures 47K. Could this be a manufacturing change as it has not been disturbed since new. The screen voltage is not too far off. I was also thinking that C32 may have lost capacity and was going to try a 10uF capacitor across it. Christopher Capener __________________ Interests in the collection and restoration of Tefifon players and 405 line television

22nd Jul 2017, 5:55 pm	#2
Radio Tech Nonode Join Date: Dec 2012 Location: Bocking, near Braintree, Essex, UK. Posts: 2,071	Re: Resistor query in Bush VHF90A What is the colour coding of the resistor in question? If the manufacture sheet says 33K then chances are the resistor has gone high in value, old style carbon ones do this. Ken

22nd Jul 2017, 5:57 pm	#3
high_vacuum_house Octode Join Date: May 2003 Location: Belper Derbyshire Posts: 1,935	Re: Resistor query in Bush VHF90A This was the manufacturers service data from Bush though could this have been published before a modification was made. Drifting up in value from 33K to 47K seems possible though it is very close to 47K which makes me think it is a preferred standard value so was unsure if it is correct. Christopher Capener __________________ Interests in the collection and restoration of Tefifon players and 405 line television

24th Jul 2017, 9:53 am	#4
high_vacuum_house Octode Join Date: May 2003 Location: Belper Derbyshire Posts: 1,935	Re: Resistor query in Bush VHF90A I have added a 10 uF capacitor across C32 (20uF) and this has effectively eliminated all of the oscillations and the sound quality is also much better. I am looking at replacing the UL84 as well as this is heavily stained on the inside of the glass. Christopher Capener __________________ Interests in the collection and restoration of Tefifon players and 405 line television

24th Jul 2017, 10:55 am	#5
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,288	Re: Resistor query in Bush VHF90A We never did find out what the colour code of the resistor was. 33k and 47k only share one colour. The value will affect the screen grid voltage. How does that compare with the service sheet? __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.