Bush VHF81 Volume funny - UK Vintage Radio Repair and Restoration Discussion Forum

cosmocat · 26th Mar 2024, 10:55 am

Hi.
I am repairing a Bush VHF81, the set is in suprisingly good nick, (even the Hunts caps seem to be fine without leaking!).
My problem is that on FM the volume will not go all the way down to quiet, and on AM there is always a noticable background hiss that sounds like the hiss between FM stations.
Looking at the circuit I see that V5 a UABC80 does two jobs, FM detector AND first stage of the audio amp. Is it likely that the FM from the detector diodes is leaking into the audio triode stage, kind of bypassing the volume control?
I've done resistance checks around the pot and all seems fine.
Has anyone had similar issue, I just thought I'd check before splashing out on a replacement UABC80

Sideband · 26th Mar 2024, 9:21 pm

I have had UABC80's (and other similar valves) cause this problem. Actually it does three jobs, AF amp, FM discriminator, AM detector. It may be leakage within the valve but it could also be a change in the valve characteristics. I have found that when one doesn't work in a particular set, it works in another....so even if you change it and the new one clears the problem, don't throw the old one out....just mark it with a question mark. It may work in another set.

The VHF81 (and VHF80) are good sets....they work where others fail. Excellent on AM with the extra RF stage and also on FM with the extra IF stage.....all done with clever switching of the UF89.

frsimen · 27th Mar 2024, 6:03 pm

The volume control circuit on the VHF81 and many other radios has negative feedback applied at its earthy end. In this case it is applied across R24 (both the trader and manufacturer's data have the same component reference). R24 prevents the volume being reduced to zero output, but at the minimum setting of the volume control the volume shouldn't be very high.

Check that resistance from the slider of the volume control to R24 is close to 0 ohms with the volume control at the minimum volume setting and that R24 is 1.5k ohms, before splashing out on a new valve.

cosmocat · 28th Mar 2024, 9:59 pm

Thanks for the replies.
I had a look around the negative feedback section, that all looked good, I also replaced the discriminator cap, but that made no difference with the old valve.
Fitting a new UABC80, all fixed, volume working properly and FM sounding great.
Valves do weird things when they wear out!

Sideband · 28th Mar 2024, 10:06 pm

It was either leakage within the old UABC80 or a change in the grid base characteristic....that's the point of cut-off of the valve. Hang on to the old valve...as I said above, it may work OK in another set.

26th Mar 2024, 10:55 am	#1
cosmocat Triode Join Date: May 2011 Location: Bristol, UK. Posts: 35	Bush VHF81 Volume funny Hi. I am repairing a Bush VHF81, the set is in suprisingly good nick, (even the Hunts caps seem to be fine without leaking!). My problem is that on FM the volume will not go all the way down to quiet, and on AM there is always a noticable background hiss that sounds like the hiss between FM stations. Looking at the circuit I see that V5 a UABC80 does two jobs, FM detector AND first stage of the audio amp. Is it likely that the FM from the detector diodes is leaking into the audio triode stage, kind of bypassing the volume control? I've done resistance checks around the pot and all seems fine. Has anyone had similar issue, I just thought I'd check before splashing out on a replacement UABC80 Last edited by cosmocat; 26th Mar 2024 at 11:15 am. Reason: More details of fault

26th Mar 2024, 9:21 pm	#2
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,584	Re: Bush VHF81 Volume funny I have had UABC80's (and other similar valves) cause this problem. Actually it does three jobs, AF amp, FM discriminator, AM detector. It may be leakage within the valve but it could also be a change in the valve characteristics. I have found that when one doesn't work in a particular set, it works in another....so even if you change it and the new one clears the problem, don't throw the old one out....just mark it with a question mark. It may work in another set. The VHF81 (and VHF80) are good sets....they work where others fail. Excellent on AM with the extra RF stage and also on FM with the extra IF stage.....all done with clever switching of the UF89. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

27th Mar 2024, 6:03 pm	#3
frsimen Heptode Join Date: Jan 2011 Location: Croydon, London, UK. Posts: 773	Re: Bush VHF81 Volume funny The volume control circuit on the VHF81 and many other radios has negative feedback applied at its earthy end. In this case it is applied across R24 (both the trader and manufacturer's data have the same component reference). R24 prevents the volume being reduced to zero output, but at the minimum setting of the volume control the volume shouldn't be very high. Check that resistance from the slider of the volume control to R24 is close to 0 ohms with the volume control at the minimum volume setting and that R24 is 1.5k ohms, before splashing out on a new valve.

28th Mar 2024, 9:59 pm	#4
cosmocat Triode Join Date: May 2011 Location: Bristol, UK. Posts: 35	Re: Bush VHF81 Volume funny Thanks for the replies. I had a look around the negative feedback section, that all looked good, I also replaced the discriminator cap, but that made no difference with the old valve. Fitting a new UABC80, all fixed, volume working properly and FM sounding great. Valves do weird things when they wear out!

28th Mar 2024, 10:06 pm	#5
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,584	Re: Bush VHF81 Volume funny It was either leakage within the old UABC80 or a change in the grid base characteristic....that's the point of cut-off of the valve. Hang on to the old valve...as I said above, it may work OK in another set. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....