Bush VHF80 valve radio

[MICK1985]

Hi Guys, I wish to ask regarding this model radio as I've bought a fully restored one from a guy. It works really well on all wavebands, but does seem to have a background hum. I've been told it's normal by the guy I bought it from. Please can anyone advise if that's correct or not?

Kind regards, Mick.

[Nuvistor]

All depends on how much but yes most of the standard radios built in the 50’s had a small amount of hum.
It’s not normally noticeable at normal listening levels when sat a few feet away from the set.

[dazzlevision]

The Bush VHF80 is an AC/DC mains set, so it uses half wave (rather than full wave) rectification for producing the DC HT supply, generating a 50Hz ripple on it, which may well be heard on the audio output. 50Hz ripple is more difficult to filter out than the 100Hz ripple in AC only sets.

AC/DC sets use series connected heater chains, rather than parallel. Consequently, the Heater-Cathode voltage differences in the valves are therefore greater and 50Hz hum may be induced into the audio via this path.

Because of the above, residual hum on the audio is invariably more noticeable on AC/DC sets.

[G6Tanuki]

I'm not familiar with the VHF80 though I have its successor the VHF81 here - and it doesn't hum.

if the 80 is anything like the 81 it will have been designed to benefit from the noise- and interference-free background that FM transmissions can offer. Bush wouldn't have wanted this to be spoiled by mains-hum!

I'd be checking for dried-out smoothing electrolytics as the #1 issue. Similarly, anything that increases the HT current draw [leaky 'that capacitor' or leaky decouplers] will mean more ripple on the HT line, and also [assuming your output transformer has a 'hum bucking' tap on the primary] imbalance of the current through this winding meaning the hum-cancellation will be less-than-optimal.

Also check the emission of the rectifier - if it's low then you'll get more ripple.

[ms660]

Hum cancelling sound output transformer in the VHF80 plus 40uF reservoir according to the Trader schematic.

Lawrence.

[Nuvistor]

Is it possible to confirm what the restoration consisted of?

[Sideband]

Originally Posted by Nuvistor

Is it possible to confirm what the restore consisted of?

Quite so.....'fully restored' might mean a good polish and a vacuum out! Unless you buy a set from a known supplier who restores sets for a living, it doesn't follow that any major work has been carried out.

I've seen original sets with no work carried out advertised as 'fully working'. The fact that it switches on, lights up and receives some radio stations is good enough for some people.

Due to the 'double smoothing' arrangement and the anti-hum winding on the transformer, the hum level on these sets is very low.

[Neil Breward]

Originally Posted by G6Tanuki

I'm not familiar with the VHF80 though I have its successor the VHF81 here - and it doesn't hum.

if the 80 is anything like the 81 it will have been designed to benefit from the noise- and interference-free background that FM transmissions can offer. Bush wouldn't have wanted this to be spoiled by mains-hum!

I'd be checking for dried-out smoothing electrolytics as the #1 issue. Similarly, anything that increases the HT current draw [leaky 'that capacitor' or leaky decouplers] will mean more ripple on the HT line, and also [assuming your output transformer has a 'hum bucking' tap on the primary] imbalance of the current through this winding meaning the hum-cancellation will be less-than-optimal.

Also check the emission of the rectifier - if it's low then you'll get more ripple.

Apart from loss of capacitance of the smoothing capacitor and leakage in the AF coupling capacitor, the other culprit might be heater-cathode leakage in the UL84 output valve. They were rather prone to this. Also make sure that nothing is connected to Pin 1 of this valve - this is an internal connection, but some manufacurers used pin one of the valveholder as an anchor point. Not a good idea as internal inter-electrode leakage is possible.

Cheers,
Neil

[MICK1985]

Hi guys.

Many thanks for all your inputs regarding this matter, I'm going find out what work has been done to it as I noticed a lot of new capacitors inside when I took the back off to have quick look.

[Aubade65]

Re post#4
The VHF80 and VHF81 have exactly the same chassis. The difference between them is the VHF80 uses a plastic cabinet and the VHF81 a slightly larger teak case. You can swap the chassis between them after changing the dial glass and its supports over-the VHfF81 has slightly bigger glass and its supports are different but it is an easy job to do.
Be aware though that the cabinet on the VHF80 is made from very thin plastic and can be easily cracked if the chassis removed which seems to give it extra rigidity.

I have restored several 80s and 81s and hum has never been a problem. For their time these were top of the range sets and give a good account of themselves even today once the normal age related issues have been dealt with. I use an 81 daily and certainly on FM there no trace of noise or hum even when the volume is turned up full.

Although I have found the main smoothing caps used in these sets to be very reliable a quick check can be made by tacking a high voltage electrolytic across the HT rail to see if it does cure the problem
In addition to the other suggestions made here you may want to look a bit more closely around the UABC80 and the volume control to make sure that cables carrying the low level AF are in good condition and have not been rerouted close to any sources of hum such as the heater chain