New member - Bush VHF80C - help sought...

[mbyarm2000]

Hi, I’m new to the forum and also to the world of valve radios and looking for a bit of help.

I have bought a Bush VHF80C that appeared to be very original, working on all bands and all features but that (in my opinion) seems to be running hot. After a few minutes, there is a ‘heat’ smell and the top of the case at the LHS viewed from the front gets warm.

Having read around the forum after buying it, I have cleared the dust from inside, cleaned the valves and visually inspected the wiring / components. Having purchased the service manual I have replaced a number of the wax-type capacitors for new film capacitors. The capacitor C61 had blown apart but all others were intact but all over the place when measured after removal.

I identified that the heat seemed to be coming from R34 & TH1 when viewed with a thermal imaging camera (luckily able to borrow from work!) which are part of the panel illumination circuit. The heat spot on the case is directly above these components. The original R34 resistor measures correctly when removed from circuit but I have replaced it for test purposes. I cannot find a direct equivalent for the VA1010 thermistor so have left it in place.

I have removed the UY85 valve to do some rudimentary continuity checks on it and have found the following: continuity between pins 4&5 which I think is correct, but also continuity between pins 1, 6 & 9 that I would welcome advice on…

I have measured the voltage at the UY85 valve base when running. As I am more familiar with modern electronics and wary of the chassis voltage I am not certain how to measure the voltages correctly, but the measurements shown here are across the live input to the pin therefore relative to 240VAC.

Pin 3 238V
Pin 4 99V
Pin 5 65V
Pin 9 27V

Can anybody comment on these results?

The set appears to be running really well apart from the heat, but I am learning that, unlike more modern equipment, there is so much in a valve radio that can be ‘wrong’ even though it still works.

Thanks in anticipation, Michael

[Nuvistor]

The voltages shown in the makers manual are with respect to chassis, see note on circuit diagram, except the valve heaters.
TH1 along with R35 will get hot, R34 less so but will still heat up if close to TH1. There is 100ma through those components so the wattage can be worked out.
Confirm the voltage setting is correct fro 230-250V input.

[Sideband]

Valves work by heating a cathode and some valves particularly rectifier and output valves run hot, very hot. Fact. Most of the signal processing valves run warm to very warm. It was common practice (in the case of your Bush) to run valve heaters in series. The heaters are connected across the mains with a series resistor (a dropper) which was usually a large wirewound type rated anything from 10 to 30 watts. Some sets like yours also had a thermistor in series to help with voltage surges etc. So when you consider the you have seven valves in that radio, a dropper (although quite a small one in the case of the Bush) and a thermistor, there is quite a lot of heat being generated. The hottest valves will be the output (UL84) and rectifier (UY85) valves both of these will be too hot to touch. Most of the others will be warm to very warm. All the valves, together with the dropper and thermister generate a lot of heat compared to modern solid state equipment. The most important capacitor to change is the audio coupling capacitor but all of the wax paper capacitors should be replaced.

Chances are everything is OK and the 'heat smell' is normal....in fact most of us veterans of vintage radio and who were around at the time these were in daily use, remember the 'heat smell' as normal. There is probably residual dust burning off the dropper as well.

If you enjoy the hobby and acquire a collection of sets, you will find that sets fitted with a mains transformer run significantly cooler since they don't have or need a dropper. However the valves will still get warm to very hot since they need to in order to operate.

[ms660]

As far as I know, continuity between pins 1, 6 and 9 is normal for the UY85.

Lawrence.

[mbyarm2000]

Hi - thanks for the responses and reassurance about the heat. I do think it is running a bit cooler since changing R34 but I was concerned about the 'hotspot' on the top of the case. The measurements I have taken show the case at >45 degC and the R34 resistor running at 80-90 degC. The R35 10W resistor is running cooler. I think the voltage selector is set correctly - 230V - 250V is visible when looking from the top down.

Good to know the valve continuity is ok - on the schematic I had seen I couldn't see a link between the pins.

Next decision is whether to repaint the cream front of the case!

I can imagine how the purchase of one valve radio leads to another...

[cathoderay57]

Hi Michael, welcome to the Forum. You should find the following link full of useful info, particularly the hyperlink entitled "Initial checks and tests". https://www.vintage-radio.com/repair...ion/index.html . Above all, keep safe until you are used to working around lethal voltages. Cheers, Jerry

[Sideband]

The world of vintage radio is far different from the modern world of solid state. Many of us olduns (like me) grew up with valves and repairing valve circuits is like second nature. I can appreciate a younger person who has grown up with transistors and solid state being uncertain about some aspects. I suggest you read the Success Stories and the link above is as good as anywhere to gain a good insight into valve radio.

As an aside, the Bush VHF80 is a great set since it has an extra IF stage on FM and on AM, one valve operates as an RF amplifier which enables it to receive weak signals at greater strength. I find mine will work where others fail.

[JimMac53]

Keep safe even after you are used to working around lethal voltages!

[Sideband]

Beware of mis-information on the Web. If in doubt, ask here!

[mbyarm2000]

Once again, thanks for the info & advice. The link to the 'initial checks' will be useful to increase my knowledge.

So far, I have only run the set for 30-40 mins because I was concerned about the heat - I will run it for a bit longer to see what happens.

I note the comment about checking voltages relative to the chassis apart from the valve heaters - what is the correct way to measure the voltage at the valve heaters? (I have looked in the linked notes but can't immediately see the answer there)

['LIVEWIRE?']

With your meter set on an AC voltage range, you can either measure the voltage at the heater pins (4 & 5 with B9A valves) or at the transformer winding which supplies the heaters.

[Nuvistor]

This set uses a series heater chain and should measure 100ma AC within I think 5%.
Voltages can be taken across the valve heater and would show a faulty valve heater say partial short. Valve heater voltages are in the valve data books or on line.
U series valves have different voltages but are 100ma, E series are 6.3v at different currents and are generally for parallel heaters.
TV sets used some E series valves eg EF80 these are standard 6.3 v but 300ma to fit with many TV series heater chains.

The manual states 45 watt power consumption, lot of heat in a small cabinet but that was standard for this type of set.

[Sideband]

Originally Posted by mbyarm2000

So far, I have only run the set for 30-40 mins because I was concerned about the heat - I will run it for a bit longer to see what happens.

Jut as well your first set was not a DAC90! Most of these got so hot, the backs have now disintegrated.

Seriously, I doubt you have a problem. The radio will get quite warm to very warm after several hours for the reasons already outlined previously. Make sure you leave at least 10 - 16cm ( 4 - 6 inches) behind the radio when in use and not in front of curtains etc. This applies to any valve radio.