Bush VHF 81 Restoration
 

On Friday last week the Bush VHF81 that I bought from member Bryan M arrived. I plan to systematically restore this radio, only replacing what is necessary as opposed to a blanket change of parts. I have done this before with two radios in the past and found it unsatisfying in the long run as I did not learn a lot from the experience. I want to use this as a learning experience.

I have stripped the set down and gave the chassis and cabinet a good clean. Resistors have been checked and none are grossly over tolerance. The internal fuse tests good.

As a minimum, I will replace the across the mains capacitor (C61, Trader Sheet) and the output valve grid coupling capacitor. I believe that the grid coupling capacitor is C49. I thought that this would be a conventional Hunts or TCC paper capacitor but it seems to be something else. Am I right in thinking that C49 is the GCC for the UL84?

I have made sure that the chassis of the radio is wired to neutral and live is wired to the switch. The radio will be powered via a lamp limiter and HT will be monitored with my Avometer.

I'm not too sure if the UABC80 is in good order as there is no silver getter flash. This is the only valve I've seen like this. All of the others look to be in good order. Is it OK?

Does anybody have any advice before I switch the set on?

Thanks

Re: Bush VHF 81 Restoration
 

UABC80 is toast, bet there is a crack between the pins.
If the g1 cap is that brown ceramic one, it may be OK.
X type across the mains.

Beware of that poly silver cap, decoupler? They can fail badly.

Re: Bush VHF 81 Restoration
 

If the cap you're referring to is the clear/shiny one with one end slightly reddish, that's a "Suflex" type - looks like a recent fitment.

They can be OK, but in the past I've come across a radio where these have been fitted for coupling/decoupling but they were not 250V-or-higher rated ! The outcome was predictable, and slightly smelly.

Re: Bush VHF 81 Restoration
 

That UABC80 may be broken, or may be OK. Some valves seem to have very little getter flash (just a bit of grey smudge) yet work OK. If what getter there is looks white then bin it; if not try it.

You will probably have to replace the electrolytic cap across the ratio detector. In the VHF81 this gets heated by nearby valves and dries out. However, it won't do any harm to run the set before doing this - you will just get distorted audio and find that there is no optimum tuning spot. The cap has positive to chassis - the opposite of most electrolytics.

Re: Bush VHF 81 Restoration
 

The silver Sufflex type is the output valves grid coupling capacitor (0.01uF) The small brown ceramic is a de-coupler (HF) for the anode of the triode, that ceramic type is usually reliable, but if it happened to go short circuit or leaky no damage would be done.

EDIT: Post crossed.

Lawrence.

Re: Bush VHF 81 Restoration
 

Would it be worth changing the 0.01uF Sufflex capacitor?

There looks to be a split between pins 1 and 9 of the UABC80. There is a milky appearance to the top of the valve.

I forgot to mention that the primary of the audio output transformer measures correctly.

Re: Bush VHF 81 Restoration
 

Looks like you have it covered, I usually check continuity between the live and neutral with the set switched on (but obviously not plugged in) to make sure there are no shorts before powering up.

John

Re: Bush VHF 81 Restoration
 

I've just measured the continuity between the live and neutral with the set switched on and there is no shorts.

Re: Bush VHF 81 Restoration
 

Reform the reservoir/smoothing capacitor before switch on, when the radios were only a few years old I had some empty their cans inside the Radio, not nice. Any question on it being good I would replace it.

Re: Bush VHF 81 Restoration
 

Would I be able to temporarily sub an EABC80 in place of the dead UABC80? I have one in my Ekco A277.

I've tested my lamp limiter and it is in good order.

Re: Bush VHF 81 Restoration
 

No the valve heaters are different voltages and current.

Re: Bush VHF 81 Restoration
 

E series valves = 6.3 volt heater but heater current varies between valve types, U series valves = 100mA heaters but voltage of heaters varies between valve types, P series valves = 300mA heaters but voltage of heaters varies between valve types.

All the valves in your set have 100mA heaters so only Uxx valves can be used.

Re: Bush VHF 81 Restoration
 

Yes, sort of; but it won't be a direct substitution, because of the different heater ratings. Otherwise, the valves are the same.

You will need to complete the radio's heater circuit, using a 270 ohm, 5W resistor (should really be 280Ω; will dissipate 2.8W), and power the EABC80's heater from a separate 6.3V supply rated for at least 450mA. You should connect one side of the heater supply to the set's chassis; so if using a mains PSU, it obviously must not be earthed.

Re: Bush VHF 81 Restoration
 

Thanks for all the information on the UABC80. I will try and find a replacement online. Its heater does not glow.

I powered the set up via the lamp limiter this evening. As expected, there was not a great deal of activity apart from a small buzz from the speaker and lit dial bulbs. HT measured (from positive terminal of C52) 110V. During subsequent switch ons the HT rose to 140V. With the limiting off the HT was 220V. It looks like the 40uf smoothing capacitor has reformed somewhat.

There is only a handful of paper capacitors in this set. Three Hunts Mouldseals. The rest are TCC Supamolds. Both types are well known to be electrically leaky.

How do I measure leakage in circuit? Is it a matter of measuring the voltage on one side of capacitor then measuring the other side?

For example:

C50 is a 0.01uf, 400V Supamold. One side of the cap measures 140VDC and the other side measures 130VDC. Should I be seeing these voltages on C50? Is the cap defective?

Re: Bush VHF 81 Restoration
 

One way is to disconnect the cap at one end...obviously if it's a decoupler and one end is earthed, you disconnect the earthy end and measure the voltage present at the disconnected end...a good capacitor won't show any appreciable voltage ideally none at all apart from an initial charge which should then drop back to zero.

However if a it's a Moldseal or a Supamold it WILL be leaky so the measurement is somewhat academic!!

You won't get much from the set with the UABC80 not working since it contains the triode AF amplifier as well as the AM and FM detectors. You might get a low hum if you touch the control grid of the UL84 but that is about it.

Re: Bush VHF 81 Restoration
 

I've just bought a NOS Mazda branded UABC80. In the meantime I'll concentrate on making sure that the power supply is putting out the correct voltages.

Re: Bush VHF 81 Restoration
 

The UABC80 arrived and was installed today. I turned the radio on via the lamp limiter and was greeted by the noise of an off tune MW station. I could pull in plenty of stations on MW and the usual R4 and RTE1 on LW.

FM works but it pulls in Radio 2 at about 92MHz. I think this is because I did not restring the VHF tuner correctly. I'll try and get this sorted.

When the radio is tuned the dial cord looks to bunch up on the control spindle making the radio difficult to tune. You find points of resistance as you tune across the band. It kind of comes and goes. How would I sort this?

Re: Bush VHF 81 Restoration
 

Try altering the number of turns of the cord wrapped round the control-knob spindle. The frictional characteristics of the replacement cord may not exactly match those of the original type for which the number-of-turns-of-wrap were designed.

You may need to do some trial and error to find the number-of-turns that give enough frictional drive to make the tuning work but which has enough slip to let the wound-round-the-spindle part move up the spindle as you tune.

Re: Bush VHF 81 Restoration
 

The original cords were often waxed so that they would slide in the spindle groove, sounds illogical making them slippy but that's how it was done.
I think the VHF81 has 2 turns around the spindle, may be 3.
Wax the cord in the length that goes past the spindle with a rub of a candle and you will find that it stops the bunching up.

Re: Bush VHF 81 Restoration
 

Also make sure the cord you have used is not too thick. Drivecords came in several thicknesses.