Bush VHF 81 Restoration

[Nuvistor]

So it is, never used one always had the AVO 8 but I was under the incorrect impression that AVO 7 was just 1000opv.

Live and learn.

[OldTechFan96]

When I switched the radio on yesterday the HT was low at 140V. After dinner I measured the HT again. It was 140V for a few minutes then it rose to 210V.

This evening the HT is still low so I'll comb through the power supply tomorrow.

EDIT: How would I know which mains tapping the (dropper?) resistor is set to?

[Nuvistor]

Low HT could be PSU or the radio drawing too much current.

[Half a Mullard]

These are nice little sets with very good sound quality for their size. I restored one a while ago and have been very pleased with it.

As Frank mentioned the low HT could be due to the radio drawing too much current or a problem with the HT supply. Have you checked the UY85 rectifier? It could be low emission, a simple check would be substitution if you have another to hand. Have you noticed anything getting warm which may indicate drawing too much current?

The voltage adjustment is achieved by the two connections on the dropper which should be labelled A and B. The dropper is just to the left of the mains switch.

One connection goes to the mains input, the other goes to the rectifier.

The connections for 230v are mains to B and rectifier to A
The connections for 250v are mains to A and rectifier to B

If this is confusing I can dig mine out tomorrow and post a photo which may make it clearer if this would help.

However I don’t think the voltage adjustment would make that much difference, so I would go with Frank’s suggestion of higher current draw or a fault in the HT supply. I think another rectifier would be well worth a try if you can lay your hands on one.

[OldTechFan96]

I don't have another UY85 to use as a substitute. R34 and R33 do look to get warm when the radio is in use. Both resistors have drifted high.

UL84 does get quite hot. All the other valves are are slightly warm to the touch.

I'll take some photos when I find some batteries.

[OldTechFan96]

I've attached a photograph of the dropper resistor. Since there is no A or B markings I cannot tell which voltage it is set for.

R34's value has increased by 18.5%. Its tolerance is 10%.
R36's value has increased by 21.5%. Its tolerance is 5%.

To check the health of the UY85 would it be a good idea to desolder the wire on pin three and measure the HT directly on the pin? Since there will little current draw I expect the HT to be higher than normal.

[Nuvistor]

Not a good idea, you will find the HT reads low due to the reservoir capacitor being out of circuit.

[ms660]

It's set for the voltage shown on the tag strip, 230-250 volts, if you undid that and flipped it over it would be set for 200-230 volts so far as I know.

Lawrence.

[Station X]

it would be very unusual for wire wound resistors to go high in value.

I suggest disconnecting the wire running from C54 to the output transformer at some point. This will leave the reservoir capacitor C54 in circuit whilst isolating the PSU from the rest of the circuit.

Then check the AC voltage at UY85 pin 9 and the DC voltage at UY85 pin 3. If the HT is OK check that it remains OK over say 15 minutes.

[Half a Mullard]

I would check the HT as Graham suggests.

I have looked at my own set to check the voltage adjustment. Yours would appear to be adjusted for the 250v setting. The connections would be reversed for 230v. The red wire connects to the mains switch and you should find that the blue wire connects directly to the anode of the UY85 (pin 9)

The other resistors you mention (R34 and R36) are in the heater chain so shouldn't affect the HT.

[OldTechFan96]

I have been monitoring voltages using the method suggested by Station X.

HT, as measured on pin 3, was consistently 260V over the two and a half hour period the radio was on. The AC voltage on pin 9 varied from 211V to 225V.

[Station X]

In that case the UY85 rectifier valve is fine off load and C54 is unlikely to be leaking electrically.

Check the resistance from the disconnected wire leading to the output transformer to chassis just in case there's a short there. Check this on all wavebands.

It's difficult to say what's been pulling the HT down. I would suggest disconnecting C49 (that capacitor) before reconnecting the HT to the set. Monitor the HT voltage continuously and switch off if it takes a dive.

[OldTechFan96]

This morning I checked the HT voltage using the same setup as last night. Now the HT is 295V! Nothing has changed since last night so we might have an intermittent fault on our hands. I'll try and source a replacement rectifier.

The resistance between the chassis and the disconnected HT wire is 29M ohms on all wavebands.

When HT is connected the audio O/P valve does get very hot. How warm should these valves be? I don't think C49 is leaking as only a tiny (18-25mV) DC voltage is measured by my DMM on the grid of V6.

Originally Posted by Station X

It's difficult to say what's been pulling the HT down. I would suggest disconnecting C49 manufacturer's sheet (that capacitor) before reconnecting the HT to the set. Monitor the HT voltage continuously and switch off if it takes a dive.

This will be the next thing that I'll check.

[Nuvistor]

Output valves get hot, you can easily measure the current through the valve.
Confirm the value of the cathode bias resistor and that the bypass capacitor is not faulty.
Measure the voltage across the cathode resistor and use ohms law to work out the current through the valve.

[OldTechFan96]

R33: 220Ω, 1/2W, 5% tolerance. Its actual resistance is 240Ω which is a 9% increase. I will replace it as soon as I can.

The voltage across R33 was initially 9.7V and it started to climb to 11.5V. The cut out button popped up on my Avometer so I switched the radio off.

To estimate the current through the valve I used 10V as the voltage.

Current through the valve: 0.042A.

EDIT: I forgot to mention that these measurements were taken with HT reconnected.

[Station X]

The cathode voltage is high, indicating that the anode and or screen current of the output valve is too high. The combined currents should be more like 30mA.

Try again with the grid coupling capacitor disconnected. If the high cathode voltage persists check the anode and screen voltages of the output valve.

[OldTechFan96]

I disconnected the grid coupling capacitor and the cathode voltage was still too high at 11V.

Anode: 190V (193V)
Screen: 130V (112V)

The voltage were measured with the radio set to MW.

[julie_m]

Have you got another, working set with a UL84? A simple swap should tell you if or not the valve is duff.

[Station X]

Anode voltage is OK, so any excess current will be via the screen grid.

Check the resistance of R30 and R32

[OldTechFan96]

R30: 5.5KΩ
R32: 1.2KΩ

I don't have a UL84 to substitute.