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Old 19th Mar 2018, 3:05 pm   #101
ms660
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Default Re: Bush VHF 81 Restoration

I've got the wife's old glasses on now and things seem a bit clearer.....

Anyways back on topic, whether tuned to a station or not, the currents derived from the measured voltages in the spreadsheet don't tally.

Lawrence.
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Old 19th Mar 2018, 4:28 pm   #102
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Default Re: Bush VHF 81 Restoration

I've replaced out of spec. resistors R31, R38 and R26. The grid coupling capacitor, C49, has also been replaced (0.01uf, 400V).

Before I remeasured voltages I wanted to make sure that the control grid of V6 had no voltage on it. After a few minutes there was 1.6V on the grid!

Would the cap be defective?
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Old 19th Mar 2018, 4:30 pm   #103
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Default Re: Bush VHF 81 Restoration

Disconnect the cap from the control only and check whether the grid voltage drops to zero.

Measure the voltage w.r.t chassis on the disconnected lead. It should be zero, but if the cap is leaky you may see a voltage there.
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Old 19th Mar 2018, 5:46 pm   #104
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Default Re: Bush VHF 81 Restoration

I've disconnected one side of C49 and made some measurements:

Grid voltage on V6: starts at 20mV then rises to about 1V.

Voltage on disconnected cap. lead: 0V

There is 55V on the anode of V5d and 0V on the end of C49 so the capacitor is doing its job of blocking DC.

My hypothesis is that there must be a leaky component putting 1V on the grid of V6. Looking at what components feed the grid, I think C45 could be suspect. Or even the valve itself?

EDIT: While making a cup of tea I realised that I could disconnect one end of C45 to test my hypothesis. If there in no voltage on the grid of V6 C45 is leaky.
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Old 19th Mar 2018, 6:26 pm   #105
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Default Re: Bush VHF 81 Restoration

Most likely component is the UL84, could be the valve base but unlikely. A leak inside the valve causing the +ve on the grid.
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Old 19th Mar 2018, 6:30 pm   #106
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Default Re: Bush VHF 81 Restoration

Even if C45 were leaking, it wouldn't put a positive potential on the output valve grid, as the grid is isolated by C49.

As you say it could be internal leakage within the valve, or across the valve holder.

I'm afraid you'll have to try another UL84.
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Old 19th Mar 2018, 6:33 pm   #107
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Default Re: Bush VHF 81 Restoration

Crossed with Frank.

My experience has been that valve holders made from hard plastic don't leak electrically, whereas those made from paxolin or SRBP can suffer in this way.
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Old 19th Mar 2018, 6:57 pm   #108
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Default Re: Bush VHF 81 Restoration

Only time I have seen hard plastic cause problems if there had been arcing due to say an o/c speaker circuit, but very rare, paxolin on the other hand.
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Old 19th Mar 2018, 7:08 pm   #109
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Default Re: Bush VHF 81 Restoration

Disconnect one end of C49 then check for voltage on the grid of V6 (put the meter across R28).

If V6 has internal leakage [not unknown] then even with a good C49 you can get an unwanted voltage appearing on the grid, which will cause excessive current to flow through the valve (and so an excessive voltage across R33).

Check that R28 hasn't drifted high in value: the "Erie" ceramic-bodied resistors with the ends plugged by a brownish putty/cement are known to drift - your V6 grid-to-earth resistor could well have wandered up to a couple of Megohms!

UL84 valves are known to sometimes have this leakage issue, though they're nowhere as bad as their earlier EL41/UL41 relatives.

It's possible - in various ways - to 'zap' a leaky valve to try and destroy the conductive deposits that cause this leakage.
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Old 19th Mar 2018, 8:54 pm   #110
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Default Re: Bush VHF 81 Restoration

With one end of C49 disconnected:

Voltage across R28 (has been replaced): 20mV and rising.

Voltage across R33: 8V

Current draw of V6 cathode: 0.036A

I'll be on the lookout for a good used or NOS UL84.
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Old 19th Mar 2018, 9:16 pm   #111
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Default Re: Bush VHF 81 Restoration

I have had read through the posts and cannot find if C49 has been replaced. I would change C49 if not already done so while waiting for the UL84.
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Old 19th Mar 2018, 9:29 pm   #112
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Default Re: Bush VHF 81 Restoration

C49 has been replaced. It was one of those 'sufflex' type capacitors.
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Old 19th Mar 2018, 9:38 pm   #113
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Default Re: Bush VHF 81 Restoration

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Old 20th Mar 2018, 2:35 pm   #114
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Default Re: Bush VHF 81 Restoration

I would not think 20mV on the grid was anything to worry about maybe the valve is ok?
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Old 20th Mar 2018, 2:47 pm   #115
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Default Re: Bush VHF 81 Restoration

In an earlier post #104 the OP states that the grid voltage starts at 20mV and rises to 1V.

That is something to worry about.
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Old 5th Apr 2018, 8:15 pm   #116
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Default Re: Bush VHF 81 Restoration

The replacement UL84 came today. There is now 0V on the grid of V6.

The anode, screen and cathode voltages are now spot on for V6.

I'll retake all other valve voltages.
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