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Old 18th Mar 2018, 2:02 am   #61
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Default Re: Bush VHF 81 Restoration

Valve base leakage?
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Old 18th Mar 2018, 8:17 am   #62
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Default Re: Bush VHF 81 Restoration

Possibly.

A couple of things to check before going down that route though.

Is the resistance of the output transformer primary winding correctly measuring 600 ohms?

R32 and R30 don't just carry the screen grid current of the output valve. They feed other valves as well. Checking the anode voltage of say V5C wouldn't go amiss.
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Old 18th Mar 2018, 11:49 am   #63
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Default Re: Bush VHF 81 Restoration

Resistance of audio output transformer: 525Ω
Voltage on anode of V5c: -3.25V

What is valve base leakage?
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Old 18th Mar 2018, 12:04 pm   #64
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Default Re: Bush VHF 81 Restoration

The material the valve base is made from should insulate each pin from the others. The insulation can breakdown and allow the voltage on one pin to leak to another.
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Old 18th Mar 2018, 12:14 pm   #65
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Default Re: Bush VHF 81 Restoration

Quote:
Originally Posted by OldTechFan96 View Post
Voltage on anode of V5c: -3.25V
That's very odd. Are you measuring the correct pin ie pin 9?

The anode is fed by R26 (check its resistance). It's isolated from the chassis DC wise by C49 which has been disconnected and C45, which being 270pF is unlikely to be faulty. You could try disconnecting one end of it though.
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Old 18th Mar 2018, 12:18 pm   #66
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Default Re: Bush VHF 81 Restoration

On the Trader sheet the anode of V5c is the AM detector pin 6.

On the manufacturers circuit V5c is the triode anode pin 9.

Presume that may be the confusion.
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Old 18th Mar 2018, 12:22 pm   #67
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Default Re: Bush VHF 81 Restoration

I'm referring to the manufacturer's sheet.

I don't care which one we use so long as we're consistent.

Which does the OP prefer?

Are we being lead up the garden path by component designations?

EDIT.

Looking back through the component designations referenced so far, they seem to be the same on both sheets apart from the sections of V5.

Pin 9 of V5 is the one to check.
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Old 18th Mar 2018, 12:47 pm   #68
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Default Re: Bush VHF 81 Restoration

What material is the insulating part of the valve holders made from? If they're hard plastic I wouldn't expect them to be leaky.
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Old 18th Mar 2018, 3:24 pm   #69
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Default Re: Bush VHF 81 Restoration

My apologies. My previous reading was taken from the wrong pin.

I'll start again.

Voltage on anode of V5c (pin 6, trader sheet): -1.5V
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Old 18th Mar 2018, 3:53 pm   #70
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Default Re: Bush VHF 81 Restoration

Try pin 9, expect about 55volts.
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Old 18th Mar 2018, 3:55 pm   #71
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Default Re: Bush VHF 81 Restoration

Voltage on anode of V5c (pin 9, Bush data): 60V

Anode feed resistor: 180kΩ. Actual: 203KΩ. Drifted by 12%. I think the resistor tolerance is 20%.
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Old 18th Mar 2018, 4:20 pm   #72
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Default Re: Bush VHF 81 Restoration

V6 anode (pin 7) is correct which points to the anode current being correct.

V6 screen grid (pin 9) is high which points to the screen grid current being low.

Yet V6 cathode (pin 3) is high which points to the combined anode and screen currents being high.

V5 anode pin 9 is OK, so volt drop across R26 must be correct.

I'm stuck. Does R26 measure correctly resistance wise?
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Old 18th Mar 2018, 4:29 pm   #73
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Default Re: Bush VHF 81 Restoration

Quote:
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Does R26 measure correctly resistance wise?
R26 measures 203KΩ.
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Old 18th Mar 2018, 4:34 pm   #74
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Default Re: Bush VHF 81 Restoration

Just in spec then, but probably best changed.

What's the DC voltage across the 525R resistance of the output transformer primary? That will enable us to accurately calculate the anode current.

Do the same for the cathode resistor R33. The difference between the two figures will be the screen grid current.
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Old 18th Mar 2018, 6:33 pm   #75
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Default Re: Bush VHF 81 Restoration

Voltage across O/P transformer primary: 21.5V

Anode current of V6: 0.041A

R33 was out of spec. and was replaced.

Voltage across R33: 8.5V

Cathode current of V6: 0.04A

Screen grid current of V6: 0.001A?
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Old 18th Mar 2018, 6:59 pm   #76
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Default Re: Bush VHF 81 Restoration

Anode current is 21.5/525 = 41mA.

Cathode current is 8.5/220 = 39mA.

Screen grid current is 41-39 = 2mA.

Cathode voltage is high when compared to the service sheet, but the anode and screen currents are well within the rating of a UL84, so it shouldn't be getting excessively hot.

Is the cathode voltage now steady? Before it was rising above 10V.
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Old 18th Mar 2018, 7:11 pm   #77
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Default Re: Bush VHF 81 Restoration

How can the cathode current be less than the anode current?

Lawrence.
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Old 18th Mar 2018, 7:14 pm   #78
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Default Re: Bush VHF 81 Restoration

Quite right I am a

Trouble is, short of inserting a meter in the screen grid lead, there's no way of measuring screen current.

Bad contact between the valveholder and pin 9?
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Old 18th Mar 2018, 7:36 pm   #79
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Default Re: Bush VHF 81 Restoration

Regardless of anything else, what was being investigated here was that the output valve was possibly getting too hot. The cathode voltage doesn't seem to support that. At least not with the coupling capacitor disconnected.
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Old 18th Mar 2018, 8:47 pm   #80
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Default Re: Bush VHF 81 Restoration

It's good to know that the output valve is OK.

Since HT is what it should be I will measure all the valve voltages and report back.
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