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Old 18th Mar 2018, 8:55 pm   #81
ms660
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Default Re: Bush VHF 81 Restoration

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Trouble is, short of inserting a meter in the screen grid lead, there's no way of measuring screen current.
In theory the expected screen current can be deduced from the voltages and resistor values given in the Trader sheet, trouble is it's a bit pointless because the values given don't tally up with the current expected....unless I've made a mistake....

...Switched to AM, HT side of R30 (5.6k) is approx. 158 volts (mixers anode = 158 volts) bottom side of R30 = 112 volts (output valve screen voltage) so current through R30 = 8.2mA.

But, V2 anode voltage = 80 volts, therefore voltage dropped across R7 = 112v - 80v = 32 volts therefore the current flowing through R7 (2.2k) = 14.5mA, by the same method the current flowing through R8 = 4mA, through R9 = 4.2mA and through R26 = 0.3mA giving a total of 23mA, that and the 8.2mA that was initially arrived at don't compute...?

Anyone care to confirm?

EDIT: Out of interest do the Trader sheet voltages tally up with the voltages in the manufactures manual? (I've only got a copy of the Trader sheet)

Lawrence.

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Old 18th Mar 2018, 9:23 pm   #82
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Default Re: Bush VHF 81 Restoration

The voltages on the UL84 and UABC80 are the same in the Trader and Bush circuits, the Trade sheet states the voltages were derived from information from the manufacturers.
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Old 18th Mar 2018, 9:28 pm   #83
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Default Re: Bush VHF 81 Restoration

What about the voltages for V2 and the mixer anode?

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Old 18th Mar 2018, 9:42 pm   #84
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Default Re: Bush VHF 81 Restoration

All the voltages are the same in each service sheet, I think the Trader probably just copied the ones in the Bush circuit.
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Old 18th Mar 2018, 9:46 pm   #85
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Default Re: Bush VHF 81 Restoration

Cheers for that Frank.

If the resistor values are also the same then something is badly wrong so far as I can make out ? (Post#81)

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Old 18th Mar 2018, 10:07 pm   #86
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Default Re: Bush VHF 81 Restoration

The measured output valve screen grid voltage was apparently 130V (post #57) which, if the 158V figure is correct, gives a volt drop across R30 of 28V and a current through it of 5mA. That makes thing look even worse! That reading was taken before the cathode resistor of the output valve was changed though.

I think we'd better wait for a full set of readings from the OP and see what's what.
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Old 18th Mar 2018, 10:20 pm   #87
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Default Re: Bush VHF 81 Restoration

I have checked the resistors on each circuit from the volume control to the output transformer, the circuit reference and values are the same.
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Old 18th Mar 2018, 11:36 pm   #88
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Default Re: Bush VHF 81 Restoration

I've measured all of the valve voltages on MW and have entered them into a spreadsheet.

They are a mixture of too high and too low. Only a few are spot on or close.
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Old 19th Mar 2018, 12:07 am   #89
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Default Re: Bush VHF 81 Restoration

Are you sure about V6 cathode? It was 8.5V earlier on today.
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Old 19th Mar 2018, 12:33 am   #90
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Default Re: Bush VHF 81 Restoration

Just had another look. It is 11V.

I've added a revised spreadsheet.
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Old 19th Mar 2018, 9:17 am   #91
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Default Re: Bush VHF 81 Restoration

Valves V2,V3a and V4 have AGC applied to the grid 1, control grid. Tuning in a station and measuring voltages on these valves will give higher readings, in some cases much higher than when no station or interfence is being received. Reference your spreadsheet.

Most circuits state voltages are read when no station is tuned in, the manual for this radio does not make that statement. Perhaps an oversight or they took it as read that it would be normal practice.

Try taking the anode and screen voltages on those stages with no station tuned in, you have given the voltage on the detector in one post as -3.5v, this will be used as AGC and alter the voltage readings on those stages.

From earlier posts the receiving stages are working quite well, I would concentrate on the output stage, although the current through the valve is well within is absolute maximum that may not be so for the output transformer.

Julie earlier suggested a replacement UL84, that may well be something to try, even if it doesn’t fix it, you will have a spare output valve.

If nothing else “tames” the output stage, an increase in its cathode bias resistor will reduce the current through it with a slight loss in volume.

The post is a bit long winded, hope it’s understandable.
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Old 19th Mar 2018, 11:21 am   #92
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Default Re: Bush VHF 81 Restoration

Re: The revised voltages, same problem as highlighted in Post#81, if the circuit is to be believed the voltage drop across the 5.6k resistor is 35 volts, therefore current should be 6.25mA...Yes/no?

The current drawn by the anodes of V2 and V5 and the screen grid current drawn by V2 and V3 = 11.74mA...Yes/no?

No compute for me, major error somewhere or the current is partly being supplied by a parallel path to the 5.6k resistor....?

Can anyone confirm my calcs?

Lawrence.
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Old 19th Mar 2018, 11:24 am   #93
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Default Re: Bush VHF 81 Restoration

Regarding taming the output valve.

Service sheet gives a cathode voltage of 7.4V (34mA). The voltage has been measured at 8.5V (39mA) and 11V (50mA). As Frank states the latter two readings are within the limits for the valve, but not perhaps the output transformer.

Does the replacement cathode resistor still measure 220R?

What about R28 and R31, do they measure OK?

We don't have a control grid voltage. It should be near zero regardless of whether the grid coupling capacitor is connected.

The voltage between the cathode and the control grid should be equal to the voltage across the cathode resistor.

With the cathode at 11V, what is the voltage across the primary of the output transformer?
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Old 19th Mar 2018, 11:51 am   #94
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Default Re: Bush VHF 81 Restoration

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Re: The revised voltages, same problem as highlighted in Post#81, if the circuit is to be believed the voltage drop across the 5.6k resistor is 35 volts, therefore current should be 6.25mA...Yes/no?

The current drawn by the anodes of V2 and V5 and the screen grid current drawn by V2 and V3 = 11.74mA...Yes/no?

No compute for me, major error somewhere or the current is partly being supplied by a parallel path to the 5.6k resistor....?

Can anyone confirm my calcs?

Lawrence.
I agree. The figures don't look right and we're now talking about actual voltage readings rather than those quoted in the service sheets.
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Old 19th Mar 2018, 12:18 pm   #95
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Default Re: Bush VHF 81 Restoration

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I agree. The figures don't look right and we're now talking about actual voltage readings rather than those quoted in the service sheets.
Thanks for that, I'm not going la la land yet then.

Just another thought re: calculating UL84 anode current by voltage drop across the output transformer, the OP gave a resistance figure of 525 ohms for the primary, maybe the OP can confirm as to whether or not that was for the whole winding 6-8 (Trader) or the anode load section 6-7 (trader) because it's the resistance between 6 & 7 that's needed for the calc.

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Old 19th Mar 2018, 1:35 pm   #96
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Default Re: Bush VHF 81 Restoration

Snippet from VHF81 Makers manual.
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Old 19th Mar 2018, 1:43 pm   #97
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Default Re: Bush VHF 81 Restoration

I think this is what Lawrence is after. Probably not a good enough copy though.
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Old 19th Mar 2018, 1:52 pm   #98
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Default Re: Bush VHF 81 Restoration

Thanks, I deleted the relevant post because I made a fundamental error....need new glasses

Lawrence.
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Old 19th Mar 2018, 2:47 pm   #99
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Default Re: Bush VHF 81 Restoration

Thanks for that helpful information Nuvistor. The measurements in the spreadsheet were taken with the radio tuned to BBC Radio Newcastle. In the future I'll know now to take these measurements with the radio off tune.

I retook the measurements you mentioned with the radio off tune. The anode and screen voltages of V2 and V4 are now where they should be. The voltages on the anode and screen are 106V and -5.3V respectively.

StationX,

R33 is still 220Ω

R28 and R31 are both 25% high and will be replaced.

I monitored the control grid of V6 and initially it was not leaky. I noticed that the voltage slowly crept up from less than 50mV to 0.6V. The grid coupling capacitor will be replaced. This would have been the first thing I would of changed, but since is was not a Hunts or TCC part I decided to leave it alone.

The voltage across the primary of the output transformer (pin 8-6, Bush data): 25.6V
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Old 19th Mar 2018, 2:51 pm   #100
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Default Re: Bush VHF 81 Restoration

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Thanks, I deleted the relevant post because I made a fundamental error
Welcome to my world, as the old Jim Reeves song goes.
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