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Old 29th January 2017, 02:56 PM   #1
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Default Getting positive voltage at the output grid

The TSE was working (breadboard) when I packed it up and brought it to the UK with me. Swapped in the 240V version Edcor XPWR131 and wired the umbilical.

At checkout with rectifier in I get

B+ 414V
B- -216V

Filament 4.96V on each

When I try to get a reading on grid (clipping to pin 3 on the socket) to ground I get no reading on one and a positive mV reading on the other. I turned the pots at R12 and 23 but they didn't have an effect.

I have checked continuity across ground (to make sure the board is properly grounded as I know someone had a similar problem before and got positive readings) and continuity between the grids and the outputs at T3-2 and T2-2.

Any suggestions on what to check next please? Where could I measure to check continuity into the grid?

As those who know me will remember, I need as much non-technical direction as possible. I'm still learning which is why it's taking me so long, I'm trying not to make any dangerous mistakes.

Thanks.
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Old 29th January 2017, 04:03 PM   #2
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Make, model schematic?
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Old 29th January 2017, 04:15 PM   #3
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Make sure mm dial is in the right position.
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Old 29th January 2017, 04:20 PM   #4
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Quote:
Originally Posted by JonSnell Electronic View Post
Make, model schematic?
Tubelab SE John
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Old 29th January 2017, 04:33 PM   #5
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Quote:
Originally Posted by mkane77g View Post
Make sure mm dial is in the right position.
I'm glad you started with the obvious mkane as these are the kind of mistakes I make. Had one set for auto VDC and one set for 200VDC and then 20 VDC then 2VDC.

Is that correct?
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Old 29th January 2017, 05:50 PM   #6
w5jag is offline w5jag  United States
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Where did you measure the -216 vdc?

Since the fault looks to be common to both channels, I'd start by taking a look at R5 and R6 in the bias supply, to see if one or both has gone open.

Win W5JAG
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Old 29th January 2017, 06:56 PM   #7
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Got it.
There should be a negative voltage, with respect to ground on the grids of the output valves.
I am a bit new to types of amplifier models as I have only been designing and repairing valve amps since 1967.
I suspect a bias issue somewhere as w5jag writes.
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Old 29th January 2017, 07:10 PM   #8
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W5JAG,B- measured at R7.

I'll check the resistors you suggest and get back.

Thank you both.
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Old 29th January 2017, 08:26 PM   #9
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Quote:
Originally Posted by moggi1964 View Post
I'm glad you started with the obvious mkane as these are the kind of mistakes I make. Had one set for auto VDC and one set for 200VDC and then 20 VDC then 2VDC.

Is that correct?
Rookie here also and seemingly it's the simple things I miss.
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Old 30th January 2017, 09:15 PM   #10
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Rookie error.

Choose the wrong pin to measure from so was measuring from pin 2 instead of pin 3.

Readings now after adjustment with the pots showing -129v on both.

5842's going in next. I'll report back when that has happened, might be tomorrow or Weds.

Thanks everyone.
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