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Old 24th April 2008, 01:14 PM   #1
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Default Help! What could possibly be wrong?

I had a pair of Leak TL12+ UL PP amps which I modified with new components (resistors/capacitors). The EF86 first stage is triode connected - the link at the junction R6-C3 is connected to pin 6. The circuit is here: http://www.44bx.com/leak/Leak/Circuits/TL12Plus3.gif

They have been working for a couple of afternoons, but the next day were dead. I've been checking/replacing components but I'm still getting wrong dc voltages (without input signal). I'm using a common, commercial DVM. Filament voltages are within spec.

The output stage (EL84s) measures OK at the cathode. At the OPT centre-tap voltage is 322V (slightly higher than specified) and the voltage across R22 is 9V.

Now, here comes the cracker... Across R15 it is 3.04V but at junction R15-R7 to chassis it is 265V.

At pins 1 and 6 of the phase splitter (ECC81) it is 3.5V and 3.04V respectively. At the cathode pins 3-8 to chassis it is 3.7V

On the driver side (EF86 in triode mode) HT only measures 3V! and at the cathode it is 0.9V.

I used different valves, changed PS capacitors and resistors, dismantled the PCB to check connections and clean it but the problem remains. It seems the problem is around the PS but so far I haven't been able to spot it.

Can some members on this forum please help. Not too technical please. Not exactly a newbie (I've built a couple of amps and understand basic valve terms). My measuring equipment consists of a DVM and analogue multimeter!

Thanks a lot for your help.
Joe A
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Old 24th April 2008, 03:07 PM   #2
oshifis is offline oshifis  Hungary
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Default Re: Help! What could possibly be wrong?

Quote:
Originally posted by sonata149
At pins 1 and 6 of the phase splitter (ECC81) it is 3.5V and 3.04V respectively. At the cathode pins 3-8 to chassis it is 3.7V
There should be 170-180V, so maybe one of the cathode resistors is broken. I would measure each passive component with the unit switched off and tubes removed.
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Old 24th April 2008, 03:41 PM   #3
SY is offline SY  United States
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Almost certainly not power supply. Try clipping out C2 and see if the DC voltages look better. If so, replace it. Also, what osh said- take out the phase splitter tube and measure the resistance from cathode to ground.
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Old 24th April 2008, 04:10 PM   #4
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Thanks oshifis and SY:

I checked the resistors with both valves removed and they all measure close except R12 (NFB) which is practically the same value as R4 (100R). But that is because (I suppose) it is connected in parallel to R4 via the OPT secondary winding. Isn't that so?

SY, I had already taken C2 off the board so the voltages I reported were taken without it.

That leaves me back to square 1. But anyway thanks a lot. I already feel relieved that I have expert friends like u to lean on.
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Old 24th April 2008, 04:43 PM   #5
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Hi there,

remove all the valves, except the rectifier and power up the amp. You should see about a 20% rise in voltages, but the scale to each other should be similar.

If the voltages remain as you report them, then remove r7 and see if they rise. This would indicate c3 as having gone closed. Not unlikely.

Bernie
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Old 24th April 2008, 05:15 PM   #6
SY is offline SY  United States
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Bernie, good suggestion, but I'd keep the output tubes plugged in.

Quote:
On the driver side (EF86 in triode mode) HT only measures 3V!
Is that at the EF86 plate or at the rail end of the plate resistor?
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Old 24th April 2008, 05:34 PM   #7
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Removed driver (ef86) and phase-splitter (ecc81) valves and the voltages remained practically the same. OK on the supply rail but very low on the phase splitter plates (ecc81).

On the driver ef86 side, voltages at the supply rail and the plate are both around 3-4V, that on the plate is slightly lower.

These measurements were taken with C2 disconnected at the cathode. c3 has been completely removed as I'm running the ef86 in triode mode.

Joe A
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Old 24th April 2008, 05:54 PM   #8
SY is offline SY  United States
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OK, just so I'm keeping up, you're saying that the voltage on the phase-splitter side of R15 (marked 280 on the schematic) is 3V? If so, you've probably got a shorted C6.
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Old 24th April 2008, 06:46 PM   #9
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The supply rail on the phase splitter side is OK but the voltage on both plates is 3-4V. Even on the cathodes, where it should be 47V it's giving me the same 3-4 volts.

Similarly on the driver (ef86) side it's even worse coz on the supply I'm getting 3v again.

I already changed C5-C6 with another dual capacitor (reformed) and the situation was the same.

What I find curious are the voltages on R15 (10K) and R7 (100K) which are 3.15 and 1.5 volts respectively whereas these should be around 35 and 80V. The HT feeding this supply rail (i.e. at C13-R22) is with spec.

Other aspects I notice are that (a) when switching on from cold state, the ecc81 valve lights up brightly but burns normally after some seconds; (b) the speakers I have connected to the output respond positively to any prodding with the DVM. But the last time I fed it with an FM tuner signal there was no output.

Thanks SY and all concerned for your assistance.

Joe A
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Old 24th April 2008, 07:39 PM   #10
oshifis is offline oshifis  Hungary
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When building tube equipment, the voltage tolerance of the capacitors is usually considered, but the voltage tolerance and power handling of the resistors are sometimes overlooked. R5, R6, R9, R13 and maybe others should tolerate at least 300V. Also calculate the power on them from the circuit diagram, and use resistors with at least 5x power handling.
I don't say this is the problem, but at the beginning you wrote that you replaced some resistors and capacitors...
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