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Author Topic: un61 power supply heater voltage problem  (Read 3556 times)

industrialarts

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un61 power supply heater voltage problem
« on: September 24, 2013, 05:03:36 pm »

Hi All,

Last time I posted it was about power supply problems.  I have another PS problem, so I guess we see where I am technically weak ;)

I have a UN61 supply on my bench and the heater voltage drops to about 1.8-2.0 volts when loaded.  To make sure it wasn't the mic I had with the supply, I loaded the heater voltage with about 40 -50 ohms (EC92 6.3 v @ 150 ma = about 42 ohms) and the supply drops just the same.

I have tacked in some different diodes for bridge just to rule out leaky diodes and have tacked in some filter caps, because at least the first one is bad and the second seems to be on it's way out too.

When I load the supply, the voltage before the choke drops to about 6.8 volts (8.8 unloaded), but the other side of the choke shows about 2 volts.  So am I looking at a bad choke?  To tshoot choke problems on a guitar amp, I can sub a 1k resistor to make sure that it's the choke - is there a value that I can use on the UN61 to insure that it is the choke?

Any other hints or advice welcome, thanks in advance,

Mark Springer
industrial arts
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klaus

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Re: un61 power supply heater voltage problem
« Reply #1 on: September 24, 2013, 08:00:43 pm »

Helathy choke coils in power supplies usually have very small resistance. Transformer/choke manufacture in the former East German "Neumann Gefell" company was haphazard, due to lack of good copper alloys, and with a few shorted coils showing up here through the years.

I cannot think of a reliable test to conclusively troubleshoot your supply form a distance, but would start by:

a.) isolating the input and output of the coil from the rest of the circuit, and then
b.) referencing the coil's winding against ground, to check for a short.
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Klaus Heyne
German Masterworks®
www.GermanMasterworks.com

Uwe

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Re: un61 power supply heater voltage problem
« Reply #2 on: September 25, 2013, 09:06:54 am »

As Klaus mentioned, you may have already some excess current draw from a possible leak to ground, either from a defective choke or leaky filter capacitor. I suggest to open one of the connections between the rectifier to the filter capacitor and insert a DC-current meter. Without load, you should see no appreciable current at all. Assuming there is no stray current leaking to ground, and if your your previous measurements are correct, then I would suspect more than just the choke. Your mains transformer's filament section may also be a problem. Here is the reasoning:
1.8V across a 45Ω load (substitute for the EC 92 filament), means a current of 40mA is flowing through the choke. Since under load the voltage at the opposite of this choke has dropped from the unloaded 8.8V to 6.8V, the choke's resistance must be 125Ω [(6.8V-1.8V)/40mA], which seems rather high. Furthermore, if the open circuit voltage drops from 8.8V to 6.8V with the 40mA current draw, this would indicate the source resistance of the transformer filament secondary and rectifier/filter section to be 50Ω [(8.8V-6.8V)/40mA].
In order to have 6.3V at the filament with 150mA (= 42Ω load), the unloaded voltage should be 32.55V, if the DC-resistance of the choke is indeed 125Ω and the source resistance of the supply is an additional 50Ω [(42Ω+125Ω+50Ω)*150mA = 32.55V.
It is very likely that the DC resistance of the choke should be considerably lower than 125Ω, and the open source voltage should be considerably higher than 8.8 V.
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mbrebes

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Re: un61 power supply heater voltage problem
« Reply #3 on: September 25, 2013, 12:25:55 pm »

You already said that the capacitor before the choke (C6) is bad.  If you have a capacitor that is shorting voltage to ground you can't tack on another cap across it for testing purposes.  From looking at the schematic, I would replace all three caps in that section (C4, C5, and C6) then see what you've got for voltage with a load.  My guess is that C4 and/or C5 is pulling down the voltage.
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klaus

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Re: un61 power supply heater voltage problem
« Reply #4 on: September 25, 2013, 01:39:35 pm »

Isolating a component under suspicion is always a good start. Hence my original suggestion to disconnect the OT's prime suspect, the choke, and test it for shorts. Then move on to the next suspect and repeat.
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Klaus Heyne
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mbrebes

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Re: un61 power supply heater voltage problem
« Reply #5 on: September 25, 2013, 06:25:35 pm »

... or you can lift leads on C4 or C5 and see if the voltage comes back up.
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