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Old 5th Sep 2017, 5:22 pm   #1
Freya
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Default Farnell L30 stablised power supply

Been looking at this power supply this afternoon, the fault is excessive uncontrollable output, with resistor R22 burning up.
A test of the OC36 Germanium PNP output transistors revealed VT7 has a C-E short.
I have fitted a tested unmarked PNP GE transistor and replaced R22 22ohm.
Now is the output is very low, in the region of 6 volts, It is slightly variable but only at maximum output range.
Could the unmarked transistor be the issue possibly not being a pair?
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Old 5th Sep 2017, 9:21 pm   #2
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Default Re: Farnell L30 stablised power supply

Farnell L30.
Vt7 is the main series regulating transistor, high power as OC35 family. Stands on it's own. Driven by Vt6, medium power, better gain, usually the OC35 family. This receives current through Vt5 from the balanced pair Vt3 and Vt4. These two should be similar, as one takes the reference from the zener Z2, and the other from the output voltage setting potiometer chain P1. Otherwise, the transistors are not particularly critical.
P2 by Vt2 sets the current limit, drawing off current drive to Vt6, the medium power driver.
So if you can't get the voltage output up enough, check this current limit/overload section and around Vt1.
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Old 6th Sep 2017, 9:20 am   #3
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Default Re: Farnell L30 stablised power supply

Thanks for the pointer Bill, sure enough, no readings can be obtained testing VT1 ACY21
Now, what do I fit as a replacement !
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Old 6th Sep 2017, 2:30 pm   #4
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Default Re: Farnell L30 stablised power supply

Farnell L30 germanium version.
Vt1 is marked as OC72. You have ACY21. So any small signal, audio, low power germanium will do.
Have you anything in your junk box. Can you raid a scrap radio. =ACY17 series, GET880 series, 2N1302 series, 2G302 series. Best keep to germanium.
I suggest you also check the zener diodes and MR4 (OA6 ge gold bonded =OA5, AAZ15 series or even a Si 1N914).
If your junk box doesn't produce anything, send me a PM and I'll send you a couple of possibles for the postage.
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Old 6th Sep 2017, 3:13 pm   #5
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Default Re: Farnell L30 stablised power supply

Thanks Bill, not found anything suitable yet but have a few more places to try.
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Old 8th Sep 2017, 1:17 pm   #6
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Default Re: Farnell L30 stablised power supply

If R22 keeps burning up, check C3, 25uF, for short!

As for replacing Ge transistors, I would hazard to replace them (all of the Ge transistors, Ge diodes as well) with Si ones, chances are that the PS will work just fine.
If you have any doubts, just plug the circuit in to a circuit simulator, then you can tweak component values if they need to be.
Having Si transistors in the PS will increase its reliability.

Good luck, Peter
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Old 8th Sep 2017, 8:57 pm   #7
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Default Re: Farnell L30 stablised power supply

Hi Peter, the 25uF tested fine but as its original it got replaced anyway. circuit simulators are beyond me, i think i should stick to valves
Bill, I have searched through mountains of boards and bags and not had any luck finding a suitable transistor, the diode MR4 also is also shot of which i don't have anything suitable for either.
I do wonder what happened to this to destroy so much.
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Old 8th Sep 2017, 10:46 pm   #8
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Default Re: Farnell L30 stablised power supply

I have an ACY19 and an OA5 if you want them. The ACY19 is very similar to the ACY21.

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Old 8th Sep 2017, 10:51 pm   #9
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Default Re: Farnell L30 stablised power supply

Thanks Ron i will send you a PM.
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Old 9th Sep 2017, 6:45 pm   #10
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Default Re: Farnell L30 stablised power supply

Stephen,
MR4 can be(?) a TVS diode, but i dont find some data over that...
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Old 12th Sep 2017, 1:04 pm   #11
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Default Re: Farnell L30 stablised power supply

Still cant get this going, transistor replaced thanks Ron, everything tests out fine on the board.

Z1 =15 volt, Z2 =5.2 volt, Z4 21.04

transistors not doing much
VT1 e 0.730 b 0.533 c 0.126
VT2 e 0.0 b 0.142 c 0.0
VT3 e 0.141 b 0.0 c 8.72
VT4 e 0.141 b 0.660 c 9.41
VT5 e 5.6 b 9.3 c 0.0
VT6 e 0.036 b 0.0 c -40.81
VT7 e 0.0 b 0.036 c -40.81

Anyone help please, I really dislike germanium
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Old 12th Sep 2017, 3:27 pm   #12
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Default Re: Farnell L30 stablised power supply

VT2?
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Old 12th Sep 2017, 6:44 pm   #13
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Default Re: Farnell L30 stablised power supply

I will wrap this up now, in a word VT2 was SHORT. It was tested along with the others on the Atlas DCA75 and was fine and I have the figures for it.
Look as I might I could not find an NPN germanium in the box so stumped for a silicon 2N3439 and hooray it works.
Thanks for the pointer Tim, also to Bill and Ron
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Last edited by Freya; 12th Sep 2017 at 6:59 pm.
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