Icom IC-PS15 repair

[framar1]

Hi All
i have just picked up a linear power supply aka:"ICOM IC-ps15"
its a 13.8v 20A power supply but it is feeling a bit sad
it has the required 13.8v with no load BUT under load drops down to 6.9V mmmmm 
i am thinking its one of the 2sd797 diodes but am not sure.
i thought i would ask here before i spend hours desoldering everything to get the buggers out if anyone has a better place to start
i have attached the manual

any help would be great and much appreciated

regards
Martin     

[voltz]

Some initial thoughts:

Check the 20A fuse is not corroded and working. Any resistance here, even tiny, will cause a voltage drop. And yes, fuses can go resistive in some cases.

Check the 2 x emitter resistors from both power transistors, are they both the correct value?

Check that the input side of the 20A fuse (from 58,000uf cap) is holding voltage under load. It should be only dropping to near 13.8v only under full 20A load. If not, then the capacitor or diodes could be defective.

Check contacts on the output connector for corrosion and a good contact (again for low resistance).

[framar1]

here is what i have got
20A fuse:23.7v
Collector
Q1 Collector 24.2v no load
Q2 Collector 24.3v no load
Q1 Collector 24.1v load
Q2 Collector 24.1v load

Q1 base 6.7 no load
Q2 base 6.7 no load
Q1 base 7.1v load
Q2 bast 7.1v load

[voltz]

here is what i have got
20A fuse:23.7v
Collector
Q1 Collector 24.2v no load
Q2 Collector 24.3v no load
Q1 Collector 24.1v load
Q2 Collector 24.1v load

Those are correct.

Q1 base 6.7 no load
Q2 base 6.7 no load
Q1 base 7.1v load
Q2 bast 7.1v load

Those are not correct. The base voltage should be 0.6v above the emitters. ie around 14.4 (to give a 13.8v output)

Something is wrong with the regulation circuit.
Check aux supply D4 and D5 with C9 and C10.
R2 = 2.2r, check that too. Do you have aux supply?

[framar1]

yes i have a converted server supply i use for my radios

[framar1]

Check aux supply
D4 24.8 on one side 8.4 on the other
D5 24.8 on one side 8.4 on the other
C9 24.8
C10 24.8
R2 = 2.2r

[framar1]

may have found the problem there is a slight discoloring on Q3 (See photos) 

[voltz]

Yes, quite possible. Looks suspect. And its the base driver to the output regulation transistors.
Pull out and test.

If you can, try to clean out all that dust while you're at it. Compressed air is good.

[framar1]

Q3 Emitter is 25.3 

[voltz]

It should be. The emitter in this case is the supply to the regulator. Dont forget its a PNP device.

However the collector should be at 14.4, and we already know its around 6. Base is fed from Q4 and the zenner diodes D7 D6 with preset variable R9.
Those actually set the voltage.

[framar1]

ok so at D7 D6 im getting 14.4V no load and under load 6.8V and 7.2V on the base of Q3

[voltz]

Those zenner diodes are 5.1 v each. They should have very close to 5.1 across each one. That will confirm the zenner reference operation.

[framar1]

sorry looks like a previous repair and they have replaced the original 2 with just 1 1N4740A and im getting 4v on one side and the 14.4 on the other
but im going to call it quits for tonight its 12:20 am here lol and my eyes are starting to burn lol and i dont want to blow myself up
btw thankyou for all the help your giving me i owe you a beer or 10

[voltz]

ok, the 1N4740 is a 10v zenner. Thats pretty close to 10.2. Should be fine.

Suspect parts are Q1, Q2 and Q3. Yes the output transistors do need to be checked. Reference seems reasonable without further testing.

Good night

[CJay]

A simple rule that's served me well many times, any electrolytic is suspect, doubly so if it's got a 4 and a 7 in the value.

Test C9 and C10 by replacement, it's not worth putting old caps of that sort of value back in circuit unless you're absolutely desperately short of parts, they smooth the voltage to  Q3 which, if not smooth, will cause you all kinds of grief.

And yes, as Voltz says, Q1,2 and 3 need to be tested but I'd go for Q3 the 2SB435, I've seen dozens of those green Toshiba transistors go intermittently open in old CB radios (shoot me, I used to repair them)

[framar1]

just placed a order for 4 2SD797 and 10pcs BD244B (they are the replacement for the 2SB435) and tomorrow i order CAPS lol

[cl5ltd]

I've also recently acquired a PS15 and unloaded voltage is steady 13.8v, voltage under load drops to 8.5.
Will work through the steps detailed in thread but there was no final result posted - wonder what resolved the issue?

I might need to find equivalent for the 2SD797 as don't appear to be available locally