Farnell E30/2 power supply

[simplifyiyuk]

Hi All

Ive recieved this power supply, I'm trying to repair it but years of being a "board swapper" have spoiled me.
symptom is:
polarity is reversed and max output is only 0.55v  (-O/P , +O/P)
fine and course pots operate in the opposite direction (P1 , P2) i.e right to left to get the max 0.55v both pots read good (1K and 5K)

The unit did work momentarily with a 15v output. "percussive" troubleshooting showed no signs of returning any functionality. I've cleaned and tinned the card edge connector as well as a couple of dryish looking joints with no fix. everything I measure appears to be spec or very close.
one resistor screened on the main PCB marked R17 doesn't seem to tally with the circuit diagram, Its a 680k resistor but is marked S.O.T on the diagram?? another R17 appears up with the 2N3055s. this r17 and r16 both read 1ohm as expected.

Overcurrent protection the problem? I'm green on electronics theory as you can probably tell (most i have to do in the day job is continuity)

if anyones got experience with these and any suggestions of likely causes that would be great. also somebody has been in for a repair, replaced power switch so imagine something could have gotten meddled with.

heres a link to the manual/schematic I'm referencing.

https://ia601700.us.archive.org/21/items/FarnellE302StabilisedPowerSupplyUnitManual/FarnellE30-2Psu.pdf

[Tinkerer]

My first best guess would be something wrong with VT5, 6, and/or 7, if you have measured voltages elsewhere and they appear correct. Any mechanical switches are also a good thing to check if you have not.

[simplifyiyuk]

quick update, just noticed somebody has replaced C1 for a 47mF 63v (it is in the right way round at least)...

[mij59]

Hi,

What's the voltage across C1, and C3 ?

Check F2

[simplifyiyuk]

checked F2 and the holder first, doing a bit of jiggling and more disassembly to get good clearance for further testing.

[simplifyiyuk]

p.s gratitude, forgot that on line unless you type it its not implied. so thanks for the help thus far.. 

[simplifyiyuk]

Hi all

measured most of the aux power just now. C1 is 9.5V and C2 nothing but could that be related to VT1 VT2 VT3? or just my bad DMM technique. best points i could get to with the probes were legs of R8 and R5.  Can't seem to find the -0.7v where should this be? Z3/D3 junction?
R4 looks damaged/chipped and is reading 1.8k could be causing issues with the reference voltages?
I'll keep playing

but would assume the issue exists in reference voltages/control and not the power side + switches etc (both main secondaries bridge rectifier smoothing cap etc solid as a rock as expected)

what do y'all think?

[mij59]

Hi,

Please post a photo of the interior of the power supply.
Capacitor C2 is 330pF , do you mean C3 ?

[simplifyiyuk]

Hiya
here's a photo as requested as-well as the better (likely the later version that I have) schematic
repair is somewhat on the back burner as Ive got hold of a Thandar TS3021
confirmed there was no voltage at C2 when measured but has intermittently gone full scale on both 15 and 30 v without any twiddling or poking.
so as I say its gone into the long term pile but would be nice to get it going again.

http://www.introni.it/pdf/Farnell%20-%20E30%20Bench%20Power%20Supplies.pdf

[mij59]

The switch SW1 has already been replaced, I think you need to replace SW2, or try contact cleaner.

[simplifyiyuk]

Yeah I believe SW1 is prone to failure its a "clicky clack" locker latch type originally. while SW2 isn't failing any continuity test I could probably hard wire it into one or other position to confirm it.

cheers