Thurlby PL310 power supply fault

[Paul Ramsbottom]

Having had a great deal of help on here over problems with an oscilloscope here is another problem that has just arisen that I hope someone can help with.

My son's Thurlby power supply has started to deliver 50v which is lowered slightly by the control but not much. The voltage display shows 0.FL which is presumably an error code. I cannot find anything helpful about this and an rather out of my depth with it. I am attaching the service manual - can anyone help please?

 

[73's de Edd]

Sir Paul Ramsbottom . . . . . . . . . . . .

Initially, in a no power on and having BLED down filters situation.
Use ohmmeter to confirm no dead shorted . . . nor leaky( C-E junctions of ) . . .Q2, Q3 and Q1 transistors.
If all OK you could initially monitor Q3 emitter with that present almost maximum voltage output and lift
Q2 base connection and the 50 should dissappear if a 1K load is on the output of the supply
output terminals.
O voltage output clears fault of those two transistors, the same could also be done by restoring their base drive and then lifting Q1 s base drive.
If still having problems follow the power flow tracing below.

Referencing to top of mark up schematic . . . . .

MAIN power supply outputs via BLACK and RED dot path with RED making it all the way to the Q2-3 Darlington power pair.
These need base drive . . . . so some Ve of the auxiliary supply is acquired and follows the PINK dot path and passes thru the R15-R3 series voltage divider set and then passes a HARD base drive to Q1 which in turn outputs at its emitter.
That BLUE dot path then presents a HARD drive to base Q2 of the Darlington pair which then has FULL voltage output at the LARGE RED dot path.

The regulation / voltage adjustment is accomplished at the R15-3 junction at the GREEN dot path that passes down to the D14-D15 that THEN permits the two halves of IC2 to conduct, to bleed down the voltage at the R15-3 junction.

. . . . . . Le Schematique:

73s de Edd

Addenda:
On the HP I have further info on more PRECISELY pinpointing its fallacy,
No problems at all with ripple as per your scopings.

 

[Paul Ramsbottom]

Thank you again, Edd.

One of the Darlington pair shows open circuit between C and E but the other shows a resistance of 430K when the collector is negative. I disconnected the wires to be sure nothing else in the circuit was affecting it. Q1 is open either way. Does that mean a simple replacement of those transistors or should I check anything else first?

I will be very interested to hear your further views on the HP.

Regards

Paul

 

[73's de Edd]

Sir Paul Ramsbottom . . . . . . . .

With my mentioned placement of a load on the output terminals, that should bleed down any static leakage through of voltage fron the Q2-3 darlington pair. If there is still any voltage presence, lift the base drive voltage that comes in from the BLUE dotted line.That should then inhibit any main voltage supply pass through from those two semis.

The other possibility of causing those two devices to conduct excessively, is leakage through the units Q1, to test it, just lift the pink dotted line from he base of Q1.

73s de Edd

 

[Paul Ramsbottom]

Thank you Edd.

I replaced the power transistors and that has reduced the output. It is now variable but will not go lower than 6v. It is also affected by turning the current adjusting knob.

I have had to put it to one side temporarily as I am overhauling a 1930s longcase triple chime clock and this has taken over my limited space somewhat! When I finally get it back together again I will resume with the Thurlby and continue the diagnostics you suggest.

Many thaks for your help

Paul

 

[Paul Ramsbottom]

Apologies for the long delay in replying. In addition to doing the clock I have been laid low for 3 weeks due to s stomach bug.

Anyway, the 6 or 7 volts were still appearing on the output at minimum voltage setting. Disconnecting the base of Q2 or that of Q1 reduced the output to around 0.3v. Q1,2 and 3 have all been replaced. Does that suggest a solution?

Thank you
Paul