Bench PSU question

[Crystaleyes]

Hi all.
Looking for some help regarding the workings of this unit.

Some time ago I picked up a digital bench power supply from the local flea market here in Rio. It's a 'Sincler SPS 3015/D' which I imagine is 30V, 1.5A 'Digital'.
Even though it would turn on and show a display, it wasn't working, and no matter how hard I have looked, I've found no reference let alone any kind of schematic. I imagine it is from the late 70's or early 80's? I can only assume that all the components are to the original specs although it had clearly had prior work done so it 'is' possible that some component is not to spec.

What keeps happening is that one or both of the 2N3055 transistors keep blowing, and I would love to know why?

Fortunately the main board is really quite basic. A dual voltage transformer, filter caps, rectifier diodes, two voltage regulators (7805/7905), a CA747 dual op-amp, two 2N3055's and various resistors and caps. Separate from that is the display control section.

Anyway, I no longer remember everything that was done, although for sure I changed the caps, and the two large 1Ω wire-wound resistors.

Does anyone have any thoughts as to what is going wrong here?



Adding a couple of photos I took shortly after getting the PSU.

[Crystaleyes]

In fact, just looking at the PSU again (first time is maybe three years), it appears that the main rectifier diodes on the 35V input are 1N400x which being 1A diodes, clearly won't work in a 1.5A PSU.

Gonna stick in some 3A diodes and see what that does.


The photo below was taken on the day I bought the PSU and those diodes D1, D2, etc. are what it came with...

[stj]

1.5A??
a 3055 is good for about 7A from what i remember.
are they in parallel?

[Crystaleyes]

Hi fella.

Yeah. They're in parallel. Did think about upgrading them to the 2n3773's but the current ratings are almost the same, only the 3773 can take higher voltages.

I have wondered if the CA747/LM747 Op Amp is responsible and what might be a suitable replacement?

Seems to me that the main reasons that these transistors would blow might be; a) something shorting, b) too much Emitter-Base voltage, c) too much Base current.

But I don't immediately know the answer as I'm not too familiar with Variable power supplies.. yet.

[stj]

are the 1n4007's used for the 2n3055's or do they have a seperate bridge rectifier?

i dont understand why you have enough transistors for a 15A output with just a 1A input.

[Crystaleyes]

The 1N4007's are used for the 2N3055's. I know they take up to 15A but surely this PSU can't be expected to handle 15A - It must be 1.5A, no?

Gonna upgrade the diodes anyway but didn't get to the shops today... I'll see what I have in the parts boxes...

[stj]

if it's only 1.5A then i dont understand why they use 2 output drivers.
infact they could have just used a single LM317!!!

show more pictures of this thing

[redwire]

I think it's a 0-50VDC rating. PCB text the transformer main winding is 35VAC and control winding is 6-0-6VAC.
So you need to not push the pass transistors that much and they used two. If it was set to 5V output so seeing a 45V drop, 2N3055 good for about 2A max.

Note- the control power (+/-5V) common is the PSU (+) output which can make these circuits confusing to figure out.

The driver circuit is not so great. Q1 (TO-92) drives the pass transistors and there is no current-limiting base drive resistor there.
I don't see any backfeed or reverse diodes either.

[Crystaleyes]

Apologies for the delayed response, but thank you both for the input.

Camera is playing up but there's a couple or few images below..

As for he voltage, I thought I remembered it going up to 30v but perhaps it might be 50v as when the 3055's fail (as now) it shows 51v on the LED display (see photo). For the brief time that the PSU does work, the display and voltage output works fine. It never lasted any decent amount of time before killing the transistors.


The story with this thing is that I bought it back when I was first tinkering with electronics and certainly didn't have the (still limited - lol) experience that I have now. I never really deep-dived into it and gave up a few years ago, after buying a cheap 1502D+ supply, so it's been on the shelf gathering dust. Although opening it up again, I see I put a heatsink onto the LM7805, so obviously that was burning hot and I attempted to keep in cool.

The photo of the separated board (posted again below) shows fairly clearly the traces on the reverse. Measuring just now the 3055's, reveals failure towards short between both the Base - Collector junctions.
Looking at the board circuitry, the 3055's are indeed driven by Q1, a BC 547, so it must be in this section that the problem lies. Could the dual Op-Amp or Q1 be wrong? Could someone have put in the CA747CE after the original different one blew?

The final picture is one I have just found which was taken on the day it was bought. Minus the removed 40v filter cap, this is how it came. The op-amp had already been socketed, for instance.


Questions, questions...?

[CapLeaker]

Maybe the 3055's aren't original and were subbed for example from a 2n2773 or 2n3773 etc.

[redwire]

The driver circuit doesn't make sense - Q1 is NPN BC547 CBE pinout? Base drive for it is pulled up to 5V and the two diodes are for the op-amp to pull down (turn off) the pass transistors.
I think you should sketch the board schematic a bit more.

[Crystaleyes]

Maybe the 3055's aren't original and were subbed for example from a 2n2773 or 2n3773 etc.

This is absolutely possible.
In that photo from when it was bought, it certainly appears that the BC547 has been re-soldered, so it is also either not original or was pulled for some reason? Looking at the datasheet the base collector max is 50v... Perhaps I should try with a 546?
I also have some 3773's here.

[redwire]

I drew a draft schematic for the SPS3020 board. I don't think the 2N3055's should be failing C-B.
What's with the 40V Siemens filter caps but you said they measure 51V? Uh they don't have vents...

[Crystaleyes]

Here is the original list I made of the board components, before any changes.

R1 1R
R2 1R
R3 1k8
R4 4k7
R5 4k7
R6 1K5
R7 3k9
R8 4k7
R9 200R
R10 18k

Q1 BC 547

CI 1 LM 7805
CI 2 LM 7905
CI 3 CA 747CE

C1 1000µF 25V
C2 1000µF 25V
C3 2200µF 40V
C4 2200µF 40V
C5 Not used
C6 472 Ceramic
C7 22µF 25V
C8 22µF 25V

D1 1N4006
D3 1N4006
D4 1N4006
D5 1N4006
D6 1N4006
D7 1N4006
D8 1N4006
D9 1N4148
D10 1N4148


*Where 'CI' is circuito integrado - as this is Brazilian Portuguese

[stj]

so the bc547 has no base resistor to limit current. hmmm...
and i see what they did with the op-amp but i dont see why a circuit with an output stage referenced to ground would need to drive the bc547 5v below ground!!!
i also think the in4148's should be checked for shorts.
i also see it has no final-feedback to compensate for the load.

[Crystaleyes]

I think I might have found the problem.

As mentioned, this is something I last messed with several years ago and only came off the shelf as I'm having a total revamp in my little overcrowded workspace. In fact, I posted here before even taking a renewed look, as I've just been putting up shelves and sorting boxes. I thought there might be a design flaw which someone might spot?

Anyway, just now I went to actually take some voltages and immediately saw that one of the two main filter caps which I installed maybe ten years ago, was ever so slightly touching the wire connecting to Collector terminal C1.

I'm imagining that as the voltage was raised via the front panel pot, that at some point it shorted across and fecked the output...


I'm off out to work now, but when back I'll sort this out and fingers crossed that all will be well.


It's nice to know that if nothing else, my eye for potential problems has significantly improved since this PSU was last opened. Just that I should probably of had more faith in my capabilities before posting here.
So let's see. Maybe this isn't the problem ?

[Crystaleyes]

so the bc547 has no base resistor to limit current. hmmm...
and i see what they did with the op-amp but i dont see why a circuit with an output stage referenced to ground would need to drive the bc547 5v below ground!!!
i also think the in4148's should be checked for shorts.
i also see it has no final-feedback to compensate for the load.

So do you think there are any minor modifications which could be made to improve this PSU?

[stj]

lets see.
if it works i wouldnt tempt other problems appearing.

[Crystaleyes]

Hi all. Thank you for all the input. I'm learning how this thing works thanks to you.


So, re-sitting the filter cap worked. At least in the sense that it no longer blows the 3055, and turned out that only 3055 had blown.

One thing that still needs resolving is the output voltage is fluctuating. Quite savagely. The unit goes from zero up to around 42v but the digital display is constantly and rapidly, jumping around. On lower voltages such as 7v it might jump between 0.3v up and down, although up around 40v it is flying around, anywhere within a 2v difference. Connecting a voltmeter to the output, still shows as unstable but nothing like the devices display.

I thought it must be that the 2200µF filter cap which had been touching the 3055 Collector wire had been damaged, however both main filter caps test fine regarding capacitance and ESR.

Is it even correct to imagine that the unstable voltage would be capacitor related?

The LED displays have seemingly identical - but separate - boards for the voltage and current, with two small caps and one large display IC in the middle of each.

My question now would be, "what could cause such an erratic voltage display if not capacitors?"

[stj]

check the opamp pins are clean and the socket is not fucked.
i only buy machined-pin / turned-pin sockets to avoid contact issues, that board is using one but it may need re-soldering.
and clean any flux off the board - it can absorb moisture and become slightly capacitive or resistive
not much, but if it's between the pins of an opamp it could effect it