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MagnetoSound

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May 12, 2008
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It's on the front panel of a PSU I need to troubleshoot. Not sophisticated but difficult to ID some possibly incorrect parts without a schem.

I got this a while ago from a chap at the BBC, it looks to be a basic console PSU but there is no mfg info on it.

There is a uA723c regulator with a trimmer, and what perhaps might be the wrong op-amp, LF351. Plus BDV64/65V pass transistors and a few other components.

Anyone familiar?

 

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That looks familiar but I can't place it.

The 723 was a popular early variable voltage regulator. I think I may have used it in my first audio kit (1976).  The LF351 could also place this design in the late 70s.

Good luck.

JR
 
Analog supplies from the 70's were quite straightforward, and this one must be quite close to a 723 datasheet application, what else?

Why do you thing LF351 is wrong?

It was a good performance age correct component, and one of the first Fet input Op Amps.

BDV64/65 are European power Darlingtons and sound reasonable as pass transistors in an ...um .... European power supply.

What is the actual problem you are trying to solve?

I bet you can lift the schematic in less than 1 hour with good light, pencil and paper.
 
For more reading (in addition to the datasheet) on the uA723 (remarkably, TI still makes it), there's "The Art of Electronics" 3rd Edition (!). Chapter 9 is on power supplies, and it happens to be the "sample chapter" you can download from the book's website:
https://artofelectronics.net/
 
Thanks fellas, your help is appreciated.

The problem I was trying to solve is that the supply was working very erratically, if at all. Someone has attempted a prior repair, placing suspicion on some of the components and/or their orientation.

The unit is a dual-rail supply, output is normally adjustable between a nominal +/- 12-18v, but the trimmer was having no effect.

The 351 is configured as an inverting amplifier after the regulator to drive the negative pass transistor. The 723 drives the positive pass transistor directly.

A few components were not quite right. There was a 78L05 acting as a reference for the 351, supplying a fixed 5v to pin 7 (pin 4 being connected to the unregulated -27v rail). The 78L05 was inserted  backwards and had failed. Not having the exact part, I substituted a 7805 the right way round, and things started to improve, although the voltages would still not adjust.

I also found that one of the electrolytic decoupling caps was reversed, so I renewed all three of those and fitted all of them in the correct orientation.

There are a couple of smaller transistors in the negative circuit that seem to be ok.

Voltage(s) are now fully adjustable, but the supply collapses under load and the base resistor on one of the output transistors is burning up. I have ordered new pass transistors and will replace those when they arrive. I have no way of knowing whether the Darlington output devices are correct, and since I will only require a couple of amps max from each rail, I have ordered TIP2955 and TIP3055 replacements to keep things as simple as possible.

If I can’t progress further I will let you know. I hate it when simple things get stubborn.  ;D
 
benb said:
For more reading (in addition to the datasheet) on the uA723 (remarkably, TI still makes it), there's "The Art of Electronics" 3rd Edition (!). Chapter 9 is on power supplies, and it happens to be the "sample chapter" you can download from the book's website:
https://artofelectronics.net/

Excellent link, thanks! 👍
 
Ok, then you NEED to lift the schematic, there is no other way.
since I will only require a couple of amps max from each rail, I have ordered TIP2955 and TIP3055 replacements to keep things as simple as possible.
WRONG: you need Darlingtons there.

Why randomly replace parts with incompatible/unfit ones?
 
JMFahey said:
WRONG: you need Darlingtons there.
I think you are probably right. The page from TAOE (thanks benb!) in the attachment below (see note 9) makes note of the use of a PNP pass device, which seems to conform to the circuit in question.

JMFahey said:
Why randomly replace parts with incompatible/unfit ones?
This thing has been hacked before I got it. I really don't know if the parts in there are even original or correct, although I am beginning to think the outputs probably are.

CJ said:
check ground from filter caps plus and minus junction
Replacing the filter caps was my first move. Ground is fine. :)

 

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Happy to report that new BDVs did the trick. I gave a good coating of silicon grease between the transistors and their insulators and between the insulators and heatsink.

I now have stable voltages on both rails, adjustable from +- 12.2v to +- 17.5v.

Glad to have carried out a full recap along the way too.

Thanks to you guys for keeping me on track. 😎

(Still would love to know who made this thing!)



 
I know I've seen that logo before... (maybe here a few days ago). ;D

Perhaps it was a company that changed their logo later.

Maybe figure out what it is the PS for?

JR
 
JohnRoberts said:
I know I've seen that logo before... (maybe here a few days ago). ;D

big A
left half a C
whole around the A a O

ACO
Acopian ?

old logo if there is one ... just a guess...

Best
Zam
 
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