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Old 15th Apr 2019, 10:37 am   #1
SteveCG
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Default Raycom Ltd Bench PSU CB524

Does anybody have a circuit diagram for this Bench/Lab type PSU? It is nominally a 0-15 Volt DC, upto 3 Amp, stabilised unit.

Recently got from a car boot sale, after tightening one of the output connectors and the panel mounted mains fuse I find it works, but only goes up to 12 Volts according to its meter. I checked the output voltage with my multi-meter and it was indeed 12 volts. I can see three preset pots inside the rather cramped unit. It has a printed circuit board that is mounted on the rear of the meter and so is not going to be easy to trace the circuit - hence my request.

Thanks.
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Old 15th Apr 2019, 1:07 pm   #2
Refugee
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Default Re: Raycom Ltd Bench PSU CB524

I had one with a loose output terminal and when I opened it up it was a can of worms.
The feedback line to the sense transistor took the full power. It was only a little transistor but the whole thing had to come apart to find and replace it.

https://www.vintage-radio.net/forum/...ad.php?t=89688
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Old 17th Apr 2019, 11:18 am   #3
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Default Re: Raycom Ltd Bench PSU CB524

I have some further information which might help.

Examining again the Power Supply Unit (PSU) in a strong light I realized that the Raycom Ltd label was stuck over another one. This label says Academy-CB. I reckon that Raycom sold this Academy-CB product model CB-524 under its own name. I found a photo on ebay in the completed listings which is identical to the one I have except for the Raycom sticker:

https://www.ebay.co.uk/itm/Academy-C...p2047675.l2557

If Raycom sold Citizens Band Radio ( CB ) equipment in the 1980s then this product would make sense as a PSU for people who used CB Radios at home - as distinct from the 12 Volt powered car usage.
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Old 17th May 2019, 4:05 pm   #4
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Default Re: Raycom Ltd Bench PSU CB524

An update:

I discovered the regulator IC, an MC1723CP, had bad contacts (corroded and pitted) in an IC holder. I removed it and cleaned them and then put it back and the thing failed!

I tried to find a replacement IC at a reasonable price but failed in that too - so the PSU went to one side. Until today that is, when by chance I found that an LM723CN seems to be identical and available at a reasonable price - ebay, China. So I took the plunge and made an order. I know from reading the Forum, that some of these replacements from the Far East can be a waste of money but modifying the circuit for a commonly available LM317 did not seem feasible. I'll update in a few weeks time.
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Old 17th May 2019, 4:28 pm   #5
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Default Re: Raycom Ltd Bench PSU CB524

I don't know hat you call 'reasonable' but RS sell the Texas Instruments version (UA723) at
2.01 each here :

https://uk.rs-online.com/web/p/linea...ators/1266368/

or a tube of 25 for 10.20 here :

https://uk.rs-online.com/web/p/linea...ators/1450100/

(both prices exclusive of VAT).

The '723' is an old and well-known regulator IC. Really a regulator building block kit . There's a reference voltage source, an op-amp and an output transistor in there. You can link it to external transistors to increase the current capacity. You can use it (as you might expect) as a linear regulator, or you can get the op-amp to oscillate and use it as a switching regulator (as DEC and HP did over 45 years ago).
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Old 18th May 2019, 11:19 am   #6
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Default Re: Raycom Ltd Bench PSU CB524

TonyDuell,

Well, does 1.06 for 5 pcs from China, post paid, sound too good to be true (Maybe)? It just about fits in with the price I paid for the PSU. I don't believe in spending more on attempting to fix a unit (for my own use) than I paid for it (good money after bad) - but I do appreciate that others take a different position.
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Old 18th May 2019, 11:28 am   #7
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Default Re: Raycom Ltd Bench PSU CB524

``723 are a common v reg I.C. and can be found in many power supplies
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Old 18th May 2019, 2:16 pm   #8
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Default Re: Raycom Ltd Bench PSU CB524

Quote:
Originally Posted by SteveCG View Post
Well, does 1.06 for 5 pcs from China, post paid, sound too good to be true (Maybe)? It just about fits in with the price I paid for the PSU. I don't believe in spending more on attempting to fix a unit (for my own use) than I paid for it (good money after bad) - but I do appreciate that others take a different position.
I would regard RS as a trustworthy supplier. Many Chinese suppliers are OK but I am told some are not, And if I have to spend hours tracing a fault that turns out to be a new component that is not what it claims to be then I am not happy!

Unlike you, I look at the total cost of the unit, what I paid for the thing originally shipping costs, replacement part costs, etc and compare the total against a new unit. So in this case, I'd be comparing the cost of the defecting PSU + the cost of a 723 against the cost of a new PSU of similar specification (which is likely to be an SMPSU, and thus harder to repair, and thus less desirable to me).
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Old 18th May 2019, 4:59 pm   #9
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Default Re: Raycom Ltd Bench PSU CB524

Is it worth the effort to try what might be dodgy parts to effect a repair. If there is still a problem you will still be unsure. Time and effort is valuable as well. Ignoring any frustration!
Raycom sold mostly amateur radio gear. I imagine back then a CB PSU was a cheap source rather than a custom Yaesu, Kenwood etc.
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Old 20th May 2019, 10:24 am   #10
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Default Re: Raycom Ltd Bench PSU CB524

TonyDuell & robinshack,

Having traced most of it (a bit of challenge - not much space), I think the circuit is simple enough that if the 723 does not do the job then I can alter it to get a similar (but not as good) functionality using a LM317.

What you both say is true, but I do have a better (Farnell) psu for more critical use. From the circuit I suspect that the initial low voltage output may have been the result of the previous owner 'tweaking' a pot...
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Old Yesterday, 11:05 am   #11
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Default Re: Raycom Ltd Bench PSU CB524

The LM723 order finally turned up.

I fitted one into the IC holder and gently wound the mains voltage up until the unit was just working. This was to minimize any faults taking out the new IC.

I found the unit worked except that the IC holder was one of those cheap ones that does not take lead solder well and that if I gently pressed on pin 6 on the underside of the ic holder with an insulated prod the output voltage went up.

Since because of its cramped construction changing the holder was not really an option, I resoldered all the ic holder pins and really checked that prodding did not produce a voltage change.

According to a DMM the maximum output voltage was about 13 Volts - not the claimed 15 - but 13 seemed quite likely from a study of the LM723's application notes.

The voltage stability under varying load is good.

Finally I reset the analogue meter voltage and current presets to agree with my Fluke DMM. These presets were not the best either - a drop of Servisol 10 helped.
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Old Yesterday, 1:44 pm   #12
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Default Re: Raycom Ltd Bench PSU CB524

I would be very tempted to remove the cheap socket and solder the chip directly into the board.
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Old Today, 12:11 pm   #13
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Default Re: Raycom Ltd Bench PSU CB524

I always use turned pin type IC sockets and have never had any problems.
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Old Today, 12:45 pm   #14
SteveCG
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Default Re: Raycom Ltd Bench PSU CB524

Yes, changing the socket or removing it entirely would indeed be good moves - however the way the pcb was connected to other parts of the psu was such that I could easily have caused other problems with doing either actions. I'll certainly keep an eye on it in operation. As I said before it is not exactly a Lab grade PSU - but looks to be able to do what it was presumably sold for - powering CBs in the 1980s.
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