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Old 7th Dec 2023, 5:15 am   #1
JohnnyAR_88
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Thumbs up Racal RA1772 power supply issue and component

Hi all,

Do any of you have experience with a RACAL 1772??

I have a recently acquired Racal 1772 that just went pOp!

The voltage meter says all the supply are incorrect (not on the green bit of the meter where they use to be, display is just "dots" and it won't interact with the controls on front panel)..she is ill.

With respect to the Power Supply regulator board- PM370 and the maintenance manual steps 1-6.

Downloaded the manual and found the 20V supply O/P was 6.6 volts on o/p pins 22/23, drilling down- IC ML3 pin 7 and earth have 29,5V so that seems OK going in.

But on pin 22 and earth only have + 6.1V (not +20V as flow chart 13, step 3 instructs)- followed step 4 (disconnected PCB from o/p and got so 29V (not +7.15V)
all seems to point to ML4 (UA 723/HM) non regulation and Tr 3 (40412) is very hotter than I would expect--I would want to replace ML4 and TR3.

So after all that my question is:-

any one experienced this issue?

and

Where can you get legit UA723/HM and 40412 transistor replacements (eBay a bit vague on this UA723 (not HM) TR6 nearest replacement 40410--so not ideal

Any talented guys and gals got an ideas?

Ta ever so much.

Johnny
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Old 7th Dec 2023, 12:27 pm   #2
vosperd
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Default Re: Racal RA1772 power supply issue and component

https://www.silicon-ark.co.uk/ua723h...5-by-fairchild

The uA723HM is listed here and I have seen an equivalent for the 40412 listed as a 2n3439.
I have had problems with the RA1772 psu but not needed to change components on the board as the faults were elsewhere, in my case tantalum caps.
Have you disconnected the wires from pins 22/23?
I'm not sure the significance of the suffixes on the uA723, I would not have thought that TR3 was that critical. Never too sure why they used some of the components that they did, but they probably did not have the choice we have today.
Don m5aky
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Old 7th Dec 2023, 12:30 pm   #3
Radio Wrangler
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Default Re: Racal RA1772 power supply issue and component

The power supply is the weak point of the RA1772, it runs hot under normal conditions and this degrades the reliability of its parts. I've fixed a few over the years but none recently enough to have any definite guidance other that faults found were scattered around with no obvious prime suspects.

David
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Old 7th Dec 2023, 8:16 pm   #4
JohnnyAR_88
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Default Re: Racal RA1772 power supply issue and component

Dear Don and David,

Thank you for the reply's,--- yes I have removed pins 22/23 as per the service manual and the issues remain the same (isolated to the PM370 PCB)


it points to ua723 (or so it seems)
but yes those tant caps are a pain, its a pity really as in the long term it makes complicated receivers like the1772 a bit of a liability for those who can fix them.

I shall follow the URL for the ua723- thank you for that.

regards

John
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Old 7th Dec 2023, 10:55 pm   #5
Sparky67
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Default Re: Racal RA1772 power supply issue and component

The only 1772 I fixed had a blown bridge rectifier, brittle wiring and some very dry-looking joints around the PSU, probably due to it being too hot for many years in its previous life. After replacing the bridge and resoldering many connections (including those vias going through boards) all was well. Just be careful of any loose cadmium plating and dust from that, particles from Cd are toxic and are best avoided, even in minute quantities.

Martin
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Old 8th Dec 2023, 12:23 pm   #6
Peter F
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Default Re: Racal RA1772 power supply issue and component

Hi Jonny
I had compete failure of 1772 PSU earlier this year. D2 had decided to short between all pins, Took out both links LK2 & LK6, ML2 & Ml5 on both IF boards.

Both the +5V & +12v supplies are reliant on the +20v, so first thing is to work out why only 6.6v. Is LK3 OK and volts on ML4/8&7.?
Have you disconnected wires to pins 22 & 23 ? If there is a fault on the +20v line excessive load may cause ML4 to current limit.

I did find replacements for all failed devices including 723. Can't find a TR6 in the PSU, so a bit confused on that.

Cheers
Peter
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Old 9th Dec 2023, 9:42 am   #7
JohnnyAR_88
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Default Re: Racal RA1772 power supply issue and component

hi sparky and peter,

Sorry i meant TR3 not TR6

thank you for your reply's, yes I have disconnected pins 22/23 and still only 6 v on o/p. when isolated from loading.

The wiring is not to brittle (nice in fact!) and all the links are ok, rectifier for 20 v side looks good AC/DC wise.

Yes as per the manual- it states if the 20v line fails so does the rest as confirmed by the front panel meter and good ole imperial measurement with my fluke., it looks like the ua XXX(ML3?) is not regulating and that transistor TR3 is VERY hot (even with the heatsink fitted)

Bits on order so will fit and report back, thank you gents for your interest and help.

regards

Johnny

I await the post man next move (not not them bills)


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Old 29th Feb 2024, 11:06 pm   #8
MWCS-Mike
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Default Re: Racal RA1772 power supply issue and component

Johnny

You left us hanging. Have you been successful with your repair? I also am having troubles with my power supply and am anxious to hear your results.

Mike
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