Racal RA3791 Rx fault

[JohnnyAR88]

Hi Gents,

First Post so Hi all.

I was lucky to come across an Racal RA 3791 Rx that had been stored in a garage for many years, after an internal/visual inspection and a bit of drying out I plugged it in turned it on.

To my joy it did not smoke or blow up, however.....It is powered by a Switched Mode Power Supply, A Bulgin SAX 64-01 which makes a ticking noise (never good)

The only signs of life on the Rx are only the center section illuminates
(the "digital" S meter) the rest of the radio is inert, but on the main PCB Two orange LEDS flashing indicating that something is up, the LEDS alas have no markings to indicate /confirm the issue.

So..Has any one a circuit diagram for the RX?...I am OK with a diagram and a soldering iron so it would be great to attempt to repair it my self if possible?

I can find no reference to the Bulgin SMPS or a Racal RA 3791 circuit diagram or any anicodotal evidence on the web as to whats its trying to tell me, just knowing what the power supply should give out and what those LEDS mean would help greatly to confirm its a PSU fault ? (or not!)

Thank you gents (and ladies) on a fine forum.

Very best regards
Johnny

[Dave757]

Hi Johnny,

I have had a scratch around but the only thing I have found
which may be of some help is the Operators Manual for the set
which can be downloaded from Radio Pics.
You may of course have this already.

You could contact Telford Electronics in the UK who appear to have
several RA3700 series manuals available although I do not know
which models are included in this series. See their website.
I would add that I do not have any connection whatsoever with this company.

Good luck with your search

Kind regards
Dave

[JohnnyAR88]

Dear Dave,

Thank you for your reply, i found a guy on the web who had dropbox link to the Racal Manual and was kind enough to share it, so I just have to print it out!!

And Just have to source the +/- 15v+5v Switched mode PSU.

Best regards

John

[Herald1360]

That'll need to be well designed to keep the noise out of the receiver!

If you just want to use, rather than restore, the set, a home brewed linear PSU might be the way to go.

[HamishBoxer]

Check all small caps in power supply.

[Jon_G4MDC]

A ticking SMPS is usually doing one of two things.

Trying to start but it can't, then retrying over and over. That can be due to some short circuit on an output rail - maybe bad capacitors as suggested?
Otherwise it did start but it found too little load (due to a blown fuse for example) and so it resorts to "burst firing" because it has not enough to supply.

It would be good to find out which of those applies here.

[JohnnyAR88]

Hi John and Herald,

The caps look physically sound, fuses OK, will try a bench PSU to confirm if it actually a power supply problem, if so a linear PSU sounds good, let you know when I try it.

regards to all

Johnny

[turretslug]

Is this set from the era of the tantalum bead capacitor? A decent switch-mode PSU may ironically be too good in terms of low output impedance and hasten the short-circuit demise of these beasties especially if they are used as incoming rail decouplers on individual cards or around sub-regulators. They don't always look cracked or toasted when they go short.

[JohnnyAR88]

Hi Turret slug and Jon,

Well despite my misgivings about the SMPS the voltages on the PCB and outward to the regulators on the first point of contact (the digital board) are correct according to the manual, so the PSU seems not to be an issue as +5/-5 are where they should be on the PCB.

The only fault indications are the two LEDS that wink orange at me on the PCD, D1 and D14
still trying to find out what that means. (Must be a big hint!)

Any one experienced on a Racal RA 3791 Analog board?? (only Ex Racal types i think?)

Thanks gents (and Ladies)
Johnny