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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 17th May 2019, 9:57 am   #1
vosperd
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Default Racal RA1772 (Ser 497) update

I hope this is posted in the right place.
A while ago the mains transformer burnt out. No luck with a replacement until a month or so ago after a chance remark on a qso on 80 led to a fellow amateur who had one for sale for at very reasonable price.

Installed it during the week and wired up yesterday and apart from an intermittent connection on the Mc/s switch all seems to be ok.

I thought I would connect the individual supplies one at a time disconnecting the DC in each case from the psu in case of any over voltages.
However, the lower voltage supplies rely on the 20V supply to be working to bring up the supplies in sequence. Trying the 20V supply on its own only produced about 6V. Apart from that due to the type of wiring any attempt to unsolder the wiring to the regulator board resulted in about 1/2" of the insulation melting.

In the end I just wired up all of the supplies apart from the 80V, not needed without RTTY, and I was somewhat relieved to find that the set burst into life rather than flames.

Also, thanks to a rewind by Ed, my BC342 is now essentially working but probably will benefit from a few out of tolerance resistor and leaky cap changes.
Don m5aky

Last edited by vosperd; 17th May 2019 at 10:14 am.
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Old 18th May 2019, 11:13 am   #2
HamishBoxer
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Default Re: Racal RA1772 (Ser 497) update

I think Tantalum caps cause trouble on this model.
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Old 20th May 2019, 7:57 am   #3
vosperd
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Default Re: Racal RA1772 (Ser 497) update

I have only had one tantalum fail recently and it went quietly short circuit. Took a bit of tracking down.
Found it eventually and replaced it with a conventional elctrolytic.
Another problem is the display dot matrix leds. Not easy to come by and very expensive.
Don m5aky
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Old 25th May 2019, 6:32 am   #4
GW4FRX
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Default Re: Racal RA1772 (Ser 497) update

Quote:
Originally Posted by HamishBoxer View Post
I think Tantalum caps cause trouble on this model.
Nowhere near as much as they do on the RA1792!

Pertinent to a thread elsewhere about PVC outgassing, many RA1772s suffer from it. The main wiring harness between the mains transformer, the rear-mounted PSU board, the reservoir capacitors and the rear heatsink carrying the PSU pass transistors was executed in PVC-insulated cable which in service became very hot. Over the years this caused the plasticizer to leach out and vigorously attack any copper it could find. The result is extensive radial cracking of the insulation and corrosion of the wiring and associated terminals.

The cure is to rewire the entire harness in modern PTFE cabling, which is a chore but not inordinately difficult and allows easy replacement of the reservoir capacitors at the same time.
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