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Old 8th Mar 2015, 3:40 pm   #1
glowinganode
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Default Plessey PR2250 restoration.

This thread is written in retrospect, having spent the last couple of months restoring this set.
I purchased a pair of these in the hope I might get both working.
Upon inspection it became apparent these had been stored in the damp.
One unit was not too bad, the other was showing a significant degree of corrosion to the mother board and flexible pcb interconnects.
At this point I decided I was only ever going made one good unit out of the pair.
Starting with the better of the two, the unit was given a good clean and internal inspection. The controls were lubricated and allowed to soak.
Whilst it was possible to remove the main tuning and meter range knobs, the others were stuck fast. The grub screws were rusted up so were left to soak for the time being.
The power supply module was tackled next. Despite the temptation to power it up and hope for the best, I thought I'd best check it over separately.
This unit sits at the rear of the receiver and plugs into the rear of the mother board. It was removed and tested for functionality.
It quickly became apparent that the mains transformer had an o/c primary and an o/c secondary winding. Luckily the tx from the spare unit was ok, so was transplanted over.
Power supply now working, it was left on soak for a few hours (no load) with no sign of distress.
The power supply module was refitted in the unit, all other modules removed, and then powered up.

TBC

Rob.
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Old 8th Mar 2015, 4:03 pm   #2
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Default Re: Plessey PR2250 restoration.

Next module to attack was the 1 MHz reference.
Nice easy one, low output due to an o/c emitter on one of the transistors.
This was left on soak with the power supply whilst I formed a plan of attack.
I was in un-charted waters now, having never worked on a digital controlled synthesised receiver.
Despite this, the RF side of things was fairly conventional architecture.
Following etiquette, the audio stage was attempted first, working my way towards the aerial.
Surprisingly little work was required on the RF chain, beside a handful of tantalum time-delayed smoke bomb caps.
A card extension ribbon cable was made up from a connector taken from the scrap motherboard, and a rectangle of double sided copper clad. The extender board is for the synthesiser, to follow.

TBC.

Rob.
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Old 8th Mar 2015, 4:33 pm   #3
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Default Re: Plessey PR2250 restoration.

I didn't see any point attempting the synthesiser until the control logic was working, so this was tackled next.
The control logic is mounted on the rear of the front door, and hinges down for access. This controls the synthesiser, agc, filters, mode and tuning controls.
This was tested as much as possible without the memory module fitted. All fairly conventional stuff, though this section took the longest to repair due to the large number of failed and intermittent / yet to fail cmos ic's. In the end I build a logic tester. Although I had a complete logic control board spare, I found around 40% of the ic's had (at least one gate) failed.
One thing I could not avoid was removing the front panel, to enable me to replace a couple of failed led's and an o/c push button.
To remove the front panel though the remaining knobs were going to have to come off.
The grub screws were still rusted up, so the knobs were either going to have to be drawn or cut off.
Luckily the knobs were made with a reverse taper design, and removal of the bright allowed access to the end of the shaft.
A puller was turned up out of aluminium and worked a treat removing the knobs with no damage to them, and only slight scoring of the 1/4" shafts which were dressed up with a fine file.


Rob.
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Old 8th Mar 2015, 4:43 pm   #4
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Default Re: Plessey PR2250 restoration.

I like your puller Rob.
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Old 8th Mar 2015, 5:07 pm   #5
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Default Re: Plessey PR2250 restoration.

All efforts to remove the grub screws failed, so the knobs were left to soak in a 20% solution of phosphoric acid whilst I got on with finishing the front panel.
The edges of the holes were cleaned up and painted gloss black with Humbrol paint and a fine brush. Any spots of corrosion were flatted down with 1200 emery. The panel was given several light blows over with gloss lacquer. Whilst not brilliant, it looks a lot happier and won't get any worse.
The whole lot was re-assembled and was working well enough to test the memory module.
Before powering up the memory module, the tants were replaced as a matter of course, as was the back-up 3 V lithium cell. I had to spend a bit of time cleaning up the pcb as it looks like the cell may have leaked or sweated a bit.
All seemed well which was a relief as the memory module contains very expensive looking CDP1802 micro-processor, RAM, PROM, D/A and A/D's.
Fingers crossed with this one.
Finally all that remained was the synthesiser. As previously mentioned an extender card was fabricated to allow access for fault finding.
The fixed (LO2) was worked on first, and didn't really need much doing. The variable (LO1) was of similar design, so made a useful reference. Again not too much effort was required to get it working, although a couple of crystal b.p. filters were found to be faulty. Fortunately the spares were good.
Beside failing control cmos adding to the confusion, the only trouble was with falling out of lock at certain dial settings.
This was found to be caused by marginal b.p. filters working right on the edge of their pass band. Jockeying round a few boards cured this.

Rob.
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Old 8th Mar 2015, 5:35 pm   #6
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Default Re: Plessey PR2250 restoration.

I now had an essentially working radio, the mean time between cmos failures was starting to settle down. I could receive several stations on 80 M and ssb resolution was good.
However on it's first outing to a local gathering it was observed to be very deaf.
Back on the bench, the sig gen confirmed this. The agc buffer amp in the RF stage had failed, backing the gain right off. This was replaced, and the entire agc set up as per the manual to confirm correct operation.
What a difference this made (about 40 dB!), in fact I now find -20dB to -30dB pre-attenuation is required for best agc action.
Had a couple more cmos failures since, one of which was an MC14490 de-bounce ic (£5 - ouch).
What about the knobs you ask?
Yes, I'd forgotten about them too until about two weeks later. I fished them out and rinsed them off. To my amazement the grubs screws came out as easy as anything, even the one I'd tried to drill out. The phosphoric acid seems to have removed the rust down the thread without eroding or damaging the Bakelite or brass bush.
Something else I forgot to mention was the rotary encoder. This was completely stripped down, cleaned and lubricated. After 20 minutes or so it stopped working, eventually it was found the IR led had failed. Fortunately Maplin sell these quite cheaply, though it took a couple of guesses to get the right sort. Setting up the two level controls (not covered in the manual) was a matter of guess work, in the end I adjusted them for as close to even msr I could manage.
On the whole I'm very pleased with this receiver and despite the grey hairs it's caused me, I'm glad I stuck it out.

Cheers, Rob.
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Old 8th Mar 2015, 5:42 pm   #7
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Default Re: Plessey PR2250 restoration.

A few more pics of the knob puller under construction.

Rob.
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Old 8th Mar 2015, 11:22 pm   #8
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Default Re: Plessey PR2250 restoration.

Absolutely amazing work!

I didn't realise CMOS logic ICs failed so readily.

Nick.
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Old 9th Mar 2015, 12:36 am   #9
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Default Re: Plessey PR2250 restoration.

Nick, look up CMOS and 'Purple Plague'

Cmos uses damn -all power, so the chips run cold. Plastic packaging eventually lets in moisture along the leg/plastic joints. the water leaches phosphorus out of the glass passivation ("Glassivation") over the die, and the phosphoric acid formed attacks the aluminium conductor layer.

It can happen to other ICs, but in use they make heat and drive off moisture, protecting themselves.

This was known about in the 80s and the military stuck with hermetic ceramic packages.

David
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Old 9th Mar 2015, 12:40 am   #10
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Default Re: Plessey PR2250 restoration.

https://www.vintage-radio.net/forum/...3&postcount=11

From another recent thread might explain this.........

Purple Plague

Edit: RW too quick!
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Old 9th Mar 2015, 8:29 am   #11
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Default Re: Plessey PR2250 restoration.

What an epic story! I take my hat off to you. Judging by the number of repairs needed, there probably won't be many of these receivers left working in the future, so well done for persevering long enough to keep this one going.

Chris
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Old 9th Mar 2015, 9:18 am   #12
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Default Re: Plessey PR2250 restoration.

Ok thanks. I hadn't heard of Purple Plague, but confirms my suspicion that dark forces were involved.
Interestingly around 90% of the failed devices were 4001 / 4011's which may have been particularly prone.

Cheers, Rob.
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Old 9th Mar 2015, 3:34 pm   #13
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Default Re: Plessey PR2250 restoration.

The bag'o'bits photo seems to show clean legged chips- were they socketed? If they were, maybe a wholesale rechip could be on the cards?
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Old 9th Mar 2015, 11:09 pm   #14
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Default Re: Plessey PR2250 restoration.

I had considered so doing Chris but there are approximately 70 ic's just on the control logic board, ranging from around £1 a go for the common-or-garden types to a fiver for some of the less common.
I've just replaced another failed 4011, one gate o/c the rest stuck high.
Maybe it's the cadmium plating, it works alright 'til I replace all the screws

Rob.
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Old 23rd Aug 2015, 3:44 pm   #15
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Default Re: Plessey PR2250 restoration.

Given that you had two sets, do you have a spare power supply?
I could possibly find a good use for it in a set sans supply.
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Old 23rd Aug 2015, 10:29 pm   #16
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Default Re: Plessey PR2250 restoration.

Hi Geoff, I still have all the parts from the second power supply.
However the mains toroidal transformer is u/s. A section of the primary, and the secondary winding feeding the 65 Vdc supply, have gone open circuit.
You're welcome to it and the other parts FOC if they're of use.
Happy to help with any other parts you may need, mine seems to have settled down and is working quite nicely.

Cheers, Rob.
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Old 19th Sep 2015, 7:29 pm   #17
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Default Re: Plessey PR2250 restoration.

Geoff, see attached photo of spare power supply and tx.
PM if you still want them.
Cheers,
Rob.
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