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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 29th Jan 2017, 10:34 pm   #1
Tyso_Bl
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Default B40D Success story, sort of!

Well what to do, I got this B40D at the Newbury rally about three years ago, it sat on the floor of the workshop drying out and acclimatising for a year before I got round to it.

The most obvious thing wrong apart from the missing wavechange was an oil leak, not the sort of thing normally associated with a radio, took the cabinet off, and removed the power supply/audio unit, the problem was that both of the small 4uF smoothing capacitors had burst and spilled their oil. Other things were a missing band change knob, stuck speaker, AGC, and limiter switches, the mains switch had been changed for a sub miniature one. The receiver function switch also goes round more positions than marked on the front panel. I can also see that the audio gain control has been replaced with one having a switch on the back.. hmm. What else, the side panel on the RF deck was missing, and the bracket and plate that locates th IF unit at the front. I should have picked a better example. Never mind it was cheap.

I can't find a Plessey mains connector, so I decide to fit a standard IEC thing while sorting out the mains switch.
I also unwired the voltage selector panel at the back of the radio, I don't like the idea of nearly exposed mains wiring like that.

Managed to find a pair of non polarised moor capacitors about the right value a bit of builders strapping and bodged them in. So powered up, no smoke lots of crackles from the in built speaker. Hmmm, the set came with a sort of home made adapter for the aerial connection, so on goes an aerial, yep we do get signs of reception, and rather a lot of cross mod. A lot of crackly switches too.

Never as easy as you hope is it?

A more detailed look round shows that the aerial input circuitry has been modified with a small disk ceramic to feed the input tuned circuit directly, enquires on this forum bring replies that this was to provide a high impedance aerial input, this a B40D which is designed for a low impedance aerial feed. OK me thinks, we'll sort that out, disconnect said capacitor, and find that whatever value it had when made it now behaves like a 10/15 ohm thermistor.
That was one little problem solved, reception is better now, no cross mod, but the calibration is all over the place.
The tuning action isn't up to much either, out comes the RF deck, chain drive and chain tensioner has been fiddled with, problem is that the upper gearbox mounting shims or whatever they were supposed to be are missing allowing the gearbox to drop enough so that there is too much slack in the chain, the chain tensioner mechanism had been modified to compensate using a self tapping screw forced in to the ally chassis casting. This was turning out to be more work than I thought, sort out the upper gearbox with some spacers and shims fron the junk box, and it all works as well as expected, while the mech was apart I dismantles and greased the lower gearbox, it was full of congealed grease so just as well.

Putting it all back together took most of an evening, the tuning mech operated end to end like it said in the manual, but calibration was badly off, reception wasn't brilliant either. Out with the sig genny and frequency meter, if you follow the procedure in the manual it will come right after a few goes. It sort of worked alright with it's tinny little speaker but it wasn't easy to listen to, after finding out the availability and prices of the matching external speaker I made one out of an old car phone hands free speaker (ripped out of my car for free), and used a 12 / 240 PSU transformer to provide the impedance transformation as the loudspeaker output of the B40 is 600 ohm.
Removed the speaker out socket from the back of the radio and fitted a 1/4 jack on a adapter plate in its place, when the plug is removed a 600R dummy load takes its place.

This worked well but the audio gain didn't do much, a poke round the wiring revealed that the pot was a linear one wired with a resistor to emulate a log one, whoever fitted it got confused and wired it wrong, a but of cutting and splicing, and it works OK now.

The radio got moved to the operating bench and got used mostly at weekends to monitor the AM intruders on the HF bands, worked well at that, and I can use it to listen to SSB on the amateur bands, need safecracker fingers though and having 40M spread over an inch of tuning scale is not ideal.

Few months down the line the radio made a bad smell and went quiet, a bit of tracing found that one of the screen decoupling capacitors in the RF deck had gone short circuit frying its feed resistor, replacing them still didn't bring back any life, more fault tracing found that the overload had taken out the smoothing choke, a small mains/12v transformer got substituted and it done the job since, the radios back on the operating bench and in use as before.

Plans for the future? I might put it back in its cabinet! Also if I can get a set of spare turret coils I'd like to investigate limiting coverage to Amateur bands and one or two sections of the broadcast bands.

I know some purists mugh take exception to the way I've dealt with it, but nothing I've done is irreversible, and at least it is doing something for its keep, it's not a valuable or rare radio and it turned up at the time I was interested in one although I don't know why!
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Old 30th Jan 2017, 12:14 am   #2
g4aaw pete
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Default Re: B40D Success story, sort of!

Thanks for the interesting write up. This all sounds a bit familiar with two of the three I have.
I put 10uF electrolytic replacements in my power units.

My D version sat in the garden for 5 years, without it's case,( not necessarily in the rain).
After a rebuild of the power supply, it actually worked! These beast can certainly take some abuse.

I think your proposed turret mod would work if you can get a spare set of four turret sections for the relevant band range.

Good luck with it
Pete
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Old 31st Jan 2017, 11:07 pm   #3
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Default Re: B40D Success story, sort of!

Found some pictures to illustrate some of what went on, should be self explanatory.
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Old 1st Feb 2017, 1:24 am   #4
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Default Re: B40D Success story, sort of!

If you have still got the original mains input connector I have the matching right angle cable end.
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Old 4th Feb 2017, 1:55 pm   #5
Phil G4SPZ
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Default Re: B40D Success story, sort of!

Well done, particularly for tracing and putting right all the defective work inflicted on this poor set by some previous ham-fisted owner. It beats me why some people hack around with classic gear when they clearly have no idea, and even less skill. Still, you've done a great job and must be very satisfied with the outcome.
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Old 30th Apr 2017, 7:42 pm   #6
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Default Re: B40D Success story, sort of!

Well its broken down again, what a surprise, only gets used around 20 hours a week and can't hack it... Still it lasted since January. Will report if it gets going again.
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Old 30th Apr 2017, 7:47 pm   #7
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Default More B40 hassles..

Yep its at it again, put the radio on this morning, had the usual listen and log, left it alone for a few hours, come back and it's very quiet, sort of working, but not.

I run the receiver with the cabinet off, looking down the side I notice one of the rectifer valves is fluorecing its glass. switch it off, the on after the cathodes have cooled, comes up as before but the fluorecence is pulsing on and off, no effect on the queit audio though.

The joys of vintage tech eh?
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Old 1st May 2017, 2:49 pm   #8
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Default Re: More B40 hassles..

Quote:
Originally Posted by Tyso_Bl View Post
one of the rectifer valves is fluorecing its glass. switch it off, the on after the cathodes have cooled, comes up as before but the fluorecence is pulsing on and off
Hello Tyso

do you mean the cold cathode stabiliser?
If it's pulsing, perhaps something is dragging the HT down?

Regards
Pete
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Old 2nd May 2017, 11:30 am   #9
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Default Re: More B40 hassles..

Have had a poke round with the meter, one of the rectifier valves is seriously low emmision, only just manages 190V off load, the other one flashed over internaly, trying pass the full current from one half of the transformer.

Now got to source a pair of 6X4 rectifiers, bother.

T
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Old 2nd May 2017, 11:41 am   #10
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Default Re: B40D Success story, sort of!

Seem to be around £10--12 each on Ebay.
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Old 2nd May 2017, 12:38 pm   #11
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Default Re: B40D Success story, sort of!

I have a couple of NOS 6X4WA's which I understand is a ruggedized version of the 6X4 down rated from 70 to 55mA.

Yours for £5 each plus postage.
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Old 4th May 2017, 10:33 pm   #12
Tyso_Bl
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Default Re: B40D Success story, sort of!

Thanks for the offer Graham,

I'm going to have a more thorough look at the radio to see if there are any other issues that maight have caused the rectifiers to fail, it may only be old age, the 6x4s are Colomor branded so not the originals, will PM you when I know.

T
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Old 7th Jun 2017, 11:55 pm   #13
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Default Re: B40D Success story, sort of!

This is sort of turning into a battle of wills, me against the laws of nature/ physics, so where have we got to now? had a few rain enforced indoor hoursthe other day so had a go at the B40.

I'd got as far as checking out the power supply, like theres no point in checking if the voltages are correct if the power supply is playing up is there? Well as reported previously, there was a problem with at least one worn out rectifier valve, a search of the valve pulls (unchecked) revealed a couple of 6x4's that had some usable emmision.

With the power supply restored to some sort of normality, guess what, the radio was not much better, what a bummer, so a more systematic aproach was needed. Poking round with a scope probe showed enough output from the detector valve, so pointing to a problem in the audio section.

Those of you familiar with the B40 will know how easily it comes apart for servicing, thing is while its apart you only get a limited degree of functionality to fault trace.. so with the power supply withdrawn we're reduced to proding round with a multimeter, first suspect is 'that cap' as its known here, cathode resistor voltage drop is a bit high, over the 10.5V mentioned in the manual, but the resistor is a bit high too, within limits tho, just.

Disconecting the anode/grid coupler cap makes no difference to the cathode voltage drop, and no leakage noted in the cap.
Now if youre plaing round with the audio/power deck while it's out of the radio, the audio driver stage will have no grid bias on on it, so you need to pull the valve out to prevent it floating into full conduction.

Next tests revove round checking for leaky caps, and drifting resistors, I've already found during previous checks that most of the resistors have gone high, only just within their limits, the exeptions were on the audio driver valve, the Anode load resistor had increased from 82k to 190k, the screen resistor from 220k to an open circuit, my fingers on the test prods had less resitance, the screen decoupler had gone leaky showin just over 5M, although in this position I don't think its critical like a coupling cap, I let it stay.
Not having replacements for the resistors the Anode load was replaced
with two 150k in parallel, and the screen with two 430k paralleled. similar size so should stand the dissipation.

The radios back in action, don't know how long it will last till another weak spot reveals itself, I suppose that a success story means your still keeping it going, rather than its 'fixed' (for now).

T
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