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Old 17th Jan 2019, 11:55 pm   #1
M6SPW1974
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Default Receiver R1155L. R42 burnt out.

Hi Guys,just here for some advice on my R1155L...

Resistor R42 which is a 2.2K and is located in the RF section has suddenly decided to burn itself out.

Does anybody have any idea as to what could be causing this?? as the radio was working perfect up until last night but now no receive.

Not had chance yet to have a good look see at it,just thought i would ask on here first.

Any help appreciated

Thanks in advance.
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Old 18th Jan 2019, 12:04 am   #2
G6Tanuki
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Default Re: R1155l r42

You say "R42" but what schematic are you using [there are numerous different ones - at least three 'official' RAF ones, several Short Wave Mag/Practical Wireless redrafts - which have different component-numbering].

Post a photo of the schematic you're using to identify "R42" and then we can relate this to our own preferred schematics.

I'm suspecting it will be a screen-dropper or HT-decoupler resistor that's let-go because its associated capacitor has decided to exit stage-left into the land of short-circuit.
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Old 18th Jan 2019, 12:55 am   #3
M6SPW1974
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Default Re: R1155l r42

Hi there,below is the best i can do,it is out of publication AP2548 VOL 1.

The second picture shows the location of R42.

From what i gather R42 is the anode feed resistor for the RF amplifier section, so do you think C38 could be the culprit then??

Thanks again.
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Old 18th Jan 2019, 11:11 am   #4
Keith
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Default Re: R1155l r42

My bet would be on C38 (just below it in the diagram) having gone short circuit.
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Old 18th Jan 2019, 1:44 pm   #5
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Default Re: R1155l r42

Quote:
Originally Posted by Keith View Post
My bet would be on C38 (just below it in the diagram) having gone short circuit.
Yes - I would agree! If replacing C38 and fitting a new resistor doesn't restore proper operation then my next suspect would be the capacitor in series between V3 anode and the coil L31 (which from memory together form an 'IF trap' of some kind - wasn't it to keep an Irish broadcast-station on 566KHz out of the IF stages?)

A failure of either of these capacitors will essentially short one side of R42 to earth - and with several hundred volts of HT on the other side of R42, smoke is inevitable!
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Old 18th Jan 2019, 3:44 pm   #6
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Default Re: R1155l r42

Wasn't Athlone right on the nose at 560kc/s? Presumably the odd IF was chosen with the set's required frequency coverage in mind and an inconvenient neutral broadcasting station got forgotten about!
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Old 18th Jan 2019, 4:18 pm   #7
M0FYA Andy
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Default Re: R1155l r42

The filter is usually called the 'Athlone Filter'.

Andy
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Old 21st Jan 2019, 8:36 pm   #8
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Default Re: R1155l r42

A conspiracy theorist might imagine that Athlone shifted their tx frequency to annoy the RAF. Rather unlikely. Athlone was one of the stations commonly marked on the dials of prewar broadcast sets.
Careless of the R1155 designers, anyway.
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Old 23rd Jan 2019, 9:05 pm   #9
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Default Re: Receiver R1155L. R42 burnt out.

It was probably going to be difficult to find any quiet IF spot for a radio whose LF coverage extended to 500kHz and where some versions had an MF band starting at 600kHz. Perhaps it was felt better to choose a frequency occupied by a station well to the west of Great Britain (that is, the larger of the British Isles), rather than one in continental Europe where the majority of R1155-equipped aircraft (i.e. bombers) would be operating. I assume that it would have been receivers in Coastal Command aircraft operating over the eastern North Atlantic where Athlone breakthrough would have been most problematic, though MF night-time propogation can be far-reaching!

There were a scattering of other airborne receivers with similar coverage but different IFs, e.g. the US-built Bendix RA-1 (1630kHz IF) but presumably the R1155 was the one that was pouring off British production lines for British aircraft in greatest quantity.
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Old 23rd Jan 2019, 10:20 pm   #10
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Default Re: Receiver R1155L. R42 burnt out.

Athlone Radio broadcast on 567Khz with 100KW from a site to the west of Dublin. It was a prominent European broadcast station aimed, in part, at the ex-pat Irish community.
Coastal Command used Wellingtons, Whitleys and Liberators and as you say, their focus was the north Atlantic, from bases in Ulster, Scotland and the west country. Thus their R1155s would have been particularly susceptible to interference from Athlone. The Liberators were no doubt equipped with US radios.
My father was stationed at an aerodrome in N Ireland for a time, and his twin brother at RAF St Eval in Cornwall, which was a Coastal Command station.
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Old 8th Mar 2019, 1:28 am   #11
M6SPW1974
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Default Re: R1155l r42

Quote:
Originally Posted by G6Tanuki View Post
Quote:
Originally Posted by Keith View Post
My bet would be on C38 (just below it in the diagram) having gone short circuit.
Yes - I would agree! If replacing C38 and fitting a new resistor doesn't restore proper operation then my next suspect would be the capacitor in series between V3 anode and the coil L31 (which from memory together form an 'IF trap' of some kind - wasn't it to keep an Irish broadcast-station on 566KHz out of the IF stages?)

A failure of either of these capacitors will essentially short one side of R42 to earth - and with several hundred volts of HT on the other side of R42, smoke is inevitable!

Checked & replaced both & R42 is still going in to meltdown.
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Old 8th Mar 2019, 6:22 am   #12
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Default Re: Receiver R1155L. R42 burnt out.

Does it do it on all bands? In case a short to ground has developed in one coil in the switched pack

Does it do it with the RF amp valve unplugged?

Then there's a series LC from the anode to ground, the capacitor may have gone leaky.

David
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Old 8th Mar 2019, 10:41 am   #13
G4XWDJim
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Default Re: Receiver R1155L. R42 burnt out.

Crumbled rubber wiring held against metalwork by clips is another favourite in those receivers.

Jim
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Old 8th Mar 2019, 10:55 am   #14
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Default Re: Receiver R1155L. R42 burnt out.

Apart from the mentioned crumbled wiring ,check C37 after that it could even be (long shot) leaky wave change switch etc.

Looking at the circuit it should not take much finding (in theory).
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Old 8th Mar 2019, 12:01 pm   #15
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Default Re: Receiver R1155L. R42 burnt out.

What's the resistance at the junction of R42/C38 to earth?
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Old 8th Mar 2019, 12:04 pm   #16
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Default Re: Receiver R1155L. R42 burnt out.

There is a cap in series with L31 worth checking too. Did not spot it first time. Sorry,already mentioned.
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Old 8th Mar 2019, 1:43 pm   #17
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Default Re: Receiver R1155L. R42 burnt out.

Valve aside I would be going for smokeless voltage monitoring.

Lawrence.
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Old 8th Mar 2019, 9:19 pm   #18
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Default Re: Receiver R1155L. R42 burnt out.

I think that second capacitor is C111.

You may be able to find a 'short' with a multimeter, but if it only breaks down at high voltage you may need to use a different technique.
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Old 9th Mar 2019, 3:00 am   #19
M6SPW1974
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Default Re: Receiver R1155L. R42 burnt out.

Quote:
Originally Posted by G4XWDJim View Post
Crumbled rubber wiring held against metalwork by clips is another favourite in those receivers.
Hi Jim,no rubber wiring in this set,it has had an REME PVC rewire at some point.
Quote:
Originally Posted by HamishBoxer View Post
There is a cap in series with L31 worth checking too. Did not spot it first time. Sorry,already mentioned.
Checked & replaced,not causing the issue but cheers anyways pal
Quote:
Originally Posted by HamishBoxer View Post
Apart from the mentioned crumbled wiring ,check C37 after that it could even be (long shot) leaky wave change switch etc.

Looking at the circuit it should not take much finding (in theory).
Cheers pal, will give C37 a look-see when i get chance & also the wave change switch,much appreciated
Quote:
Originally Posted by Radio Wrangler View Post
Does it do it on all bands? In case a short to ground has developed in one coil in the switched pack

Does it do it with the RF amp valve unplugged?

Then there's a series LC from the anode to ground, the capacitor may have gone leaky.
Hi David,not checked out the issues you mentioned,i will look into it asap.

Again thanks for your advice guys!!
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