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Larkspur A14 transistor problems


AlanM
 

Hi,

I am thinking of acquiring a Larkspur A14 radio, however I wondered if A14 owners have had problems with the germanium transistors employed in the circuits,

in particular the dreaded OC171which tend to develop internal short-circuits.

Thanks,

Alan M

 

 


richard.moys
 

Hi

Some on mine were duff (tin whiskers), I did replace them with silicon, surprisingly I didn't have to change the biasing. A couple of boards are a bit difficult to get at. Remember there are two versions of PCB, one with pins for soldering components to (early type) and a fully designed PCB (later type), both are described in the manuals

73 Richard M0YSR


On 19/11/2020 16:56, AlanM wrote:

Hi,

I am thinking of acquiring a Larkspur A14 radio, however I wondered if A14 owners have had problems with the germanium transistors employed in the circuits,

in particular the dreaded OC171which tend to develop internal short-circuits.

Thanks,

Alan M

 

 


Andy_G8JAC
 

The army certainly had a lot of trouble with the original transistors in the A14 and several of them were changed to different types during service.  You will need the modification instructions (there were 30) to follow what was done. At least one of the transistors was a double transistor package which will likely be Unobtainium if you need one.

OC171s and similar which have grown internal shorts can often be rescued by shorting together the Emitter, Base and Collector and applying a DC pulse between them and the screen to “blow” away the tin whiskers that cause it. The A14 also has lots of AFZ11s that are similar construction.

 

The A14 was not part of the Larkspur range.  They were privately developed by British Communications Corporation as the BCC30 and adopted by the army for specialist operations after a good performance in trials against the high power version of the Plessey A13.  The Navy also bought them and labelled them Type 635.

 

Andy  G8JAC

 

Hi,

I am thinking of acquiring a Larkspur A14 radio, however I wondered if A14 owners have had problems with the germanium transistors employed in the circuits,

in particular the dreaded OC171which tend to develop internal short-circuits.

Thanks,

Alan M


GordonB
 

I had to change them all in my set. Seemed worth the effort as reports on the net say blowing the whiskers by a voltage pulse is not a permanent solution.
Not the easiest set to work on.

Gordon
G7KNS


Henk_Parasetguy
 

Hello Group

 

I happen to have many hundreds NOS Germanium and even more other transistors. About 88 pcs AFZ11. If you are looking for old types, feel free to contact me.

 

 

Kind regards,

Henk - Parasetguy

http://www.paraset.nl/index.htm

http://www.paraset.nl/hut12_warehouse/index.htm

 

 

 

Van: wireless-set-no19@groups.io <wireless-set-no19@groups.io> Namens GordonB via groups.io
Verzonden: vrijdag 20 november 2020 10:54
Aan: wireless-set-no19@groups.io
Onderwerp: Re: [wireless-set-no19] Larkspur A14 transistor problems

 

I had to change them all in my set. Seemed worth the effort as reports on the net say blowing the whiskers by a voltage pulse is not a permanent solution.
Not the easiest set to work on.

Gordon
G7KNS


Michael O'Beirne
 

Gordon

 

The weakest part of the A14 is likely to be the mixer(s).  Old germanium ones are pretty useless when the signals get large. 

At the risk of offending purists, is there any mileage in rebuilding that part of the set with decent mixers such as the SBL-1 with appropriate matching components to match the 50 ohms needed by the SBL-1, or may be an FET?  I have never seen the innards of an A14 but I suspect there’s not much spare space inside for any extensive modifications.

 

73s

Michael

G8MOB

 

From: wireless-set-no19@groups.io <wireless-set-no19@groups.io> On Behalf Of GordonB via groups.io
Sent: 20 November 2020 09:54
To: wireless-set-no19@groups.io
Subject: Re: [wireless-set-no19] Larkspur A14 transistor problems

 

I had to change them all in my set. Seemed worth the effort as reports on the net say blowing the whiskers by a voltage pulse is not a permanent solution.
Not the easiest set to work on.

Gordon
G7KNS

_._,_._,_

 


AllanIsaacs
 

I noticed mention of mixers and as I’d replaced the duff mixer in my R216 I’ll add my 2 pennyworth.

The R216 mixer is a CV291 and these diodes like many germanium microwave diodes cannot easily be tested as they can be destroyed by many multimeters.

I had an HP5082-2835 in my junk box which is like a 1N5712 Schottky diode rated at 8 volts which worked fine but there are loads around these days for example the SD103 that should work OK.

Allan G3PIY