Burnt Resistor Approx Value?

[Sparky67]

Hi all,

I am attempting to repair a low power BCC32 HF transceiver from the 1980s, also known as the SSB32. The only info I have is an incomplete and blurred cct diagram.

The set is 12v on Rx and 12/24V on Tx, with just the RFPA operating at 24V and a couple of relay coils between the 12V and 24V rails in the PSU switching cct.

Apart from an o/c transistor and dry joint in the Rx module the main issue was a short between the 12V and 24V supply in the Tx module (an internal wiring issue). This has burnt a resistor in series with the 12V supply to the Tx module. It now measures about 820 Ohms out of cct but I suspect it was originally a lot lower than that? Can anyone suggest a ball-park correct value for this R please?

Many thanks.

Martin
G4NCE

[G6Tanuki]

Given that it looks like it's being used as a decoupler between two sections of a capacitor? its resistance needs to be on the low side.

I'd suggest starting with something like a 5-Ohm wirewound one and monitoring the voltage on either side when under load.

Use a high-wattage resistor so if the downstream side of the circuit does do any nasty short-circuit things again the resistor will limit the current but without self-immolating.

[Chris55000]

What might have been barely readable is made far worse by the forum compression algorithm! Can you upload it as a zip file please?

The first band is brown, and I can make out two very blurry figures on the circuit diagram, so I would say that 10 Ω @ 2 W metal–oxide or carbon–film is a reasonable starting point to begin with!

Is it a British made unit? It looks like UNO Stencilled Upright Lettering on the bottom right of the diagram!

Any Maker's name?

Chris Williams

[Chris55000]

Further to my first reply, the BCC32 appears to be a modified export version of the Larkspur A14 BCC30, and you can download the A14 Part 1 and Part 2 Technical Manuals from the VMARS site free of charge!

Any help?

According to:– http://store.cbeagle.co.uk/army-mili...ceiver-sn-2661

the set you have appears to be lettered in something that looks to me like Hebrew or Arabic!

The Larkspur A14 manuals I can lay my hands on are dated 1968, so your set might be older than the 1980s – UNO Stencil Lettering tended to disappear from British Technical Documents from the early 1970s!

The store I provided a link to has a BCC32V for sale parts or repair at the moment!

Chris Williams

[Sparky67]

Ah, yes, the upload certainly hasn’t improved the readability! I will look at sorting out a better blurred version later today. Thanks for the thoughts on the R value too. I have ordered a selection of 1W resistors to start me off.

Sadly the A14 and the BCC32 are very different radios Chris and the information on the C Beagle web site stating they are similar is incorrect. I emailed them after heading off down that route previously. Although they are both green and use the same ATU, and are a little bit similar in appearance…!

I was told these V-suffixed sets were part of a blocked export destined for Iran. They were part of an AFV fit, hence a different PSU arrangement and a vehicle harness connector. Probably explains the lack of info too.

Thanks for passing on the info though.

Cheers,

Martin
G4NCE

[murphyv310]

Originally Posted by G6Tanuki

Given that it looks like it's being used as a decoupler between two sections of a capacitor? its resistance needs to be on the low side.

I'd suggest starting with something like a 5-Ohm wirewound one and monitoring the voltage on either side when under load.

Use a high-wattage resistor so if the downstream side of the circuit does do any nasty short-circuit things again the resistor will limit the current but without self-immolating.

Hi.
Not good servicing procedure IMHO start with a higher value and work down, going low first could cause a major catastrophe.

[Chris55000]

Surplus Sales of Nebraska list a BCC32 Service Manual but they want 100 USD for it!

Somebody on Racal Groups.io posted a Dropbox link that's now dead – so somebody on there has got the BCC32 manuals and data!

The fact that that store selling the BCC32V incorrectly attributed the A14 and BCC32 as being similar explains why I couldn't match up the layout and circuits with what you posted!

Chris Williams

[Sparky67]

The correct resistor value to start with would be nice, but I do take your point. It is visible with the radio out of its case, changing it means quite a lot of work to remove and dismantle the Tx head to get to its soldered connections.

I had the info I possess on Dropbox some time ago for someone else to download, so that .io link may have been mine?

I did the same, compared the A14 cct to the BCC32 parts I have and scratched my head a bit. I do have the user manual as a .PDF, hadn’t seen the service manual listed at 100 USD. That’s 10dB up on what I would be interested in paying.

Slightly less blurry zip file attached.

Martin
G4NCE

[Chris55000]

My eyes aren't the best by any means, but looking at the zip file you posted I can see enough of the numbers to read your resistor as 10 Ω !

The original was, I think, an Iskra 1 W component!

If you plan any serious work on this set I would try and get a better diagram or more of the Original EMER – I believe it matches the Army A15 Manpack set!

Chris Williams

[Sparky67]

That was a good lead Chris, however, after a bit of research elsewhere, it turns out the Larkspur A15 is the American AN/TRC-77. Also it appears this radio was also made in Portugal under licence, but a good copy of the cct or other servicing info is still proving impossible to locate, so far.

As a reasonable compromise for starters I fitted a 15Ω resistor in place of the burnt one. The transmitter worked on AM, correct output with clean but very low mod, was unstable on CW, varying in power each time it was keyed up, and produced very low output on SSB, so that was a start. Test-point voltages I have details of weren’t too far off. Knowing the correct Tx drive level would be a huge bonus. Then the Rx became intermittent, the lightest touch on the Rx head stopped it working. Then the Tx failed completely...! So a bit more work to do!

Cheers,

Martin

[Sparky67]

An update on this query. The resistor has now been identified as 15 Ohms (thanks Bruno!). The Rx intermittent cleared after resoldering component connections on the 1.4MHz IF module, then resoldering the module connections to the motherboard. Some of the solder work in this set is very minimalist, that’s the second dry joint cured so far. A fault in the Tx has been identified as what I take to be a bias-control transistor feeding one of the tuned RF amp stages, which is o/c - the LH transistor in the pair at the top of the cct diagram. I’m also guessing the diode in its emitter cct, which I also need to check, is a zener?

Martin
G4NCE

[trh01uk]

Martin,

yes, I think that diode you mention is a zener. However the operation of this bias control circuit is not intuively obvious, so I am far from sure on that point. If that one is a zener, then the one in the base circuit of the first transistor (PNP) is also a zener - simply based on the schematic symbol used.


Richard