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Old 28th Oct 2020, 8:50 pm   #1
Sparky67
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Default Racal VRM4156 AGC Fault

Hi all,

I bought a Racal VRM4156 HF transceiver recently which has non-working AGC. The AGC IC, a CR711476, is u/s and the output has been set to provide mid-range AGC volts with a skeleton pot. The IC is a 10-pin thick film device.

I would very much like to return this to a working state, so I have two questions...

Does anyone know of a source of this IC, or an equivalent please?

Does anyone know of a suitable circuit which could replace this IC? The DC output range on a working set is 0.4V with no antenna connected, 4.3V with an antenna connected to 4.75V with a strong signal.

Any thoughts would be very gratefully received.

Diagrams attached.

Many thanks.

Martin
G4NCE
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Old 28th Oct 2020, 10:03 pm   #2
PeterRoberts
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Default Re: Racal VRM4156 AGC Fault

Hi Martin,

The Racal thick film devices were usually made up of a number of active and passive devices and without access to the actual circuit diagram of the CR711476 I don't think its going to be an easy job to replicate it accurately without knowing what is inside.

I've had a look through my info and junk drawers and I am unfortunately unable to help, but can say that the same thick film was used in other radios, such as the PRM4021, PRM4031, if that is any help.

73,

Peter
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Old 28th Oct 2020, 10:13 pm   #3
trh01uk
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Default Re: Racal VRM4156 AGC Fault

Martin,

without knowing the specific device at all, my understanding of thick film devices are that they are all custom made, are not ICs as such, but may contain ICs/semiconductors without packages, and directly printed Rs and Cs. I think they had their heyday before surface mount on PCBs became common place, back in the 60s and 70s I think. And I guess that means you are going to have a tough time finding out what's in the device, and producing a replacement. Maybe a scrap set with a working device is your best bet.

Richard
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Old 28th Oct 2020, 11:21 pm   #4
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Default Re: Racal VRM4156 AGC Fault

Just look around the periphery of the device and try to write a set of specs for its signals and functions. Then you start completely from scratch to design a little surface mount PCB to replace the dead hybrid. It's usually do-able, but a fair amount of work.

David
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Old 29th Oct 2020, 6:35 am   #5
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Default Re: Racal VRM4156 AGC Fault

David,

Thats clearly the way to go, but it isnt going to work if martin is correct that the device he has in his set is faulty. He will need to find someone with a good set, and then study the thick film device in that set.

Richard
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Old 29th Oct 2020, 9:23 am   #6
PeterRoberts
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Default Re: Racal VRM4156 AGC Fault

Martin,

Having had a read of the manual as to what the thick film actually does, it does sound very much like the same as a Plessey SL620/SL621. Its unlikely that in those days in particular that Racal would have used a Plessey device, but they may have made something that did the same job and created it as a thick film circuit. Mind you, finding a SL620/621 is not going to be easy either.

If the old thick film is dead you could try to carefully scrape away the encapsulation and see what is inside. You may be able to reverse engineer it or even find a fault. Some of these thick films did include tantalum capacitors and they don't age well.

If you are a member of the RacalRadio group on groups.io it is well worth asking there to see if anyone has this thick film or a gash board.

73,

Peter
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Last edited by PeterRoberts; 29th Oct 2020 at 9:26 am. Reason: add info
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Old 29th Oct 2020, 3:14 pm   #7
Sparky67
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Default Re: Racal VRM4156 AGC Fault

Thanks all.

I do have some voltage readings either side of R62 sent to me by another owner, so have some info on what volts are needed for AGC action. Knowing the 4021 series uses the same AGC circuit is also useful. I did wonder how the PRC320 and VRC321 achieve the same thing, and whether that method could be adopted. No doubt there are a few other HF radio circuits to peruse too!

Yes, the Plessey building-block chips do look like they might provide AGC for this set, thanks. I may also pull the thick film and see if there is a possibility of getting into it.

I have just joined the Racal group on Groups.io and will have a search through their posts this evening.

Thanks again

Martin
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