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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 19th Aug 2019, 8:54 pm   #21
dave walsh
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Default Re: AR88 - New keeper

The Output Trans was a bit of an obsession at one point for the reasons just outlined and it was commonly said that if yours had gone nothing could be done as it was a rare item. Then one or two people found that a "normal" Op Tr could be a substitute [with suitable protection of course]. Sean Williams had a number of new and used ones when he helped with the sale of a large amount of War Surplus gear. Some people wondered why there would be a flaw in the Output Stage of such a Rolls Royce Radio? One answer possible answer was just the wartime haste. It was also pointed out [I liked this one] that they were used in fixed positions or arrays and a large supple of spares would probably be easily available, so they would just get one from the stores perhaps?

Dave W
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Old 5th Sep 2019, 10:26 pm   #22
ribbonmicsrus
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Default Re: AR88 - New keeper

Andy
It looks as if yours is around 200 higher than mine in serial number so we have the AR88. Above sn 3000-10000 the designations D and LF became normal.
Also note the o/p transformer arrangement is different from the D and LF models in that it uses a 2.5 ohm output and 20 ohms for headphones output.

The advice to fit two NE2 neons in series is a good idea, alas too late for mine as it had expired. I just unpotted the original and fitted in the can a small single ended transformer for a 6V6, works well.

I have nearly finished refurbing mine, opened up all the bathtub capacitors with care as the oil if any left is PCB based and now banned. I fitted modern Polyester caps with a 630v rating, bit overkill but to hand. I also replaced the PSU smoothing capacitors, C96, C67 and C98 with 4.7uF 500v polyesters.

Quite a few resistors had gone high and some of the capacitors in the RF section were changed.

The performance of the receiver with new components is amazing, with only about a foot of wire in the aerial socket even on Range 1 is really good. they are very stable as well. You might have to increase BFO injection a bit to make ssb reception a bit easier. I decided against fitting a product detector.

Be careful of the wiring in these models, being basically lazy decided against PVC rewire but had to re-sleeve one or two places with heatshrink sleeving to be on safe side.

My paxolin board with the bias resistors etc. had had a hard life and was brittle, so made up new board with stand offs and wire wound resistors in the biasing section.

Mine did not have S meter but have been given a similar meter and reprinted scale using a meter / scale drawing program called Galva 2.0.

Oh, and one final point, if you have quite a high noise level with no aerial connected, it was know early versions had an instability problem with V7 the 3rd IF stage. It was caused by rather long leads to the decoupling capacitors associated with that stage. In mine it was the screened grid decoupling that was the problem, so fitted a short length of RG174 coax to the decoupling capacitor C103. Later receivers had this factory fitted.
Also if fitted make sure C119 is disconnected, these used to go s/c and another reason the output transformers failed. I fitted a 3.3nF 400V across the primary, this is safe.

Have fun (and a bad back!!!).
73
Andy
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Old 6th Sep 2019, 11:45 am   #23
Silicon
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Default Re: AR88 - New keeper

The HT voltage can go up to 500V DC when the power switch is in the 'Trans' position.

Interruption of the current through the primary of the output transformer could generate very high voltages.

I would choose a 1kV rated capacitor for the component placed across the primary.
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Old 6th Sep 2019, 4:28 pm   #24
ribbonmicsrus
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Default Re: AR88 - New keeper

Yes, its a bit tight I agree.
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