RCA AR88D Comms Receiver

[Radio Wrangler]

Yeah They must have seen R1's foibles coming, or why else would the folk at Camden have put it such an awful position?

David

[Radio Wrangler]

Should be fine. Modern non-polarised capacitors are dramatically more reliable than such things used to be, with the exceptions of faked electrolytics and audiophile parts.

There are probably plenty of equally fine ones sitting in stock in the UK.

David

[carnivalpete]

Sorry if this seems a bit too basic. I have been following this thread with great interest and have found some things I understood and some things I have found more difficult.

I am not at all convinced about the experience of the questioner in a forum of the very very experienced.

Should we not start with absolute basics, just in case? That is, examples like point voltages should be checked with the Rx warmed up and running. That great care should be taken with potentially lethal voltages. Resistor and/or capacitor values should be tested only after unsoldering at one end. Some types of known troublesome caps and res should be removed on sight.

Sorry if you think I have put a negative note in here, but just a reminder. Perhaps its a keep it simple plea. Unless we can be completely sure how much is being understood by a keen newcomer, lets take a little care. best wishes, pete

[Radio Wrangler]

Indeed!

With the AR88 there are a couple of things which experience has taught people to change on sight, with power off, and before any power is ever applied. This can be done safely, then the closed-up set can be tried on-power to see what signs of life it produces, and things can be taken from there.

The OP did put power on, and it seems like there is some activity but the tightest three bandwidths are not working.

He's going to need someone to show him how to handle valve gear without taking any risks. Who is there in the vicinity?

David

[HamishBoxer]

The screen grid reses are known to cause problems and I think are 33K?

[TrevorG3VLF]

Just Google. As an example I tried '6V6 data' and found a page by duncanamps.com which is always a good site.

You will get base connections and normal voltages.

[ian rose]

for valve pin numbers and parameters go to the National Valve Museum: http://www.r-type.org/index.htm

As for the tone correction capacitor it is probably safest to remove it and not replace it. The audio will be quite satisfactory without it.

cheers
Ian rose

[ms660]

Originally Posted by ian rose

As for the tone correction capacitor it is probably safest to remove it and not replace it. the audio will be quite satisfactory without it.

It will, but if going full bore and the output transformers load becomes disconnected or goes O/C there will be nothing to damp the output transformer primary from the high voltages generated, the insulation might break down.

Lawrence.

[HamishBoxer]

Dont forget that easy mod with the neons across the o/put tx.

[ms660]

When I replaced the tone correction capacitor in the one I had I connected the new one across the primary winding rather than anode to chassis.

Didn't do the neon mod but from all accounts it works very well.

Lawrence.

[frank rabbets]

Lawrence would you be kind enough to give me the transformer tag number where I should connect to as opposed to the earth ?

I've just managed to get hold of an original RCA manual which is clearer than the copies on line. Also a "restricted" 1953 military manual giving the exact procedure for alignment together with loads of other stuff I don't understand. About 30 pages in all. Complete double Dutch to me. The Man I bought it from is ex RAF and speaks Russian, Mandarin and some other languages. He worked at Bletchly park after the war.

[ms660]

The audio output transformer is located on the right hand side of the chassis next to the mains transformer as viewed from the top, the tags underneath should be visibly numbered, tags 3 and 4 are the primary winding connections, connect the replacement tone correction capacitor across those two tags.

Lawrence.

[Radio Wrangler]

I thought the OP's receiver had a different-pattern replacement transformer fitted? and that there were some lose wires around (suspacted from original 600 Ohm windings)

David

[ms660]

A photo of the transformer and it's configuration might help.

Should be able to identify the primary with a meter anyways.

Lawrence.

[frank rabbets]

It's a lot to take in but it's sinking in. Just 2 more questions please.

1. This high voltage thing. Is that with the on/off switch in "Trans" mode ? It would be ok to keep it turned on in "Rec" mode. What's this "Standby" mode? With the switch "Off" is everything in the receiver actually off.

2. Nearly all the caps are "Solar" ones. I think these are the more reliable mica ones aren't they? I can't see any "Micamolds".

[G6Tanuki]

The problem with the "trans" position on the power switch is that the HT supply is unloaded so the smoothing-capacitors charge to the peak-voltage of the power-transformer's output - then if you switch to either "rec" setting you dump this peak-voltage into the receiver's circuitry - which stresses the insulation and can cause shock-excitation of the output transformer - which unduly stresses the insulation.

As well as the neons-across-the-OPT-primary trick, when powering-up I quickly turn the switch straight to rec-mod or rec-cw so there's no time for excess HT voltage to build-up [even though the rectifier's directly-heated it still takes half a second or so for the filament to come up to emissive-temperature].

Switching between rec-mod and rec-cw while listening isn't a problem. When powering-off, go straight to "off" and don't turn things back on for a while (to allow the charge on the capacitors to die away).

[frank rabbets]

When "Off" I have to pass "Trans" to get to "Rec". This should be done quickly then and is OK ?

Question 2 anyone?

Thanks very much in advance.

[ms660]

Originally Posted by frank rabbets

It's a lot to take in but it's sinking in. Just 2 more questions please.

1. This high voltage thing. Is that with the on/off switch in "Trans" mode ? It would be ok to keep it turned on in "Rec" mode. What's this "Standby" mode? With the switch "Off" is everything in the receiver actually off.

2. Nearly all the caps are "Solar" ones. I think these are the more reliable mica ones aren't they? I can't see any "Micamolds".

What's already been said really.

OFF: No power is applied to the mains transformer, therefore no HT and no heaters.

TRANS: (transmit) Power is applied to the mains transformer and all the valve heaters, HT is present at the HT reservoir and filter capacitors but will not be present at the valve electrodes (except the rectifier of course)

REC MOD: (receive modulated transmissions) Able to resolve AM transmissions.

REC CW: (receive continuous wave transmissions) Able to resolve CW transmissions.

The manual I'm looking at states that the AC input to the rectifier is 345 volts per anode, that will be the RMS voltage, this means that when rectified and fed into a capacitor only load that the DC voltage (the HT) will rise to the peak of the RMS value which, with the above input voltage, will be approx 490 volts DC, this is what will happen in the TRANS mode.

In the REC MOD or REC CW position the HT at the reservoir capacitor will be approx 300 volts DC according to the manual.

In the REC CW position the BFO is activated so that CW transmissions can be resolved (and also SSB transmissions if you've got the knack)

So... REC mode...HT at reservoir capacitor approx 300 volts DC...nasty enough, TRANS mode...HT at reservoir approx 490 volts DC...much nastier.

I would suggest the TRANS switch mod as suggested earlier by RW.

Stay safe.

EDIT: Post crossed.

Lawrence.

[Radio Wrangler]

I have an AR88 which was rebuilt by the RAF in about 1953. They not only fitted a nice PVC wiring harness, they also fitted a separate toggle switch for mains on/off. I used to think this was standard practice because of problems with reliability of the mains switch that's part of the mode switch, but now I wonder if it wasn't to allow the set to be turned on and off with the mode switch in either receive position?

If you turn the AR88 on from cold and move the switch straight through the 'Trans' position, there is no risk, because it takes the rectifier valve a few seconds to get warm enough to start making any HT.

Turning off is a different matter because going through the 'trans' position, you abruptly disconnect the whole receiver from the high voltage power supply, the rectifier valve is up at full running temperature and the power supply voltage shoots up. For added stress, the stored energy in the smoothing inductors comes out as a high voltage spike.

On mine, I leave the switch in 'rec' or 'rec CW' and flip off the toggle. There's no nasty surge, and the radio discharges the reservoir and smoothing capacitors gently as its valves cool.

When I rebuild it (when= operative word!) I will bridge the mode switch so 'trans' doesn't break the HT connection.

David

[ms660]

Originally Posted by frank rabbets

It's a lot to take in but it's sinking in. Just 2 more questions please.

1. This high voltage thing. Is that with the on/off switch in "Trans" mode ? It would be ok to keep it turned on in "Rec" mode. What's this "Standby" mode? With the switch "Off" is everything in the receiver actually off.

2. Nearly all the caps are "Solar" ones. I think these are the more reliable mica ones aren't they? I can't see any "Micamolds".

I think Solar types were made with paper or mica, it depends on the value, post some photo's maybe.

As has been said before, replace those two "trouble" capacitors then I would tackle any faults on a case by case basis, it's easy to screw up doing lots of component changes just for the sake of it, you can learn a lot better about how things work by tackling faults as they crop up, replace the part(s) then if you want long term reliability think about changing other components if that's what you want to do.

Lawrence.