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Old 27th May 2020, 7:43 pm   #1
warspoils
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Default RCA 88, R1556 receiver help

Hi I have modified 88, and hope someone can recognise the modification, it seems to be aerial/RF related, as you can tell I have no electronic Knowledge, just a collector
I have fired up the receiver with a variac and speaker attached but no aerial or earth and it seems to be receiving, there were 2 coaxial cables attached, one to the normal tag board, the other going through the rear chassis to a single turn pot, is it possible this had two separate aerial inputs, any help or thoughts appreciated
regards Jeff
http://pic80.picturetrail.com/VOL814.../414433466.jpg
http://pic80.picturetrail.com/VOL814.../414433467.jpg
http://pic80.picturetrail.com/VOL814.../414433468.jpg
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Old 27th May 2020, 8:50 pm   #2
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Default Re: RCA 88, R1556 receiver help

There are several modification underneath, on the front and on the back panel.

Some typical modifications could include :-

a) Product detector
b) IF output
c) Improved BFO
d) Separate RF and IF gain controls
e) Voltage stabilised supplies.
f) Modified RF amplifier
g) Filter circuits

I am not suggesting these modifications are necessary, just that others may consider them desirable.

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Old 27th May 2020, 8:55 pm   #3
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Default Re: RCA 88, R1556 receiver help

Some info here:-

https://www.vintage-radio.net/forum/...ad.php?t=50017
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Old 27th May 2020, 9:13 pm   #4
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Default Re: RCA 88, R1556 receiver help

Ooh, there's a feast of guesswork to behold there! The die-cast box attached to the RF box screen with crystal and trimmer might be a calibrator, feeding a sniff into the RF circuits. The ECC81 might be some form of IF product detector, or maybe an AF circuit associated with what looks like a Radiospares transformer on the side of the chassis. Something indiscernible is going on adjacent to the aerial input terminals with a small transformer and maybe a small circuit board.

Is there any labelling/documentation associated with all those extra knobs on the front panel?
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Old 27th May 2020, 9:40 pm   #5
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Default Re: RCA 88, R1556 receiver help

That isn't an R1556, at least it isn't any longer!

The 1556 looks externally like a standard AR88D, though it might have an on/off toggle switch on the front panel. On the rear drop of the chassis there is a little bakelite 2-pole connector with dummy plug (yours is missing)

So it's grown three extra knobs on the front panel. I assume one of them is the crystal phasing control brought out and to the front.

Under the chassis I think you might have an added product detector as a highly likely to be chosen mod. A crystal calibrator would also have been desirable in the sixties. The added 1-turn pot is a modern (late sixties onwards) consumer economy grade one.

Is the front panel engraved for the legends on the original controls? That would show an early panel. It has the black-yellow-black tuning dial which is later.

The UK government had all ages of AR88 converted to R1556 spec. I have one with the R1556 modification sticker intact with engraved panel, all yellow tuning scales and they fitted a PVC wiring harness at the time of modification. I think it says the mods were done by REME.

David

Are both 6H6s still in positiion.
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Old 27th May 2020, 9:58 pm   #6
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Default Re: RCA 88, R1556 receiver help

The previous Air Ministry info you put up a link to Graham, was that during the war or later I'm interested really because I didn't know about the 1156 and this thread coincides with the one recently started by theyer454 on 3/5/20, re the AR80, although that seemed to be about early, not later, versions of the AR88. Apologies if I've got any of this wrong.

Dave W
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Old 28th May 2020, 10:54 am   #7
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Default Re: RCA 88, R1556 receiver help

That transformer looks similar to a Radiospares one.

Lawrence.
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Old 28th May 2020, 11:00 am   #8
warspoils
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Default Re: RCA 88, R1556 receiver help

Many thanks for all your thoughts
David, the three extra knobs, the top one is a six position switch, the middle one is a single turn pot, and the bottom one a two position switch, it has both 6H6s. the front panel has printed legends, the serial number is 002582 so is most likely a D model
the only markings are on the bottom shield see pictures
any thoughts on the two seperate aerial/ coaxial wires, could the white one be an output ? or from a separate aerial
regards Jeff
http://pic80.picturetrail.com/VOL814.../414433693.jpg
http://pic80.picturetrail.com/VOL814.../414433694.jpg
http://pic80.picturetrail.com/VOL814.../414433695.jpg
http://pic80.picturetrail.com/VOL814.../414433696.jpg
http://pic80.picturetrail.com/VOL814.../414433697.jpg
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Old 28th May 2020, 4:29 pm   #9
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Default Re: RCA 88, R1556 receiver help

It *was* a R1556 but it's been modified quite a bit since!

[to me, apart from the front-panel mods in the bottom left corner, the other 1556-giveaway is the large circular hole with four smaller holes at the corners on the rear chassis, which would have once housed a large diecast-alloy antenna-socket of obscure RAF-designation and now long since extinct.

Looking at your front-panel shot, above the upper chrome-strip there would appear to be an engraved plate - all I can see on it is "Modified by [3 letters?]"

Any chance of a better shot of this plate?
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Old 28th May 2020, 6:41 pm   #10
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Default Re: RCA 88, R1556 receiver help

Thanks again for the observations, I know this set has been extensively modified, since the air ministry r1967 upgrade, that is why I asked for help, hoping someone would recognise the add ons, and what purpose the two potentiometer, added to the rear panel and connected with coaxial
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Old 28th May 2020, 6:50 pm   #11
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Default Re: RCA 88, R1556 receiver help

So it's an R.1967 rather than an R.1156?

https://www.vintage-radio.net/forum/...d.php?t=151328
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Old 28th May 2020, 7:06 pm   #12
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Default Re: RCA 88, R1556 receiver help

I wonder who - in this context - "B.T.R. Ltd" were?

I'm familiar with BTR as British Tyre & Rubber : part of my family sold their declining business to BTR back in BTR's expansionist phase under Sir Owen Green : but am not aware of them having an electronics-industry involvement in the 50s/60s.
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Old 28th May 2020, 7:06 pm   #13
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Default Re: RCA 88, R1556 receiver help

R1967.....AP 116E-0706-16 (Formerly AP 2566D, Vol.1)

Apparently.
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Old 28th May 2020, 7:08 pm   #14
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Default Re: RCA 88, R1556 receiver help

It looks like it started out as a plain AR88D, with alternating tuning scale and printed labelling.

THen it may have been modified to R1556 standard. There would be a label on the outside of one of the front panel bracing gussets. The label on mine gives a date in the mid 1950s.

Then it got modified to R1967 by BTR ltd.

Then someone got their hands on it and did all sorts of modifications.


So does anyone know what changes constituted the R1967 mods? That would give us a starting point to what got done in private hands.

I don't think the audio output transformer is original. I've only seen cylindrical canned ones similar to the choke cans, but a different size.

David

Then i
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Old 28th May 2020, 7:20 pm   #15
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Default Re: RCA 88, R1556 receiver help

That square-can output transformer type illustrated has featured every now and then in forum posts- my supposition is that it was a type specified by and sourced for the UK military, possibly the fragile nature of the original had become a problem, maybe it had extra windings for say 600 ohm feeds in the way that the RA17 transformer had a generous array of secondaries. It appears to be of high quality, a typical sealed/potted type with ceramic post terminals (the latter possibly from the famous KLG concern?).

https://www.vintage-radio.net/forum/...d.php?t=138320
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Old 28th May 2020, 7:47 pm   #16
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Default Re: RCA 88, R1556 receiver help

Quote:
Originally Posted by ms660 View Post
R1967.....AP 116E-0706-16 (Formerly AP 2566D, Vol.1)

Apparently.
Apparently the R1967 wasn't that far removed from the R1968, the R1968 schematic can be found in here:

https://www.blunham.com/Radar/Signal...68andR1475.pdf

R1967 layout and schematic at the end.

Lawrence.

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Old 28th May 2020, 7:50 pm   #17
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Default Re: RCA 88, R1556 receiver help

It's always been recognised that the OP Tr was vulnerable to excess current if not feeding the LS. It was once suggested that, rather than altering the circuitry, it was maybe convenient to just get another one from the nearby stores. Perhaps this tougher version was was another possible solution as suggested by ts? In operation you'd want to keep the set running at all costs! Not so long ago, I used to see people advising that if the Tr failed on an 88 there wasn't much doing, if you couldn't find another one. hey became sought after! Then others experimented with non standard OT's, which worked ok and there was a circuit modification to prevent runaway I think.

Dave W
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Old 29th May 2020, 12:22 am   #18
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Default Re: RCA 88, R1556 receiver help

There's a crappy capacitor arranged in just such a way as to take the output transformer with it. So if either this one or the 'that' capacitor went, it was bye-bye transformer.

David
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Old 29th May 2020, 12:08 pm   #19
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Default Re: RCA 88, R1556 receiver help

Thanks for that link, Lawrence- that made for an interesting browse, it confirms that the output transformer was a UK MOD special with a second 600 ohm winding. I note that it retains the low-impedance speaker tapping of the original but this is hard-wired to a 5 ohm wire-wound load, simulating the situation of headphones being permanently plugged in in the original scheme of things. It seems that R1967 and R1968 were modified AR88D and AR88LF respectively, maybe the AR88LF had the advantage to the RAF of being usable as an NDB check as well as a capable HF receiver.

One or two other tweaks to the original design, including potting down the AGC to the 1st RF stage to one-fifth of full value, applying a slight sniff of DC from the 150V line to the AGC line (overcoming diode contact potential?), adding an IF gain control in the cathodes of the 1st and 2nd IF amps in order to balance diversity gain, and fitting a 300uA tuning indicator meter in series with the AF detector diode load. (I have to confess that I tried this with an AR88 long ago as a tuning indicator, but decided that it was a makeshift bodge with upper-scale compression (predictably). Subsequently, I find that not only did the RA17 do this but some of the RAF's AR88s too! If it's good enough for the professionals....)

The manual does comment that the new output transformer results in slightly impaired minimum sensitivity due to its wider frequency response- an audiophile AR88?

Colin

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Old 29th May 2020, 12:22 pm   #20
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Default Re: RCA 88, R1556 receiver help

That cap if fitted goes from primary anode to deck,all you to do is put across both primary connections of transformer.
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