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Old 21st Dec 2019, 2:56 pm   #101
Richard - F4VPR
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Default Re: RCA AR88D Comms Receiver.

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Originally Posted by G6Tanuki View Post

Anyone who's fluent in Italian will be familiar with "Mille" as 1000.
And French too !

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Old 21st Dec 2019, 2:56 pm   #102
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Default Re: RCA AR88D Comms Receiver.

Cracked it!!!
It is the slugs that have lost something.
Rob's osc slug gives me a Q of 210 at 24mHz in the band 6 coil. Better than the Advance slug.
Band 4 & 5 track perfectly over the whole ranges. Steady amplitude.
Because I had to rewind the band 6 coil, and I think the coils are coated with silver, I have to experiment with the turns ratio for the bare copper wire. The osc collapses at 22.5 mHz, almost there.
Going to cross check the rf slugs next, then the if's, ect..
John.

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Old 23rd Dec 2019, 4:42 am   #103
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Default Re: RCA AR88D Comms Receiver.

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Cracked it!!!
It is the slugs that have lost something.
Rob's osc slug gives me a Q of 210 at 24mHz in the band 6 coil. Better than
John.
It is quite a finding. Somehow the slugs have increased their losses. Since the coils are still able to resonate at the correct frequency range, presumably the slug's B-H relationship must still be about right, so it must be circulating eddy currents in the slug, suggesting somehow the magnetic particles of the mix were once electrically isolated and now many are connected.

Does anyone have a theory how a slug could degrade, one would think with such a brittle substance that any of the particulate matter in it wouldn't be moving around much. Maybe its the base substance that holds it all together. It doesn't seem likely that they are made out a modern style ferrite mix where this would be very unusual. Great mystery though.
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Old 23rd Dec 2019, 9:44 am   #104
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Default Re: RCA AR88D Comms Receiver.

They'll be carbonyl iron particles pressed together. Modern materials will have better binding agents. They won't be ferrites. Ferrites have too much permeability and lower stability.

Dust-iron cores going off with time is a new one on me. Something to watch out for when I get round to restoring my AR88.

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Old 23rd Dec 2019, 11:30 am   #105
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Default Re: RCA AR88D Comms Receiver.

Allow yourself a few hundred hours of time David.
I'm not finished yet.
John.
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Old 23rd Dec 2019, 12:13 pm   #106
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Default Re: RCA AR88D Comms Receiver.

Well done John. Most people would have given up.
The slug degradation is interesting. I'm not a chemist, but I wonder if the material used for insulating/binding the iron particles contained a minute quantity of water or liberated minute amounts of oxygen or both causing the iron particles to oxidise and form a conductive path?
If so then the water percentage may have been dependent on the time of the year the slugs were manufactured. That would explain why some are bad yet others seem OK. Breaking one and looking at the particles under a microscope may reveal signs of rust?

Those AR88 oil filled condensers with the solder tags do fail eventually but the 0.1uF Sprague cylindrical types seem to go on forever. I tested some recently and they were in perfect condition with virtually zero leakage. Watch out for the output transformer as the condenser C119 at the anode of the 6K6 can go short circuit. Just clip it off and you'll get a bit more top in the audio.
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Old 23rd Dec 2019, 2:44 pm   #107
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Default Re: RCA AR88D Comms Receiver.

Hi Alan.
I might try crushing one to see if there is anything odd in it. I have a Philips stylus microscope so I may see something.
I have changed c116 for a 630v cap. I am reluctant to omit it as it is there for a reason, maybe to stop hf oscillation in the stage which could damage the o/p tx winding.
There were no sprague caps in mine. My set is an early one with the engraved front panel.
It also has a pre-set pot at the rear of the chassis marked R21, but there is no R21 in the component list.
The pot is in series with the cathode load res of the last if stage. the two leads that are bolted to the lamp case on the front panel short out the pot to chassis. So I am assuming that the two leads are for the S meter when fitted and the pot is the setting control for the meter.
john.
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Old 23rd Dec 2019, 3:48 pm   #108
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Default Re: RCA AR88D Comms Receiver.

This has been an intriguing thread to follow- I suppose the elimination trail had been pointing towards tuning slugs for a while but it was still a bit of an eye-opener to find that they were affecting things so much. Definitely a future possibility to file in the general AR88 knowledge bank, at least the one here seems to oscillate merrily across the ranges- at the moment. Now that things have come to a conclusion, I had had second thoughts and held back from applying a testmeter to slugs, just in case passing even a bit of DC through them could be counter-productive- at least until I have some dependable substitutes! I couldn't help thinking that that might be a bit like sticking an ohm-meter across tape heads or microwave diodes- just don't, unless you're very certain of what is to be achieved.

Yes, the "R21" chassis apron hole is for a 100 ohm variable resistor shunting the meter to set FSD, the parallel combination being in series with the first IF amp cathode resistor (R20, 100 ohm). Being intended for a RHS zero meter of course, "set FSD" ends up being "set zero". I used a 2700pF 1600V Philips polyprop in the C119 position, connected across the O/P transformer primary rather than between anode and chassis- the latter connection seems to be asking for trouble in the event of capacitor failure.
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Old 23rd Dec 2019, 4:16 pm   #109
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Default Re: RCA AR88D Comms Receiver.

Of course if that C119 goes short then wave goodbye to the output transformer, It goes from bottom primary to chassis. Much safer if needed across the primary.
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Old 23rd Dec 2019, 8:30 pm   #110
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Default Re: RCA AR88D Comms Receiver.

Another problem solved-I noticed that sometimes the amplitude of the band 6 osc waveform was varying, traced to bad chassis connection on the earth tag under the trimmer mounting, the padder and the fixed cap are connected to this tag. Cleaned the base of the trimmer, cleaned up both sides of the earth tag, and the chassis around the mounting hole, refitted with box spanner on nut, tested ok.
Even though the nut was very tight, there was still a bad connection due to slight corrosion on the tag.
Advice taken, I will connect C119 across the o/p tx pri. Better safe than sorry.
John.
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Old 23rd Dec 2019, 10:26 pm   #111
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Default Re: RCA AR88D Comms Receiver.

Hi guys found this may be of interest.

In the AP 116E-0724-16 RECEIVERS.1968

Well done John
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Old 23rd Dec 2019, 11:33 pm   #112
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Default Re: RCA AR88D Comms Receiver.

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Originally Posted by gezza123 View Post
Hi guys found this may be of interest.

In the AP 116E-0724-16 RECEIVERS.1968

Well done John
gezza123
Wow ! Somebody knew about this slug deterioration in 1968. Some must have deteriorated relatively rapidly to have figured that out. Its almost as they were thinking of the slugs as a limited life part, a concept I have never thought of or heard of before.

I have a vintage 1939 Meissner TV set, with some of the earliest known dust cores (Meissner helped pioneer the development of them) when most other MFR's at the time for TV's used air core coils in the USA. I'll have to be on the lookout for any possible issues related to the cores.
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Old 24th Dec 2019, 12:21 am   #113
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Default Re: RCA AR88D Comms Receiver.

Well, well, good spot there! So old radios, like us, really are mortal and doomed to fade away. I suppose that anything involving finely divided metal is gambling against entropy anyway. It's just as well that, in this case at least, the cores aren't deteriorating in a way that involves becoming crumbly as would be quite conceivable with chemical changes.
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Old 24th Dec 2019, 10:58 am   #114
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Default Re: RCA AR88D Comms Receiver.

I think the dust core problem is well known but not by us users. If you look up the subject it seems two topics come up... thermal ageing and the effect on iron dust by the organic binder.
Also, it seems, high pressure water is used to produce the powder and by its nature iron dust will be prone to oxidise so storage of the powder before making cores is important. Although thermal ageing is characterised by manufacturers over fairly short periods, given 70 years or so, the temperature need not to be too great for deterioration to occur. The organic binder may be the problem though. I saw a very verbose patent for improving dust cores but I noticed it was abandoned.
Turretslug mentions entropy... there's the answer...you can't beat nature doing its damnedest to return dust cores back to iron ore. It'll happen eventually.
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Old 24th Dec 2019, 12:16 pm   #115
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Default Re: RCA AR88D Comms Receiver.

Not all that I have are to the same standard of packaging as this one shown! It took some work with a scalpel to cut through the sealed thick wax coated fine mesh reinforced paper.
All are certainly packed well, even if not so good. A few are just loose, without packing.
Rob
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Old 28th Dec 2019, 5:06 pm   #116
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Default Re: RCA AR88D Comms Receiver.

More news on the slugs.
I am more sure now that moisture has somethig to do with it.
I baked the 4, 5, & 6 slugs in the oven over night, low heat, and the 4 slug tunes the 4 coil ok. The 5 slug would tune but the scale alignment would not come in.
I used Bob's slug for the 5 coil and it is ok.
So I thought lets give one a bit more heat with a blowtorch, no good, wrecked it.
So did the oven heat for to long or to short?
I have made some measurements on band 5 16.40mc/s with my super more modern, very expensive, National sig gen, VP8177A.
If the attenuator can be believed I get a signal to noise ratio of 10dB with an input of -10dB below 1uv, with no mod I get 15mv noise at the det.
There is a bit of mismatch as the o/p of the gen is 50ohms terminated and input to the set with one side earthed.
Has anyone made any sensitivity measurements on an AR88 to come anywhere near to the published spec?
There is a guy on ebay selling parts for an AR88L. He is selling a set of 8 IF coils for this model. Can any one tell me if the coils for the 2 models are the same part numbers? I have the part numbers for the AR88D model.
John.
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Old 28th Dec 2019, 5:56 pm   #117
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Default Re: RCA AR88D Comms Receiver.

The IF coils for the 88LF are different to those used in the D - the "LF" has an IF around 730KHz as opposed to the 450-ish of the D.
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Old 28th Dec 2019, 6:13 pm   #118
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Default Re: RCA AR88D Comms Receiver.

Thanks G6
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Old 28th Dec 2019, 9:19 pm   #119
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Default Re: RCA AR88D Comms Receiver.

I have a very sad 88LF in my shed... No idea if it has anything much left in it.
It is pretty stripped.
I'll look next time I'm in there.
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Old 29th Dec 2019, 10:50 am   #120
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Default Re: RCA AR88D Comms Receiver.

Thanks Jon.
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