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Old 16th Nov 2019, 5:57 pm   #1
theredhouseinn
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Angry RCA AR88D Comms Receiver.

A local oscillator problem that is now driving me mad.
The osc. is packing up on the bands 5 & 6 at the low freq. end.
measurements as follows.

HF LF
Band 1 12.0v 7.0vv
Band 2 10.0v 6.0v
Band 3 8.0v 5.5v
Band 4 4.0v 1.3v
Band 5 3.0v 0.0v goes to 0 at 16.6mc/s
Band 6 1.5v 0.0v goes to 0 at 26.0mc/s
Voltages are p to p
Measurements taken via a 5pf cap from the anode, to scope probe X10, 6.8pf to 1st det disconnected.
I bought this completely untouched in the original wooden box, with ops. manual, rack mounted version. Complete with mouses nest under the chassis bottom plate.
serial no. 1304 engraved front plate
I have tried 4 nos valves, all the same. All ok on the MHSVT.
The 10k anode load and the 56k grid leak res have been changed.
the anode voltage at 110v is ok, this falls to 70v when osc fails, grid v go to 0.
I hav'nt changed the valve base yet!
All resistors and papers in the rest of the set have been changed.
After 65 years repairing radios, you would think that I could fix a simple osc. fault. Help.
Has anyone else had this problem?
John.
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Old 16th Nov 2019, 6:13 pm   #2
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Default Re: RCA AR88D Comms Receiver.

"Mouse's nest" suggests poor storage conditions- the ceramic band-change wafers and polystyrene coil-formers ought to be reasonably resistant to humidity but components like oscillator padders/fixed trimmers could have absorbed moisture over years and be killing Q. Padder values get bigger as range frequency increases, so more dielectric to deteriorate.

Leave it by a radiator for several days and come back to it?

Colin
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Old 16th Nov 2019, 6:24 pm   #3
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Default Re: RCA AR88D Comms Receiver.

Some of the RF capacitors in these aren't real mica types, they're wartime economy paper jobs packaged like moulded case micas.

David
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Old 16th Nov 2019, 6:25 pm   #4
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Default Re: RCA AR88D Comms Receiver.

There lies the answer,damp!
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Old 16th Nov 2019, 7:12 pm   #5
theredhouseinn
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Default Re: RCA AR88D Comms Receiver.

No damp, it has been in my heated man cave for two months.
All padders and fixed caps across the trimmers have been changed
the fact that all wavebands are exhibiting this problem rules out tuned cct's. I think.
I have sprayed the wafers and scrubbed them with toothbrush, spotless now.
However I will stick the fan heater on it for a few hours and see if any change occurs.
I know I am missing something, it's just spotting it.
I am starting on the road to daft theories now.
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Old 16th Nov 2019, 7:22 pm   #6
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Default Re: RCA AR88D Comms Receiver.

That is worth try anyway a good bit of warmth.Guess HT is ok plus the oil filled caps have been changed/re stuffed regardless?
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Old 16th Nov 2019, 8:39 pm   #7
turretslug
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Default Re: RCA AR88D Comms Receiver.

Gang capacitor shaft grounding fingers making good contact? Fixed vane bracket screws tight (but not over-tight)? Full 6.3VAC at the 6J5's pins? Bit of straw-clutching but things to eliminate! Good luck.
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Old 16th Nov 2019, 8:49 pm   #8
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Default Re: RCA AR88D Comms Receiver.

Could it somehow be the mixer-stage presenting an excessive load on the osc?

I'd suggest pulling-out the mixer valve and seeing if things improve.
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Old 16th Nov 2019, 10:42 pm   #9
theredhouseinn
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Default Re: RCA AR88D Comms Receiver.

Yes G8JET all done. HT ok. Oil filled restuffed with 630v poly's.
Yes Turretslug. 6.3 ok. Var. cap sprayed and cleaned thoroughly, Will recheck fixing screws.
Yes G6. Dissed the 6.8pf from the osc anode to the mix grig. Osc running on it's own.
Tried pulling out the mix first, no change much, slight increase in amp.
John.
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Old 17th Nov 2019, 2:17 am   #10
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Default Re: RCA AR88D Comms Receiver.

If you changed everything around, go for the coil. It has probably lost its Q somehow.
I had the same problem on the next highest band in my Trio 9R59D. As last thing I did, I took out the coil and warmed and soaked off the remaning wax coating and bruched it with polypropyl alcohol.
It solved the problem. On the highest band I had to take off the winding and clean the former and rewind it with new wire, as the wax was stuck between the turns of the thicker wire, to make it work properly.
It’s not many turns on these higher frequencies, but count the turns and double check the total length of the unwinded wire. The tap seen in the diagram, can be the form of a link between the turns of the main coil or a soldered tap.

I think the Q-value of the coil is most important at the low end of the bands when the parallell capacitance is largest.
My oscillators were also fluttering when working higher up in frequency with less capacitance, due to the low Q. All caused by the Q-destroying wax coating.
There may also be some other resin coatings than wax that will deteriorate the Q by age.


I’m also thinking of that 10K anode resistor you changed. It maybe also had some self-inductance to help deload the oscillator at higher frequencies. Could be tested with a small inductance of arond 100uH in series with it.

Also see my thread on the 9R59 in this same section of the forum.
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Old 17th Nov 2019, 9:46 am   #11
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Default Re: RCA AR88D Comms Receiver.

There is a similar effect at the relatively low frequency of 50kHz in the small HT transformer in Tektronix oscilloscopes. At one stage in the late 50's they used epoxy potting, and that degrades over the decades until the oscillator fails as the scope warms up. Again a Q-destroying effect.

Like JohanBee says, the only solution (certainly for the Tektronix transformer) is to rewind it, and wax pot the rewind with new wax.

Craig
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Old 17th Nov 2019, 3:49 pm   #12
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Default Re: RCA AR88D Comms Receiver.

just a thought. Has the frequency changer/osc valveholder become conductive or leaky. I have experienced this a few times with paxolin holders in television RF units.
It takes more than just a few hours heating to deep dry oscillator/IF coils and particularly their formers and some never recover. You appear to have checked everything else. Good luck with it! John.
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Old 17th Nov 2019, 5:19 pm   #13
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Default Re: RCA AR88D Comms Receiver.

As built, all valves have decent quality ceramic IO sockets, so these should be OK, though there could always be a contact problem- but substituting the 6J5 several times should have cleaned the mating faces up assuming that the tangs haven't become so fatigued as to engender a problem that way!

The oscillator coil formers for the HF bands are of the-then new-fangled and low-loss polystyrene, with windings held in place by spiral grooves, so no need for wax on these, though the many turns of fine wire for the LF band coils do have a waxy coating. The polystyrene formers can go cloudy-looking though- perhaps micro-cracking which could be hygroscopic. Ceramic is also used for other components such as the bandswitch wafers and gang capacitor insulators, so they should be pretty blameless.

There is some black sleeving over much of the interlinking wiring around the switch wafers which could conceivably have deteriorated and gone leaky if in contact with chassis or metallic things- I had an intermittent ticking as a result of HT leakage to IF bandwidth switch screen because of this in an AR88.

All a bit of a puzzle, really- given the quality of most components in this area, it's one of those sets that should just run and run. Whilst the pink lozenge low-value caps can certainly be suspect, it sounds as though these have been changed. Keep us informed, there are probably a few AR88 owners both intrigued and concerned!

Colin
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Old 18th Nov 2019, 4:05 pm   #14
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Default Re: RCA AR88D Comms Receiver.

Hmmm, so what else is involved with the oscillator.....? Is it conceivably possible for adjuster core material to oxidise/corrode/be affected by water vapour and become very lossy? I also wondered if it was possible to substitute the VR150, just as an "edge of likelihood" elimination more than anything. If they age, or are allowed to stray outside the recommended operating current window, they can behave in peculiar ways including HF oscillation.
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Old 18th Nov 2019, 4:35 pm   #15
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Default Re: RCA AR88D Comms Receiver.

I only refurbed one AR88 and that was enough...…. David in Post #3 makes a good point that has possibly been overlooked? I disconnected and tested for leakage all of the "Micamold" caps inside the mixer-oscillator box. They are fiddly to get at but all of mine were leaky and were replaced. You mention that you've addressed the PIO bathtub jobs but what about the Micamolds? Cheers, Jerry
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Old 19th Nov 2019, 4:46 pm   #16
theredhouseinn
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Default Re: RCA AR88D Comms Receiver.

Thanks for the tips guys.
I had a metal oxide 10k at first, so I changed this for a good carbon 10k, no change.
I realise that the loading of the scope probe may cause problems so I unhooked the cathode bypass on the mixer and used this as a cathode follower to feed the scope and frequency counter, waveforms a bit distorted but ok to work the counter, still no change.
I will give it some more heat from the hair drier as I know that it has been in damp storage as the screened leads have mould on them. The wiring is in very good condition with no visual hardening or crumbling.
No hf osc on the 150v ht line, my scope goes up to 50meg.
All resistors and caps were changed before I applied the mains to it, I didn't want a facefull of molten wax and oil. Not the ones inside the i.f. cans yet.
I have not ruled out human error yet, but for the life of me I can't see what I may have done wrong.
I shall perservere.

John.
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Old 20th Nov 2019, 3:21 pm   #17
allan
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Default Re: RCA AR88D Comms Receiver.

The AR88D local oscillator is a 6J5 and uses a number of high value resistors which means capacitor leakage will have to be zero.
In case any are bad If you haven't done so fit new resistors at R4, R11 and R14 then new capacitors at C13 (=82pF) and C14 (220pF) and C53 (=6.8pF)
Different examples of the 6J5 metal/glass have varying stray capacitances which may also come into play.
Also check all the ground connections around the 6J5 in case one has oxidised and you're inadvertently adding a section of chassis or resistance into the oscillator circuit.
That range of RF voltages looks to be too great to me. Remember a scope probe may be not all it seems to be in terms of bandwidth.
Allan G3PIY
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Old 20th Nov 2019, 6:53 pm   #18
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Default Re: RCA AR88D Comms Receiver.

HI. I had something like this on my Ar88d.
Have you stripped out the two plunger caps C32 and C27 across the
Oscillator tuning coils, they get furred up with crud and kill the osc.

How I did it.
Put a signal generator on 27Mhz and set your dial to 27Mhz and lock it,
then you can remove (C32 BAND 6) to clean it.
After you put it back do not remove the lock until you tune C32 back to the 27Mhz.
If this works you should do them all.

Note1; the two plunger caps do have some interaction with each other so do one at a time.
Note2; they have to come out at the bottom of the receiver.
Hope this helps gezza123
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Old 21st Nov 2019, 4:30 am   #19
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Default Re: RCA AR88D Comms Receiver.

One good trick to avoid loading the oscillator under test is to clip the scope probe onto the outside of the insulation of a piece of wire in the circuit. It makes a fraction of a pF gimmick capacitor and if you wind up the scope gain you will still be able to see the oscillation (but the amplitude data will be meaningless) but at least it won't alter the circuit you are testing to any significant degree.

The fact it is bombing out at the HF end, means likely there are frequency dependent losses somewhere in the feedback circuit/pathway, assuming the main resonant circuit has not lost Q. Possibly a feedback pathway in the socket from plate to grid & phase cancelling the + feedback sustaining oscillations.

The main resonant circuit is easily checked at all frequencies if you break the feedback to stop the oscillator and feed a generator signal in instead.

Possibly mouse urine has contaminated the ceramic valve socket, that ionic compound won't clean off well with contact cleaner either, needs deionised H2O, soak for some hours, or replace socket.

Last edited by Argus25; 21st Nov 2019 at 4:37 am.
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Old 22nd Nov 2019, 5:06 pm   #20
theredhouseinn
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Default Re: RCA AR88D Comms Receiver.

Thanks for all the tips folks.
I have been through them all, no change.
During all this, one end of the coil broke off, damn it.
Rewound it, the original wire was 18swg, rewound it, 7 turns centre tapped with 18 swg, one strand 6mm power cable, wire pressed nicely into the former groove.
Switched on, no change, same as before.
one thing I do not have is a 3900pf mica cap. for the padder. I have tried various style caps. all produce varying results. The worst one was a polystyrene sufflex style cap, this one would not osc. at all. I now have on order a 3900pf silver mica cap.
I will proceed when this arrives.
john.
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