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Old 23rd Nov 2019, 10:30 am   #21
allan
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Default Re: RCA AR88D Comms Receiver.

Have you tried a 6J5G or GT rather than a metal type John?
I had a similar problem to yours with a DST100. This used a glass 6J5 with a metal can so I tried the all metal valve and it stopped oscillating like yours.

I also had trouble with my RA17. The EF91 oscillator cut out mid-band and I had to try others until one worked OK but the original was just slightly down on emission.
A third instance was in an R1132. This turned out to be bad earthing but VHF oscillators are more prone to this but... the AR88 tuning condenser might have a dirty contact forcing the RF current to adopt a different path. If you use switch cleaner do not get it between the vanes.
Finally there's always an option to use switch cleaner on the wavechange switch which might be resistive.

Padders are a good candidate to blame but I must say I've never come across a bad one.

Can you alter the HT to see if higher or lower voltage improves or makes matters worse.
Also.. can you measure the LT voltage with an RMS meter at the 6J5 pins in case this is low which will reduce emission and gain?

Allan G3PIY
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Old 26th Nov 2019, 10:16 am   #22
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Default Re: RCA AR88D Comms Receiver.

Many thanks for all the tips guys.

My friend, a radio ham across the road lent me his 6J5 form his AR88, and a spare glass GT type, no change, still the same fault. TheGTt showed low emission on the MHSVT, but still worked in the set.

I hard wired the coil and tuning cap to the valve base missing out the switch, fault still there, switch OK. As the fault only occurs on the top 3 bands, the only common part left is the tuning cap. HF section.

I am now going to try replacing the tuning cap with a selection of fixed caps across the range, this should eliminate this part. I will also try a small var. cap, on order, when it arrives.

If I have missed anything, please keep shouting.

John.
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Old 26th Nov 2019, 12:45 pm   #23
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Default Re: RCA AR88D Comms Receiver.

Next phase.
I have disconnected the var cap, tried various caps to tune, still have the fault.
I put the var cap on the bridge and it varies from 20pf to 140pf, the cct says 8pf to 128 pf, no problem here then.
I have cured one intermittent fault on range 5, the padder was intermittent when flexing the lead at one end, the capacitence changed from 3000pf to 300 pf.
The only common factor that is left is the coil former material that is killing the Q at LF.
the 3 high band formers are different to the 3 lower band formers.
Does anyone have any more theories?
John.
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Old 26th Nov 2019, 1:00 pm   #24
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Default Re: RCA AR88D Comms Receiver.

Here's the circuit diagram for those following the thread.
If the osc cuts out at the same point on the dial could it be a short in either C49 or C50, whichever is in circuit or the grounding tab going high resistance?
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Old 26th Nov 2019, 1:26 pm   #25
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Default Re: RCA AR88D Comms Receiver.

A longshot, just wondering if the composition of the 10k anode resistor has anything to do with it.

Lawrence.
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Old 26th Nov 2019, 2:30 pm   #26
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Default Re: RCA AR88D Comms Receiver.

No Allan, if I tune the coil core it cuts out in a different place on the dial. Also I have checked the VC on the bridge and it is smooth from low to high, I also checked the other 3 gangs and they are identical. The grounding connections are all perfect.

No Lawrence, I have tried both metal oxide and carbon composition, no change.
Remember the other 3 low bands are perfect, it is only the 3 bands that use the nylon type formers that are the problem.
Winding a new coil on a new former is my next job.
John.
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Old 26th Nov 2019, 3:06 pm   #27
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Default Re: RCA AR88D Comms Receiver.

More info.
You could say that this is the drowning man clutching at the last straw.
I have measured the resistance at the point in the center of the tuning core slug to chassis of all coils.
The resistance of the 4 slugs of the osc coils bands 3,4,5 & 6 measure greater than 2meg
The resistance of all other slugs including the RF ones measure between 40k and 250k.
Although the band 3 osc runs over the whole band the amplitude is greatly reduced.
Has anyone out there got an AR88 to measure the slug resistances?
My pal is going on holiday this week so it will be 2 weeks before I can get to his AR88.
I used my Avo 8 on the high res range.
Any comments on this?
john.
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Old 27th Nov 2019, 1:27 am   #28
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Default Re: RCA AR88D Comms Receiver.

The former composition must have changed making it conductive.
Hygroscopic perhaps?
Allan
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Old 27th Nov 2019, 11:14 am   #29
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Default Re: RCA AR88D Comms Receiver.

Not necessarily a former leakage problem, the adjustment cores do not touch the inside of the former as adjustment is by a fine-threaded brass rod passing through the former's brass securing boss and tensioned by a steel spring- thus adjuster electrical contact with chassis is good.

I'm working at the moment, otherwise I'd be happy to invert the chassis and get the meter probes out!
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Old 27th Nov 2019, 12:50 pm   #30
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Default Re: RCA AR88D Comms Receiver.

I have taken the 3 slugs out of band 4, 5, & 6 coils.
Using an old MW ferrite rod the same dia, sliding it in from the underside end of the coils I can get perfect tracking over the 3 bands end to end with a good waveform.
The ferrite rod material looks different to the slug material. The ferrite rod has no measureable resistance, even with the probes mm's apart.
So, we have a problem with the coils. The formers and / or slugs have deteriorated due to damp. My experiments at the moment seem to suggest that the former's are ok, and that something has happend to the slugs.
If Turretslug could measure his slugs and post the results I can take it from there.
john.
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Old 27th Nov 2019, 3:09 pm   #31
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Default Re: RCA AR88D Comms Receiver.

Sometimes in damp conditions a slug can rust and jam in place then get broken when being adjusted. I've had to replace several in my Moreton-Cheyney receiver IF coils. Being only 465KHz mine shouldn't be too critical.

Is it possible someone removed the slugs in your AR88 and either replaced them in the wrong coils or fitted some with the wrong characteristics from another equipment?
Allan
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Old 27th Nov 2019, 5:45 pm   #32
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Default Re: RCA AR88D Comms Receiver.

Hi Allan.
Apart from one cap replaced in the rf section, 1950's era this one was untouched. All covers are in place with the two tuning rods clipped on.
I heated up the slugs on the iron and the res went down to 50k. When they went cold two went back to 2meg and one stayed at 50k.
I measured the slugs in my friends set today and band 1, 2, & 3 slugs measured 50k, the 3 slugs in 4, 5 & 6 coils mesured > 20meg.
I think I have got a definite problem with the slugs.
John
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Old 27th Nov 2019, 5:52 pm   #33
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Default Re: RCA AR88D Comms Receiver.

A pulse of current through the coil can kill some types of ferrite as can a permanent magnet brought too close.

https://www.vintage-radio.net/forum/...4&postcount=56
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Old 28th Nov 2019, 8:49 am   #34
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Default Re: RCA AR88D Comms Receiver.

It is essential to find out if the tuning slugs are made from powdered iron or from ferrite.

They have different characteristics. My guess is that they are made of powdered iron.
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Old 28th Nov 2019, 9:49 am   #35
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Default Re: RCA AR88D Comms Receiver.

Your guess could be right.
They do look different, the slugs are black, and ferrite is a dark blue colour.
Also ferrite has a greater magnetic attraction.
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Old 1st Dec 2019, 1:33 am   #36
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Default Re: RCA AR88D Comms Receiver.

Quote:
A pulse of current through the coil can kill some types of ferrite as can a permanent magnet brought too close.
I've seen this first hand, I was once working on a Pye Westminster and accidentally shorted the collector of one of the receiver RF amp transistors to ground, pulling a lot of current through the coil. The receiver sensitivity dropped by more than 10dB and only recovered when the ferrite slug was replaced.
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Old 1st Dec 2019, 8:14 am   #37
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Default Re: RCA AR88D Comms Receiver.

Quote:
Originally Posted by AlanC View Post
Quote:
A pulse of current through the coil can kill some types of ferrite as can a permanent magnet brought too close.
I've seen this first hand, I was once working on a Pye Westminster and accidentally shorted the collector of one of the receiver RF amp transistors to ground, pulling a lot of current through the coil. The receiver sensitivity dropped by more than 10dB and only recovered when the ferrite slug was replaced.
In the case of powdered iron and ferrite, they have really low magnetic retentivity, meaning a very poor memory for a strong applied magnetic field. Even it they did, you could not permanently damage them anyway, you could always degauss them with a TV de-guassing wand.

I'm skeptical that the slugs are the issue here, I think there is probably something else going on that has not been identified, but trying new slugs will help confirm it.
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Old 1st Dec 2019, 11:54 am   #38
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Default Re: RCA AR88D Comms Receiver.

I'm currently overhauling my Moreton Cheyney receiver. It uses a 6K8 mixer and a set of standard Wearite coils covering 5 wavebands.
The local oscillator works on only part of the medium waveband.
The HF end 800KHz to 1600KHz (osc=1265KHz to 2065KHz) works with the higher RF voltage having a greater value and completely fizzling out by 1265KHz. None of the other 4 wavebands have any local oscillator output.
The 6K8 voltages all look OK. As the receiver was missing its mixer it could have been an X63/6A8 or similar but trying that shows it performs exactly like the 6K8.
None of the coils uses a dust core. Changing the HT voltage from 150 to 300 hardly makes any difference. Clearly another case of nonoscillatoryitis.
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Old 1st Dec 2019, 1:13 pm   #39
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Default Re: RCA AR88D Comms Receiver.

Quote:
Originally Posted by allan View Post
None of the coils uses a dust core. Changing the HT voltage from 150 to 300 hardly makes any difference. Clearly another case of nonoscillatoryitis.
Allan G3PIY
Yes, its a little hard to blame the dust iron cores when there are none.

Obviously to sustain oscillations there has to be enough + feedback and enough gain.

One wonders what could have changed over the years. In the absence of dust iron or ferrite to blame then;

Assuming:

1) the valve is good and not gassy (all of our valves are slowly letting in gases you know).

2) the resistors are fine.

3) the capacitors are ok

4)the applied voltages (HT & heater) are normal.

5) the Q of the tank circuit is normal and its not losing excessive energy.

.... then it must either be some losses in the dielectric/insulation materials of the supporting tags/sockets have increased, or some other pathway conducting signal has developed which is phase canceling some of the + feedback or absorbing energy somewhere.

Thinking along the line of dielectric losses it is known to happen with some types of wax impregnated coils.

Of course if the permittivity Ko and permeability Uo of space had altered since the radio was built, we would know by now because the speed of light would have altered since it equal to root (1/KoUo), fouling up everything, so it cannot be that, joking of course !
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Old 1st Dec 2019, 4:11 pm   #40
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Default Re: RCA AR88D Comms Receiver.

I've now discovered the mixer isn't fully home in its socket. A couple of other valves are the same leaving a 5mm gap between the base and the socket surface. Because the mixer socket is mounted 30mm below the level of the chassis I can't push it too hard but whatever is stopping it going home is pretty solid. I had to change one socket already because the contacts had sprung open and weren't gripping the valve pins.
The sockets are cheap and nasty (what we used to refer to as "muckite") so I'll fit a standard black IO socket at the mixer for starters (top right in picture).
My stock of 6K8/X65 etc are all in very used condition so I wouldn't be surprised for most to have poor emission or a bit gassy.
Allan
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