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Old 1st Dec 2019, 5:59 pm   #41
G6Tanuki
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Default Re: RCA AR88D Comms Receiver.

In the AR88 context it's worth remembering that the tuning-capacitor actually has two separate gangs for each stage, in order to give 'relatively sensible' tuning-rates across the entire set of wavebands.

My official RCA schematic from 2/16/1943 shows two oscillator-tuning capacitors - C49 [8-128pF] and C50 [10-370pF] - which one is used depends on the bandswitch position.

This presents the opportunity for a bit of divination - is the fault associated specifically with the circuit's use of one or the other of these capacitors?

Can you check that there's nothing 'compromising' the RF-insulation of C49 and its associated switching?
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Old 1st Dec 2019, 6:51 pm   #42
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Default Re: RCA AR88D Comms Receiver.

The 8-128 var cap is used on the bands 4, 5, & 6'
I have hard wired the coil direct to the anode and grid of the osc valve, leaving out the var cap and the switch, using fixed caps to tune the osc. Still have the fault.
All that I can see is left is the coil former material.
Today I heated up the osc compartment with the anglepoise lamp and chased the osc with the tuning control and from cold to quite hot I chased the osc from 30.5meg down to 29.5 meg on the scale. Something is very tempreature concious, but I don't have another one to compare.
Next step is to buy another one for spares.
Keep the theories coming, I will not give up!!
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Old 1st Dec 2019, 9:50 pm   #43
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Default Re: RCA AR88D Comms Receiver.

...just double checking that the valve socket was eliminated by trial with another socket, or when you did the altered hard wiring, was the osc valve still in its original socket ?
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Old 2nd Dec 2019, 11:03 am   #44
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Default Re: RCA AR88D Comms Receiver.

The valve socket has not been elimnated yet, but as the socket is of the same material as the wave change switch, I am asuming for the present that it is ok. My next step is to build a external osc assy using just the coil and all new components. I have now started working through the IF alignment proceedure to see if I have any more problems. The alingment proceedure is in the operating manual that came with the set.
This is the biggest refurb job that I have done to date.
john.
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Old 3rd Dec 2019, 12:23 am   #45
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Default Re: RCA AR88D Comms Receiver.

John... You did say that you tested the oscillator but did you check that the heater voltage actually at the valve pins in the AR88 was up to the mark and not low. Half a volt down and the feedback loop may not sustain oscillations when emission is low.

Allan G3PIY
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Old 3rd Dec 2019, 1:55 am   #46
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Default Re: RCA AR88D Comms Receiver.

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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
Not being too familiar with the AR88, seeing the chassis in this picture, I never thought RCA used a valve socket shown in this picture.
But then, I read a little further that the chassis shown is not an RCA.
Dave, USradcoll1.
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Old 3rd Dec 2019, 10:47 am   #47
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Default Re: RCA AR88D Comms Receiver.

Sorry Dave. My receiver is the rare Moreton Cheyney which I'm attempting to resurrect. It has a similar problem with the local oscillator to that in this discussion ie. it only works on one of its 5 wavebands and only partly on the that one.
The chassis is roughly the same size as the AR88.
I've been down the same route trying to get my 6K8 oscillating as the 6J5 in the AR88 in this thread. Like 2-heads are better than one... 2 receivers may be better than one or a trouble shared is a trouble halved.
I'm now suspecting a bad earth contact. The 6K8 earthing point is a solder washer at the valveholder fixing screw but that is into a brass post secured to the steel chassis by another screw with some corrosion throughout. Lack of oscillation may be due to a combination of several minor problems.
Allan G3PIY
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Old 3rd Dec 2019, 12:39 pm   #48
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Default Re: RCA AR88D Comms Receiver.

As far as I can recall, all AR88 valve holders are white ceramic, mounted with the top surface about flush with the top of the chassis. If the tines are still springy and there's not muck or arc-tracking on the surface of the ceramic, they ought to be OK.

David
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Old 3rd Dec 2019, 1:03 pm   #49
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Default Re: RCA AR88D Comms Receiver.

Mine are all ceramic for certain.
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Old 3rd Dec 2019, 1:41 pm   #50
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Default Re: RCA AR88D Comms Receiver.

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If the tines are still springy and there's not muck or arc-tracking on the surface of the ceramic, they ought to be OK.

David
Generally I would agree. But many ceramic valve sockets have a flat porous surface, not a glazed ceramic surface, though some types have that.

If it is the former porous type, I would be very suspicious of the socket (which has not been eliminated yet) for the reason that there was a Mouse nest in the radio.

If a Mouse urinated ( that is far too polite, pissed is better) on the valve socket, it would have absorbed into the porous ceramic and could easily create a negative feedback pathway between the anode and the grid, not only upsetting the DC conditions of the valve, which should be running in a negative grid bias mode due to rectified grid current, but also from the AC perspective, phase canceling some of the + feedback that sustains oscillations.

That is why the socket should be eliminated from the investigation in a manner that a Police Detective would: Assume nothing, believe nobody (especially the Mouse) and check everything.

It may well turn out to be a fanciful notion that a Mouse was responsible, but so far, everything else has led to a dead end. So at least the valve socket needs eliminating from the investigations.

Of course if something gets urinated on it might well increase its value, I'm sure I remember an episode of the Young Ones where Neil remarked that a speaker cabinet was valuable because Jimi Hendrix once pissed on it.
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Old 3rd Dec 2019, 2:12 pm   #51
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Default Re: RCA AR88D Comms Receiver.

Well,that is one way of increasing the value of our collections!
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Old 3rd Dec 2019, 4:43 pm   #52
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Default Re: RCA AR88D Comms Receiver.

As a fellow AR88 user, I have been following this thread with interest.


Have I lost the plot?


I thought the OP had found back in post #30 that the problem was due to (damp affected?) cores in the oscillator coils..
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Old 3rd Dec 2019, 4:49 pm   #53
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Default Re: RCA AR88D Comms Receiver.

Two sets with similar faults running here. One is the AR88 the other is something different.
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Old 3rd Dec 2019, 5:04 pm   #54
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Default Re: RCA AR88D Comms Receiver.

Because the covers are on, the mouse could not get near the base.
I checked the cores, and there was some residual magnetism present, I connected a coil of 28swg across the mains and dipped the cores down the center, result, magnetic flux zero.
Made no difference to the osc..
If I push a ferrite rod down the coil I can tune down to the LF end, no problem.
I am going to try and cut a piece of rod and glue a piece of studding into the end and see if I can eliminate the slug from the equation.
Meanwhile, yesterday, I went through the IF alignment proceedure. It was a long way out, but tuned up ok, giving me a 20dB increase in gain. 20 micro volts into the grid of the mixer gave me an audible signal at the detector. This leads me to think that the 680pf's may be failing in the cans. The crystal phasing was way out, but adjusted ok.
By setting the genny to FM and adjusting the deviation to match the IF bandwidth giving me a perfect envelope on all 4 positions of the bandwidth switch, no adjuustment necessary. I hope.
I have ordered some IO valve bases to construct an external osc cct..
One day I might actually get to listen to some stations on this.
John.
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Old 3rd Dec 2019, 5:25 pm   #55
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Default Re: RCA AR88D Comms Receiver.

Quote:
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Because the covers are on, the mouse could not get near the base.

Ha! - that was what I thought, but the little blighters had got in behind the back apron and eaten most of the power supply wiring. I now leave the extra rear cover plate fitted permanently.

PS Like the idea of using and FM signal generator as a wobbulator. Brilliant!
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Old 3rd Dec 2019, 6:49 pm   #56
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Default Re: RCA AR88D Comms Receiver.

I cut off a piece of ferrite rod, same length as dust core, no change, so dust core looks ok.
Because the valve base pins 4 & 6 are used as stand offs for the anode and a screen supply, I lifted the components off into mid air so no connection to the valve base pins, no change. Down now to the valve base and coil former. I have some short wave coils on paxolin formers on their way to me in the next day or so.
Thanks keith, for the praise on the genny idea.
John.
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Old 3rd Dec 2019, 10:12 pm   #57
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Default Re: RCA AR88D Comms Receiver.

Quote:
Originally Posted by theredhouseinn View Post

Meanwhile, yesterday, I went through the IF alignment proceedure. It was a long way out, but tuned up ok, giving me a 20dB increase in gain. 20 micro volts into the grid of the mixer gave me an audible signal at the detector. This leads me to think that the 680pf's may be failing in the cans.
John.
It is heartwarming to read remarks like this, in that clearly before you tried re-alignment you made a gain measurement. One major error that often gets made in receiver alignment protocols is that the performance is not documented first before the alignment is altered. So in the end the person doing it has no idea if they have improved the performance, not changed it, of perhaps made it worse than it was. So Well done !

One clue if its the capacitors in the cans, is that each stage should have a similar sharp peak, of one has become broad its a little suspicious.
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Old 4th Dec 2019, 2:00 am   #58
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Default Re: RCA AR88D Comms Receiver.

Greetings all.
I have been watching this thread with a great deal of interest as I have a very early model AR-88LF sitting on my bench at the moment. I also have another AR-88LF waiting for restoration and an AR-88D that is fully restored but just needing alignment.
I do not have a sweep generator to do an effective IF alignment on my D model and I have been using it for communications with the IFs just peaked. It works well but lacks the famed audio bandwidth.
I was interested in Johnís use of a sig gen set for FM with a suitable deviation to align the Ifs.
Hadnít thought of that one! So simple!
I really need to go play in my workshop but need to get rid of other jobs first.
Thank you all for this thread and I wish you all the best John with your AR-88.

Robert.
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Old 4th Dec 2019, 9:22 am   #59
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Default Re: RCA AR88D Comms Receiver.

It will be interesting to ultimately find out why the oscillator is failing, since most of the obvious suspects have been eliminated and we know that it once worked properly.

Hopefully it will be "one defect" responsible.

One horrible outcome that can occur in a case like this, is a collective accumulation of small losses that conspire to give the defect, like dielectric losses, core losses , low gain or reduced transconductance in the valve, slightly low heater voltage etc etc. It's like the remark where someone says: "That's a lie" and the response is "No, its just an incremental accumulation of half truths"

Then in that case its hard to know what to target for a definitive repair, so I hope it turns out to be "one thing"
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Old 4th Dec 2019, 10:49 am   #60
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Default Re: RCA AR88D Comms Receiver.

Bearing in mind the age of the AR88 there could be dozens of minor problems all conspiring to defeat the repairer. It's ages since I last overhauled one of these (I have 4 at last count but they're so heavy and cumbersome to work on they're not very inviting). I'd recommend removing the capacitor C119 at the anode of the audio output valve because when this fails it can open circuit the primary winding of the output transformer.

Allan

Last edited by AC/HL; 4th Dec 2019 at 1:55 pm. Reason: OT aside edited
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