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Old 7th Jan 2020, 12:58 pm   #141
theredhouseinn
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Default Re: RCA AR88D Comms Receiver.

Thanks for the pics Gezza.
For the purpose of identification, the bottom of the coil is the end that is fixed to the chasis and the top is the end open to the atmosphere.
Have a close look at Gezza's pics. The top of L56 goes to the trimmer, and the anode wafer, the top of L55 goes to the grid wafer. When I first installed Rob's coil, 3.5 & 8.5 I was tuning the 8.5 section, this explaines why it was oscillating at 11meg, instead of 22meg. All tops went to the grid wafer. I now have this with the top connection to the anode wafer and the bottom connection to the grid wafer. I can now tune to 22meg at LF and 32meg at HF, almost.
I had to take out the 75pf across the trimmer to get it to oscillate. I have an air spaced trimmer connected across the trimmer to try and find the optimum value to get the tracking to 22meg and 32meg.
If I can get this done then I will go back to the original 3.5 & 3.5 coil with the reversed connections and see if I can get the same results.
John.

Last edited by theredhouseinn; 7th Jan 2020 at 12:59 pm. Reason: errors
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Old 7th Jan 2020, 1:30 pm   #142
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Default Re: RCA AR88D Comms Receiver.

You are most likely right Allan,I might just do a Gerry Wells and put full mains on.
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Old 7th Jan 2020, 1:53 pm   #143
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Default Re: RCA AR88D Comms Receiver.

Quote:
Originally Posted by theredhouseinn View Post
When I first installed Rob's coil, 3.5 & 8.5 I was tuning the 8.5 section, this explaines why it was oscillating at 11meg, instead of 22meg.
John.
I thought that might be the case as the 8.5T coil seemed too long for the tuned band and 3.5T more realistic.

Maybe though, some of these coils had 8.5T rather than 3.5T for the feedback winding to help sustain oscillations. So it might be that the design was "borderline" from the start and that could explain why they altered the coils along the way.
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Old 9th Jan 2020, 6:49 pm   #144
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Default Re: RCA AR88D Comms Receiver.

Latest results.
I have tried 3 coils on my osc test rig, various turns ratios, various cores, various caps on anode and grid. Using the 3.5 & 3.5 turns ratio, 220 pf caps on anode and grid, FM TC with trimmers I can get it to osc anywhere between 16meg and 42meg. Min to max cap on the TC.
So, there is still a fault on the main chassis.
Strap a coil and TC to the grid and anode caps on the chassis, Osc from 16meg to 42meg. Getting close now.
Bypass the band switch, no go osc dies at 27meg going down.
Bypass the trimmer and fixed cap across it. Still no good.
Diss the wire from the TC and strap in my FM gang, 42meg down to 14 meg.
There is somthing wrong with the TC.
Moving vanes to chassis, 0 ohms over the range.
Fixed vanes to chassis, greater than 20meg ohms.
cap reading on the bridge, 18.5 to 138 pf. CCT says 8 to 126 pf, could be long leads on the bridge.
It is though there is a dilectric failure in the TC, but it is air spaced.
Has anyone got any theories before I remove the TC from the chassis?
I have finished going round in circles.
John.
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Old 10th Jan 2020, 4:13 am   #145
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Default Re: RCA AR88D Comms Receiver.

Just maybe:

Drawing out the circuit, it relies on their being absolutely no losses in the connection between the padder capacitor's (C17) ground and the TC's ground. If there was, this loss would be effectively in series with the oscillator tank circuit. Connect a heavy conductor between the padder capacitor's ground connection and the TC's ground as an experiment. The circuit config also relies on a perfect padder that has zip ESR, if there is something wrong with the dielectric there it would also cause it, but I think you have replaced that by substitution, but of so, did it have the same earth connection ? because it could just as easily be that as the padder cap itself.

If its not that maybe something has happened to the insulators that support the stator vanes on the TC, even though the ohm meter cannot measure it.
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Old 10th Jan 2020, 11:27 am   #146
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Default Re: RCA AR88D Comms Receiver.

Is it worth doing a high voltage insulation test on the tuning cap? I’m thinking of something like metal whiskers which look open circuit at low (ohmmeter) voltages, but break down at valve oscillator voltages.

Stuart

Last edited by stuarth; 10th Jan 2020 at 11:27 am. Reason: Typo
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Old 10th Jan 2020, 5:08 pm   #147
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Default Re: RCA AR88D Comms Receiver.

Today.
Measured from the top of the fixed vanes to the wire that comes through the chassis to the switch, 5 ohms on the avo.
In the process of taking out the gang assy.
Suspect the screw connections that fix the tag to the vane assy mounting.
I remember cleaning the top screws, should have checked the bottom at the time, silly me. Convinced myself that the bottom screws were ok, so I needn't take out the assy..
Taking the gang asy out is a major job, going to take a few days.
I will put the gang on my test rig before remounting it on the chassis.
John.
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Old 11th Jan 2020, 12:50 pm   #148
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Default Re: RCA AR88D Comms Receiver.

Removed the gang assy.
It came out quite easy.
Disconnect the switch connections, cut the earthing braid, remove the filter cap box, 4 grub screws on the flexi coupling, and slide along the gang shaft, 4 screws at the front of the cradle, one screw at the rear of the cradle, lift of the cradle from the chassis. Take out the black head screws holding the gangs to the cradle, unsolder the rear RF gang support from the side of the cradle, lift out the gangs.
I measured the res between the bottom tag to the top of the gang before starting work, measured 10 ohms. Removed the tag, filed it, and the screw clean, refitted, 0 ohms.
I am going to clean up the rest of the tags, even though they show 0 ohms.
John.
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Old 11th Jan 2020, 6:47 pm   #149
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Default Re: RCA AR88D Comms Receiver.

I am sure you will sort this John, you don't give up easily! Good luck!
Rob
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Old 11th Jan 2020, 7:15 pm   #150
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Default Re: RCA AR88D Comms Receiver.

The phrase is,like a terrier with a rat!Pretty sure you must be very near now.
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Old 12th Jan 2020, 5:40 pm   #151
theredhouseinn
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Default Re: RCA AR88D Comms Receiver.

Soldered new leads and braid to the TC. All tags cleaned, fitted back into the tray.
Connected the TC to my osc rig, and with the 3.5 & 3.5 turns coil it tunes from 20meg to 34meg, good enough. I also connected the rig to the next RF gang down and it performed the same, so I think that this is good enough now to remount in the chassis. Easier said than done.
I have had to order a 200w iron to clean and solder the braid to the earth tags, they take a lot of heat.
John.
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Old 12th Jan 2020, 7:27 pm   #152
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Default Re: RCA AR88D Comms Receiver.

That certainly is a large iron.
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Old 13th Jan 2020, 11:36 am   #153
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Default Re: RCA AR88D Comms Receiver.

A couple of comments.
I use a variable HT supply to check old receivers for the first time (with no valve heater supply). Often you get a pretty high initial current at say 50 volts which drops steadily, then you can increase the voltage keeping an eye on HT current. Bad wax condensers will get warm and sometimes one might suddenly fail with a sudden increase in current followed by a puff of smoke.
There appears to be two types of degradation. One where the leakage increases with voltage ending with an avalanche effect, and the second where leakage drops and then remains low.

I'm now at the stage of aligning my old Moreton Cheyney receiver and meeting much the same issues as John. Some RF coils do not seem to peak or will peak intermittently. Also, I found the local oscillator on two HF ranges stopped working at the same point on the dial. That was a bent plate on the tuning condenser. One TC is used for LF and MF bands and a smaller TC for the two HF bands. Once I started hearing broadcasts they were intermittent. I found corrosion at every point on the tuning condensers that relied on metal-to-metal contact. I also found the (8) mounting screws were all loose as well as rusty and no earthing braid provided. The slow motion drive is too complicated to allow removal of the two tuning condensers but a new can of switch cleaner is working its magic.
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Old 19th Jan 2020, 3:25 pm   #154
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Default Re: RCA AR88D Comms Receiver.

Mounted the TC back in it's frame with new solid wire and braid to the tags on the TC.
Carefully positioned the frame back on the chassis, feeding the leads through the holes as I went. refitted the 4 front screws and the 1 rear screw, slid the coupling into place, aligned the TC and the scale, and tightend the 4 grub screws.
I finally settled for a Sealey 100w iron, as this fitted between the switch wafers. I had to move 2 trimmers to one side in the RF stages to get the iron to the earth tags.
All 8 braids now soldered to the earth tags. The fixed vane wires soldered to the switch tags, this was tricky, but just made it with the Weller iron.
Refitted and connected the smoothing cap block back to the chassis.
Checked all connections have been made to the correct tags.
Switch on and let it warm up for 20 mins.
Went through the alignment proceedure band 1 to 6, perfect tracking and good osc. waveform on all bands. I think I have cracked it at last.
Now for the IF coils.
Removed the final IF before the det. there are 3 180pf's in this can. They are the pink types. I checked them on the bridge, (Marconi TF2700), the values were quite close, within about 20pf or so, measured them on the Avo, no indication of any leakage >20meg.
However, on the bridge I had to use the loss adjust control between 3 and 7 to get a balance on the 3 caps. Three new caps I measured the loss adjust control was on 0 for all 3. I asumed that all was not well with the original caps., so I fitted the new ones as I had the can out.
Replaced the IF can, fed in the 455kc/s aligned ok.
I am now going to check the rest of the IF TX's.
Best of luck on your Moreton Cheyney receiver Alan.
John.

Last edited by theredhouseinn; 19th Jan 2020 at 3:28 pm. Reason: Small addition.
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Old 26th Jan 2020, 4:16 pm   #155
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Default Re: RCA AR88D Comms Receiver.

I checked the BFO signal, very weak. Anode voltage just under 20v, no good
removed the can, and both resistors high in value, replaced these, and the 4 pink caps while I was at it. BFO would not osc without the scope probe on the anode. Replaced the 6J5, good beat when BFO selected.
Got 680pf's on order for the IF's, Will then do a complete alignment of the set and do a final report on this thread. (Assuming no more problems)
John.
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Old 30th Jan 2020, 4:21 pm   #156
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Smile Re: RCA AR88D Comms Receiver.

Success at last.
I removed the IF cans one at a time, measuring the improvement in gain each time.
At the start, for 40uv input at the grid of the first det. I got 40mv p to p at the final det.
When I had finished I had 200mv p to p.
All the caps across the coils were the pink variety. Although they were not far out of tolerance, and no leakage, I had to use the loss adjust control on the bridge to get a null on all of them.
A final signal to noise check on all bands is better than 6dB with 10dB below 1uv input.
I can't prove this as I don't have the gear to go that low, so I have to believe the attenuator on the sig gen.
I can pick up stations around the world on a bit of wire in the workshop.
Now to get the long wire up between the trees.
Many thanks to you all for the help and sugestions that you gave me to enable me to succeed.
John.
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Old 30th Jan 2020, 5:02 pm   #157
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Default Re: RCA AR88D Comms Receiver.

Well done and excellent work looks lovely.
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Old 31st Jan 2020, 2:16 am   #158
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Default Re: RCA AR88D Comms Receiver.

Hi.

Whilst I have been in no way experienced enough to be able to contribute in any way to your thread, I have been following it with great interest and intrigue.

Well done John, that must have been a heck of a challange, hopefully you will now be able to enjoy many, many years of fun and use to come.



Paul
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Old 31st Jan 2020, 10:08 am   #159
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Default Re: RCA AR88D Comms Receiver.

Congratulations.
I'll start my AR88 refurbishment next John, although none of mine look as good as yours.
If you're going to put up a long wire I recommend trying an "unun". I made one a few weeks ago and it works really well. It's an auto-transformer wound on a toroid with about 9:1 step down. The larger the toroid and the more turns of wire the better the LF performance, and adding a ground wire to a metal plate stuck in the soil adds extra performance. Lots of constructional details around and I put mine in a small plastic box. It took 30 minutes to make it. I might try a 16:1 version next for VLF.
I think I have three AR88LF and one AR88D. See if you can spot them.
http://www.radiomuseum.co.uk/plans.html
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Old 31st Jan 2020, 11:26 am   #160
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Default Re: RCA AR88D Comms Receiver.

That Alan is one serious collection and like me you will need another life time to restore.

Looks like a very full garage.
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