RCA AR88D Comms Receiver

[frank rabbets]

Thanks for replies. I am changing the 2 caps and take your advices about changing just those components needed. I have thought about leaving the set in Rec mode and using the
wall socket switch to turn on and off. Perhaps it will work not too badly when I get a good aerial up.

Thanks very much indeed. I will post a pic of a typical Solar cap.

[Radio Wrangler]

Sounds good!

If you just pop back to the photo in post number 27, the lower capacitor being pointed to, the doubled-up pair... those are the dreaded 'micamold' type. they're in the shape normally associated with mica capacitors (usually nice and reliable) but for economy in wartie reasons these were paper types designed to be used in place of mica. Like any paper capacitor, they are susceptible to water ingress.

The shiny block thing just going off the right edge of the picture is one of the 'Bathtub' paper capacitors. They are filled with oil and as they age, some leak. In other cases the rubber seals (bright red) break down chemically and mix with the oil, leaking out as red goo. If you remember the old "Red Hermetite" gasket compound, it looks like that. That one looks OK. Your chassis looks good and clean. There's another bathtub at bottom left with just one seal in the photo. It too looks OK.

David

[Craig Sawyers]

Not sure if there is anyone from the National Museum of Computing (on the Bletchley site) on the list, but they are past masters with the AR88 - they have a period rack full of them, all on-song, all restored by the volunteer team there. They have another on the bench right now going through restoration.

Could be worth giving them a call.

[frank rabbets]

Morning chaps,

I've put a 50 metre single wire aerial up 20 feet average above the ground and the AR88D works well on all channels apart from Number 5. I wonder if someone could tell me where to start looking for the fault please. There is a slight hissing from the speaker.

Stripped and rebuilt the gearbox. Pictures to follow.

Thanks.

[Radio Wrangler]

There is a weak point in the AR88 gearbox. In all but the earliest sets, the gear on the capacitor driving shaft which operates the tuning scale is only a sector, not a full circular gear. It's also a sprung pair to remove backlash.

These sector gears are prone to cracking and breaking. Unfortunately people tend to spin the flywheel tuning from end to end, crashing it into the end-stops, which is probably when the damage happened because the end-stops are implemented via these gears.

David

[ms660]

If nothing on band 5 then I would check the relevant switch contacts and coils in both the RF and Osc stages, also check first for -ve grid volts on the Osc's grid.

The gearbox can suffer from backlash if worn, most noticeable when trying to tune CW/SSB signals, sometimes increasing the offset of the anti backlash gears might improve matters but if it's beyond it then it's beyond it, I used to own a fully working AR88D but it wasn't my favourite receiver, give me an HRO any day.

Good luck with it all though.

Lawrence.

[frank rabbets]

The blank 5 band was nothing to do with the gearbox. Two unrelated items sorry.

I stripped the gearbox to clean and adjust it. The big wheels were stuck together so did not clamp the pinion teeth sufficiently. Cleaning and thin oil cured this. Then I turned them one tooth against their springs and clamped each pair together with clamps I made. Two teeth is no good as the wheel springs fly out. That way I could assemble the gearbox with the wheels loose to adjust the spindle bearings properly. They WERE overtightened. There is a fault in all the AR88 gearboxes which I found. If you turn tuning knob round until the output wheel stop is engaged and the clutch jut slips you will see one of the gearbox plates bends to an alarming extent. So much in fact that the spindle ball nearly falls out. I think they were "overtightened" so that this bent the plate in the "to be" bent position so that the ball could not fall out. Both my 88's are like this. Anyway I just put minimal preload on the grub screws and the ball just stays in place. The grub screws are hardened and brittle so I had to make and harden a replacement. I would never scrap a gearbox even if I had to cut new wheels.

P.S. when all together and adjusted I pulled off the 4 clamps so the wheel teeth sprung in to position and clamped the pinion teeth.

[frank rabbets]

My approach to bath tub repacking.

[frank rabbets]

Ah...........sorry for brief post. No picture shown but it obviously shows.

It was easy to slit the back of the bath tubs and bend the back over. Then I drilled central holes in the connectors to feed the one wire through from the caps. I like the original look and the original can provides an adequate mounting. Each bath tub restuffing improved overall performance.

Now I have a big ask please. Would someone kindly tell me what resistors to check? I have bits of information but no complete list.

I have a spare AR88D for parts. I cracked open a capacitor which had "MICAMOLD" embossed on it. Well it's a nicely made Mica one so it does not seem logical just to replace them all, but how to tell the difference.

Scale 5 is empty with no hiss from the speaker. I have a friend who says he can trace the fault and says he understands the circuit diagram and realignment !! So he's borrowed the booklets to study.

I have an original output transformer which I shall change as a last job. Should I run it with 2 x 4 ohm speakers in parallel ?

I picked up an Australian station Sunday a.m. in UK. They changed to speaking Chinese as their verbal introduction described them as serving China. Would that transmitter be in Australia? If so quite impressive reception in UK.

Thanks for eveyones help.

[ms660]

With resistors it depends on ones approach, some would replace the lot, some would replace the usual suspects.

Most can be checked in circuit with the receiver cold.

Screen feed resistors (RF/IF etc) would be worth checking, so would resistors in the AGC line, also check the three resistors in the bias chain that's in the HT-ve line, some of them have been known to change value and give problems.

"Micamoulds" I've never opened them up, I just changed them.

The manual I have says 2.25 ohms load for the output transformer so two 4 ohm loudspeakers in parallel would be ok if you wanted to.

Good luck.

Lawrence.

[Silicon]

The Impedance of a speaker is not the same as the Resistance.

A standard 8 Ohm HI-Fi speaker often has a DC resistance of 6 Ohms.

Some service manuals don't make it clear if they are referring to impedance or DC resistance.

[ms660]

Quoted as impedance in the AR88 Manual (2.5 ohms)

Lawrence.

[Kevin Hoyland]

Hello Frank.
Keep the good work up and give us an up date on the Ar88.

Regards Kevin.

[frank rabbets]

Hello Kevin and my other friends on here to whom I'm most grateful for all the help.

With regard to the dead band 5. One of my friends and I are Toyota Landcruiser specialists. What I did not know was that he was also "good" at radios as well. Last night he came round with an Oscilloscope and decided to test the Osc stage for signal for each of the 6 bands. Started at 1 with a nice big wave then smaller waves until there was just a flat line on 5 then a small wave on 6. Whilst he looked at the diagram I looked at the switch in position 5 and decided there could only be a fault on one of 4 components :-

1. C29. Micamold type.
2. C28. Hollow condenser.
3. L55. Coil (unlikely).
4. C27. Air condenser (unlikely).

I changed C29 first and got no result. I changed C28 and the Oscilloscope sprang into life and band 5 now works perfectly !!

I now just have two last pieces of advice please:-

1. Can I put the tone correction cap on what were terminals 3+4 of the output transformer as this is a replacement one but someone has labelled the leads kindly.

2. Reception on band 2 (1.6-4.5) suffers distortion when tuned in to a station and also noise increases when tuning in to a station rather than decreasing. Sometimes there seams to be 2 stations at once. Is there anybody who could suggest how to improve or is this normal?

As a summary I now have an almost perfect AR88D. I think it must have been inside a nice warm dry place all it's life. Apart from the swapped OT all was completely original and very clean and shiny on top and underneath. I scrubbed the cabinet in the bath with very hot soapy water and the inside and outside have come up like new with a really bright crackle finish.

Thanks again.