Paxolin board under chassis of AR88

[G6Tanuki]

I use the rivet-type turret-tags along with a piece of non-copperclad FR4 to make up little panels as replacements for the bathtub capacitors. Mounted using the original bathtub-holes, and with 2mm brass tubular spacers to keep the back-sides of the turret-tags clear of the chassis, then yellow Vishay capacitors.
In times-past I went to the effort of etching a single-sided piece of FR4 to have little islands of copper round the turret-tags, and flood-soldering round the base of the turrets to secure them but that's just too much hassle; a small brass drift to 'peen' the back over works just as well and takes much less time.

[Radio Wrangler]

And FR4 is much more stable and tougher than Paxolin.

David

[thoyer454]

Originally Posted by gezza123

Hi Alan.
don,t know if this may be what you want ,never seen a photo of the audio board
so hope this is what you need.
gezza123

Need to watch that chassis layout pdf, L49 and L50 are swapped. L49 should be towards the rear of the chassis.

Tom
W3TA

[Alan Bain]

Originally Posted by G6Tanuki

I use the rivet-type turret-tags along with a piece of non-copperclad FR4 to make up little panels as replacements for the bathtub capacitors. Mounted using the original bathtub-holes, and with 2mm brass tubular spacers to keep the back-sides of the turret-tags clear of the chassis, then yellow Vishay capacitors.
In times-past I went to the effort of etching a single-sided piece of FR4 to have little islands of copper round the turret-tags, and flood-soldering round the base of the turrets to secure them but that's just too much hassle; a small brass drift to 'peen' the back over works just as well and takes much less time.

I like this idea. What did you do with the earthy ends of the capacitors - to solder tags on the mounting bolts? I'm half tempted to produce a simple PCB layout (without solder resist or silk screen) for one of the cheap Chinese PCB sources (the MOQ of 5 might be just right) but it could have islands of copper around the bases of the turrets on the lower side and a ground plane on the upper to which the capacitor earthy ends could be soldered.

[Alan Bain]

Since there seem to be a number of people who know the AR88 well, I was wondering if anyone could help with these holes in the front panel near the phone socket. It's clear what goes in the hole marked phone but the other two are more of a mystery. They have been added very carefully as if a workshop modification, not like the typical amateur addition of a BFO or S-meter to some sets which I have seen.

On size grounds one looks like it could be a fuse holder (although the AR88 design philosophy seems to be "fuse holders are for wimps" or maybe the HT chokes double as fuses). Not sure about the other, a power lamp seems gratuitously unnecessary since the scales light up.

[Radio Wrangler]

Toggle on/off switch because the ones integrated with the mode switch weren't considered reliable. There was also an added second phones jack so a second person could listen. No Idea whether that would have been a wandering supervisor, or a trainee.

The switch mod also allows the receIver to be powered up/down without crossing the 'trans' position which breaks the HT. This allows the HT stress to surge. Never use the 'trans' switch position.

Have a look on the back panel, is there a 2-pole bakelite connector mounted sideways on the outside face?

The switch and the added phone jack were common mods when rebuilt in the early 1950s

The added socket on the back (along with a big REF coax antenna connector) was for conversion to R1556 specification. If you're really lucky, you may have a PVC wiring harness that was also sometimes fitted.

David

[G6Tanuki]

Originally Posted by Alan Bain

Since there seem to be a number of people who know the AR88 well, I was wondering if anyone could help with these holes in the front panel near the phone socket. It's clear what goes in the hole marked phone but the other two are more of a mystery. They have been added very carefully as if a workshop modification, not like the typical amateur addition of a BFO or S-meter to some sets which I have seen.

On size grounds one looks like it could be a fuse holder (although the AR88 design philosophy seems to be "fuse holders are for wimps" or maybe the HT chokes double as fuses). Not sure about the other, a power lamp seems gratuitously unnecessary since the scales light up.

Those holes show that the 'AR88' you have is actually an ex-RAF R1556 variant. Toggle-switch for on/off, a jack-socket wired for high-impedance headphones [fed by a capacitor from the output-transformer primary] and there should also be signs of a big coaxial-connector being mounted on the rear drop.

When these receivers were returned to civilian duty, the RAF removed the front-panel mods, fitting steel machined bungs to blank the big holes. The smaller holes, and the coax-socket on the rear drop were left in place.

[G6Tanuki]

Originally Posted by Alan Bain

I like this idea. What did you do with the earthy ends of the capacitors - to solder tags on the mounting bolts? I'm half tempted to produce a simple PCB layout (without solder resist or silk screen) for one of the cheap Chinese PCB sources (the MOQ of 5 might be just right) but it could have islands of copper around the bases of the turrets on the lower side and a ground plane on the upper to which the capacitor earthy ends could be soldered.

Turret-tags for both ends of the capacitors. The turret-tags I used were tubular and so I fed wires up from below, flood-soldered into the tubular bit, then linked these together essentially 'under' the little panel, to a bus-bar of 16G wire which ran between two tags that fitted, again below the panel, to the two mounting bolts.

The bus-bar and under-board wiring was invisible.

[thoyer454]

Here is one of my AR88's done in my typical fashion of using terminal strips (US wording) and solder lugs for replacing the capacitors. No soldering to the chassis.

Tom
W3TA

[Station X]

Originally Posted by G6Tanuki

Those holes show that the 'AR88' you have is actually an ex-RAF R1556 variant. Toggle-switch for on/off, a jack-socket wired for high-impedance headphones [fed by a capacitor from the output-transformer primary] and there should also be signs of a big coaxial-connector being mounted on the rear drop.

Some info and pictures here:-

https://www.vintage-radio.net/forum/...ad.php?t=50017

[Alan Bain]

This sounds very likely. On the back panel there are clear signs of a five hole drilling pattern which I'd assumed had been for an N connector or one of those shielded 4mm jack plugs which seem to appear on transceivers, but actually it is rather larger.

The on-off switch seems a good idea, but I think given the general leakiness of the capacitors in the set that using the hi-Z phone socket doesn't appeal to me...unless it was for an annoying supervisor.

As a final (?) question, most of the obvious leaky caps have been found, but in the oscillator section there are some largish brown square "micamoulds" which ground the centre tap on the coils (up to 3900pf). These have an interesting colour code which I have failed to fathom despite the helpful arrows (it includes pink?) and don't look obviously distressed unlike some decoupler, but then they didn't have HT across them. Do they need replacement and if so what with - polystyrene? I'm not totally sure I follow the circuit design topology here, it's basically a shunt Colpitts but why the tapped inductor and the small cap directly across part of the coil? Part of me suspects that some of the capacitors are chosen for temperature compensation so random replacement "in case they leak" might be a bad plan for stability.

[Alan Bain]

Originally Posted by Radio Wrangler

[Cut]
The added socket on the back (along with a big REF coax antenna connector) was for conversion to R1556 specification. If you're really lucky, you may have a PVC wiring harness that was also sometimes fitted.

David

Sadly it looks like the wiring insulation is some kind of black natural rubber covered by woven cotton coated in shellac, which I can honestly say is a type of insulation I am too young to be familiar with! Except in the vicinity of the audio output stage where it seems things got hot, it has remained flexible and seems functional. The wiring round the 6V6 and the bias resistors has become brittle and will undoubtedly need replacing.

[Radio Wrangler]

That's what they were all built with.

With the receiver out of its cabinet, you may find modification labels stuck to the outside faces of the gusset plates bracing the front panel to the chassis sides. Mine recorded a rebuild by REME and conversion to R1556 spec. It's a very early serial number with all-yellow tuning background, engraved front panel and an original S-meter. It's actually an AR88 without a suffix letter, from before the D/LF split.

David

[Alan Bain]

It has come to my attention that the CR88 (single ended not the B) manual has a really useful wiring diagram (Fig 21) which describes the wiring loom of an almost identical receiver (although one has to imagine the bright reds and blues instead of the muted browns one sees today).

I've got a couple of scans of this (including from BAMA) but the scan resolution isn't great and some of the wire numbers are hard to read. Does anyone have a real manual that they could scan at a decent resolution?

The corresponding scan of the RF section (Fig 22) is much crisper but still has some puzzles e.g. pin 1 of X1 is connected to something labelled as BASE rather than to the expected convenient ground tag on the chassis.

[Alan Bain]

Thanks to the excellent PCB material suggestion here is a replacement bathtub alonside the original. The board has extra holes because to fit in certain locations it needs shortening. This PCB works in such variants for all but the 0.22uF bathtub next to the audio bias board.

[Alan Bain]

Thanks to all the helpful information and photographs here I was able working very slowly to reproduce the missing paxolin board see photo (with modern components since all the original ones either leaked or were the wrong values) and bring the set back to a state in which it could be aligned and is basically a radio.

That now leaves the giant mystery problem of the AF output stage. There seem to have been various design changes as even the valve choice 6K6 or 6V6 seems uncertain. Bias seems to be such as to suggest about 35mA DC bias current and given that the optimal ac load is around 7K, it would seem that a 30:1 output transformer with an 8 ohm speaker might do the trick. I found an old Wharfdale OP-3 which looks like the right ratio although the power handling is probably only 2-3W (and clearly I don't need the 600 ohm output). I couldn't find any specifications of the AR88D OP trans.