Car-boot-sale "Barker 88"

[ms660]

I woudn't be too bothered that one resistor feeds two screen grids, the HRO had one resistor feeding the screens of the first RF amp, second RF amp, first IF amp and the second IF amp.

Lawrence.

[G6Tanuki]

I can understand it working on chained RF/IF-amps, but including a mixer in this must surely be a recipe for 'pulling' of the LO as the AGC affects the current-draw on the IF stage and this reflects back on the mixer?

[There again, I'm one of the kind of people who think applying any kind of AGC to a mixer-stage is pure evil anyway].

[Boater Sam]

Drill 3 holes in the electrolytic can seal end, slot in 3 cheap Chinese "budget" caps and it will go well at minimum cost.
It may be a budget set built to a low cost but I agree, it is worth fixing. They do sound well too.

[ms660]

Originally Posted by G6Tanuki

[There again, I'm one of the kind of people who think applying any kind of AGC to a mixer-stage is pure evil anyway].

Yes, I agree to a certain extent but it's basically a bog standard broadcast radio not a comms receiver.

RDH4 comments that the 6K8 is fair when it comes to stability on shortwaves due to varying supply voltages, which is neither good nor poor.

Lawrence.

[G6Tanuki]

You're right it's not a comms receiver: must admit I tend to come to these things from the amateur/SWL/MW-DXer angle, where I'm more interested in hearing/identifying stations than actually *listening* to them.

I've found a 3-way-plus-ground-tag tagstrip that will fit just fine on to one of the securing-bolts for the leaky 'can' - now to find some suitable electrolytics from old PC PSUs. (I don't like sticking them inside the old can; wired beneath the chassis they'll run cooler, and you can't see them anyway).

Given time, the Barker-88 will "Bark" once more! [this is one of the last series, with all B7G/B9A valves apart from the rectifier and magic eye].

or leave it in place and wire some modern components to a tagstrip beneath?

Modern bits underneath, that is what a period repair would be like.

[David G4EBT]

Repairers (rather than restorers) were under time pressures to complete the job and get on to the next one. I've mentioned before that I have a chum who spent some years working for Radio Rentals back in the 1980s, and was under pressure to complete 12 jobs a day. He rarely switched on his soldering iron - just snipped out the old component, twisted the wires of the new one to the remnants of the wires of the old one, then off to the next job. Given that we're hobbyists, mercifully, we're not under those time pressures and the job can take as long as it takes. Though it's true that the set will work just as well if new caps are tagged underneath the chassis, it really isn't a big deal to gut and re-stuff the old can, though that of course is each person's own decision on which I wouldn't presume to dictate.

Modern miniature high ripple-rated electrolytics are so compact that it makes it a doddle to re-stuff the old can. I know some restorers do this by drilling down into the cap to push the new caps into the cavity, but personally, I cut the can open just above the base, remove the innards fit the caps then re-seal the can with an aluminium sleeve fixed with 'Super Steel' 2-part epoxy.

The caps aren't expensive - much cheaper in fact to stuff the old can than to fit a new unit of the same size.

With the Barker 88, two 18uF caps and one 10uF cap, both 450V rated would do the job fine, and would fit snugly in the old can. The 18uF ones are 10mm diam x 25mm long and have a ripple rating of 400mA at 100Hz, the 10uF ones are 10mm diam x 16mm long, and have a ripple rating of 350mA at 100 Hz, so are well up to the task. They're rated at 105C (221F!), so the question of them not being in free air doesn't enter into the equation if the get slightly warm. They're certainly not going to get to above the boiling point of water, but in the very unlikely event that they ever did, they'd cope with it.

RUBYCON 450BXW18MEFC10X25 Electrolytic Capacitor, Miniature, 18 µF, 450 V, BXW Series, ± 20%, Radial Leaded, 10 mm are £1.42 each + VAT.

http://uk.farnell.com/rubycon/450bxw...rad/dp/2342047

RUBYCON 450BXF10M10X16 Electrolytic Capacitor, Miniature, 10 µF, 450 V, BXF Series, ± 20%, Radial Leaded, 10 mm are £1.20 each + VAT

http://uk.farnell.com/rubycon/450bxf...rad/dp/1831326

I'm not inferring that's what you should do - it's your set not mine - I'm simply saying that if you wished to, it would cost about £4.00 + VAT for the three caps, so it's still a low budget option. That said, there's loads on space under the chassis of a 'Barker 88' to mount them 'outboard' if that's your preference.

Interesting sets - no two '88s' seem to be the same. I believe the set was (allegedly) named because it used 8 valves (if you count the magic eye as a 'valve' rather than a 'tuning indicator'), and had an output of 8 Watts from its push-pull output stage. The one that I had went to forum mebmer Colin Wood, ('Retired'), who promptly veneered in in burr walnut and French polished it. Must be the poshest '88' on the planet! There's an thread on the forum covering that.

Good luck with the restoration.

[G6Tanuki]

Revisiting this - I found in a scrap chassis a TCC "Lectropack" 16+16uF 450V-rated can-type electrolytic which after a few minutes being fed 450V through a 10KOhm resistor settled out nicely with less than 0.2mA leakage. And another junkbox find yielded a "Jamicon" 450V wire-ended electrolytic.

Fitting these in place of the dead 16+16+8uF Hunts [it just had to be...] can - the two 16uF being closest to the rectifier, the Jamicon 47uF being wired on a tagstrip a couple of KOhms of resistor away - let's just say that the hum issues have gone.

It's still rather insensitive - I can receive a few stations at the HF end of medium-wave using a 5-metre throw-out antenna, and on LW there's Radio4 and buggerall-else. Shortwave does a bit better - I was listening to Radio Romania around 6MHz.

Looking at the signal-frequency frontend, it's really odd.

The antenna terminal feeds an input winding on the shortwave coil, the bottom-end of which is connected to earth through a 3000pF capacitor.

This capacitor is also wired between the bottom-ends of both the LW and MW tuned-circuits.

See attachment...

I guess it's intended as some sort of bottom-capacity-coupling on MW and LW. Deeply strange! Why did they do it this way rather than having proper antenna-coupling windings on MW and LW ??

I've also done some case-polishing....

[Boater Sam]

Don't know why, but Cossor did it with many of their sets in the same period. Suppose it was cheap and if they could sell sets, why not? The adjustment of the aerial trimmers is critical to get the sensitivity up.

Went mad and bought a fresh tuning eye for my pre-war Barker 88, not new but shown lit and working on a tester. As it was the UX6 based early one which went straight in and works well, I don't begrudge spending a few pounds, it finishes the set. The cabinet is done with Danish oil and looks good.

[G6Tanuki]

The odd "bottom capacity coupling" thing is something I've only otherwise come across when using frame-aerials [the Pye P75 does it...] - I guess perhaps it avoids stray-capacitance detuning-effects on frame aerials or something.

Either way from a design perspective it's deeply odd to me!

I'm wondering whether I should spend four times what I paid for this Barker-88 to buy a new magic-eye. Or perhaps replace the ME with a piece of ground-glass with a green neon behind it to give a pseudo-magic-eye glow? Given how insensitive the receiver seems to be I doubt that a tuning-indicator would have been much use anyway.

[turretslug]

I'm guessing that the designer was aiming at a degree of aerial input LPF action on MW/LW with that combination of SW coupling coil and 3nF? Low harmonics of the LO on these bands might have bought in interfering burble/whistles from some of the quite monstrously powerful and far-reaching BC stations on 49/41/31m, a "simple" front end set would always be prone to receiving more than it was meant to! Also, that 3nF presumably represents a fairly low impedance even at low frequency RF, perhaps it was intended for a traditional random wire of the "as long as possible...." variety. I'm also guessing that this arrangement would result in better signal transfer as input frequency went lower with that simple, randomly longish piece of wire compared to low-impedance coupling windings? I liked the arrangement on the CR100 (and probably other similar sets) where there was a choice of low-impedance balanced input coupling or high-impedance unbalanced with a few pF between aerial socket and aerial tuning gang section- the latter would give something with a few feet thrown out (admittedly with a front panel aerial trimmer to optimise any de-tuning) and is easily added to most sets.