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Barker 88 post war version.

 
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Barker 88 post war version.

Post by sideband » Sun Nov 22, 2015 7:27 pm

Picked up a really nice Barker 88 today from someone in Horley. My CD's only have the pre-war version. Was there a circuit issued for the post war version or are the two so similar that the pre-war will do? I doubt I'll be able to do much to it before Christmas so it will most likely be a New Year job. Push-pull 6V6....can't wait to get it sorted.....

 
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Re: Barker 88 post war version.

Post by Cathovisor » Sun Nov 22, 2015 8:28 pm

What's the dial like on this one? There was a version with thermometer tuning I understand.

I can't imagine the circuit will have varied much between pre- and post- war models on this.

 
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Re: Barker 88 post war version.

Post by sideband » Sun Nov 22, 2015 8:45 pm

It's this one.
Attachments
js640_SAM_0513.jpg
Barker 88

 
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Re: Barker 88 post war version.

Post by Cathovisor » Sun Nov 22, 2015 8:56 pm

I'll have a chat with a mate of mine to see if there are any differences, he knows a lot about Barker sets...

 
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Re: Barker 88 post war version.

Post by sideband » Sun Nov 22, 2015 9:59 pm

In case it's of any help it has 6K8 6K7 6Q7 6J5 2x6V6 U50 and an EM34 (probably), all octal types.

I doubt it's a million miles from the pre-war version circuit-wise although the chassis layout looks completely different.


Cheers

 
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Re: Barker 88 post war version.

Post by sideband » Sun Jan 03, 2016 10:24 pm

I've decided to put this up on the bench next. A close examination shows that it looks completely original....no signs of previous work at all. Lots of dust....just as I like them. First I removed the chassis from the cabinet, then removed all valves, cleaned them and then vacuumed the chassis. The tuning indicator is a 6U5...not come across one of those before but it's probably duff anyway. The scale is filthy but not cleaned it yet.

So far all I've done is had a good look underneath. The only circuit I have is the pre-war version. This one does seem to have a few differences, notably in the output stage. It uses a PM speaker rather than an energised one as shown on the circuit. Secondly the 6J5 is used as a phase splitter for the output pair. The Trader sheet I have shows an alternative output stage but this one is different again. I spent half-an-hour tracing out the wiring of the output stage and phase splitter and converting it into a circuit diagram which I have attached. It all looks perfectly conventional although I think the centre of the output transformer would connect to what I have marked as HT 1 rather than to HT 2 as shown. There will also be a decoupler from HT 2 to chassis which I haven't shown. So the triode of the 6Q7 functions as a simple voltage amplifier in this version with the 6J5 as phase splitter.

Next stage will probably be the replacement of the paper caps and possibly the smoothing capacitors. The rubber end cap is bulging slightly and I'm thinking that it may have started gassing when last used (40 odd years ago). I have a couple of small 15uF 450V caps that will fit nicely inside the can.

So plenty of work to keep me occupied for a week or so.
Attachments
js1024_Barker 88 output stage 002.jpg
Just a quick hand drawn circuit of the output stage. The centre connection of the output transformer should connect to HT1 rather than HT 2. The unmarked resistor between HT1 and HT2 is 1K.
js1024_SAM_0558.jpg
Nice dusty chassis
js1024_SAM_0560.jpg
The scale is very dusty and needs a good clean behind
js1024_SAM_0559.jpg
Completely original underneath

 
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Re: Barker 88 post war version.

Post by sideband » Wed Jan 06, 2016 11:23 pm

I'm changing capacitors at the moment. There are a number of circuit differences in mine and I'd like to confirm a capacitor value. For convenience I'll use the references in Trader sheet 747 which is the only circuit I can find. C11 is shown as the cathode bypass cap for V1 and V2 with a published value of 0.1uF. My set has a similar circuit BUT the value of the capacitor is 1uF. It's actually marked 1.0uF so no chance of mis-reading it. I can't for the life of me see why a 1uF would be used in this position. It's original so not a mistake that someone else has made, unless someone fitted the wrong value at the factory. It's obviously worked OK in the past.

If anyone has a later version Barker 88 would it be possible to confirm the value for me? I have no problem fitting a 1uF if it's correct but I can't see why it needs to be so high. Most sets use .05uF to .1uF in this position.

Cheers!
Attachments
Barker 88 under.jpg
The capacitor in question. 1.0uF or 0.1uF?
Barker 88 under.jpg (48.71 KiB) Viewed 2896 times

 
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Re: Barker 88 post war version.

Post by Cathovisor » Fri Jan 08, 2016 7:54 pm

Okay... I spoke to my friend the Barker expert, who opened up a couple of post-war 88s that match yours in appearance to examine; and the answer to your question about the value is - whatever they had to hand! Seriously; one of the sets had a 0.25µF @ 150V, the other 0.5µF @ 150V in that position. So I think it's up to you what you choose to fit!

 
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Re: Barker 88 post war version.

Post by sideband » Fri Jan 08, 2016 8:03 pm

It's only an RF bypass.......I think it will be the published value 0.1uF!


Cheers! :thumb

 
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Re: Barker 88 post war version.

Post by sideband » Sun Jan 10, 2016 12:53 am

...and it's working! I'll post some pictures later but just to fill in the details. The smoothing cap wouldn't reform so I made the decision to restuff it and removed it from the chassis. However the weather was foul and all my drills are in the shed so I'll leave that for another time. In the meantime I've made up a temporary substitute using three (it's a triple) 15uF 450V caps and a small piece of tagstrip. Once I'd wired these up, I was ready for a first test. Fitted all the valves and a couple of pilot lamps, did a few basic resistance checks....and found that the mains switch was O/C! The control is pretty well sealed but I found a small hole in the casing and squirted some switch cleaner in to try and free the switch but to no avail. In the end I had no option but to short it out (it's only a single pole). Further resistance checks now showed mains continuity, no shorts across the HT and the output transformer seemed to measure OK as well. Now the speaker and output transformer plug in to a socket on the chassis so I had to remove the speaker and transformer from the cabinet and place it loose on the bench so the leads would reach.

So the moment of truth.....mains applied, pilot lights came on, valves lit and HT at the reservoir read 340V, this dropped to around 280 as the valves warmed up. There was an encouraging crackle from the speaker when I connected an aerial to the socket and a quick turn of the tuning brought in Talk Sport. A check around the dial showed half a dozen or so stations (I think a lot of MW stations went off at midnight on New Year). There seems to be a fair amount of background hiss on the stronger locals so maybe the aerial tuning needs a tweak.

There didn't seem to be much volume for a push-pull output stage although the quality was OK. A check of voltages around the 6V6's showed that one wasn't drawing any current. I then noticed its heater wasn't alight. I experimentally waggled it in its socket and the heater started to glow. After around 15 seconds, the volume increased considerably. Voltage checks showed that both valves were pretty much the same so they seem to be well matched. I left it running for around half an hour and it worked without any problems.

So jobs left to do, restuff the original smoothing cap, sort out the volume control/switch, sort out the heater connection on the 6V6, check alignment (as far as possible without correct information), clean and refinish the cabinet, reassemble, enjoy!

Oh yes...the 6U5 tuning indicator is dead so no surprises there.....!

 
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Re: Barker 88 post war version.

Post by Terrykc » Mon Jan 11, 2016 12:51 pm

sideband wrote:So jobs left to do ...  check alignment (as far as possible without correct information) ...
Who needs the 'correct information'?

Manufacturers instruction may vary slightly from set to set (and manufacturer to manufacturer) but a generic routine will produce identical results. The IF adjustments will be obvious and it shouldn't be much trouble to identify which band the RF adjustments belong to.

A set of this vintage will almost certainly have an IF of 465kHz but, if an initial check shows a peak at 470kHz, then use that. Set the signal generator accordingly and set all the IF adjustments for maximum output, remembering the golden rule - turn the volume up and continue to reduce the RF input to the lowest level possible as alignment progresses to minimise AGC action.

You can, of course, disable the AGC all together, but the same rules apply because the ear is much more sensitive to volume changes at low levels. You can disable the AGC and use a meter to measure the output with a meter if you wish but I prefer to keep my eyes on the trimming tool and my ears on the output!

Before touching the RF alignment, make certain that the pointer is set exactly on the LF datum mark (where one is provided) or at the very end of the scale markings as appears to be the case here - the clue being that the dge of the 49m SW band coincides with scale edge.

Carry out MW alignment at 600kHz and 1500kHz (500m and 200m), adjusting first the local oscillator followed by peaking the RF circuit, then repeating both sets of adjustments until no further improvement is possible.

For Long Wave, use 150kHz and 300kHz (2000m and 1000m). Note that the coverage extends some way above the highest frequency broadcast transmission (279kHz) so there is no point in using the top end of the band for alignment as for MW. The frequencies and wavelengths I've suggested will be clearly identifiable both on the radio dial and the generator but can be varied for other radios which have different scale markings.

The picture isn't clear enough to be certain but I think the lowest  marked SW wavelength is 13m. Taking this and 45m as convenient points to tune to, 6.7MHz and 23.1MHz would be suitable frequencies to use although absolute accuracy will depend both on that of the signal generator and resetting accuracy of the dial pointer. If the alignment has never been disturbed, a simpler method might be to assume reasonable tuning accuracy of the receiver and content yourself simply to peaking the RF adjustments to match.

I've described this in some detail as the procedure may be of help to others who are not as well versed in these matters as you!

 
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Re: Barker 88 post war version.

Post by sideband » Mon Jan 11, 2016 1:12 pm

I've already given it a basic tweak and improved it immensely without using a signal genny. The problem is knowing what coil/pre-set cap does what. When you have no layout it's a question of tracing wiring. Fortunately it was easy to find the MW aerial and oscillator caps/coils. I tweaked the oscillator pre-set cap to bring in Gold on 1548kHz then found Sunrise at the other end of MW (around 600Khz) and peaked that and just worked between the two, adjusting the aerial trimmers and coils as necessary. MW is now quite lively and there is stacks of volume. Now that I know where all the adjustments are (now suitably and subtly marked) I can do a proper job when the scale is re-installed and I have some markers to work with!

P-P 6V6 is pretty powerful.......!

In the meantime, has anyone got a good/reasonable 6U5 they are prepared to part with? The one in this set is just about visible in total darkness (and I mean total)! There are some used ones on eBay but no guarantee that any of them are any good. Otherwise I'll search the usual suppliers.

 
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Re: Barker 88 post war version.

Post by Cathovisor » Mon Jan 11, 2016 2:13 pm

sideband wrote:In the meantime, has anyone got a good/reasonable 6U5 they are prepared to part with?

Rich, for the avoidance of doubt, do you mean 6U5 or 6U5G? The former has a UX base, the latter International Octal.

 
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Re: Barker 88 post war version.

Post by Michael Watterson » Mon Jan 11, 2016 2:21 pm

There are cheap NOS Russian magic eyes. The Octal ones are slightly different wiring to EM34. See Radiomuseum for details.

 
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Re: Barker 88 post war version.

Post by sideband » Mon Jan 11, 2016 2:56 pm

Cathovisor wrote:
sideband wrote:In the meantime, has anyone got a good/reasonable 6U5 they are prepared to part with?

Rich, for the avoidance of doubt, do you mean 6U5 or 6U5G? The former has a UX base, the latter International Octal.


Good point. It's the 'G' type with IO base. Looking on line I didn't realise there were several variants.....!

 
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Re: Barker 88 post war version.

Post by Cathovisor » Mon Jan 11, 2016 3:12 pm

Michael Watterson wrote:There are cheap NOS Russian magic eyes. The Octal ones are slightly different wiring to EM34. See Radiomuseum for details.

That'll be the 6E5C (6E5S), yes? The new ones don't seem very cheap, it has to be said.

Langrex currently have 427 6U5G in stock Rich - give 'em a call.

 
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Re: Barker 88 post war version.

Post by Michael Watterson » Mon Jan 11, 2016 8:37 pm

Cathovisor wrote:That'll be the 6E5C (6E5S), yes? The new ones don't seem very cheap, it has to be said.

Langrex currently have 427 6U5G in stock Rich - give 'em a call.

You mean NOS 6U5G?

There are very cheap Russian 6E5C, they are a bit more sensitive. That can be fixed with a couple of 1M resistors. They are not plug and play. You do have to swap two wires I think. Or if a purest you can make an adaptor. I have them on my Pye 39JH/E and Pye PE 80
http://www.radiomuseum.org/r/pye_39_jhe.html
http://www.radiomuseum.org/r/pye_cambri ... nal_p.html

The NOS EM34 are rather dear.
Iv'e used Russian & Chinese replacements EM8x series side view "angel wing" pattern types.

 
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Re: Barker 88 post war version.

Post by Cathovisor » Mon Jan 11, 2016 8:50 pm

Michael Watterson wrote:
Cathovisor wrote:That'll be the 6E5C (6E5S), yes? The new ones don't seem very cheap, it has to be said.

Langrex currently have 427 6U5G in stock Rich - give 'em a call.

You mean NOS 6U5G?

Yes, NOS ones - Langrex are long-established valve dealers, have been in business for at least forty years.

A lot of the 6E5C I found on a Europe-wide eBay search were actually used: new ones seemed to be around the £20 mark.

A long, long time ago I bought some FIVRE 6E5GT from Tudor Rees which were very good but had rather a short grid base compared to similar 'eyes'.

 
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Re: Barker 88 post war version.

Post by Terrykc » Mon Jan 11, 2016 9:31 pm

Cathovisor wrote:[A long, long time ago I bought some FIVRE 6E5GT from Tudor Rees which were very good but had rather a short grid base compared to similar 'eyes'.

I think you'll find that's what Michael's pair of 1M resistors are for ... :qq1

 
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Re: Barker 88 post war version.

Post by sideband » Mon Jan 11, 2016 9:55 pm

sideband wrote:
Cathovisor wrote:Rich, for the avoidance of doubt, do you mean 6U5 or 6U5G? The former has a UX base, the latter International Octal.


Good point. It's the 'G' type with IO base. Looking on line I didn't realise there were several variants.....!


I've asked Langrex for a price. While I was browsing their list I looked at stock levels of some of the more sought-after valves. I note that they no longer have any UL41's and they are down to the last few EM34's...

 
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Re: Barker 88 post war version.

Post by turretslug » Mon Jan 11, 2016 10:27 pm

I assume that what's meant is to replace the typical single AGC diode load resistor with two series resistors adding up to the same value and feeding the ME grid from the tap, rather than put 2M across the AGC line? You could even SOT the values, or use a pot, if you want to put the ME into a satisfyingly picturesque operating region.

 
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Re: Barker 88 post war version.

Post by Michael Watterson » Tue Jan 12, 2016 10:25 am

Some sets have 1M series and then capacitor to 0V feeding magic eye anyway, so that it changes slower than AGC line volts.

Search 6E5S magic eye
6E5C ... dreaded Cyrillic ambiguity. Russian / Ukrainian sellers will transliterate the C to S (CCCP = SSSR)
Loads of NOS in Russia, Ukraine, Moldovia, Bulgaria. I see the price has risen about 50% since I last bought them. About £11 plus postage (the postage is often the same for even four mixed tubes).

The eye has only one deflector, so a single > like Y63 rather than >< like EM34.

 
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Re: Barker 88 post war version.

Post by Cathovisor » Tue Jan 12, 2016 10:32 am

One thing to remember is that magic eyes are more frequently fed from the detector rather than the AVC line. This allows the eye to be useful where the signal is below the delay potential. Where this occurs, the R/C combination is there to remove the AF component, not to alter the rate of change of the eye.

 
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Re: Barker 88 post war version.

Post by Michael Watterson » Tue Jan 12, 2016 10:59 am

Yes, that's true too. Some sets though, strangely even with dual diodes, don't use a separate AGC and Audio detectors.

A couple of battery models achieve "delay" by biasing the AGC line from the output tube grid bias, though magic eyes are rare on battery sets (DM70 only really anyway).

 
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Re: Barker 88 post war version.

Post by turretslug » Tue Jan 12, 2016 11:58 am

Oh, yes, time constant.... At least the ME grid can be stood off from the pick-off point with a good high resistance, to keep C sensibly low and compact, and prevent too much influence on basic circuit time-constant. One thing that sticks in my mind as a nipper was the ME wings (EM81?) of my grandmother's Baird radiogram (invariably tuned to 200kHz) gently fluttering on modulation peaks. One of those things that gave me a fascination for the amplifying light-bulb, just when the prevailing attitude of the time was, "What, it's got valves in it!!! Quick, throw it away and buy a nasty piece of flimsy music centre (probably with lurking Lockfits....) before the neighbours find out!".

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