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Old 21st Aug 2018, 10:05 pm   #21
paulsherwin
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Default Re: Bush TR82B

If the set is working well and the circuit voltages are broadly correct then I would leave the rest of the caps alone.
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Old 22nd Aug 2018, 12:56 am   #22
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Default Re: Bush TR82B

If you replace those red and black capacitors you might be able to squeeze a bit more use out of an old battery at the expense of low volume.
It will not gain you a great amount of value though.
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Old 22nd Aug 2018, 8:07 am   #23
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Default Re: Bush TR82B

Every one of these Bush sets that I have bought have worked straight off. As you have found its mainly cleaning that they need, though I did have one with a ratty speaker.
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Old 22nd Aug 2018, 2:16 pm   #24
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Default Re: Bush TR82B

Echoing Sam's comments, back in the day it was rare to have on of these turn up on the bench, although we sold them in large numbers.

If the problem wasn't a dirty wave-change switch or pot, the chances were that it had been dropped! The combination of metal chassis and integral loudspeaker resulted in quite a weight so that, if dropped, when the cabinet hit the deck, the chassis kept on going!

The chassis ripped the spring steel mounting brackets out of the cabinet fixings, snapping the fixings so that there was no way to refit the chassis. The solution was simple - remove the screws holding the front on and order a new body shell from Bush - it was quite cheap, I recall - slide off the chrome trim, fit it onto the new cabinet body and reassemble.

Depending on how far the set had fallen, some turned up with a broken ferrite rod, so that had to be replaced as well.

We did have one set that also had a bent tuning gang spindle - the slow motion tuning part had bent where it entered the main spindle and the gang had to be replaced.

I can honestly say that, in the nine years I worked there, I never repaired a genuine fault in a Mark I TR82!
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Old 22nd Aug 2018, 2:43 pm   #25
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Default Re: Bush TR82B

They are a very conservative design, with no attempt to cram everything into a small space. There is a four transistor audio amp so no need to squeeze every last bit of gain out of it. They work with almost any transistors. The last one I fixed had been stored so badly that the OC71 leads had actually corroded through, but a new junk box transistor, a squirt of Servisol and off it went.

The switchgear does benefit from an annual cleaning though.

Of course, they weren't a cheap set when new.
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Old 22nd Aug 2018, 3:46 pm   #26
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Default Re: Bush TR82B

Many thanks for the advice folks, it's good to know they are such a reliable set! In view of this and the fact that it's working well and current draw appears to be within published parameters I think I'll leave well alone! It might be worth checking the current from time to time in case those Plessey caps do become leaky although I guess after 60 years they've done OK!

Thanks again for all your help, it's much appreciated
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Old 22nd Aug 2018, 3:46 pm   #27
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Default Re: Bush TR82B

I have done a couple that had been dropped. I had to rebuild the fixings with plastic that had been melted with solvent. One was an AF117 one and even they worked again.
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Old 22nd Aug 2018, 3:56 pm   #28
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Default Re: Bush TR82B

Any AF117s are trivially easy to whip out in these - you don't even need to remove the chassis. Any Ge type with an ft above 2MHz seems OK as a replacement. Si types seem to be fine too, but I've never had to use them as any low spec Ge type from the junk box will work well including old industrial switching types.

The glass OC types normally soldier on forever unless physically damaged by shock or corrosion.
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Old 22nd Aug 2018, 4:20 pm   #29
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Default Re: Bush TR82B

I have only recently acquired one of these radios myself. The first thing I noticed was the unusual audio output transformers with the windings for the emitters of the output transistors and an unusual bias method for the output transistors. What is the reason behind this design ?
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Old 23rd Aug 2018, 10:27 am   #30
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Default Re: Bush TR82B

Bearing in mind that early power transistors were limited in power output - 200mW max from a pair of OC72s, I think - it looks like a clever way of increasing the overall efficiency by combining power from both collector and emitter currents.

I'm fairly certain it was based on a Mullard design - perhaps somebody else will know more?

It is rare, but that is because of the added cost of the transformer design but, as has been said earlier, these sets were not cheap!
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Old 23rd Aug 2018, 11:00 am   #31
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Default Re: Bush TR82B

Thanks Terry.

I had wondered since there are no bias diodes or thermistors and it is just collector to base bias resistors, whether it might have been a way to create emitter resistors made from copper wire, that have a positive temperature coeffient and increase the emitter voltage with heating and have the advantage, unlike a real resistor, that the currents and ampere.turns in the emitter windings add to the collector windings and increase rather than lower the efficiency of the output stage. So it improves the situation for AC signals and helps stabilise the situation for DC with increases in temperature.

But there could be more behind the idea that I might find when I get it on the test bench. I expect it will sound pretty good with the celestion speaker. I guess another feature of this is it makes best use of the full battery voltage, which is always reduced to an extent by emitter degeneration resistors.

It is the only transistor radio I have ever seen that is wired like this, though there may be others. I still have to restore this TR-82, it is the version with AF117 transistors. I picked it up from an antique shop in York.
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Old 23rd Aug 2018, 11:02 am   #32
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Default Re: Bush TR82B

I recall the first 'faulty' TR82 I ever had any knowledge of. The model was introduced at around the same time I started work and, about 3 months later, a TR82 appeared in the workshop one morning for service under guarantee.

We apprentices - myself and Ray, 2 years my senior - were not allowed anywhere near this strange new beast, filled with these mysterious new transistor thingies (technical term) while the Service Manager and two field engineers formed a protective shield around it.

The fault was clearly audible - low frequency insatiability (motor boating).

Ray and I got on with our work whilst the trio pored over the circuit diagram and there were several mumbled 'what ifs', although whether they tried changing anything, I don't know.

An age passed before one of them thought to take an AVO prod to the beast.

I was then promptly dispatched downstairs to the shop for a new PP9!

Shortly after I returned, the sweet sounds of music and three sighs of relief drifted across the workshop with no trace of motor boating.

In labour terms, a very expensive no-charge 'repair', though I'm sure that the customer would have been charged 3/9d for the new PP9!

I never experienced another TR82 with instability problems and, over the years, I've often wondered if it occurred to one of the trio to check the 100uF electrolytic across the 9V supply ...
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Old 23rd Aug 2018, 11:25 am   #33
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Default Re: Bush TR82B

Quote:
Originally Posted by Argus25 View Post
It is the only transistor radio I have ever seen that is wired like this, though there may be others. I still have to restore this TR-82, it is the version with AF117 transistors. I picked it up from an antique shop in York.
Barring dirty switches and controls, if the problem is more that faulty AF117s, I would be surprised (although some of the electrolytics may have dried out after all this time).

I remember when Bush announced that the original TR82 was going to bne discontinued and discounted the price to clear existing stocks. There was uproar from the trade because it had been such a good seller.

Bush received so many complaints, it was said, that they were forced to rethink and redesigned it to use the current AF117 in place of the obsolescent OC4x range. However, I don't recall much of a gap before the Mk.II version appeared, including a new tan colour scheme for the TR82D, so perhaps the story was apocryphal?

One thing that has always puzzled me: There was a TR82B and a TR82C, plus the later TR82D, so why was there no TR82A?
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Old 23rd Aug 2018, 1:30 pm   #34
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Default Re: Bush TR82B

Hi Terry,
Was always lead to believe the B & C refered to the Brass and Chrome trim versions not sure about the D though!
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Old 23rd Aug 2018, 2:03 pm   #35
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Default Re: Bush TR82B

Well, they certainly didn't refer to the cabinet colour or there would have been two Bs!

Hadn't heard your explanation before, though.

Another thing about the TR82 was that it used the cabinet designed for its predecessor, the Mains/Battery valved MB60 and still had the circles moulded into the inside of the back denoting the positions of the sorbo-rubber supports for the valve pips.

Most of the MB60s that I saw had the later revised valve layout, so the positions of the pads were completely different but the circular ridges mere never moved.

The TR82 also retained the depression in the back which had a hole cut out in the MB60 for the mains connector.

We had several requests to fit tape recorder outputs on TR82s and, on a cabinet with all curved surfaces, this flat depression was the ideal place to fit a 3-pin DIN socket!
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Old 23rd Aug 2018, 2:16 pm   #36
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Default Re: Bush TR82B

Understandable that Bush used the same tooling for the back, the cost of injection moulding tooling is very high. The quality was also very good, the moulds must have been well finished and maintained.
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Old 23rd Aug 2018, 3:07 pm   #37
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Default Re: Bush TR82B

Quote:
Originally Posted by terrykc View Post
I'm fairly certain it was based on a Mullard design - perhaps somebody else will know more?

It is rare, but that is because of the added cost of the transformer design but, as has been said earlier, these sets were not cheap!
Similar circuits using OC78 and OC81 transistors are on pages 102 and 108 of the Mullard Maintenance Manual Second Edition ©1961. The turns ratios and resistances of the transformers are specified:
  • Phase splitting transformer: 2:1+1, primary <120 Ω, each secondary <50 Ω
  • Output transformer with 3 Ω loudspeaker: Loudspeaker N and <0.2 Ω, collectors 3.45N+3.45N each <1 Ω, emitters 1.15N+1.15N each 5 Ω ±10%.
Frustratingly, there is no explanation of the circuits. There is some information in Transistor Superhet Receivers by Clive Sinclair:
http://mirror.thelifeofkenneth.com/l...ivers_text.pdf

Page 12 states:
“In the circuit shown the transistors are used in the common emitter mode. Although this form of connection incurs greater distortion than the common collector type it is almost invariably used because of the increased gain.”
Page 48 has the circuit of the (very rare and collectable) PAM Model 710 which has a common collector class B amplifier. It states on page 46:
“V7 and V8 form a common collector Class B push-pull amplifier. Although this type of output stage gives considerably lower gain than the common emitter type the quality is superior, furthermore, the low output impedance makes it simple to direct couple into a centre tapped speaker.”
The unusual design of the TR82B may have lower distortion than a standard common emitter design because, to some extent, the a.c. voltage to the loudspeaker will be a product of the a.c. voltage at each phase splitter transformer secondary and the 1.15:1 turns ratios of the output transformer (neglecting variations in Vbe).

It will be interesting to lean what Argus discovers when the TR82B is on the test bench.

David
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Old 23rd Aug 2018, 5:10 pm   #38
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Default Re: Bush TR82B

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Similar circuits using OC78 and OC81 transistors are on pages 102 and 108 of the Mullard Maintenance Manual Second Edition ©1961.

Frustratingly, there is no explanation of the circuits.
Thanks for confirming my memory, David.

And I think you've also identified why I have no memory of the circuit although, had I read one at the time, it's doubtful I would have remembered much by now!

Intimate details of the design would really have only been of value to a circuit designer and, as you've pointed out, the important design figures for the transformer had already been done by Mullard.

The two descriptions which you have found relate only to individual common emitter and common collector design whereas this one is both!
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Old 23rd Aug 2018, 5:33 pm   #39
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Default Re: Bush TR82B

These are a good old reliable circuit, and an iconic design, I have had a few pass through my hands.
The last one, a couple of weeks ago, I bought for £1, I just couldn't resist it. All that was wrong was an intermittent connection "inside" the tone control capacitor, oh, and I changed the two 10k resistors on the OP, but that was only to see if it helped the volume a little, which it did only slightly. I think the largest improvement was caused by time, the more I used it the better it sounded as the electrolytic caps reformed.


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Old 24th Aug 2018, 3:12 am   #40
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Default Re: Bush TR82B

I have often wondered if the idea was inspired by the Leek 2 output stage.
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