Valve radio repair series starting in Practical Wireless

[ms660]

Originally Posted by David Simpson

Personally, I would have used a simple one-valve amp., before delving into a 3 or 4 valve radio.

They don't get much simpler than the one valve UL41 output stage in a Bush DAC90A 5 valve receiver....move across the schematic and there's a very simple audio amplifier stage preceded by a simple detector and AGC stage if you want to fiddle around with either of them... left again and there's a very simple IF amplifier stage....left again and there's a very simple local oscillator and mixer stage (they are also amplifiers) and the handy thing is they are all on one chassis served by a common power supply....

Lawrence.

[Sideband]

I'm not getting into a safety argument here but I grew up with valves and AC/DC sets were as common (if not more so) than AC only sets.....and remember virtually every TV sold in the UK up until the mid-70's or so had AC/DC techniques and we were also assailed with (in my opinion) the even more dangerous 'half-mains' chassis and it didn't matter which way round they were connected to the mains.

The point here is that none of these sets ever killed anyone and they were used every day by the general public. Even more importantly, the engineers that repaired these set often had to do so in customer's houses in cramped conditions with nothing more than a meter and a soldering iron. No isolation transformers in the average living room and we lived to tell the tales. I personally NEVER had a shock from any live-chassis equipment although it is true to say that I have had 'nips' from HT rails when exploring fingers have inadvertently touched the primary of an output transformer...but that could have happened in ANY radio, AC/DC or not. Remember an isolation transformer only enables you to connect earthed test equipment safely to an AC/DC set. It doesn't make the set any safer.

The first vintage radio I repaired at 14 was a Pilot Little Maestro Model 10 AC/DC and I still have it. That was followed (at 15) by a Ferguson T283 14" TV (admittedly with help from my brother) but again AC/DC, both fixed at home from raw mains and all I did was check that the chassis wasn't live by using a neon screwdriver....

[ms660]

Originally Posted by Sideband

It doesn't make the set any safer.

A common misconception.

Lawrence.

[Station X]

Forum founder Paul Stenning had a series of vintage radio restoration articles published in ETI back in the 1990's. You can find them here:-

https://www.vintage-radio.com/repair...ion/index.html

[SiriusHardware]

The series is titled 'Valved Radio Repair' and so I suppose the target of any such repair is going to be something which was commercially made, therefore very unlikely to have been a one-valver. It seems to me, without going too deeply into it, that the author has picked one of the best known vintage valve radios of all time as his starting example.

As with other past discussions about books and articles here, I would encourage interested parties to read the article before having a steadfast opinion about it.

If you are a member of a library a lot of library services now allow you to 'borrow' electronic copies of periodicals, including Practical Wireless, so even if you can't reach the bottom shelf in WH Smiths there may be a way for you to read it anyway. I admit I have not worked out how to do this yet: I was only made aware of this when I joined SWMBO's local library in Whitby recently.

[Nickthedentist]

Originally Posted by ms660

A common misconception.

Lawrence.

Sorry Lawrence, are you agreeing or disagreeing with Sideband?

[john116]

ummm - for all the concern about safety and all the other negative comments on here, no one under 40 is going to be buying a magazine, therefore alleged crisis averted.

[Neil Purling]

The mains transformer in my CR100 gave out with a loud whooshing sound and a spectacular geyser of thick waxy smoke.

Back in the day I had a CR100 too and I didn't know valve-specific stuff that well.
I was listening to All India Radio one night. As was my habit I had my feet on the table either side of the rig to take advantage of the free heat.
It seemed to go off-tune so I looked up to see a plume of smoke coming out of one of the vent holes. I uttered an expletive & struck the mains toggle switch to shut it off.

I never thought that the set was ever a danger to my health. Not unless I had to carry it any distance. One time I got sensible & borrowed a sack barrow from a greengrocer. This set really needed a rebuild. These days i'd have sought help from the Forum.
A WW2 comms receiver is the last sort of thing a newcomer to vintage wireless should look at even the CR100 was hideously complex. Had 11 valves in total including the rectifier.

[Radio Wrangler]

Anyone reading this forum has access to a lot of info in the 'stickies' as well as the ability to search for threads on specific models or subjects. On top of that, there is Chas Miller's book, and all the past magazines in world radio history's archives.

So there's no need to wait for PW's series to get to any juicy bits, and because on the page count available for it, it's unlikely to go very deep, nor is it likely to produce any wondrous revelations.

We aren't its target audience. What it might do is to encourage some new people into trying old valve radios. It's awfully easy for people already experienced in a field to stand at the side, observing, and throwing rocks at anything they disagree with, but they can be demolished easily by pointing out that they don't need this info, it's aimed to encourage new beginners, and if they're so good, why didn't they write it?

If the articles encourage a few people, good! and if they soon reach the limits of the articles, then there are some good books, better articles and the forum all ready and waiting for them.

The authors and publishers carry some responsibility for making the risks clear and for providing adequate guidance. Remember that as a commercial operation, a publication is expected to be more trusted than something soma amateur said on the internet. Coroners can ask for people to justify actions an inactions.

Anyway, this thread is circling the familiar old safety whirlpool (sluggishly rotating anti-clockwise in the Northern hemisphere...) which has swallowed so many prior threads and never yet reached a conclusion all can agree on. (hint)

David

[SiriusHardware]

Having now read the article myself, I can say that part 3 assumes the reader is already an experienced technician looking to learn specifically about valve radio servicing. As someone in exactly that category, I found it as interesting and informative as it could be in the space available.

[ms660]

Originally Posted by Nickthedentist

Sorry Lawrence, are you agreeing or disagreeing with Sideband?

From a servicing point of view disagreeing.

Isolation transformer = Mains Live & Neutral in, and neither of those out.

Lawrence.

[David Simpson]

For a magazine article, or in fact a hands-on training session, on valves - I would have thought that basic diode, followed by a triode, and so on, would be the best option. Remind me(I've only ever restored a DAC10), does the DAC90's dropper hive off 40 or 50V for a series heater chain ? And, with separate valves needing 6, 12, 20 volts VH ? Jolly confusing for a complete novice. Even more confusing would be explaining a multi-function T/Hex or a DDT or T/P.
We cant expect novices to splash out a wad of dosh for an over priced/over rated cult object like a DAC90. A simple 3 valve TRF, AC mains - with a decent 3 core flex, with earth conductor to the chassis & fused at 1A in a modern BS 3 pin plug - should be jolly safe. Particularly if their house is up to IET spec with an RCD/MCB Consumer Unit. Assuming that the article author(who the heck is he ?) is technically competent, he will tell aspiring valve/VR novices that his demonstration set(and in fact all his sets & equipment), fully passes a PAT Test.

Regards, David

[Paul_RK]

Originally Posted by David Simpson

We cant expect novices to splash out a wad of dosh for an over priced/over rated cult object like a DAC90. A simple 3 valve TRF, AC mains ...

Looking at recent auction results, £30 - £50 seems the going rate for an "untested" DAC90A, with cream versions at the higher end of the scale - three or four hours' pay doesn't seem too formidable an investment for anyone on the verge of a new enthusiasm. And anybody struggling with the concept of a series heater chain delivering different voltages to different valves would be a beginner indeed in matters electrical, and far outside the intended readership of the article given what's been related of it.

Once more, a problem with the "simple 3 valve TRF" is that there's no single readily available model: indeed my guess would be that most in existence are either early '30s sets, likely to be scarce and with their own demands, or post-war kit sets using a wide variety of valves and which may or may not ever have worked to a passable standard. Probably my memory's going, but I'm struggling to recall a single post-war commercial UK AC mains TRF - the few TRFs that come to mind (Champion, Portadyne Minx...) are all AC/DC.

Paul

[David G4EBT]

Originally Posted by ms660

Isolation transformer = Mains Live & Neutral in, and neither of those out.

Lawrence.

True, and it's worth stressing that it's vitally important that if any earthed test equipment such as a 'scope, signal generator or frequency counter is to be connected to a live chassis radio, the radio must be powered via an isolation transformer - not with the radio connected directly to the mains.

As to servicing articles in PW, really we have to go back to the days when Practical Wireless was both 'practical' and about 'wireless' before it changed its editorial policy under Rob Manion to be an exclusively amateur radio magazine, very thin on editorial content.
In the late 60s early 70s there were several excellent series in P.W. on servicing both valve and transistor radios. To mention but three:

April - September 1967. First Series: 'Repairing Sets' - a 6-part series on valve radios. Authors H.W. Hellyer & Gordon King.

April - September 1968. Second Series: 'Repairing Radio Sets - 6-part series on transistor sets, covering fault-finding, servicing procedures and workshop practice.
Authors H.W. Hellyer & Gordon King.

May - September 1971: 'An introduction to fault-finding'.
Authors H.W. Hellyer & Gordon King.

This too, is useful and informative:

Oct 1971 - Dec 1972: 'Transistor Circuitry for Beginners' - 13-part series.
Author H.W.Hellyer.

There were lots more articles in the 60s and 70s, but the above cover just about everything we need as hobbyists.

The Practical Wireless Archive from its inception in 1932, through to 1999 can be viewed and downloaded from this link:

https://worldradiohistory.com/Practi...s_Magazine.htm

From 2000 onward, the WRH advise that CDs of back issues should be obtained from P.W.

Hope that's of interest.

[ms660]

Originally Posted by David G4EBT

True, and it's worth stressing that it's vitally important that if any earthed test equipment such as a 'scope, signal generator or frequency counter is to be connected to a live chassis radio, the radio must be powered via an isolation transformer - not with the radio connected directly to the mains.

Back in the day without an isolating transformer the usual solution was to connect both leads of the signal generator to the receiver via an isolating capacitor in each lead (signal and ground/earth) eg: the service instructions for the IF alignment in a Bush DAC90A say the same.

Lawrence.

[HamishBoxer]

Had a brief look at the article, so may have missed it. Is there any mention of an isolation transformer when working on these "live" chassis?

[David Simpson]

Yep, Sideband & other older chaps here will have masses of experience with AC/DC tellies & radios, & will know all the precautions to take. But some young Johnnie, in his late teens/early 20's say, just beginning to peruse PW & other hobby magazines, probably wont have a clue about basic electrical safety & the safety issues relating to AC/DC valve sets.
Perhaps a bog standard 3 or 4 valve DC portable from the 50's/60's, with their 90V/1.5V battery packs, would be a much safer option. There must be heaps of cheapo EverReady's & their ilk floating about. They always seem to pop up at junk sales I've been to, and photos of sale tables at Retrotech & BVWS venues reveal dozens stacked on tables(or under them). More prolific, I would say, by a long chalk, than DAC90's. Making up a 90V pack with PP3's, & 1.5V pack with U2's, etc. is dead easy, & has been covered in several forum threads.

Regards, David

[Tractionist]

And ...... the very first article dealt with 'safety' and the desirability of employing a mains transformer. I'd have thought that anyone purchasing this magazine has a rudimentary knowledge of electronics ... if not valve technology (and has read the first article)?

[Tractionist]

That's 'mains [I]isolating[I]' transformer!

[G6Tanuki]

Personally I would say something like a Pye P75 would be a better set for a valve-virgin to gain experience witj.

Proper isolating transformer, easy-to-understand circuitry, no hard/impossible-to-get parts, works well once refurbished.,

Battery-valve portables, in my experience, have often been left with the batteries in them and the effect of a few decades of escaping corrosive gases leads to open-circuit IFTs/antenna-coils/output-transformers all of which can render a radio beyond-sensible-repair.