Golborne Vintage Radio

I've listed this under 'home-brew' albeit the project is based upon a little Velleman kit, which I boxed up and made a couple of probes for, so I guess it was a team effort between me and Velleman! I made if for a chum as a 'thank-you' and thought that a few others might like to know about this neat little kit, and a few notes on signal tracing/injecting.

We all have our own approach as to how we go about fault tracing is non-working radios, and my approach ever since my teens more than fifty years ago has been to first of all look for obvious faults such as loose connections, broken wires, do some voltage checks, continuity tests on speakers, OPTs etc, then to use a signal tracer/injector approach to test the audio stage to see if the fault lies there or in the earlier stages. (As an aside, from long experience of dead audio stages in transistor radios, the culprit is often an open circuit earphone socket. It's only supposed to silence the speaker when the earphone is plugged in, but the spring contacts weaken over time and often go open circuit even with the earphone plug removed).

By injecting an AF signal of about 1kHz (actual frequency unimportant) at the slider of the volume control, if there's a loud tone from the speaker, that shows that at least the audio stage is working. But suppose that the AF stage isnt working - what then? If the RF/IF stages are working, a probe from a signal tracer (which is basically, just a small amplifier), placed on the slider of the volume control will show whether those stages are working. The tracer basically substitutes for the AF amp, so if the RF/IF stages are working, tunable signals should be heared on the speaker of the tracer.

But what if there are no signals at the volume control slider?

By applying an RF probe (a diode detector attached to the tracer input) and working towards the front end of the set, the faulty stage can be found. EG, if there are no signals getting to the detector valve (or diode in a transistor radio) the second IF stage may be faulty, but if a signal from the injector is squirted into the grid of the IF valve, and the tracer probe applied to the detector, and a loud tone is heard, that suggests that the second IF stage is probably working and the fault lies closer to the front end of the radio. If the tracer probe is applied to the grid of the first IF valve and tunable signals are heard, that proves that the mixer/oscillator (frequency changer) is working. However, if no signals are heard, that stage is faulty. If the signal injector probe is applied to the grid of the first IF valve (or base of the first IF transistor in a transistor radio) and the tracer probe is placed on the slider of the voume contorl, and a loud tone is heard, that's a good inidication that the IF stages are working, thus confirming that the fault lies in the front end. If could be the valve not oscillating due to the valve itesle or associated curcuitry, open circuit coils, faulty wavechange switch connections etc.

If you place the RF probe at the grid of the hexode section of the frequency changer valve, you are basically turning the signal tracer into the equivalent of a crystal set by connecting the probe just to the tuned cirrcuits of the radio, and if you don't hear any signals, there must be an open circuit in the coils, such as a frame aerial, aerial coils, ferrite rod or whatever. In fact if you just connect the RF probe to an aerial and earth with no radio attached to it at all, you'll hear several stations all on top of one another!

It may take a little time to get the hang of adopting this technique to anyone not already familiar with it, but once adopted, it saves hours of headscratching peering into a rats nest of wires and components wondering just where that elusiver fault might (or might not!) be.

The Velleman Kit K7000 consists of a PCB and all the board mounted components including the volume controls for the tracer and injector, both of which are mounted directly on the PCB. In addition, you need a small speaker, switch, LED (if desired) sockets, a box of some sort of project box in which to house it. I made a little comb-jointed oak box as I do for most of my litle test gear items, and as a front panel I used a scrap piece of PCB sprayed white.

You'll need to make a couple of probes. For signal injecting and AF signal tracing I just used an empty marker pen, with a probe tip made from brass rod sharpened to a point, and a 0.1 630V cap inside simply as a precaution to prevent damage to the tracer if the tip was placed on a high voltage point in a set. It's kinder to apply the tracer probe to the grid of a valve after the DC blocking capacitor than at the anode of the previous stage, but at least the cap in the probe will protect the tracer. For the RF probe, I used a little plastic pill container and made a simple diode detector on veroboard, (again with a 0.1 630V blocking cap to protect the probe and tracer from high voltage), but the few components could jut as well have been hard-wired to each other 'ugly style'.

I've attached the simple circuits of these two probes - the AF probe is used both for AF signal tracing and injecting - the RF probe is used only dffor signal tracing in the RF/IF stages prior to the detector.

The K700 Kit info and contraction manual can be found here:


http://www.vellemanprojects.eu/downloads..._k7000.pdf

You can buy the kit from ESR Electronics at this link:

http://www.esr.co.uk/velleman/products/index_kit.htm

The attched pics show the one that I made. (The speaker was glued on with a hot melt glue gun).

Hope that's of interest and these wordy notes make sense!

I must say it is a fantastic piece of kit! Beautifully made in a fine cabinet, a must for every bench!

cheers Mark

(25-11-2011, 05:46 PM)Mark Ryding Wrote: [ -> ]I must say it is a fantastic piece of kit! Beautifully made in a fine cabinet, a must for every bench!

cheers Mark

You should know Mark!

Don't I just, as the honored recipient of this I have to say it is a must!

cheers Mark

Thanks for posting this David. I did buy this kit several months ago after reading another one of your postings but at that time I was just getting into the hobby and working on transistor sets for safeties sake. As I've now not touched a transistor set in months it went away in the drawer and I forget it was there. Though I've never used one on a mains valve set I will get some probes made up and give it a go.

Re: the box, do you use a router or a gentlemans back saw for the joints?

Andrew

(25-11-2011, 09:47 PM)Andrewausfa Wrote: [ -> ]Re: the box, do you use a router or a gentlemans back saw for the joints?

Andrew

I made a comb joint router jig from scrap as they're quite expensive to buy but not difficult to make. I've attached a few pics to give you an idea what it looks like and how it's used, which I hope may be of interest. The jig consists of two main parts - a table under which the router is fitted with the bit protruding through a hole in the centre of the table, and a 'sled' which slides forwards and back. The sled has a 1/4" peg at its base, and the workpiece is held vertically againt the sled up against the peg, then moved forwards for the router to cut the slot. As each cut is made, the notch in workpiece if placed over the peg and the sled moved forwards to make the next cut. In use, the hardest thing to remember is how to make the next piece mate up with the one that's just been cut, but after a few tries, you get the hang of that! (I used any old scraps of oak that I can find from old furniture, oak flooring offcuts etc, which I cut to size on my bandsaw, then plane them smooth by hand).

I hope the pics help to clarify how it works.

I made an excellent audio signal tracer out of an old pair of PC speakers. The type I am talking about have both amps in 1 cabinet, the second having only the speaker. What I did was to find the input wire that feeds the amp and speaker in the cabinet and cut it where it joins the PCB. Put a wire link from the speaker output of the other amp to where the wire was cut. Use the remaining input wire as the signal input. The addition of a DC blocking cap and a BNC socket completed the job. Doing this mod puts the amps in series.
Tests show that I can easily resolve 1mV RMS without too much internal hiss. One advantage of doing it this way is that, because the signal inputs go directly to the volume controls, each amp still has its own gain control. This gives a wider range of signal handling(assuming you don't overload the input amp. The volume control for the second amp also acts as a DC bleed for the previous speaker coupling cap.
The volume controls on mine are 50kohm so not too bad an input impedance.
I am doing mostly audio work these days so I have not got round to building a diode probe and trying it yet.

Al

Thanks for reading the thread Al, and for your comments. Yes, those little computer amp/speakers are excellent in that role, and though it's easy to derride them due to their ludicrous ouptut power claims (50Watts RMS etc!) their reproduction is actually quite repectable. I've got a few which I've picked up at car boot sales for a pound or so, for which you get a pair of reasonable speakers, an amp, and if you're lucky, the 'wall wart' PSU to power them. As you say, a DC blocking cap is essential because if you inadevertently squirt 150V or so into them from say a valve anode, they don't like it up 'em!

As to signal injectors, I've attached a couple I made long ago - one in a Woolworth's icing syringe on a piece of perfboard in the late 1950s in my late teens. It's still going strong as the scope trace shows! I made the other in the late 1980s, built into a felt-tipped marker case with the few components soldered together 'ugly style' so they'd fit into the case, along with a small battery. There are scores of circuits for injectors on 'tinternet. I do have a soft spot for the Vellman K7000 injector/tracer kit as it's so small and neat, and the PCB is self-supporting behind the front panel with the two on-board volume controls.

Over the years, I've found the injector/tracer technique by far the quickest and easiest way to trace faults. I think the quickest fault I traced was on a dead Bush TR82C for a neighbour. A signal injected at the volume control slider showed that Audio amp stage worked fine, and there were tunable signals up to the IF stage at IFT3. It had to be the detector diode and it was. In the TR82, the diode lurks inside the IFT3 can, which I asume was the Bush designer's little bit of mischeivous fun to hide it there as there's ample space for the diode to be out in the open on the chassis. They were having a laugh at our expense, surely! Two minutes to find the fault, ten minutes to replace the OAA90 diode. Without an injector/tracer I could have been faffing around for ages, checking voltage readings and generally poking around at random, suspecting transistors and so on.

Hi,

Thanks for adding this thread David; it gives a lot of useful advice to novices such as me.

As usual you've made this to the highest standards and it looks very professional indeed. My chum Mike Phelan kindly put me a kit together and I must get around to assembling it as these appear to be a must have piece of test gear.

Regarding making the comb jointed boxes; I can heartily recommend making the router comb jointing jig as David very kindly sent me construction details of the jig a while ago.

I made mine from offcuts costing me nothing and once assembled and set up it only took a couple of sample pieces before I was cutting perfect tight fitting comb joints. For anyone owning a router this is yet another very useful bit of kit.

I made mine larger to suit the offcuts I had and also added a length of 4"x2" softwood screwed to the underside of the table to allow the jig to be mounted in the vice.

I used to cut comb joints on both my band saw and table saw but this jig once set up is so convenient and rapid to use.

Sorry to hi-jack your thread David but here is a box I made using the jig. I've not tried it but I think with a bit of determination and using the correct cutter dovetail joints might even be a possibility?

Kind regards, Col.

[attachment=3422][attachment=3423][attachment=3424][attachment=3425]

Whatever epithet people may use to describe you Colin, 'novice' isn't it!

By no stretch of the imagination can the mahogany comb-jointed, French polished box, the brass nameplate with its hand-cut lettering, and the coil winding gears that you designed and made, which the box contains within, be described as the work of a 'novice'. And no - you're not hi-jacking the thread - you're adding to it Colin. Hopefully the pics might encourage others to have a go. Once the jig is made and set up, it's surprising how quick it is with practice to make nice box from offcuts.

As to dovetails, I think you've set youself a challenge!