Golborne Vintage Radio

I got this curious device sometime ago off a fellow forum member. It is a Level Measuring Set #30A and Oscillator #132A. There was conjecture at the time that it was BBC , but it looks military being painted Navy grey, though why would the Navy etc be wanting an AF osc and recording meter? It is also in a 19" rack size chassis. I can't find anything at all about it despite numerous attempts.

I've finally got it working after a few attempts feeding it 22v DC, anything less results in clipped sine OP. The osc goes from 20hz - 20khz and the dial is bang on according to my frequency counter and goes from about 100mV - 6v P-P. The OP does have stability problems though, turning the freq dial sometimes kills the oscillation and it has trouble reviving. The OP also seems a tad touchy to various loads, my distortion meter attenuates the OP by about 90%, my AC meter by 20% ish. Also the frequency response isn't flat being attenuated at low frequencies and at the extreme HF end.

I got a THD reading of 0.03% yesterday, but got readings of 10% today @ 2khz. It could be that I have it powered incorrectly, it may need split rails, +15v 0v -15v say. Unusually chassis/ground is positive.One odd feature is the meter, it reads +/- 1dB, not the actual level of the OP.

The rec level meter records the OP in dB, but the attenuator doesn't match that of the osc. IE it stops deflection as the osc level decreases.

Both are very well constructed, from the dates on caps circa 1976 and both use germanium Q's and diodes. The frames are made of that big lads Meccano, sorry not sure of correct name. Both also use Sifam meters, gold plated connectors and decent pots as well as having custom made tfmrs/coils/inductors - I presume.

I can't indentify two components - RY1 & 2, RY1 looks like a point contact diode, RY2 a light bulb. Could it be a Wein bridge osc as the OP is boingy?

Any ideas or info would be welcome. I doubt I'll keep it as I have several better sig gens, drop us a PM if you have any similar gear, I may consider passing it on.

Andy.

PS, I'll be posting a video on Youtube at some point that looks at it in more detail.

The BBC did use that colour for some of its audio kit though the type numbers look wrong for BBC. Possibly GPO?

Can't see RY1, RY2 might be a bead thermistor?

Lawrence.

RY2 almost certainly a bead thermistor. Haven't found RY1 yet.

The once common R53 thermistor, often used in wien bridge oscillators, looks rather bigger than RY2 in the photo:
https://www.silicon-ark.co.uk/r53-thermistor-by-itt

(19-01-2018, 11:55 AM)Diabolical Artificer Wrote: [ -> ]Unusually chassis/ground is positive ...

... both use germanium Q's and diodes.

Not unusual at all! The germanium transistors will be PNP devices. If built in conventional manner with emitter connected to ground and collector to HT (via other components, of course!) then the HT will be negative and ground positive. Look at just about any early transistor radio circuit and you will find exactly the same arrangement.

NPN transistors, of course, work the other way up so the introduction of silicon transistors saw the return to a 'conventional' valve type arrangement with a positive HT rail.

Then they brought out the VTR103:

https://www.doctsf.com/documents/schemat...VTR103.PDF

Lawrence.

The first thing I would do is to go in with an analogue volt meter or an electronically amplified milivolt meter and locate the capacitors that smooth the feedback from the two banks of diodes next to the silver can in the big chassis and the smaller grey box in the smaller chassis and any others.
It will most likely be a thermistor controlled AGC arrangement. It looks like there may be more than one thermistor based regulator in there.
28 volts is common on air force gear and many other bits of off line DC kit. Most kit will run from a transformer and minimal smoothing from the internal regulator.

Thanks chaps and ta for the links Jeffrey and Lawrence. RY2 does indeed look like a R53 thermistor. funny that the thermistor is worth more than the whole thing. Your right Terry, of coarse + ground makes sense if using germanium devices as they're PNP, didn't think of that. Why do you think they used germanium devices, surely silicon was cheaply available in 76?

So you think it needs an AC power source rather than DC Simon?

Video here - https://youtu.be/RG-SJm-FxqQ, you'll be able to see RY1 & 2 clearer and understand a bit more (hopefully) what i've been waffling about. : )

A.

I would expect it to be DC unless there is a hefty transformer in it.
Air force stuff was either 28 volts or AC mains up to 400hz. If it had an internal rectifier there would in almost all cases be a mains transformer.

(19-01-2018, 03:08 PM)pwdrive Wrote: [ -> ]Then they brought out the VTR103:

I hadn't realised that before, Lawrence - or I'd forgotten - but I did say: 'Look at just about any early transistor radio circuit' , not every circuit. I was aware of some exceptions but couldn't recall any of them off hand.

Doesn't that VTR103 circuit look odd, with everything hanging off the earth rail like that?

I would agree, Andy, that germanium transistors were mostly forgotten by 1976 but, if the design worked and the units were produced over a span of several years there would be no reason to change it. These units would only be produced in small number so were hardly likely to have any impact on remaining germanium stock levels.

By the way, are you familiar with the geranium diode?

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