Level Measuring Set 30A - Oscillator 132A ??

[Diabolical Artificer]

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.

.

[Diabolical Artificer]

Video here - https://youtu.be/RG-SJm-FxqQ.

A.

[ms660]

The knobs on that unit look similar to what Wayne Kerr used on some of their stuff.

Lawrence.

[MotorBikeLes]

Is your "light bulb in fact a glass bodied thermistor? That would also explain a "bouncing" waveform.
Les.

[Diabolical Artificer]

Whoever made it Lawrence there must have been a few made. We think it is Les, an R53 no less.

Any ideas why the osc OP meter is a centre type with only a two dB range anyone? I can't see this being used in a recording studio for reel to reel setup/calibration but what about computer tape? just riffing.

A.

[ms660]

Any ideas why the osc OP meter is a centre type with only a two dB range anyone? I can't see this being used in a recording studio for reel to reel setup/calibration but what about computer tape? just riffing.

It looks like it's for "Set Level" Similar to what you would find on some RF signal Generators, Avo CT378a, Marconi TF144 etc, setting the reference level for the attenuator input.

Lawrence.

[barrymagrec]

Looks BBC ish to me, for line transmission work - or just possibly, BT.

[Radio Wrangler]

It's made to produce only certain levels one for each position of the oscillator's attenuator switch. It isn't designed to produce levels between those. So the levels on the knob will relate to some specific test or alignment procedure.

You probably can get intermediary levels by turning the pot, but the meter will be in unmarked territory and the level will not be calibrated.

The format of the model number string engraved at the bottom may be recognised by someone.

The military, the BBC, and the Post Office had the clout to get a special like that made for them. Audio companies used off the shelf test gear or made their own. Ferrograph made their own and sold some of it... RTS1, RTS2 for example. They were also part of the same group as Wayne-Kerr.

Collet knobs like those cropped up all over the place... my old AVO CT38 had them with black caps.


David

[ms660]

Here you go Andy, more info on the oscillator here, not the whole story but it might help:

www.mw0llo.com/audiooscillator.aspx

Lawrence.

[Refugee]

It has got an AF117 in it

[Radio Wrangler]

Hmm, the ability to run off -50v does suggest telephone system use.

Probably no AF117, but it does look like it has a couple of small bead thermistors, like the R53. and those components are getting scarce.

David

[ms660]

If we knew of a manufacturer that uses VT,MR & RY for those circuit component identifiers it might narrow it down, I tried Wayne Kerr which was ok for VT & MR but no RY.

Then there's the KCC.

Lawrence.

[bc312]

We used these in Post Office telephone repeater stations (TRS) in the 1960's. The pictures show a 19" rack mounted version, but there was also a cased version which was more common and portable; used batteries or the 24 volt repeater station supply. Used for 'lining-up' 2 or 4 wire transmission circuits and designed to take 600 ohm terminated or through measurements. As suggested the Level control is adjusted to read zero depending on the attenuator setting. Post Office instruments usually had a 'calibration' sticker to show when the instrument was last calibrated by the labs, and a date when the next calibration was due. This instrument was superceded by the Measuring Set 44C in the 1970's - a Wayne Kerr manufacture, and an example of which I own.

[g8jzyian]

Hi
It looks a lot like some STC ( Standard Telephones and cables) equipment that i still have in the attic.
Ian

[dseymo1]

bc312 - Was there an earlier valved version of this set, made by Solartron?
I ask because I had an incomplete instrument with much the same functions, and was never able to confirm its original application.

[bc312]

The only cased (portable) valve instrument in my TRS was an Oscillator No.28A - although this was manufactured by ST&C. Not an audio oscillator, but used on 12/24 circuit carrier systems and 4MHz coaxial line systems. The 28A had six selectable bands covering 50kHz to 5.2MHz. Valve line-up included four SP41 (ST&C branded) and a CV1078. 75ohm impedance, 24 volt TRS supply or separate mains PSU. Was one of the many test instruments I grabbed when the station closed. There were 19" rack mounted instruments for audio frequencies, but no idea of model or manufacturer now; all painted battleship grey of course!

Mike

[Diabolical Artificer]

Brilliant Lawrence, that's the kiddy, don't know how you do it, (probably spending too much time at the PC when you should be out with the chainsaw : ) Gobsmacked, thank you.

"We used these in Post Office telephone repeater stations (TRS) in the 1960's.2 But this one was made in 76?

Think I've seen RY used in Marconi gear Lawrence , just looked, no, but they use MR. Again, good thinking.#

So the consensus leans towards it being for telephone line testing and possibly PO.

Thanks, Andy.

[keithwicks]

I own an Oscillator No. 132A and a Level Measuring Set No. 30A. I had failed to find out anything about it because my search included the name Farnell Instruments Ltd. I believed they were the makers as their name appears on a label on the back of the oscillator case. Another label of theirs appears on the inside of the unit. But were Farnell the makers or just users of this equipment?

Although it is called an Oscillator No.132A Spec RC5480 on the front panel, the Farnell labels identify the unit as Model F 2227. Interestingly, they give the serial number as 000024, so either it's a "rare" early model, or very few were made. Or perhaps it was just Farnell that didn't make (or use) many of these instruments.

My oscillator is very similar to the one shown by Andy and that pointed out by Lawrence (many thanks for that link — the circuit will be very useful), but mine has two output jacks on the front panel and a switch to apply a 600 Ω load. Also, the meter showing output level is labelled at ±1.5 dB, not that that's of much importance. A label bearing the 1980-1991 BT logo and the words Calibration Services was attached to the cover (and had to be broken to get inside). Three large capacitors at the top of the unit had been replaced by more modern, smaller ones. A hand-written note on one edge of the oscillator chassis states Farnell 1973/1.

The Level Measuring Set does not bear any Farnell labels, but the front panel identifies the unit as a No. 30A Spec RC5480, so it appears to have been originally designed for use with the above oscillator. A hand-written note on the chassis reads GEC (1972). Like the oscillator unit, it had a Calibration Services label attached. The measuring set contains at least three PNP transistors, which explains the positive ground connection. My version of this unit has front-panel terminal posts for the input.

The cover of my measuring set appears to have been professionally modified after manufacture. It now contains a small mains power supply to provide separately fused -24 V supplies to the two units. I say "professionally" because the quality of construction is excellent. But, strangely, the fuses are in the positive lines, whereas they should be in the negative lines in this case. I've not spent much time with this equipment, but a brief test showed it gave a steady output and the level calibration was spot on.

I didn't come across any similar instruments when I worked for the BBC in the 1960s. Everything was 600 Ω then, but the BBC generally used their own equipment (PTMs — Programme Test Meters) for measuring audio levels in those days. Things could have changed later, of course, so I'll check with some old BBC technical people and see what they say.

If I can help with any more information, such as voltage measurements, just ask. When I've had time to investigate further, I'll probably have a question or two of my own.

Keith Wicks

[Diabolical Artificer]

Thanks Keith, interesting. Perhaps these were PO and yours ended up at Farnell in the testing dept. Farnell AFAIK only made scopes ( re badged Scopex? ) sig gen's and famously PSU's. The construction isn't Farnellish, but who knows.

Your probably right that only a few were made, the construction doesn't look like a commercially made product.

Hopefully any other owners, will in future find this thread and contribute more info on these cracking little oscillators.

Andy.

[keithwicks]

Andy: Since I wrote about my own equipment, I found that you had posted the promised video on YouTube. Interestingly, the construction of my units is entirely different from yours, although I imagine the circuits are almost identical. I don't have much free time at the moment, but I'll eventually get around to posting some photographs of my units.

Note that my measuring set doesn't contain a toroidal transformer. That looks rather out of place behind your meter, so I suspect it's a later add-on — but what for?

I have the same 16-pin Mcmurdo plugs and also the 16 soldering posts (is that what they are called?) on the backs of the units. Posts that have internal connections to them are paralleled to the corresponding pins on the plugs. What I'd like to know is what all the connections are for and also what links are normally made, and why. (I think your links differ from the ones on my equipment.) It would take time to work this out, so I'm hoping someone else can provide the information.

On my oscillator, the 24 V supply is fed to post 1 (-ve) and post 9 (+ve). There is a connection from +ve to chassis. Posts 10 and 11 are linked externally.

On my measuring set, the 24 V supply is fed to post 2 (-ve) and post 9 (+ve). There appears to be no connection at all to chassis in this unit. External links have been made between posts 10 & 11, 7 & 15 and 8 & 10.

I was thinking of linking the two units using Mcmurdo 16-pin sockets — until I discovered that they now cost over £30 each! So I'll use something simpler to connect the PSU (added to the measuring unit) to the oscillator.

One last question: under your video, you mentioned that this equipment resembled "the famous BBC oscillator set". Which set was that? Do you have its official name and number? I was not aware of it, but I had left the BBC by the 1970s.

I'll keep my comments confined to this forum, but you might want to alert YouTube users to the information on this site (if that is permitted).

Keith Wicks