Oscillator, Test, No1. CT212

[QQVO6/40]

Greetings all. I have on my bench now a CT212 sig gen.

I was wondering if it is worth going through it and getting it going.
Not a question of it's monetary value but purely it's value for the intended function of radio repair.
I do already have other sig gens that are quite up to the task of radio alignments and such so this one is just another challenge to keep the brain active.
37 caps of various types to replace. Mostly waxies.
Just looking at it I would say it is in quite good shape internally. Seems to have been owned by a radio tech somewhere in it's life as the meter has been replaced by a small Asian meter and had a warning label attached over the otherwise exposed mains terminals on the power transformer. I would guess the previous owner has been bitten by the exposed terminals.

Silly question really isn't it? I am asking if it is worth doing it! Where am I? Vintage Radio Forum. What is the classic answer I am going to get?
Just do it of course!

I will get it going.
Just was wondering if it is a worthwhile bit of test kit or do these have bugs and foibles enough to keep them off the regular test equipment list.

Just wondering.

Thanks everyone for reading this and a special thank you to those who tickle the keys to reply to this post

Cheers, Robert. VK2BNM (VK2 Broke No Money)

[ronbryan]

I was given my CT212 by HKS of this forum some years ago and use it for alignment of AM radios and FM IF transformers. As the maximum frequency is 32MHz you need to use harmonics for FM RF alignment. The generator must have cost a fortune to make as every component is stencilled with its circuit diagram reference number. I think they were part of the Larkspur military radio project, hence the diecast case.

I had to change some of the metal cased TCC capacitors and eventually the HT smoothing caps to deal with some problems on various ranges, but did not do a blanket replacement of capacitors. Some of the capacitors are quite hard to get at, far down inside the case.

The RF output waveform is distorted with harmonics at 85kHz, but is much cleaner from 270kHz upwards. At least one other example of the CT212 belonging to another forum member performs the same at those frequencies. I put it down to excessive feedback in the oscillator section at low frequencies as the buffer amp and output stage seem to be ok.

Ron

[Refugee]

I have got one.
It is the highest band that has the harmonics that will most likely go up to 100Mhz.
I have never tried to power it from DC.
For an FM radio front end it may be worth using a parallel tuned circuit between the output and a bit of wire next to the aerial on the set under test.

I have a photo for anyone who does not know what one looks like.
It is a perfectly good RF generator so I see no need to obtain anything else.

[QQVO6/40]

Greetings all.

Thank you for the speedy replies.
I did look up the other posts on the forum and took note of the comments.
Not one person has rubbished this sig gen so it will be a good one to get going for the test equipment lineup.
Refugee.
I have other sig gens for hf, vhf and uhf tasks as well as various Marconi sig gens with lf capabilities so I don't "need" the 212 but what else am I going to do with it. I am just inducting our son into the antique radio world and it would be a good one for him to get used to doing lineups on simple broadcast radios. Classic 455kc ifs and the like.
Ron.
I have quite a bit of Larkspur stuff so it will fit in just nicely.
I have read comments from other people as well about the harmonics below about 300kc so I will keep that in mind. Will not be a problem as I have a HP audio sig gen that works nicely up to nearly 10Mc. The only thing I am thinking where I will need a low freq sig gen is a Marconi B29 that is waiting it's turn on the bench.
All good!
There are so many potential waxie failure points that I might just shotgun the caps and just replace the lot.
Yes; it is a juggling act doing it as it really is better to do it one at a time or cut and test but I am thinking in this case I might give it a try.
What could possibly go wrong?

All part of the fun.
Thank you all for the comments.
Cheers and 73.
Robert, VK2 Broke No Money.

[QQVO6/40]

Greetings everyone.

My little beauty.
Now undergoing repair as I have found a lot of out of spec caps and a few resistors.


Plus one weird wire wound resistor.
R58, 10k ohms 6 Watts.
Someone else had been there before as this resistor had been removed and then just tack soldered back in place. Not very professional.
I removed one end and it measured 6.47 ohms with 2 digital meters and a good Avo8 analog meter. All of the meters are in spec.
Ok the resistor is u/s.
Removed it and tested it on the bench and it measured 9.97 ohms ??
Nothing I could do would make it go back to the lower reading. Hot or cold no difference.
Weird.
How many wire wound resistors do you see go low? High or open circuit yes but low?
The resistor shows no sign of stress.


I have replaced it with 2 22k ohm resistors in parallel.

The fun continues.

Cheers all.
Robert.

[ronbryan]

I've never seen a CT212 with the control knob caps still fitted. Were they picked off by bored squaddies, or were they never fitted?

Ron

[QQVO6/40]

Hello Ron and everyone.
This and the other one owned by Ray who did the restoration published on his website and the Royal Signals website are the only two I have seen. Both of which are here in Australia.
We don't have "squaddies" here in Australia so that rules them out.

I would say the CT212s have come from the factory as we see them, sans knobs.

The knobs themselves seem to be quite solid and functional so adequate for the job.

Medical stuff for me tomorrow so I will get back to the sig gen when I can.

Cheers all, Robert.

[2000 type]

Well worth restoring for what it is, and to keep another bit of vintage kit in working order. But, bear in mind that it is designated as a Test Oscillator and not a signal generator which would normally imply a rather higher standard of accuracy.

The intended role for the CT212 (and its VHF companion the Oscillator, Test No.2, CT210 which covered 20 - 80MHz) was for mobile first level Field workshop units who used them to perform basic functionality and simple repair tasks.
The frequency readout is not very accurate and suffers inevitably from parallax error, and there is no provision to feed a frequency counter or wavemeter. The item under test was used to provide the accuracy with the army equipment being catered-for usully having internal crystal calibration and accurate enough frequency display to suffice.

One poor design point is that the tuning drive consists of a rubber wheel running on an aluminium disc. Any hint of oil causes it to slip, or more pertinently given its age, the rubber has hardened and loses its grip.

Your one has had a meter replacement with a wrongly scaled commercial meter. It should be an un-scaled centre-zero meter to give a simple set point for the level controls.

I don't understand the comment about "control knob caps", the round ali caps cover the mains and 12v power input plugs.
Which illustrates another design problem - there is no supply changeover facility, so it is physically and electrically possible to connect mains and 12v DC at the same time. As both supplies use the same transformer the result isn't good! The caps are supposed to prevent this.

The design originated with the Signals Research & Development Establishment (SRDE), manufacture was by Airmec who designated it Type 871 and 872. The sealed case style goes back to the Wireless Set No.42 of 1943.

[ronbryan]

Collet knobs often have a cap pressed in to cover the screw that tightens the collet.

Ron

[2000 type]

They are not collet type knobs. The end of the control's shaft is tapped and the knob is held on by a 6BA screw. "Rotational drive" is by a cross pin through the shaft which sits in a groove in the under side of the knob.

The knob style is the same as used on the R209 receivers and other war-end origin kit.