CT212 Oscillator Test No1

[Refugee]

I have begun doing one of these ex Royal Navy signal generators. The build quality is very over engineered by today's standard but for us fans of well engineered stuff it is a fantastic example.
I dived in and put my Capacitor reformer on and immediately it came up with a false alarm. I found a potential divider that would account for it and once I allowed for that we were on the Variac.
   

It was the potential divider going from R24 to ground biasing V5 that did it.

I soon had it up to full power and everything works but with some work to do as there is clipping on the carrier signal even with the correct termination resistor.
I then went for the cap reformer and lowered the limiting resistor to below 1K from 100K and checked most of the grids to see if there was a leaky capacitor in there but came up with no conclusive result.

I have had the scope on V2 and V3 and found that it gets worse around V3.
Next session will be with the X10 probe and the scope on DC to see of the waveform is getting close to the power rail or ground and then look for a biasing problem.

I have two of them to look at so at worst I will have to clear the bench and get the other one open.

Does anyone know what the output connector is called as I only have one and could do with another.

[pwdrive]

Possibly a Burndept connector ?

Lawrence.

[AlanBeckett]

Maybe. Have a look in here - http://www.electrojumble.org/Connects_Coax.htm

Alan

[Refugee]

It looks like they have the chassis part but not the cable mounted part.

[AlanBeckett]

Ask him - Roy. He's usually very helpful and not everything is on the Website.
I've had some pretty obscure stuff off him in the past.

Alan

[Refugee]

I will give him a go but first armed with the connector type it may be of value to me to do a much wider search.
All potential sources are worth a good look.
I did once try him for a 4 pin Plessey MK4 connector and ended up doing a swap with another forum member for a 3 pin one just like the mains input one on the CT212 These also being used on valve testers and comms receivers.

[Refugee]

I did say in post two that I have another one of these signal generators and it has come to the head of the do do pile while the internal layout and diagrams are to hand.
This one has a report that the meter is open circuit.
A meter of the same outline with a scale showing 100ua FSD was with this instrument.
For some strange reason there was an EC90 taped in to the space between the smoothing choke and the chassis. This is not related in any way to any of the valves in the instrument?
Also some notes have been scribbled onto the chassis.
Before replacing the meter the fault that damaged it must be found so the cap reforming started with an allowance for a potential divider across the HT.
R24 (first photo in post #1) started to get hot during the second phase of cap reforming.
This was very intermittent.
Previously an engineer or twiddler had been probing the center tap on the audio oscillator coil L7 to such an extent that the wire that leads into the core had become damaged. I had to dig all the varnish out of the tag on the coil in order to get a good connection again.
This work revealed another intermittent short circuit between the center tap of the coil and ground.
C19 was getting hot and making a noise so that was next and also one end of C74 came off its tag so I tested it and found it reading 3 meg so I replaced both of them.
The intermittent short in C19 would account for pulses getting into the meter causing it to fail.
Some how the diodes had survived and did not need to be replaced.
With a DMM attached I powered up with the Variac and all was fine so success was just a matter of replacing the meter. I had to open out the solder tags due to the terminals on the new meter being larger.
The meter Is reading slightly high so a check of C20 and C105 will have to be done before the covers go back on.

[AlanBeckett]

What's the Output Waveform like on this one Lawrence?

Alan

[Refugee]

The output waveform is the same and also another owner of one has posted his output waveform and it is also the same so they are all like it.
I will have to look for some connectors and make some external filters.
The waveform is a lot better on the higher bands where it would have been used the most in its day.

[ppppenguin]

The output waveform is the same and also another owner of one has posted his output waveform and it is also the same so they are all like it.
I will have to look for some connectors and make some external filters.
The waveform is a lot better on the higher bands where it would have been used the most in its day.

That waveform you've shown us is very rough indeed. I would have expected better from a serious piece of professional kit.

I suppose the valves just aren't quick enough to muck up the waveform at higher frequencies. I'm sure this is a daft question but I'll ask it all the same. Are you using a scope with sufficient bandwidth to show harmonics of the higher frequencies?