Station Radio C-12

[IK0MOZ Mario]

I just added to my collection a British station C-12. I am satisfied for this last arrival but on my set I noted :
1) No sidetone in CW
2) With the GAIN controll to minimum the audio output is too high and demodulate the SSB is quite difficult or impossible.
This is normal or my set is faulty ?

[ex seismic]

Usual advice with the 19 Set for SSB is CW mode, AF gain up high and adjust RF gain to suit. The C12 is a development from the 19 so this advice may work with it.

Gordon

[G6Tanuki]

Yes, demodulating SSB can be tricky as regards getting the gain balance right.

I have a fond memory of using a SR C12 (powered by two enormous steel-cased 12V batteries in series) as part of my school cadet-force signals unit. Managed to get a 200-plus-foot longwire antenna put up, one end being at about 70 feet high. With this we worked all over the UK on the CCF 5MHz allocations and once as far as Germany.

The only problem we had was speech-breakthrough on the intercom between the headmaster's study and his secretary!

[HamishBoxer]

Just a point I cannot see an RF Gain on this set.That could make trying to resolve SSB a bit tricky.

[G6Tanuki]

From what I remember, when set to "CW" the gain-control acts as a RF gain.

[Peter.G0DRT]

I also have a C12.There is no side-tone facility. I found the same problem with too much gain even when set at minimum. I found that increasing the value of the IF screen feed resistors cured the problem and copying SSB is no longer a problem.

Regards, Peter.

[G6Tanuki]

Remember that the likes of the C12 were designed to be used mobile in tanks with just a single unloaded 12 or 16-foot whip antenna, rather than a longwire or dipole. So by design they have plenty of front-end gain! Hooking one up to decent antenna like a longwire or a 1/2-wave dipole can be expected to cause overload.

It may be worth seeing if you can increase the amount of BFO coupling to the detector-diode: there's usually a [very small value] capacitor for this - replacing it with a trimmer-capacitor would allow you to vary the BFO injection until you found the right amount.

[IK0MOZ Mario]

HI, TNX to all for suggestions and comments. I just downloaded manuals from VMARS archive and hope to solve all problems. In pratical use seems that the sidetone facility is working only in Voice operation and the same happens with Gain control because on Voice at the minimum the set is pratically mute. This is very strange because on this way seems that also if the CW mode is in the set possibility, working in this mode it is no easy. The receiver operates in break in so when the key is up arrive on the phone a strong signals/noise without the possibility to decrease for the Gain problem. Looking the manual the C12 is designed to work with rod antennas using an ATU. I have not the ATU and in any case I will use a standard dipole or inverted V. Any suggestion fot this operation will be appreciated. TNX de Mario
PS: I have many British-CommnW sets all perfetly working (WS18, WS48, WS58, WS19MKIII, ZC1MKII, C11/R210, C12) and in turn used on OM bands. No commercial set on my shack.

[trh01uk]

As shown, your C12 is missing a critical part of the set - the ATU. Its only a roller-coaster inductor so if you can't find an original any HF roller coaster should substitute. Study the schematic and you will see what I mean - without the ATU there is no matching circuit or any selectivity at all. The set is designed to operate with the ATU straight into a 8' or 16' whip - and really nothing else.

Even then, you will find the C12 has a serious deficiency which is that it has virtually no front-end selectivity. Breakthrough from broadcast stations can render the set unusable - particularly if you hook it up to a long wire or dipole.

Pye tried to improve on the WS19 - and it was an improvement in that the PA has high level modulation. It gives a far better quality signal than the older WS19 - and it was thought equivalent in "talk power" to the WS19HP - that is the WS19 plus its Amplifier RF No.2. But lack of progress in technology defeated the designers - there was simply not enough room in the box to pack in everything they wanted to include. They didn't help themselves by abandoning the WS19s clever flick system, for a duplication of tuning elements, which take up far more space.

Richard

[IK0MOZ Mario]

@ Peter : TNX means that my set is correctly working
@ Richard : Right, I homebuilt an ATU using parts of the ZC1MKII aerial tuner. See attachments. After that I have on antenna (inverted V) around 7-8 W on 80 and 40 m. I noted that on 80 m adjusting the ATU there are 2 power picks, one on 80 (more coil) and the second on 40 (less coil). With the ATU the Gain control works better and demodulate the SSB on CW Mode now it is possible.

[trh01uk]

Yes, your additions will work fine. What you have now is a pi-coupler, when your inductor and variable capacitor are combined with the tuning capacitor in the PA anode of the C12. That will give you plenty of flexibility to match various load impedances.

Richard

[G6Tanuki]

Remember though that a pi-coupler is essentially a low-pass-filter. At frequencies below the frequency it is tuned-to it will provide less than ideal selectivity, allowing a rather unrestricted path for signals on such frequencies to get straight into the RX RF-amp.

If you're using a longwire-type antenna and are near a powerful medium-wave broadcast station you may still have crossmodulation issues.

[trh01uk]

Yes, its a tricky problem to get a C12 to work well. The cross-modulation is severe when there are large broadcast signals about. I have literally had to abandon using the set because all I could hear (regardless of the tuned signal frequency) was a local AM broadcast station.

One solution might be to keep a PI network on transmit, but to switch in a pre-selector on receive (only) that has at least a pair of coupled resonant tuned circuits. The loss through such a pre-selector is not particularly important on receive, though you would not want that loss on transmit.

Richard