A 19 Set Overhaul

 Looking at the date attached to these photographs I'm amazed to see that this overhaul was carried out about 10 years ago.

I thought that finding my technical report may pose some difficulty, but in the meantime here are some pictures of the set.

I eventually found the report which follows the pictures ! It was dated September 2004.


Below is a copy of my invoice

For: Repair of Wireless Set No.19 MkIII, S/No 65861, Rebuilt 7/59

 The set is in very good general condition and from their appearance the valves are in new condition. The top corner of the front panel is badly bent and the meter glass is cracked but otherwise the set is in very good order.
The tuning controls were rather sticky and improved considerably when I'd refitted them. I had to file off about a sixteenth of an inch from the rear of the main tuning knob as this was rubbing on the mechanism.
I temporarily removed the components in the area of the dent and straightened the metal using a heavy bench vise as a press.
Before connecting the set with its ancillaries I tested the resistors to ensure that these were within tolerance. In the event no resistors needed to be changed. The decoupling capacitors are metal-cased and none of the critical components showed any significant leakage.
The set was then connected to its various ancillary parts and a quick test was made to ensure it was generally functional running from a DC supply. To carry out further work I had to make a test rig.
I identified the power input connections at the top connector and fitted temporary cables for carrying 265-volts DC for the main HT and 12-volts for the valve heaters.
A cable was clipped to the audio output connections at the lower connector and this was connected to an audio amplifier and speaker.
The set received reasonably well on its two wavebands but performance improved considerably when the IF strip and front-end tuning were aligned.
The aerial tuning control was noted to be slightly out, as if the inductance of both the tuning coils was a little high. As the dial is set for optimum reception this is of little consequence.
I connected the set to an RF power meter and checked its performance.
The set employs a mixing arrangement so that there are several frequencies present within the RF amplifier. It's important to select the correct frequency when tuning up the set as it's possible to tune to the (lower) image frequency produced at the output of the mixer. I identified the correct signal and aligned the set for best output on the 40m and 80m amateur bands.
To produce RF output one needs to connect an HT supply to the RF amplifier. In the test rig I merely connected this to the 265volt receiver HT supply.
I noted an RF output of around 6-wattts or so which will improve when the full transmitter HT is supplied.
During testing I noted that in the CW setting the receiver produced what sounded like a continuous side-tone that completely masked the audio. This turned out to be audio feedback resulting from insufficient loading at the audio output valve and is a known design fault. I connected a damping resistor across the audio output as recommended in the Army modification instructions and this cleared up the problem.
The BFO is switched on in the CW setting and it may be varied by the front panel control however, in this setting, the bandwidth is significantly reduced for Morse code reception and this is not optimum for receiving SSB. SSB can be resolved using the net switch in the normal AM/RT setting but in this mode the BFO is not adjustable.
The BFO wasn't apparently working when I first tested the set and I discovered that the tuning slug in the coil was jammed. This had resulted in loosening of the entire plastic former, which was rotating inside the coil and not producing any change to the frequency. I dismantled the BFO parts and removed the coil. I was able to remove the tuning slug and after reducing its size with emery paper it fitted and I was able to refit the coil and reset the compound securing the plastic former in place. I set the centre frequency of the BFO to correspond with the IF. During testing I noticed hum on signals whenever the BFO was switched on although this appeared to have been coming from ripple on the HT supply I was using for testing and shouldn't arise when the set is working with its correct ancillaries.
I noted that the meter reading for RF output was not registering but from the circuit description I think this is because the rectifier for the monitoring circuit is contained in an external unit, which I didn't connect in the test rig.
MCW appears to work properly. Note that in CW and MCW the key has to be removed for reception. I used a metal jack plug in the test rig and found to my surprise that HT is carried on what is normally the ground connection of the lead.
I found that there is what seems to be a regular AM net on 80m and I listened to this for some time but didn't have a microphone in the test rig so could not join in. The setting of the centre-tuning knob should be adjusted for maximum RF output rather than best reception.
Because the set employs very high gain in its front end you may find it advantageous to make use of the RF gain control for best signal to noise.

Price: £85

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