1946 HMV 1804.

[Murphyv310]

Looking good David.
Intrigued as to how sensitive the set will be.

[Geordie McBoyne]

Hi Trevor,
In due course I wound the variac up to full mains voltage but the HT had risen to 425 volts, too high.
Consequently, the variac output has been reduced so that the HT is about 325 volts.
There is some response from the audio amplifier but that's all. I'll hook up the signal generator to the IF amplifiers. Will report back my findings.

Geordie McBoyne.

[Geordie McBoyne]

Connecting up the signal generator to the grid coil of the W61 IF amplifier reveals that the sound IF is 11Mc/s, that's a long way from the original EMI frequency of 4.5Mc/s.
So what's the vision IF? Before that could be determined the 33Kohm screen feed resistor of the Z66 final vision IF amplifier valve was found to be open circuit, likely the result of a leaky 0.047mfd decoupling capacitor.
That fault put right he signal generator was connected to the mixer grid. It turns out the vision IF is 14.5Mc/s, it is 8Mc/s in the original EMI circuit. As the sound IF is below the vision confirms that the local oscillator operates below the carrier frequencies. Later on I'll turn my attention to the RF and frequency changer stages.
The local oscillator is working. This was proved by connecting a voltmeter via a 22Kohm resistor to the grid of the EC52 triode. A reading of 15 volts negative was noted.
This morning more exciting pyrotechnics. lots of smoke pouring out from the underside of the chassis. This time it is one of the vision IF transformers that has broken down. Later today the transformer will be removed from the chassis for examination.

Geordie McBoyne.

[Geordie McBoyne]

The burn up was caused by the 500pF mica coupling capacitor gone short circuit. Rare for such capacitors to fail.

Geordie McBoyne.

[ppppenguin]

David, you've certainly needed your asbestos and steel lined underpants for this set:-)

There's always an exception to every rule and this set has provided one. Mica caps never fail short circuit. Except when they do.

[Geordie McBoyne]

Hi Jeffrey,
The mica capacitor was rated at 350volts, very close to the HT voltage. For the time being I'm operating the set without the EHT transformer in the circuit. Come the time when the timebases are to be tested I have no other choice but to reconnect the EHT transformer, the same transformer carries the heater windings for the timebase valves and CRT.
Very early production models had the facility to switch off the timebases and CRT for sound only reception. The BBC transmitted the National Program from AP. Early Hi-Fi radio.

Geordie McBoyne.

[ppppenguin]

Come the time when the timebases are to be tested I have no other choice but to reconnect the EHT transformer, the same transformer carries the heater windings for the timebase valves and CRT.

External heater supply?

[Geordie McBoyne]

External heater supply?
A good idea and in fact I have a huge heater transformer that was removed from a 1953 Pilot TV87.
Actually, the EHT voltage in the 1804 is surprisingly low, only 3.5KV. Of course that doesn't make it any safer. After the Emiscope 3/4 CRT became obsolete EMI service supplied the TA10. This tube requires a higher final anode voltage so a special EHT booster was supplied. This device rectified the flyback pulse present at the anode of the KT44 line output valve and added the resultant voltage to the existing mains derived EHT.

Geordie McBoyne.

[Geordie McBoyne]

At last I'm getting some positive results from the signal circuits.
This morning the signal generator was connected to the aerial socket and believe or not signals are getting through, there's hope for this set after all.
The sound peaks up at 50Mc/s and the vision at 54Mc/s. Those frequencies are above channel B2 which are 48.25Mc/s sound and 51.75Mc/s vision.
To monitor the vision signal level a voltmeter was connected across the detector load resistor.
I'm sure it will be possible to retune this set to channel B1.

Geordie McBoyne.

[Geordie McBoyne]

The 1804 is now supplied with channel B2 signals from an Aurora standards converter. The local oscillator has been readjusted for max video at the grid connector of the Z66 video amplifier valve. Also peaked up is the grid coil of the mixer valve.
The scope trace shows the video developed across the detector load resistor, a very impressive >12volts P - P. Even more video will be present at the detector when the aerial and first RF amplifier anode and coils have been tweeked up. To tune the receiver to channel B1 the metal tuning slugs will be replaced with iron dust cores.
Even with 12volts video across the detector load resistor there is still no sync crushing. I wish I had same amount of video to drive the CRT grid in my Marconi 702.

Geordie McBoyne.