1946 HMV 1804.

[Brianc]

I read your other posting on another site before reading this one. basically, it says that I have a spare chassis which you can have and any number of spare components as I have scrapped one chassis and the rest (3+) are still complete-ish. a PM is probably necessary!

[Geordie McBoyne]

The first attachment shows the original signal paths and the second shows how the set was modified for what we suppose was done to improve the receiver's weak signals performance.
The set now has two EF50 pentode RF amplifier stages and a frequency changer consisting of an EF50 pentode mixer and an EC52 triode local oscillator.
The receiver now has only two vision IF amplifier stages.  So were all these modifications worth all the effort?  Or, simply done by a dabbler with nothing else to do but fiddle about with old TVs.

Geordie McBoyne.  

[Geordie McBoyne]

Further examination of this receiver reveals that the RF amplifier is not an EF50 but in is fact an EF54, the giveaway is how the valveholder wired. In the EF54 the anode is connected to pin 2 and the screen grid to pin 3. The cathode is connected to no less than four pins, 4, 5, 7 and 8. The developer of this valve (Philips in Holland or Mullard in the UK?) made sure that the cathode inductances were reduced to the minimum. Considering the choice of this valve as the RF amplifier it would seem the guy who did the modifications to the 1804 really knew what he was doing after all.
Info about the EF54: http://www.r-type.org/exhib/aaa1093.htm
To reduce the noise factor the EF54 employs the aligned grids technique. This must be the only known application of this valve in a television receiver.

Geordie McBoyne.

[Murphyv310]

Hi.
It makes sense to have two RF amps rather than have two IF amps in this set. The IF frequency is low so to get the bandwidth the stage gain is low, I'm sure Brian will confirm this.
At Band 1 RF frequencies it is easier to get the bandwidth and useful gain so I would believe the "engineer" doing the mod realised the issues of keeping a reasonable IF band with and trying to extract useful gain, especially with the choice of valves around at the time.

[Geordie McBoyne]

So it turns out that both RF amplifier stages employ the EF54. The valves are ex-services types, a VR136 and the other is a CV1136. The mixer is a VR91 = EF50.
http://www.radiomuseum.org/tubes/tube_cv1136.html Worth noting that the valve is dated December 1976. There must have still been a demand for this valve by the military as late as the seventies.
I'm becoming more convinced that these RF and mixer-oscillator circuits should be preserved. The interlace unit will not be refitted, however, a similar circuit can be wired on the existing D63/6H6 interlace valve base.
No decision what to do with the vision detector, should I retain the octal base EB34/6H6/D63 double diode or return the detector back to the original EMI design using the 4volt heater D43 single diode?

Geordie McBoyne.

[Geordie McBoyne]

Been checking the waxie capacitors, all are useless, big recapping job coming up.

Apart from the different type of oscillator triode, the original Hartley circuit and components have been retained and operate at 37Mhz. The local oscillator is coupled to the mixer by a single turn loop on the mixer grid coil. All the RF coils have metal cores.

Geordie McBoyne.

[Geordie McBoyne]

And now we know why the RF tuning coil cores are metal instead of iron dust cores. After further examination of the RF circuits it would appear that the set was retuned to receive the Birmingham transmitter.
The RF tuning cols have less turns than are required for 45Mc/s and the tuning cores are metal. Also, the big giveaway to confirm that the set is tuned to channel B4 is that local oscillator tuning capacitor has been changed from 40pF to 22pF. The local oscillator will be tuned to 53.75Mc/s instead of 37Mcs.

Geordie McBoyne.

[Murphyv310]

I wonder then if these mods were done for the higher frequency and fringe use at the same time?
I'd have been expecting a picture by now David you need to keep the 405 line customer happy and put those work of the devil flat screens aside for now!!

[Geordie McBoyne]

Hi Trevor,
I'm refusing to repair flat screen TV's anymore. Can't be many folks repairing those things anymore.
Some progress with the HMV 1804. A suitable frame blocking oscillator transformer has been found to replace the Pye type transformer with three windings, that component is now fitted in my Pye D16T. The replacement transformer was removed from a scrap Ekco TMB272 and in order that it would fit in the HMV an adaptor plate has been made for it. The frame oscillator and output stage have been isolated from the rest of the circuit and supplied from a separate power source, and it works, there's a 50volt P - P sawtooth waveform present at the anode of the KTZ63 oscillator valve. The waveforms present at the anode of the output valve are bit strange though.
So later today I'll turn my attention to the line timebase blocking oscillator.

Geordie McBoyne.

[Murphyv310]

Looks like your making good progress on the set David. Like you I enjoy a challenge, the Eddystone was one but now it's 95% complete you get a good feeling of satisfaction.
This set your doing is one I'm watching very deeply as I'm keen to see the end result.