[Argus25]

One interesting thing about my HMV 904 was that the setup instructions provided by the manufacturer were not ideal. Although they clearly favored gain over bandwidth.

After setting up my set and then sweeping the antenna input, I found that their intended video bandwidth was only about 1.4MHz. Of course on a smaller CRT, depending on the focused spot size, this can be not as severe as first thought. Obviously they were wanting fringe area reception too.

However, there is plenty of gain to spare, so what I did was set it up according to the attached bandpass characteristic. This is an exact transcription of what was obtained on the scope face. I used a vintage valve sweep generator applied to the input with a dummy antenna pad and used the Philips PM5326 for the accurate markers.

(It is easy to couple the marker signal in simply by clipping the lead onto the insulation of the signal wire from the sweep generator to create a fraction of a pF gimmick capacitor without altering the bandpass response and reducing the level to just see the marker as a small blip on the bandpass response).

As noted on the attached record, I documented the projections of the screws above the round brass nuts they run in.

(I have mentioned this before, post #134 and noted that the brass nut & screw threads needs to be immaculately clean and later seal with conductive paint, not wax, because the capacitance of the brass slug can alter the tuning independently of the inductance lowering effect of the brass).

With the bandwidth pushed out to about 2.4MHz the picture is much better (attached). The set is still fine receiving a signal on about a minimum of 700uV to 900uV input (peak white carrier level) but it needs about 1mV to just get out of the snow. Generally I run the RF output from my "Alexandra Palace transmitter" at about 2.5 to 3mV peak white carrier level. My set was fitted with NOS valves so I think this is about as good as it gets.

The 5FP4 I was forced to use (not able to get the 3/1), was designed by RCA for a small sharp focussed spot which helps a little too. With this CRT you can clearly see the scanning lines on less than a full height scan.

Of course the 904/5 etc the detected video appears directly out of the video amplifier (anode bend detector) so the sweep voltage seen there (using a x10 probe) as noted, is quite high in the range of 20v. The contrast control has some effects on the bandpass characteristic so it is best to have it set as noted.

Another thing about this set, because of the high overall gain, if the tuned stages are incorrectly peaked too close to the same frequency, it becomes unstable and oscillations occur. I spent some hours getting the stage tuning as perfect as it could be, it is much more of a challenge in the 904 than the usual TV IF alignment which is a walk in the park.

I would recommend setting it up initially at least with either the MFR's instructions, or use the screw projections I documented, then with the sweep and marker generators make small adjustments to get the bandpass characteristic as I have documented and it will get you a long way toward a satisfactory end point.

Also note that when the video adjustments are made the sound ones need re-adjusting. When it is correct, notice on the sweep the relatively low response in the video output at the sound frequency.