Marconi 702. Sound problems.

[FERNSEH]

The CRT has been reinstalled and the chassis positioned for servicing, let's see if that low video drive problem can be solved.
That work was completed at 12:30am last night. Too late to be fiddling about dangerous gear like these old TVs.

DFWB.

[Panrock]

Originally Posted by Peter.N.

Sadly we were scrapping them, great lumbering things when there were nice compact sets coming onto the market weighing less than half a hundredweight.

I tried weighing my 702 when I had it, using a plank and two bathroom scales.

The best measurement I obtained was around 13 stone. That's over 1½ hundredweight !

Steve

[Jac]

I've used a similar method Steve, and came to an identical weight: 82 kg.
The power pack alone is over 18 kg.

Jac

[FERNSEH]

Not much progress I'm afraid to say. The max video drive to the grid of the CRT is still only 6 volts. Valve voltages are OK, resistors reasonably within tolerance.
The scope trace shows the video present at the slider of the contrast control.
2volts/div.

DFWB.

[Jac]

Good evening David,

Yes, it should be possible to get the video amplitude a bit higher.
This is at 5V/div:



Here are the sync pulses quite a bit more than 30%, but I can't explain that.
It is just the output of the Aurora SC with TCC.

Is the last valve (before the D42) an MSP41?
In my TRF section the valves seem to draw a bit less current than per diagram.
I tried an other MSP41, but that makes no difference.
The valves do not seem to be critical.

Is the TRF already properly aligned? Does the output not rise further with increased input level, or do you already use the full Aurora output?

Jac

[FERNSEH]

Good evening Jac,
in fact your scope trace is very similar to the HMV 901 which always displays a well contrasted picture. More than 10 volts video amplitude is required for good contrast.
I can explain the reason for the "high" syncs. As the sensitivity control is advanced the active video part of the waveform becomes crushed, at the same time the sync amplitude will increase to the point where only the syncs are apparent if the sensitivity control is advanced further.
On negative modulation vision carriers it is the syncs that become crushed if the circuits are overloaded with strong signals.

I will continue focus my attention around final RF amplifier and vision demodulator.

DFWB.

[Jac]

Thanks David,

After I posted it, I remembered how the video-signal looked when I used a stair-case signal, that clearly demonstrates your explanation:



(The irregular step in the middle is due to the signal generator.)

Jac

[FERNSEH]

The circuit of the final RF amplifier and the vision demodulator. An interesting observation is the presence of 2 volts of video across the cathode bias resistor of V6. R14 500ohms. You don't expect a demodulated signal across a cathode resistor of an RF amplifier, the scope waveform was taken from the HMV901 but it is almost the same for the Marconi 702.
The second attachment shows the video across R14.
The third attachment is from the HMV 901 and shows the video waveform present at the grid of the CRT. 10 volts of active video, note the elongated syncs, just like Jac's Marconi 702.

The fact there is video present across the cathode resistor of the final RF amplifier indicates that must be some non linearity in the characteristics of that amplifier. Demodulation of the signal is taking place.

DFWB.

[FERNSEH]

Returning to the Marconi 702. The video waveform across R14 has much smaller amplitude than in the HMV 901, it's just over 1 volt including syncs.
Here's where the strangenessness comes in, when the RF gain control is advanced the waveform remains at the same amplitude, it's like if there was some kind of AGC action.
What's going on?

DFWB.

[Herald1360]

Is the screen decoupling and dc feed OK?

[FERNSEH]

Checked them by bridging another capacitor across them. The decoupling capacitors are assembled in a block of four 0.002mfd components.
Interesting to note I also scoped the cathode of previous RF stage, V5, and sure enough the video waveform was present across the cathode resistor (R11 350ohms) as well.
Connecting another capacitor across the decouplers effects the picture quality, it seems that the values of those capacitors is somewhat critical.

The attachment shows one of the capacitor blocks.

DFWB.

[Jac]

Sometime in it's life the set has obtained a different antenna cable:



The coilformers in your 702 are the earliest used type.

Jac

[Jac]

Just fantasizing aloud:

I suppose the decoupling capacitors were intended for the 45 MHz range.
Due to non-linearities in the valve characteristics there will always be some AM-detection.
And the small value capacitors are way less effective in decoupling the video frequencies.

Does this makes sense in a way?

Jac

[FERNSEH]

Originally Posted by Jac

Just fantasizing aloud:

I suppose the decoupling capacitors were intended for the 45 MHz range. Yes indeed the capacitor values were chosen for the band frequencies the amplifier is working in.Due to non-linearities in the valve characteristics there will always be some AM-detection. Perhaps because of shortcomings of the MSP4 in it's role as a VHF amplifier? It's a very early HF pentode.
And the small value capacitors are way less effective in decoupling the video frequencies.
That's agreed. Actually increasing the values of the capacitors degrades the performance.
Does this makes sense in a way? ]Jac

Indeed it does.

DFWB.

[FERNSEH]

Originally Posted by Jac

Sometime in it's life the set has obtained a different antenna cable:

Attachment 115151 Attachment 115152

The coilformers in your 702 are the earliest used type.

Jac

The connector shown in the attachment is the sound take off. It's different from the aerial connector.
The 41.5Mhz sound is taken off after the first RF amplifiers stages. V1 and 2.

DFWB.

[Jac]

Good evening David,

It looks like the antenna cable connection to me though.
I see the first coil can ? - there is a mounting bracket for the coil can cover bar in the photo.

It would be a good idea to mount a connector for the RF cables if you have to take units out more than once.
I desoldered the cable to the sound unit a few times
(at the sound unit side, but nevertheless...)
A connector on both ends (or one end at least) would have been helpful, though not original. Or are they?

Strange that the picture degrades if a decoupling capacitor is increased in value.
Was the additional one also a mica capacitor?

Jac

[FERNSEH]

Hello Jac,
I tried various types of capacitors, mica and polyester, also different values but it is likely the designer chose the values for optimum performance.
Fiddling about with alternative values of capacitors can be a bad thing, we have to consider other influences in High frequency circuits such as valve interelectrode capacitances, all these would have been factored in during the design of the circuit.

It would be much easier if the TRF unit was taken away from the set altogether and serviced on the work bench.

DFWB.