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Old 24th Feb 2020, 2:56 pm   #101
Heatercathodeshort
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Default Re: 1938 Murphy A56V television restoration

I believe the CRM92A is exactly the same CRT as the CRM92 other than the A series tubes gun assembly/neck is designed to operate at a higher EHT voltage. If the CRM92 is fitted in place of a CRM92A [with higher EHT] a single sleeve of polythene was fitted around the neck positioned under the scan coil/focus assembly. Other than that they appear to be identical. There should be no loss of brilliance if either CRT is fitted with 5kv EHT.

It's quite amazing the brilliance obtained with these notorious Mazda tubes at such low EHT voltages. The example at the Dulwich museum is certainly bright enough for daylight viewing but owners of pre war receivers would have expected to lower the lighting when viewing. The original short CRM91 was replaced in the past by a good CRM92A.

Good luck with the completion of your restoration. At first light the A56V appears to be easier to get at than some pre war models but the drop down chassis can be very frustrating. Regards, John.
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Old 24th Feb 2020, 3:10 pm   #102
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Default Re: 1938 Murphy A56V television restoration

Quote:
Originally Posted by Heatercathodeshort View Post
I believe the CRM92A is exactly the same CRT as the CRM92 other than the A series tubes gun assembly/neck is designed to operate at a higher EHT voltage. If the CRM92 is fitted in place of a CRM92A [with higher EHT] a single sleeve of polythene was fitted around the neck positioned under the scan coil/focus assembly. Other than that they appear to be identical. There should be no loss of brilliance if either CRT is fitted with 5kv EHT.
That's useful to know John, I had wondered why a few of my sets had this sleeve with the CRM92A tube.
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Old 24th Feb 2020, 10:23 pm   #103
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Default Re: 1938 Murphy A56V television restoration

The exception is the CRM93. [Ekco TMB272 and Spencer West 'TEEVY'] This has an entirely different tetrode gun assembly similar to the CRM141/2, CRM124 and the CRM171 series. It is also aluminised and will give it's best picture with around 9kv EHT. It also requires around 300-350v to tickle it's first anode and a 12.6V heater supply. The tetrode requires a much less focusing field than the triode.
The Mazda tetrode gun assembly tended to have a better service life but even so many suffered early low emission and O/C cathode connections.
Mullard, GEC and Ferranti just sat back and watched the fun! John.
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Old 28th Feb 2020, 8:56 am   #104
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Default Re: 1938 Murphy A56V television restoration

Obviously, after obtaining the test-card picture in post #91, the next step was to identify and fix the faults. Looking at the test-card picture, the most obvious fault is vertical non-linearity (*). This could be caused by a number of issues, but, the most likely is failure of the framebase linearity correction circuit.

Background aside (for non-experts): the standard way to generate the scanning waveform (or "the saw-tooth" waveform due to the way it looks) is by the charging/discharging of a capacitor, but this charging characteristic is not completely linear, and so it has to be corrected by some means.

The Murphy A56V doesn't employ a feedback circuit for linearity correction, which is perhaps the most common technique. It instead uses a resistor-capacitor combination in parallel with the saw-tooth waveform capacitor. The resistor-capacitor combination integrates the saw-tooth waveform, which generates a "correcting waveform". This correcting waveform is combined with the saw-tooth waveform to give a linear waveform. As stated in the Murphy service manual, this is over-corrected to allow for transformer distortion.

Helpfully, the Murphy service documentation gives the expected waveforms (but these are standard except for the over-corrected waveform), and so to diagnose whether this was indeed where the problem lay, it was simply a case of connecting up the oscilloscope at the stated trace points and comparing.

Photo 1 shows what should be the "over-corrected" waveform, but, while it is fairly linear, it is not over-corrected (especially after the flyback discharge, where it should be flatter, and which is where the distortion is evident in the picture).

Photo 2 shows the saw-tooth waveform from the waveform capacitor, which is exactly as it should be.

Photo 3 shows the "correcting waveform" which should be the integrated saw-tooth waveform. It is not correct.

So, the culprit was the linearity correcting circuit. This circuit consists of 4 resistors in series with a capacitor. The reason for 4 resistors is explained in the Murphy service documentation "The chain of resistances is to allow for adjustment in the factory, for variations in the primary inductance of the transformer".

The capacitor had already been replaced (as part of the general waxy restuffing), and so the 4 above resistors were removed and checked. Three out of the four were over-spec by 22.5%, 45%, and 42%, which is obviously bad news as this changes the resistor-capacitor time constant for the circuit, and hence the waveform.

As these values are fairly critical, I decided to replace with modern components, but with high wattage values, to ensure the resistors were of a fairly similar size to the originals (Photo 4).

As expected, with the replaced components, the vertical linearity was fixed (Photo 5).

Footnote (*) The other obvious fault is the picture is reversed horizontally, but, this is simply the case of having connected up the line-deflection coil connections reversed (where the labelling had fallen off), and where it was 50-50 getting it right. It needed to be fixed, but as the fault was obvious, it wasn't a priority.
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Old 14th Mar 2020, 6:13 am   #105
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Default Re: 1938 Murphy A56V television restoration

To continue with the write up of the Murphy A56V restoration …

The previous post dealt with the fixing of the frame linearity issues. This produced the test-card in Photo 1.

As can be seen the image is still stretched or non-linear on the left hand of the picture. This could be caused by a failure of the line time-base linearity correction circuit (as was the case for the frame time-base), but, it could also be caused by the failure of the "damping circuit" in the line output stage.

The "damping circuit" was employed by early mains derived EHT televisions (see footnote) to damp the effect of oscillations in the line output circuit produced by line fly-back (due to back emf, see footnote 2). If those oscillations are not damped and allowed to continue into the next scan line, it will cause variations in the speed of the scan. In more severe cases this can cause lighter and darker stripes to appear at the left of the picture. In this case the oscillations are speeding up the scan causing stretching.

The damping circuit in this set consists of a fixed resistance, capacitor and variable resistance across the secondary of the line output transformer. The variable resistance is normally called the "FORM" control.

Turning the FORM control should normally have a significant effect on the left hand of the picture. Turning it too far can cause over-damping where the left hand folds over (due to delayed fly-back of the spot).

In this case the FORM control did nothing. This implied there was a failure in that circuit.

Checking the components showed that the high wattage fixed resistance had gone open circuit (see Photo 2, it is the large green resistor). It is of a high wattage because most of the fly-back energy will be dissipated through this circuit.

I replaced it with a high wattage NOS component from the 50s/60s, which I had "in stock". In the future I may replace it with a more contemporary NOS component if/when something comes available. See Photo 3.

Fixing the damping circuit produced the test-card in Photo 4, which has much improved linearity. But it still isn't completely correct, there is still a small amount of horizontal non linearity.

Footnote: obviously in later designs, the fly-black energy is captured to produce the EHT voltage etc. and the circuits employed are very different.

Footnote 2: at line fly-back all the stored energy in the line deflection coils has to go somewhere, and if not prevented, it flows back into the line output circuit re-energising it, which flows back into the deflection coils, which flows back to the line output circuit in a cycle of decreasing or damped oscillations.
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Old 18th Mar 2020, 6:10 am   #106
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Default Re: 1938 Murphy A56V television restoration

Quote:
Originally Posted by Catkins View Post
Fixing the damping circuit produced the test-card in Photo 4, which has much improved linearity. But it still isn't completely correct, there is still a small amount of horizontal non linearity.
On re-reading, one thing I perhaps forgot to mention, is that the remaining horizonal non-linearity isn't that apparent on the test-card (last photo on the previous post). I noticed it on slowly left-right panning video where objects slightly shrink and expand when they move across the screen, which suggested there was still some non-linearity present.
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Old 19th Mar 2020, 1:08 pm   #107
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Default Re: 1938 Murphy A56V television restoration

Compared with what some people were watching in the '50s that's brilliant.

Peter
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Old 16th Apr 2020, 4:28 am   #108
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Default Re: 1938 Murphy A56V television restoration

The previous posts dealt with the discovery that the main cause of the line timebase non-linearity was due to the failure of the "damping circuit". But this still left a small amount of non-linearity, mostly only discernible on slow left-right panning video where objects slightly shrank and expanded when they moved across the screen.

Given the failure in the "damping circuit" had been fixed, the next obvious cause of the failure was in the line timebase linearity correcting circuit (which is where the frame timebase non-linearity was found to be). Again this consists of a resistor and capacitor in parallel with the waveform capacitor, which produces a correcting waveform. Checking the resistor in that circuit showed it was very slightly over-spec (20% tolerance). Replacing it with a modern high wattage resistor (for size), fixed the remaining non-linearity.

But, as can be seen from Photo 1, this resistor is in a very visible part of the circuitry. Whenever I looked at the set my eye would invariably be drawn to that resistor (no doubt because I knew it was there).

The problem is those low wattage carbon resistors are impossible to replace with original NOS components due to their nature (they suck up moisture and go out of spec). NOS wire-wound resistors don't tend to go out of spec, but as they're usually high-wattage, they're too big to fit into the available space.

I decided to fake up a resistor, to look like the original carbon resistor. I removed the "legs" from the original out of spec resistor, then I wrapped a modern resistor in card and shrink wrap to produce a body of the right size, and then glued the legs from the original resistor on to it. I then painted it to look like the original carbon resistor. Photo 2 shows the result, which I think is reasonably good.

Photo 3 shows the resistor in-situ.
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Old 16th Apr 2020, 10:34 am   #109
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Default Re: 1938 Murphy A56V television restoration

Excellent work!

Peter.
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Old 16th Apr 2020, 11:33 am   #110
Leon Crampin
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Default Re: 1938 Murphy A56V television restoration

Brilliant job - I wish I had your patience. When I restored my Murphy A30C radio, I had a similar problem with these resistors, and didn't want to change appearances, even though some of them were inside cans.

I shunted the offending resistors with very small metal film resistors, tucked under the original component. Quick to do, and almost invisible. The dissipation in the shunt is very small.

Not for the purist - but a practical fix. I think the old "carbon rods" will have stabilised by now...

Leon.
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