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Old 18th May 2017, 1:00 pm   #41
dtvmcdonald
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Default Re: Marconi 702 advice

I've tried the usual test image DVDs and they look OK, especially
the frequency response. There are modest white lines after black areas,
see the boy's right hairline. This would be expected from a modest
(30%) peak in the RF response at about 1 or 1.3 MHz away from 45 MHz,
unequal on the two sides.
The tonal response is OK.
I have no "test card C" source. I have the test DVDs from this and other British
forums but cannot play them on my players. I can however put that image you
just sent on a USB key and play that. Note that it will be played to NTSC and
converted to 405 by the Aurora. This is unoptimal. I could generate System A
directly using my HackRF, but only static images.

I tested the first three audio tubes and they appear OK. The X41 triode is clearly
OK as it oscillates nicely; I tested the multigrid section with the triode grid
connected to the cathode.The audio output is, as I said, soldered in. I can get
a scope probe to its socket to see if it is working. I should check the speaker.
The three tubes fit loosely in their sockets, but wiggling them does not
seem to make any difference so its possible they are contacting. The ones that
did not get hot in the set didn't get hot in the tube tester.

What I really need is some idea of the signal voltages through the audio chassis.
I really really really do not want to remove the CRT to get at it. That is really
bad design. I don't think that a max of 0.2 volt audio to the top of the volume
control can be right. I am going to try as hard as I can to at least diagnose the
problem with the audio chassis in place.
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Old 18th May 2017, 1:46 pm   #42
dtvmcdonald
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Default Re: Marconi 702 advice

A test of the MHD4 on a different tester shows an intermittent heater cathode short.
I thus need a new one. However, if vertical and not bumped it is OK, so that's
not the cause of the problem. That's why the first tester didn't find it.

In any case a grounded cathode would
increase gain, not ruin it. Putting 4 volts AC on the cathode
would result in an enormous hum and might blow the cathode electrolytic
bypass.

I suppose I could temproraily bodge in a 4AV6 which would be the "usual suspect"
American electrical equivalent and would work perfectly. Luckily these are cheap
as water because nobody used them.

Last edited by dtvmcdonald; 18th May 2017 at 1:52 pm.
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Old 18th May 2017, 4:20 pm   #43
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Default Re: Marconi 702 advice

If I inject 30mV p-p at 1kHz into the audio input on my Aurora I get 20mV p-p at the volume control wiper when set to maximum volume. The sound output with this is what I'd describe as loud.

Peter
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Old 18th May 2017, 4:48 pm   #44
dtvmcdonald
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Default Re: Marconi 702 advice

Thanks very much Peter, that's what I need to know. This implies that the
audio amp system, after the diode detector, is bad. I will try investigating the
output stage. As I said, I can reach it with a scope probe without removing
the chassis, and probably can reach it with a special soldering "gun" tip.

It would help, of course, if I were still the size I was at 10 years old and just as limber.
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Old 18th May 2017, 7:50 pm   #45
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Default Re: Marconi 702 advice

Most DVD players can be made multi-region by entering a code on the remote, so you should be able to persuade yours to play Region 2 discs. It does not even matter about the colour system (NB: "auto" often means "NTSC when playing a Region 1 disc, PAL for any other region") though the higher colour carrier frequency of PAL might be safely beyond what your set can see, meaning less dot crawl.

Or you could (get someone who knows about computers to) use FFMPeg to re-encode them from 625 to 525 lines and burn to a DVD which will be playable on any region player.
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Old 19th May 2017, 12:43 am   #46
dtvmcdonald
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Default Re: Marconi 702 advice

It appears that the audio problem is a dead speaker.

Connecting it to another ordinary radio's external speaker jack
(in parallel with a scope) resulted in no sound and a near short
circuit on that radio.

It also appears that the speaker will come out fairly easily.
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Old 19th May 2017, 2:06 am   #47
dtvmcdonald
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Default Re: Marconi 702 advice

Here is the test card image.
The dark defects that look like somebody was at work with an eraser
probably are exactly that. The picture was made in the mirror. Is it front surface?

There is some difference in focus quality across the screen that changes
with the setting of the push-about coil. Its set so the picture
is centered.
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Old 19th May 2017, 8:34 am   #48
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Default Re: Marconi 702 advice

Excellent!

You must work on the RF alignment though. Your set is capable of much better resolution. The 2.5 MHz bars are not too difficult to achieve and 3 MHz is also possible.

Peter

p.s. You might be interested in 240 line operation too.
http://www.vintage-radio.net/forum/s...066#post945066

Last edited by peter_scott; 19th May 2017 at 8:57 am.
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Old 19th May 2017, 10:03 am   #49
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Default Re: Marconi 702 advice

Just thought I should say, on the Aurora that I have, when there is no video input to the Aurora, it will automatically output 'test card C'.

Regards,
Lloyd
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Old 19th May 2017, 1:09 pm   #50
dtvmcdonald
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Default Re: Marconi 702 advice

When there is no video on my aurora, it outputs standard NTSC color bars,
converted to whatever is output.

As to alignment ... I worked very hard. One setting however is frozen and
PB_Blaster has not yet worked, it can take a week. Still, its actually
quite wide, 2.5 MHz is much less than 6 dB down. It is also the focus, especially
with push-about activated. I think I need to check R30 and R31 because best focus,
while very broad, is all the way down.

If I redo, what should it look like? There are pictures in the omnibus, but
they look awful. I do not know if I have the "transformer". My response is a
bit double-humped.

Do you know the impedance of the speaker? It seems to be very low,
or perhaps shorted. I cannot do more on the audio without a good MHD4.
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Old 19th May 2017, 3:46 pm   #51
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Default Re: Marconi 702 advice

Mine looks as below but Jac on this forum managed to get rather better results than I did. You'll see in my web page if you click on the test card image that I didn't manage better than 2.5MHz when fed from Aurora but Jac has very nice 3MHz. Having said that when I send it a 405 line signal with Test Card C using the PC and graphics card I can register a bit of 3MHz although you'll note that due to the crude sync combiner that I'm currently using there is no interlace.

I found I could achieve better response with a little hand and eye test card tweaking after adjustment of the response curve.

I removed the "optional" push about coil because it messed my focus and picture centring was easily achieved simply by pushing the tube neck back and forth.

The speaker will be very low resistance but a scope would tell you if it is shorting the output.

Peter
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Old 19th May 2017, 7:52 pm   #52
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Default Re: Marconi 702 advice

I'll jump in here:
Doug bought a WC-01 converter not a SCRF405A since he wanted to come from an NTSC source. There are several default images stored for each output standard.

You can change the default image in the Quick Settings menu. Black, standard color bars, the promotional video and two user images are available. You probably have it set to the standard color bars image as I don't save the 405NTSC (Queen's) test pattern in these units. For the 4:3 405 format I save Test Card C and for the 5:4 405 format (which is what would have been sent to your set pre-war) I store test Card A.

BTW, fantastic job getting that set up and running in such a short time. I'm glad that the crt works as we had no way to test it before the convention. The story was that a gentlemen brought the set with him from England back in the early 1960's to Florida. It was given to the museum by his son. What he was doing with it in America in 1960 I have no idea!

Darryl
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Old 19th May 2017, 8:34 pm   #53
dtvmcdonald
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Default Re: Marconi 702 advice

I measured the EHT and focus and they are 3.75 and 0.8 kV.
Not great, but I did notice that after an hour running the
EHT transformer was just barely warm.

Other effects make really improving this moot. First and most important,
there is 60 Hz interference from magnetic fields from the two big transformers.
This results in "wiggle" in the picture which shows in blurring on the
posted picture which was over 1/10 second.

My "test" permanent magnet has visible effects from four feet away!
The focus has a severe astigmatism, visible on that test card.

I looked at the frequency response using Digital Video Essentials
chirp pattern and it is clearly visible "in person" at 2.5 MHz, which
would be "big signal" tuning back in the day.

Since I didn't do the restoration and am afraid to remove the tube,
its good enough for me. Remember ... the previous owner never
turned it on. Had he done so, he would have got a picture pretty
much the same except that the frequency response was really
really bad.

I'm more interested in getting the sound working. I'll ask again ...
does anybody know the speaker impedance? It measures 3.7 ohms DC.
Is this an ordinary radio speaker of the time? Five volts RMS peak
(on a Simpson meter) into it does not result in "loud". That's
seven watts into the resistance, and the tube in the TV can't
put out much over one watt.

How are the knobs supposed to work? The correct ones are there
but I disbelieve that the holding mechanism is original. The controls are
slotted. The is a screw hole in the knobs, but no threads. The knobs are
held on by putting a sheet metal screw in the hole and mildly
jamming it in the slot in the shaft.
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Old 19th May 2017, 8:45 pm   #54
dtvmcdonald
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Default Re: Marconi 702 advice

Quote:
Originally Posted by tubesrule View Post
I'll jump in here:


BTW, fantastic job getting that set up and running in such a short time. I'm glad that the crt works as we had no way to test it before the convention. The story was that a gentlemen brought the set with him from England back in the early 1960's to Florida. It was given to the museum by his son. What he was doing with it in America in 1960 I have no idea!

Darryl
This set had nothing to do with the museum as far as I know. It was
a private sale just delivered at the convention. It was known to be working to the previous owner, who never turned it on.
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Old 19th May 2017, 8:54 pm   #55
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Default Re: Marconi 702 advice

It does rather sound as if your speaker is faulty.

EHT is certainly on the low side. What tapping are you using and what supply voltage are you using?

The knobs are fixed to the shaft by the screw passing through the slot and I guess splaying it a fraction.

Disconnect and remove the push-about coil for a more even focus.
Unfortunately astigmatism tends to result from a slight gasiness but you may find this improves after a few hours of use.

Peter
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Old 19th May 2017, 9:47 pm   #56
tubesrule
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Default Re: Marconi 702 advice

Doug just reminded me that his set was a private sale at the auction. We had a 705 for sale at the museum this year.

Back to the set...

Darryl
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Old 19th May 2017, 10:01 pm   #57
dtvmcdonald
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Quote:
Originally Posted by peter_scott View Post
...

EHT is certainly on the low side. What tapping are you using and what supply voltage are you using?


Peter
Its hard to see the tap setting as its soldered in under the chassis.
I think its 250, but that might be the wire to the EHT fuse.

I have my stepup transformer set so that at our usual voltage the
filaments are 4.00 and the B+ is 280. The brightness is limited by blooming,
which of course would be helped by more EHT. Running ac up
so filaments are 4.4 makes not enough difference to matter.
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Old 19th May 2017, 10:37 pm   #58
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Default Re: Marconi 702 advice

The voltage tapping is set by screwing the knurled screw in the appropriate labeled hole. You don't need to look under the chassis. The EHT primary is connected to the middle setting so it uses the HT transformer as an auto-transformer.

Peter
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Old 19th May 2017, 11:17 pm   #59
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Default Re: Marconi 702 advice

Quote:
Originally Posted by dtvmcdonald View Post
I'm more interested in getting the sound working. I'll ask again ...
does anybody know the speaker impedance? It measures 3.7 ohms DC.
On the circuit diagram for the 702 sound unit (omnibus edition) it's listed as 4 ohms.

I don't have a 702 but the speaker for the HMV 900 is listed as 4 ohms too (and is presumably the same or a similar part number).

My HMV 900 speaker is currently out of the case (the HMV 900 is currently dismantled), and so it was easy to measure that.

Readings using Peak LCR45

5.750 ohms @ 15 kHz
1.970 ohms @ 1 kHz
26.84 ohms @ 200 kHz

DC resistance 1.52 ohms

My AVO DA116 gives the DC resistance as 1.8 ohms.

Hope that helps.

Catkins.
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Old 20th May 2017, 2:30 am   #60
dtvmcdonald
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Default Re: Marconi 702 advice

Well, the speaker really won't easily come out. So far no luck getting
a MHD4. Another guy found one on a web site, but untested and
no guarantee. I really don't like that, especially when a few lines later
in much larger type it says "Tested and 100% guaranteed".

If I could get a correct tube base (from a broken tube, for example)
I could attach it to the corresponding American tube, a 4AV6,
and try that out with a known good speaker. If that works I may just
stick a small 4 inch speaker behind the bad one to get it running.

I tried realigning with no luck: trying it wider resulted in overload in
the middle stages. Trying like the picture someone posted above
gave ringing. I then aligned it to the official specs for wide which still
gave ringing. Then I tried tweeking just the last stage for the best
transient response and that resulted in no ringing and some response at
2.5 MHz. A really good picture resulted.

I played with the push-about coil: its needed, there is no way the tube
could be coaxed to a useful position, and it really has only a modest
effect on focus. Using my monster permanent magnet for centering
resulted in the same defocus. And of course it really IS authentic.
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