UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Television and Video

Notices

Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

Reply
 
Thread Tools
Old 27th May 2021, 8:00 pm   #1
dtvmcdonald
Pentode
 
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 203
Default Marconi 702 sound problems

I've been working on my Marconi 702 TV. This set is not "restored" per se.
It was "repaired" when I got it. It had a nice new EHT transformer and
EHT filter caps that tested perfect. I replaced all the flaky looking
electrolytics and a couple of early film caps with my usual stock of
Panasonics and Nichicons. One problem is that many old wax caps
are still in place, though all the ones I could get at were already
disconnected and early film ones used.

I realigned the TRF with my new sweep generator. It has a problem in
that the third stage can't be tuned due to a stuck core. However, it was close
and I have it in a state producing an excellent video waveform.

The speaker was bad. I am loathe to remove the audio chassis since
it (apparently) requires removing the picture tube. So I replaced the
speaker with a modern one just stuffed in place. It works fine with enough signal.
But enough signal means that the audio bothers the sync.

I measure about 5 mV p-p at 41.5 MHz at the grid cap of the X 41 frequency
changer (with the audio carrier at a level that does not bother the sync, and
produces weak audio). I get a few volts signal at 1.5 MHz on the plate cap
of the IF amplifier. The audio tuner tunes correctly.

The AC voltage on the volume control is less than one volt. This produces weak sound.

Do these voltages seem correct?

Oh yes, did I mention that the audio output tube is soldered in place.

Does anybody have any suggestions of what to try without removing the audio
chassis? OR, can the audio chassis actually be removed, safely,
without removing the picture tube?

I cannot test the removable audio tubes as I don't have the test data.
Are there American equivalents as could use as test templates?

These are 4 volt tubes ... is that right?

If I can't fix it I think I will simply add a tiny solid state amp I have
between the speaker output and the speakers.

Doug McDonald
dtvmcdonald is offline   Reply With Quote
Old 28th May 2021, 8:16 am   #2
peter_scott
Nonode
 
peter_scott's Avatar
 
Join Date: May 2003
Location: Edinburgh, UK.
Posts: 2,835
Default Re: Marconi 702 sound problems

Removing the CRT is no big deal. Disconnect the mirror lid prop and fold back the lid. Remove the mask/safety glass frame by pushing it up from below. After disconnecting the scan coils, EHT connector and base connectors the CRT just lifts out cleanly. I must have done it dozens of times. Yes, the tubes are 4 volt.

Peter
peter_scott is offline   Reply With Quote
Old 28th May 2021, 10:05 am   #3
FERNSEH
Dekatron
 
FERNSEH's Avatar
 
Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 6,656
Default Re: Marconi 702 sound problems

Sound IF is 1.5Mc/s in this receiver. With the EHT supply disconnected and the CRT taken out it should be possible to carry out servicing work on the sound receiver unit.
My 702 still has that low CRT video drive fault, being only 12V P - P to the tube grid.

DFWB.
FERNSEH is offline   Reply With Quote
Old 28th May 2021, 2:01 pm   #4
dtvmcdonald
Pentode
 
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 203
Default Re: Marconi 702 sound problems

I can't see a way to get the sound chassis out without first removing the CRT,
nd then the metal structure that holds the CRT and magnetically shields the
CRT from the speaker magnet. Its not clear whether one or both of the RF and
sync chassis has to be removed also.

I've been working as best I can with the chassis in place. Working underneath
is very very difficult.

Last edited by dtvmcdonald; 28th May 2021 at 2:08 pm.
dtvmcdonald is offline   Reply With Quote
Old 28th May 2021, 2:06 pm   #5
peter_scott
Nonode
 
peter_scott's Avatar
 
Join Date: May 2003
Location: Edinburgh, UK.
Posts: 2,835
Default Re: Marconi 702 sound problems

You only need to remove the CRT, its metal shield and the cradle that the shield sits in. It might make it easier if you also removed the timebase but I don't think it's essential. It's only held in by four bolts in the side frame although the knobs and panel need to come out too.

The volume and tuning knobs and shafts can remain in the cabinet if you just split them at the couplers at the sound chassis.

Sorry, you also need to remove the speaker, but again only four bolts.

Peter
peter_scott is offline   Reply With Quote
Old 28th May 2021, 3:11 pm   #6
dtvmcdonald
Pentode
 
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 203
Default Re: Marconi 702 sound problems

I have kludged up a fix for the original problem. It seems to be a short in the original
detector circuit. I have replaced the detector circuitry with a new one using a 1N914.
Due to the impossibility of getting a soldering iron in the necessary spots, its connected
with alligator clips.

This will delay the need to remove the audio chassis until something else happens.

BUT ... when that happens it will need three new sockets, which I will have to find
somewhere. One valve is partially soldered in, and two others have flaky contacts
on the sockets.

But now it is happily showing a rerun of Dennis the Menace. It does feel
odd to be changing channels on it with a remote control.
dtvmcdonald is offline   Reply With Quote
Old 30th May 2021, 8:40 pm   #7
dtvmcdonald
Pentode
 
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 203
Default Re: Marconi 702 sound problems

A few odds and ends.

Another thread I was reading noted that the video drive was too low.
I've been running about 10 to 11 volts p-p, though the TRF can generate up to 25 volts with no serious clipping. This was because it always lost sync if it was over 11.

I decided authenticity be damned, and tried adding a 22K resistor from the
output end of R33 (TRF chassis) to ground, and this fixed the problem. It
now produces a much brighter picture.

I noticed that the focus is bad in the corners. Another thread attributed this to a push-about coil. Mine had one, but it was unneeded and so turned off.
I tried physically removing too, with no change. I can reach good focus in two opposite corners, but the centre and the other two corners get worse.
I can make those other two corners get a bit better, but not really correct, by turning the focus the other way.

I tried a small magnet to see what it would do, but it just pushed the image about without changing focus. I checked for AC voltages on the focus electrode but there is none. It's not AC mains hum on the CRT voltages since being 60Hz that would cause going in and out of focus at 10 Hz. The whole picture does wobble a millimetre or so, but not go out of focus.

I never saw a solution to this on the other thread. Has anyone had this problem and found a solution?
dtvmcdonald is offline   Reply With Quote
Old 31st May 2021, 7:44 am   #8
peter_scott
Nonode
 
peter_scott's Avatar
 
Join Date: May 2003
Location: Edinburgh, UK.
Posts: 2,835
Default Re: Marconi 702 sound problems

I couldn't get my edge focus any better than the attached. Turning up the brightness tends to worsen the edge focus. As to brightness I did find that I could get more by strapping an ion trap magnet on the neck. I assume that emission from the cathode that would not normally pass through the exit hole has now been deflected so that it does get through.

Peter.
Attached Thumbnails
Click image for larger version

Name:	P1050049 (Medium).jpg
Views:	119
Size:	111.2 KB
ID:	235116   Click image for larger version

Name:	P1080767 (Medium).jpg
Views:	124
Size:	78.9 KB
ID:	235118  

Last edited by peter_scott; 31st May 2021 at 7:56 am.
peter_scott is offline   Reply With Quote
Old 31st May 2021, 2:02 pm   #9
dtvmcdonald
Pentode
 
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 203
Default Re: Marconi 702 sound problems

Peter:

How do you get that image to your screen? I can sort of do it by taking
a .jpg, converting it to NTSC using my Blu-Ray player, and then converting it
to 405 lines using my Aurora. But this ruins the fine resolution stuff.
In particular, the two finest vertical bar patterns don't appear well
and the diagonal ones, while correct in shape, are only about 40%
modulation.

However, a 1-pixel wide crosshatch in NTSC converts nicely and that's
what I use.
dtvmcdonald is offline   Reply With Quote
Old 31st May 2021, 7:52 pm   #10
peter_scott
Nonode
 
peter_scott's Avatar
 
Join Date: May 2003
Location: Edinburgh, UK.
Posts: 2,835
Default Re: Marconi 702 sound problems

For quite a number of years I was under the (mistaken) impression that the Aurora was incapable of reproducing the the top two gratings but I was wrong and it was simply that the Test Card C image that was in my Aurora was of limited resolution. It's very simple to write images into the Aurora. I've sent you a decent image to your .illinois.edu email.

Peter

User Pushbutton:
The User Pushbutton is used to store a default image into the internal FLASH
memory. On the SCRF525M-SC, it is additionally used to control the color phasing
of the output, rotating by one field each time the button is pushed momentarily.
To store a default image, the converter must be in it’s normal operating mode,
with a valid video input connected, and a Solid Status LED. If a valid video input is
not connected, the button will have no effect. With a stable, stationary video input
applied, the user button may be pressed and held for 2 seconds by inserting a paper
clip, or other small tool into the hole in the back of the unit. The Status LED will
begin to flash quickly, indicating storing of the image FLASH is taking place. This
process can take about 15 seconds, and the video signal must not be disturbed during
this process. Once the image is stored, the Status LED will return to solid.
Once an image is stored, whenever there is no video input connected to the unit,
the default image stored in the FLASH will be outputted.

Last edited by peter_scott; 31st May 2021 at 8:03 pm.
peter_scott is offline   Reply With Quote
Old 31st May 2021, 11:38 pm   #11
dtvmcdonald
Pentode
 
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 203
Default Re: Marconi 702 sound problems

We must have a different convertor box. My Aurora is the universal model.
I of course have to feed it NTSC. The NTSC viewed on my large LCD set or my
TRK120 looks just fine, whether the slide you sent or the ones I have.
The only way I have of seeing the "true" nature of the 405 line output is
by viewing the video output on the delayed "B" sweep of my analog scope, which I have done. The conversion of the four slanted line patches looks excellent, as are the two lowest frequency bars. The third bar is about 90% amplitude, the fourth bar about 60% and varies across it.

Looked at at the video output of the Marconi, the next to highest two frequency bars are perfect, the third at about 75% amplitude, the fourth
at about 35% amplitude, and the last bar shows only spikes at the start and finish.

On the screen, at a bit reduced brightness, the first two bars are perfect, the third quite clear, and the last two show no modulation at all.

The CRT is quite bright, but at turnon, before sweep starts and the focus is
good, shows signs of less than first rate splotches on the cathode surface.
Test patterns with 50 shaded gray bars actually look quite good, though the gamma is a bit high. This is better than just OK for an 80 year old tube.
dtvmcdonald is offline   Reply With Quote
Old 1st Jun 2021, 12:05 am   #12
beery
Heptode
 
beery's Avatar
 
Join Date: Jun 2006
Location: Ware, Hertfordshire, UK.
Posts: 887
Default Re: Marconi 702 sound problems

Hi Doug,
Going back to the problem of the valve sockets...
The guy who "restored" the set didn't tell me that he had soldered the valves in. It was a nasty thing to do, but the difficulty of accessing the sound chassis may explain why he did it.
Anyway, that is by the by.

If the valve sockets have suffered from de-zincification then the contacts will have lost their springiness and will be fatigued. If that is the case then they will need to be replaced. I've found that EMI valve sockets have an unusually large spacing between the mounting points, which is a pain when trying to find replacements.

If the contacts are simply tarnished then they can be cleaned up. I'm currently working on a prewar set with paxolin Mazda octal sockets (slightly different from American octal). All the postwar Mazda octal sockets I have seen are moulded Bakelite, so I couldn't use them. My only course of action was to clean the contacts. I used a drill bit with the same diameter of the valve pins. I spread the blunt end of the drill with T-CUT (original formula) and inserted the bit into a contact hole, moving it in and out and twisting it a few times. Then I cleaned the T-CUT off with a paper towel soaked in IPA. I did this for each contact of each valve socket and the results were very good. This method might work for your set.

Cheers
Andy

Last edited by beery; 1st Jun 2021 at 12:08 am. Reason: Paragraphing
beery is offline   Reply With Quote
Old 1st Jun 2021, 9:56 am   #13
peter_scott
Nonode
 
peter_scott's Avatar
 
Join Date: May 2003
Location: Edinburgh, UK.
Posts: 2,835
Default Re: Marconi 702 sound problems

Quote:
Originally Posted by dtvmcdonald View Post
Looked at at the video output of the Marconi, the next to highest two frequency bars are perfect, the third at about 75% amplitude, the fourth
at about 35% amplitude, and the last bar shows only spikes at the start and finish.

On the screen, at a bit reduced brightness, the first two bars are perfect, the third quite clear, and the last two show no modulation at all.

The CRT is quite bright, but at turnon, before sweep starts and the focus is
good, shows signs of less than first rate splotches on the cathode surface.
I should say that the alignment instructions in the EMI service manual should just be regarded as a starting point. I spent many happy hours making tiny changes whilst monitoring the effect on the television screen. After a while you learn which trimmers do what iterating to get the best bandwidth and step response.

Before I switch off the set I have always turned the brightness to minimum and have not turned it up again at switch on until I hear that the sound section has woken up.

Peter
peter_scott is offline   Reply With Quote
Old 1st Jun 2021, 6:22 pm   #14
dtvmcdonald
Pentode
 
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 203
Default Re: Marconi 702 sound problems

Because the focus is so nonuniform across the screen I can't use its appearance as a criterion for alignment. I am using a chirp pattern
from Digital Video essentials fed into the Aurora, looking at it on my scope
(viewing the CRT drive with a probe with a 4 pF capacitance). I look at both the frequency response and the transient response of the line sync.

As I said, one of my coil adjusters is frozen so I had to experiment. I tried adding a peaking coil/ sound trap in series with the CRT lead. The CRT lead measures 26 pF to ground if disconnected from its drive terminal. I settled on 120 uH in parallel with 17 pF and 8.2K. The result is excellent.

I'm using a pad in series with my Aurora that gives 5 mV p-p drive to the set input (set turned off).

The actual picture is very good at five feet away despite bad focus.
dtvmcdonald is offline   Reply With Quote
Old 18th Jun 2021, 8:07 pm   #15
dtvmcdonald
Pentode
 
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 203
Default Re: Marconi 702 sound problems

The sound remains fixed. So I'll continue this thread with the focus problem.

I've made no progress. I have no idea what to actually do except make tests.

I tried using my big degauss coil very carefully over as much of the set as I could reach, getting it as close as possible. No change in the problem. Looking down at the top of the tube, not in the mirror, the problem is symmetric along a line from upper right to lower left. At a certain setting to the focus control the center is in focus. Turning it CCW, the lower right and upper left get to excellent. Turning it more, the center gets worse, those two areas get worse again, and the upper right and lower left get a bit better, then it runs out of range.

I got a big piece of sheet steel, 6" x 24", 1/20 inch thick and degaussed it.
I tried moving it around to see if that changed the focus. This was limited by my fear of the EHT wire, whose insulation I suspect is not in great shape. In any case, no effect.

For a long time I suspected the push-about coil, but its now removed as
its not necessary.

I have noticed that the yoke is not exactly oriented parallel or perpendicular to the scanning direstions, its off a few degrees. But the picture is oriented properly.

I suspected the electric field of the EHT wire, but I checked that by attaching a foot of nylon twine to it and moving it around, to no effect.

I looked at the cylindrical electrodes in the tube, and they appear aligned correctly.

Any suggestions?

Doug McDonald
dtvmcdonald is offline   Reply With Quote
Old 19th Jun 2021, 12:34 am   #16
beery
Heptode
 
beery's Avatar
 
Join Date: Jun 2006
Location: Ware, Hertfordshire, UK.
Posts: 887
Default Re: Marconi 702 sound problems

Hi Doug,
If you are able to rotate the scan coils (you may need to loosen the clamping screws) a bit then you can see if that also rotates the focus errors. If the focus errors rotate with the coils then that would indicate that the scanning yoke could be magnetised or that the yoke is not properly centred on the scan coil windings.
If the focus errors do not rotate then it could be down to the alignment of the electron gun alignment.

My limited experience of viewing these sets is that the focus is not as good as with a magnetically focused crt.
I've not played with the controls myself though, but I assume the set's I have been set up for optimum average focus across the screen.
I can tell you that the CRT used in thec15" Baird T5, which is also from 1936, gives a much sharper picture and I'm sure this is down to the CRT design rather than anything else. The 15MW2 crt used in the T5 is a simple directly heated triode tube with magnetic focus.

Cheers
Andy
beery is offline   Reply With Quote
Old 26th Jun 2021, 3:16 pm   #17
dtvmcdonald
Pentode
 
Join Date: Mar 2016
Location: Champaign, Illinois, USA.
Posts: 203
Default Re: Marconi 702 sound problems

Quote:
Originally Posted by beery View Post
Hi Doug,

I can tell you that the CRT used in thec15" Baird T5, which is also from 1936, gives a much sharper picture and I'm sure this is down to the CRT design rather than anything else. The 15MW2 crt used in the T5 is a simple directly heated triode tube with magnetic focus.

Andy
I think that's the case.

Yesterday I tried my HackRF in transmit again and lo and behold, a recent
Windows 10 update has made it once again work reliably in transmit. It can now reliably generate a 20 MHz wide signal. I'm running it at a 12.5 MHz rate for convenience. The result is that I'm running at 52.08 Hz vertical scan rate so I can use Test Card C images designed for a 6 MHz pixel clock. This requires the minutest tweeks to scan controls. Starting from scratch you could of course run at 50 Hz.

All the test card images I have have hopelessly badly converted frequency test patches which look terrible in the data files. The set pretty much reproduces these correctly.

But I can computer generate test patterns with a 12.5 Mhz pixel clock and have the HackRF filter them digitally at baseband at cutoffs of 1.75, 2.5, 3.5, 5, 5.5, 6, 7, 8, 9, 10, 12 ... MHz. I'm using 3.5. This means I can make really good correctly synced signals with no artifacts or aliasing.

So I tried various patterns and find that, looking directly at the signal to the CRT with a scope (which adds a few pf to the intrinsic value of 26 pf),
the signal is flat to 2 MHz and drops smoothly to zero at 3 MHz, as it should, and as I thought using an old sweep generator. The signal is still
very clear on the scope at 2.7 MHz.

But the CRT is barely capable of, at the sharpest spots, 2 MHz, and over most of the screen more like 1.7 MHz. This is clearly defocus. Its not greatly dependant on brightness up to a fairly light gray or modulation amount so long as its not brighter than that. Above that brightness level the resolution drops very rapidly.

I modestly recommend the HackRF for this purpose. As a general purpose receiver at its absolutely abysmal due to low dynamic range. Most folks are using it at 1 to 6 GHz with Linux. But you have to write your own test files and play them using the command line program hackrf_transfer, a pain. Its NRFPT. Its also expensive.
dtvmcdonald is offline   Reply With Quote
Reply

Thread Tools



All times are GMT. The time now is 4:34 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2021, vBulletin Solutions, Inc.
Copyright ©2002 - 2021, Paul Stenning.