VT73DA Loss of picture - any advice? - UK Vintage Radio Repair and Restoration Discussion Forum

DoctorWho · 17th Jul 2007, 12:25 pm

Well I guess it was only a matter of time before such a problem occured, but it's got me stumped, so I'd like to ask for some help please as to what to look for or what may be causing this problem.

Initially the picture started vanishing and then coming back, I thought at first it may be due to a loose aerial lead, and so double checked this, all was well for a short while, then it started again, when the picture disappeared there was still perfect sound and, by turning up the brightness control, you could see something going on, and this was altered by the contrast control too, but it looks as though there's not enough of the vision signal getting through. With the contrast control fully advanced the indistinct mush on the screen just gets a bit more contrasty.

I removed all valves in the RF section and cleaned every valve holder and valve base and put them back, but no change. I now have an indistinct mush instead of a picture, as mentioned before it looks like there's simply not enough vision signal getting through. Wiggling the valves makes no difference, and now I'm a bit stumped. It started off as intermittant but now is permanent.

Any suggestions as to what the problem may be would be very much appreciated, or any tips as to where to look.

Many thanks,

Peter.

Steve_P · 17th Jul 2007, 1:18 pm

I knew you'ld be back on here with this soon. Actually, you lasted longer than I thought you would! Don't fill your hotel with these....

Check the Anodes of all the IF and RF valves against the circuit. Also, the resistors to the HT. Follow the vision through (you've got an RF probe?) and the problem should become evident.

The VT73 version is a superhet, and uses V16 as a frequency changer. The sound is taken off at L5/6. The vision at L25, to the control grid of V2.

Because this was intermittent before it went, I'd check soldering and cores.

A picture speaks a thousand words though, so can we have some?

Cheers,

Steve P

Heatercathodeshort · 17th Jul 2007, 4:15 pm

Ha ! ha! The thing has had another wobbly! I was counting the hours..
Hello Peter,
I think yours is the superhet version with a X78 valve as the frequency changer. It struggles at 60mc/s and may have stopped oscillating. If you have a spare it would be worth a try. As Steve suggests it may also be a faulty decoupling capacitor probably on the screen grid of one of the Z77 [EF91] i.F. amplifiers. The set will have burst into oscillation [the wrong sort] and wiped everything out. A signal generator will help. Also have a look in Cockings book under I.F. instability. This may guide you in the right direction. Regards, John.

DoctorWho · 17th Jul 2007, 6:52 pm

Steve - Thanks for the advice here. The set lasted longer than I thought too! I haven't got an RF probe, but I can check for HT. The thing that puzled me was the fact that sound is perfect, it's only vision that is effected. I'll try and get a picture or two. Interestingly the Line Hold seems to increase/decrease the brightness when under this fault condition (as well as altering the line hold position), it's as though whatever fault has developed is resulting in the Line Hold being outside of the set's range.

John - I knew you'd be so shocked that this set had developed a fault

This is indeed the Superhet version. I'm not sure if I have a spare X78 or not, I'll look and see. I did replace all of the wax condensers when I initially worked on the set, so am I right to presume that they should be alright?

I'll try and take a picture or two and let you know what I find.

Thanks again for the advice.

Peter.

Heatercathodeshort · 17th Jul 2007, 9:39 pm

Hello again Peter,
First check the pin voltages on the vision i.f. amp valves type Z77 [equivalents, EF91, 6F12 etc] You should find around 150v or thereabouts, its not critical on Pins 5 the anodes and pins 7 the screen grids.
You may have a voltage missing here due to a shorted decoupling capacitor burning out the feed resisitor.
The decoupling capacitors gave a mountain of trouble with this series of models. The originals are .001uf but EMI issued a replacement kit with the values increased to .003uf. They don't need to be exact and any value from .001 to .003 is suitable. You really need to bridge a good one across each in turn to find the faulty one and deal with them seperately. The replacements should be positioned as close as possible to the original components position on the chassis. I would suggest you do each one in turn, tedious I know but you will get in a mess if you do a mass cull. The fault may be in the video detector or amplifier but you really do need to apply a test signal from a generator to trace the fault through. The capacitors by the way are ceramic and not the usual wax type. Regards John.

DoctorWho · 17th Jul 2007, 11:11 pm

Thanks for the help here guys, I really appreciate it.

I can now confirm, 100%, that I will not have ANY further problems with the VT73DA, I can be 100% confident in this, however my "cure" has been somewhat radical. I'd never in a million years normally do what I've just done, but a person can only put up with so much!!

Up until this point I had a VT53DA which works superbly well, has not yet failed, but had a very tired tube that could only just about be seen in a dark room, then I had the VT73DA which had a replacement Cossor CRT which was up to the original spec as it would have been when it wasnew and gives a superb picture, in fact, one of the best on a tube of such a date that I have, it's really that good.

I started work on the VT73DA and then I lost sound, I continued on and then I lost the vision signal (mushy though it was) and just had a blank raster, I then started tracking this, next the frame collapsed in to a single line.

As some may know, this set has been trouble from day one, and after a considerable number of hours on this set yet again I'd reached my limit, and so I decided that I would make one really good set out of the two, I removed the CRT from the VT53DA and replaced it with the Cossor CRT from the VT73DA, along with it's heater transformer, I then fitted the required resistor to complete the heater chain. After a little while the set was re-assembled and the results stunning! The VT53DA is now giving an absolutely fantastic picture and is totally stable, it can be viewed in a well lit room too.

The VT53DA is the older set, it was in much better condition externally and has the original transfer etc., the VT73DA had been "got at" at some point in the past and so was really just having one problem after another.

For the moment I'll keep it, I may use it for spares or I may have another go at it at some point, but for the moment it's relegated to the loft! It's rewarding to finally have one of these sets working extremely well and giving a superb picture.

Thanks again to all who helped with these sets, it's very much appreciated.

All the best,

Peter.

Duke_Nukem · 18th Jul 2007, 11:35 am

Aww, don't give up. Put the poor tube in the VT73 and persevere ... if nothing else it'll be good practice for when the VT53 breaks

.

I've got three of these chassis all awaiting repair ... along with three duff CRT's :-( Shame as I'd really like to get one of these "masterpieces" working.

TTFN,
Jon

DoctorWho · 18th Jul 2007, 11:01 pm

I know what you're saying Jon, and I agree with you, but the CRT is really very porr indeed, the picture is just visible in a completely dark room, and an increase in brightness simply gives a negative image. I've since spoken with another collectir friend to-day who has a very rare radio, a Marconi/HMV model 1250 which uses the RF chassis from one of these sets and so it's been agreed between us that he can have the RF chassis and I'll use the rest of the set for spares. I would never in a million years normally dismantle a set of any era, but really and truly I have only got one good set here, that is, one good chassis and one good CRT. The VT53DA chassis must have been a good one all the way through as the CRT is very worn, with the replacement CRT the picture cannot be faulted, so, as much as I hate to see a set go, in this case I feel it's the right decision. The chassis of the VT73DA had been so heavily bodged and modified with extra resistors and modifications that I think it was doomed from the start! The VT53DA also had a superb cabinet whereas the VT73DA had a very poor cabinet. Having said all of this I sincerely hope that I will find another VT73DA in the near future which has not been "got at" and then restore that to a good working set. I'll also try and track down the HMV models in this series if I can.

At least I do have a VT53DA which works extremely well and gives absolutely fantastic picture and sound!

Is there any way of getting better results from these tired Emiscope CRT's, such as raising the heater voltage perhaps? If so, what to?

Thanks again for the help.

Peter.

17th Jul 2007, 12:25 pm	#1
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	VT73DA Loss of picture - any advice? Well I guess it was only a matter of time before such a problem occured, but it's got me stumped, so I'd like to ask for some help please as to what to look for or what may be causing this problem. Initially the picture started vanishing and then coming back, I thought at first it may be due to a loose aerial lead, and so double checked this, all was well for a short while, then it started again, when the picture disappeared there was still perfect sound and, by turning up the brightness control, you could see something going on, and this was altered by the contrast control too, but it looks as though there's not enough of the vision signal getting through. With the contrast control fully advanced the indistinct mush on the screen just gets a bit more contrasty. I removed all valves in the RF section and cleaned every valve holder and valve base and put them back, but no change. I now have an indistinct mush instead of a picture, as mentioned before it looks like there's simply not enough vision signal getting through. Wiggling the valves makes no difference, and now I'm a bit stumped. It started off as intermittant but now is permanent. Any suggestions as to what the problem may be would be very much appreciated, or any tips as to where to look. Many thanks, Peter.

17th Jul 2007, 1:18 pm	#2
Steve_P Dekatron Join Date: Sep 2003 Location: Bolton, Greater Manchester, UK. Posts: 6,644	Re: VT73DA Loss of picture - any advice? I knew you'ld be back on here with this soon. Actually, you lasted longer than I thought you would! Don't fill your hotel with these.... Check the Anodes of all the IF and RF valves against the circuit. Also, the resistors to the HT. Follow the vision through (you've got an RF probe?) and the problem should become evident. The VT73 version is a superhet, and uses V16 as a frequency changer. The sound is taken off at L5/6. The vision at L25, to the control grid of V2. Because this was intermittent before it went, I'd check soldering and cores. A picture speaks a thousand words though, so can we have some? Cheers, Steve P __________________ If we've always had it, why is the Car Boot open? You're not sneaking another Old TV in are you...?

17th Jul 2007, 4:15 pm	#3
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: VT73DA Loss of picture - any advice? Ha ! ha! The thing has had another wobbly! I was counting the hours.. Hello Peter, I think yours is the superhet version with a X78 valve as the frequency changer. It struggles at 60mc/s and may have stopped oscillating. If you have a spare it would be worth a try. As Steve suggests it may also be a faulty decoupling capacitor probably on the screen grid of one of the Z77 [EF91] i.F. amplifiers. The set will have burst into oscillation [the wrong sort] and wiped everything out. A signal generator will help. Also have a look in Cockings book under I.F. instability. This may guide you in the right direction. Regards, John.

17th Jul 2007, 6:52 pm	#4
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: VT73DA Loss of picture - any advice? Steve - Thanks for the advice here. The set lasted longer than I thought too! I haven't got an RF probe, but I can check for HT. The thing that puzled me was the fact that sound is perfect, it's only vision that is effected. I'll try and get a picture or two. Interestingly the Line Hold seems to increase/decrease the brightness when under this fault condition (as well as altering the line hold position), it's as though whatever fault has developed is resulting in the Line Hold being outside of the set's range. John - I knew you'd be so shocked that this set had developed a fault This is indeed the Superhet version. I'm not sure if I have a spare X78 or not, I'll look and see. I did replace all of the wax condensers when I initially worked on the set, so am I right to presume that they should be alright? I'll try and take a picture or two and let you know what I find. Thanks again for the advice. Peter.

17th Jul 2007, 9:39 pm	#5
Heatercathodeshort Dekatron Join Date: Jun 2005 Location: Warnham, West Sussex. 10 miles south of DORKING. Posts: 9,147	Re: VT73DA Loss of picture - any advice? Hello again Peter, First check the pin voltages on the vision i.f. amp valves type Z77 [equivalents, EF91, 6F12 etc] You should find around 150v or thereabouts, its not critical on Pins 5 the anodes and pins 7 the screen grids. You may have a voltage missing here due to a shorted decoupling capacitor burning out the feed resisitor. The decoupling capacitors gave a mountain of trouble with this series of models. The originals are .001uf but EMI issued a replacement kit with the values increased to .003uf. They don't need to be exact and any value from .001 to .003 is suitable. You really need to bridge a good one across each in turn to find the faulty one and deal with them seperately. The replacements should be positioned as close as possible to the original components position on the chassis. I would suggest you do each one in turn, tedious I know but you will get in a mess if you do a mass cull. The fault may be in the video detector or amplifier but you really do need to apply a test signal from a generator to trace the fault through. The capacitors by the way are ceramic and not the usual wax type. Regards John.

17th Jul 2007, 11:11 pm	#6
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: VT73DA Loss of picture - any advice? Thanks for the help here guys, I really appreciate it. I can now confirm, 100%, that I will not have ANY further problems with the VT73DA, I can be 100% confident in this, however my "cure" has been somewhat radical. I'd never in a million years normally do what I've just done, but a person can only put up with so much!! Up until this point I had a VT53DA which works superbly well, has not yet failed, but had a very tired tube that could only just about be seen in a dark room, then I had the VT73DA which had a replacement Cossor CRT which was up to the original spec as it would have been when it wasnew and gives a superb picture, in fact, one of the best on a tube of such a date that I have, it's really that good. I started work on the VT73DA and then I lost sound, I continued on and then I lost the vision signal (mushy though it was) and just had a blank raster, I then started tracking this, next the frame collapsed in to a single line. As some may know, this set has been trouble from day one, and after a considerable number of hours on this set yet again I'd reached my limit, and so I decided that I would make one really good set out of the two, I removed the CRT from the VT53DA and replaced it with the Cossor CRT from the VT73DA, along with it's heater transformer, I then fitted the required resistor to complete the heater chain. After a little while the set was re-assembled and the results stunning! The VT53DA is now giving an absolutely fantastic picture and is totally stable, it can be viewed in a well lit room too. The VT53DA is the older set, it was in much better condition externally and has the original transfer etc., the VT73DA had been "got at" at some point in the past and so was really just having one problem after another. For the moment I'll keep it, I may use it for spares or I may have another go at it at some point, but for the moment it's relegated to the loft! It's rewarding to finally have one of these sets working extremely well and giving a superb picture. Thanks again to all who helped with these sets, it's very much appreciated. All the best, Peter.

18th Jul 2007, 11:35 am	#7
Duke_Nukem Octode Join Date: Jan 2003 Location: Birmingham, West Midlands, UK. Posts: 1,268	Re: VT73DA Loss of picture - any advice? Aww, don't give up. Put the poor tube in the VT73 and persevere ... if nothing else it'll be good practice for when the VT53 breaks . I've got three of these chassis all awaiting repair ... along with three duff CRT's :-( Shame as I'd really like to get one of these "masterpieces" working. TTFN, Jon

18th Jul 2007, 11:01 pm	#8
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: VT73DA Loss of picture - any advice? I know what you're saying Jon, and I agree with you, but the CRT is really very porr indeed, the picture is just visible in a completely dark room, and an increase in brightness simply gives a negative image. I've since spoken with another collectir friend to-day who has a very rare radio, a Marconi/HMV model 1250 which uses the RF chassis from one of these sets and so it's been agreed between us that he can have the RF chassis and I'll use the rest of the set for spares. I would never in a million years normally dismantle a set of any era, but really and truly I have only got one good set here, that is, one good chassis and one good CRT. The VT53DA chassis must have been a good one all the way through as the CRT is very worn, with the replacement CRT the picture cannot be faulted, so, as much as I hate to see a set go, in this case I feel it's the right decision. The chassis of the VT73DA had been so heavily bodged and modified with extra resistors and modifications that I think it was doomed from the start! The VT53DA also had a superb cabinet whereas the VT73DA had a very poor cabinet. Having said all of this I sincerely hope that I will find another VT73DA in the near future which has not been "got at" and then restore that to a good working set. I'll also try and track down the HMV models in this series if I can. At least I do have a VT53DA which works extremely well and gives absolutely fantastic picture and sound! Is there any way of getting better results from these tired Emiscope CRT's, such as raising the heater voltage perhaps? If so, what to? Thanks again for the help. Peter.