1949 Motorola VT-72

Hi Lloyd, what an interesting little set, nice to see something out of the ordinary, makes a change from the normal UK sets we see.  

For those interested in following along with Lloyds work and have access to the data library, I have just placed the Motorola service data there.

Look forward to your next update Lloyd.  ? 

Cheers Chris, certainly is different to our UK sets, never worked on electrostatic CRT sets before, there’s EHT everywhere!! The set is like an angry crocodile, snapping away merrily if there’s even the tiniest spike on a solder joint. 

I’ll get those caps ordered sometime this week, Just Radios sell them, so it could be a while before I get back to it.  I did something silly and temporarily fitted some 2KV rated ceramics where the 6KV ones should be, they haven’t gone bang yet, but they will be replaced correctly. 

I’m in 2 minds as to whether to re-stuff the original Motorola caps, or just to keep them in a bag with the set and leave the replacements naked. Problems might arise with insulation on the 6KV ones.

Regards,

Lloyd

Hi.

I'm sure Ian (IJK2008) had one of these, I've lost touch with Ian which is very sad indeed, i used to really enjoy his visits as he was really keen and top chap. I hope he still reads the forums and I'd love to hear from him, I tried his number a few times with no success.

Posted by: MurphyV310

I'm sure Ian (IJK2008) had one of these

Indeed he did, a VT-71 and he logged its repair here, now 6 years ago! time does fly.

https://www.radios-tv.co.uk/community/black-white-tvs/motorola-vt71/

Posted by: Chris

Posted by: MurphyV310

I'm sure Ian (IJK2008) had one of these

Indeed he did, a VT-71 and he logged its repair here, now 6 years ago! time does fly.

https://www.radios-tv.co.uk/community/black-white-tvs/motorola-vt71/

 

Time does fly Chris and at times I think its scary,  I can't believe I'll be 65 in just a few short months. I remember now that Ian brought the set to me as it had an odd instability issue causing a patterning on screen,  it was very good and great set to have,  I'm looking forward to Lloyd's progress,  something tells me that the set needs a little more smoothing for 50Hz,  one other suggestion is to use a cap dropper or diode and resistor for heaters,  an ordinary dropper gets really hot and there's not a lot of room to dissipate the heat.

Cheers for pointing me to Ian’s thread, his set does look good! The frame linearity is certainly better on his, not sure how I missed that thread! 

The set does need more smoothing, it’s not noticeable when playing 625 line material, but if I switch the SD card player over to 525 NTSC the picture looks like it’s being reflected off a lake on a windy day... it’s good I can use it on both standards though!

I read on another forum that the high voltage caps need to be film caps rather than ceramic as they can affect the linearity, and as can be seen in the photo of it playing in its case, the top is severely stretched, that was 16:9 video! And there is some patterning too, I thought it was just down to my RF setup, but I may be wrong.

Regards,

Lloyd

Hi All

Just signed up - hope you'll have me back Chris!

I haven't really done anything with tellys for ages so time to start again.

The Just Radio caps will do the job perfectly Lloyd. That's a nice set and well worth carrying on with.

PS Nice to see Trevor here.

Ian

Hi Ian

really great to see you back, as you can see your account is still intact with all your previous topics and posts from the old forum. Look forward to you getting back in the saddle with some TV repairs, Trevor will be pleased you're here. ? 

Brilliant News. ? 

I'm really so glad to see you are back Ian, I really have missed the visits, chats and your dedication. I really hope we can catch up whenever you are up to it. ☕  ? 

Chris again I have to thank you for your kindness and thought. Appreciated ? 

Good to see you back Ian!

I still haven’t got back to this little set, there’s an EKCO T161 on the bench at the moment, which I think is just about ready to be evicted back to the shelves, just need to get to the bottom of some annoying sync problems then it’s ready, well, ready for the cabinet to be refinished anyway. 

Regards 

Lloyd 

A bit of an update!

Just ordered the caps from Just Radio's, and I also ordered a modulator which will hopefully work with this set, it's on it's way from China, so might arive before Christmas! Last look at the tracking said it had landed in the destination country.

Regards,

Lloyd

Excellent news, look forward to following something a little different from the norm.  ? 

Santa dropped by a week early! Caps arrived today from Just Radio's, and the modulator arrived last friday.

First job, try out the modulator, so out came the TV and some coax with an F connector on it. I rigged it up, and spent a few minutes trying to work out how the heck you set it up, the included instructions aren't much help, I even tried reading them upside down, but it still didn't make any sense! Eventually I had worked out how to set the spacing to 4.5mhz, and then set it to channel 12 VHF. The little Motorola had no problem tuning in to it, and for the first time, I had sound, albeit not very loud. Anyway, here's a couple of shots of the set..

As I said, the caps have arrived too, so looks like I'll be doing some work on the set over Christmas (yay!! time off work=fix telly's!!!), I also got some of the rather pricey black audiophile grade capacitors, not for the Motorola, but for an Ekco RS3 radio, since they look almost identical to the original parts in the set, and it means I can get away with not re-stuffing the old ones! I'm getting lazy...

The chassis has been extracted and put on the bench, I've left the tube in the cabinet for now. So here's the TS-18A chassis in all its glory, please forgive the unsightly bodgery underneath, it will get tidied!

I've already fitted the EHT cap, and got the circuit loaded on an old iPad, one of the few things a first generation iPad is still useful for!

Regards,

Lloyd.

Bit more progress to report, I've fitted the 6KV caps, pulled the CRT from the cabinet and connected it up whilst balancing it on a cardboard box with a towel on it (its more secure than it sounds, honest!!). The picture before replacement was seriously stretched at the top, possibly because of my under-rated and wrong value ceramic bodge. I was quite surprised by just how much of a difference the new caps made, the linearity is damn near perfect! I wasn't expecting that at all, I thought there was going to be more work to do in the vertical stages, at least with replacing some out of tolerance resistors.

Next fault to hunt down is what is causing the picture to snake around really badly, I thought smoothing, but a quick check by connecting a new cap across all the others made no difference to it at all, and it seems to vary all on it's own, sometimes its barely noticeable, and other times its almost as if the smoothing has been disconnected. It could be because of the close proximity of the isolating transformer and the variac, but I'll need to do some more work on the chassis before I can move it away to check.

There is also drifting frame sync to look into as well, although the control has plenty of range on it where it will stay locked once it is, but if it goes out of sync it goes mental and requires quite a good turn up near one end to restore it.

Oh, and the speaker I'm using for test is the reason for low sound output, well some of it anyway, because its rubbish!

regards,

Lloyd

Looking good, well on the way to being sorted out.

Excellent progress Lloyd, nice tidy work too ? those 6kV caps look to be very good quality which we've come to expect as the norm from 'Just Radios'.

That's great progress Lloyd.

I really like these little American sets, they are great fun and once all sorted out, work very well. 

Perhaps one day I'll get my hands on one.

Right, lets see if I can post without windows update getting in the way again! Automatic scheduled restarts are a stupid idea, and whoever came up with them needs a slap...

I've been working on the set throughout the week, although I didn't get very far on Christmas eve, I ended up spending 2 hours trying to dig a piece of wood 1/4" long out from behind my thumb nail, I can see why interfering with people's fingernails can be used as a form of torture, it would certainly be very effective on me! Christmas day we spent with family down the road in Horncastle, and Boxing day we went to the coast! Dropped in at the North sea observatory at Chapel st. Leonards, which only opened this year. Back to the little Motorola, I've come to the conclusion that the picture snaking around was mostly down to the isolating TX and the variac being fairly close to it, I tried tacking in various electrolytics in place of the smoothers to no avail, but moving the set on the bench seems to have reduced it considerably. I did also think maybe a valve has a heater-cathode leak, which could be causing it, as it does seem to be more present when the sets been on for a while.

Whilst looking at the circuit diagram I noticed that C4 should be a 10uf electrolytic, but I couldn't find it in the set itself, After much head scratching I found a 0.05uf connected in it's place, I had previously replaced it like-for-like. It seemed odd to me that it was so different from the diagram, so I stuck in a 10uf to see what it did. It killed the sound completely, that's what it did! The sound has always been a bit naff on this set, with annoying interference on it, as well as being not very loud. Why would someone change it for such a different value? Well after a check of resistors I checked C50, a 500pf silver mica cap, firstly by checking voltages either end of it, which were identical, turned out the cap was dead short! So it looks like back in the past the sound went on the set, so it was taken to be repaired, and the shorted cap was missed by the repairer, who took out the 10uf and swapped it for a 0.05uf, and twiddled the alignment... It also seems that the speaker and audio output transformer were changed too!

I replaced C50 with a new 470pf ceramic, and put in the 10uf as well, then ran through the audio IF alignment as best I could, using my Fluke DMM in place of a VTVM, since I don't have such a thing. I now have good loud sound, and the circuit around the ratio detector matched the diagram once again. There is still the annoying feature of vision-on-sound, which doesn't seem to go away with fine tuning, it probably wants the rest of the alignment doing, but I don't fancy getting involved with that as it requires equipment I haven't got, such as a sweep generator, got some at work! But I don't think they'll let me borrow it.

There was also one of those mica caps in the frame stage which wouldn't measure properly on the capacitance range of my DMM, just kept bouncing around all over the place, so I've replaced that too, it was C62 on the circuit. There are 3 more of them in the set, so I'll be checking them all now, as they don't appear to be any good!

Regards,

Lloyd