Sinclair mtv1
 

Anyone know the Sinclair mtv1 tv , I just picked one up on eBay so I had a power pack and it turned right on and had sound but just a hint of a pic so I shut it and this morning took it apart to replace the ni cads and did so and now it has just a hiss like no antenna for sound and no hint of a picture. Maybe they are known for something specific I don't know. I changed caps in the IF board and made no difference. The schematic shows voltages on the IF board as well as the audio board and are close to where they should be including the 33 volts for the tuner. Any help would be great.

Pls elaborate on hint of pix. Also did it have normal sound like from
a source ?
A normal good running TV should have nice strong clean snow & a
nice waterfall with no signal. Weak snow or a blank screen points
to the IF most the time. Depending on the design the 33 V may
go right to the tuner or be divided down before. Also band switching
can be external or internal to the tuner. Newer sets will do everything
inside the TNR & have data, 5V, & clock pins........

BTW https://elektrotanya.com/sinclair_mi.../download.html

73 Zeno:smoke:
LFOD !

Make sure that all plug connections are in right and also check for broken solder joints around the connectors, they are easy to break on that little beast. Also no see'm gunk buildup on the boards can stop them in there tracks. Had those problems with my MTV1 and MTV2. All the best, Tom.J

Well the pic was bearly a shadow of something but really couldn't make it out. The sound was clear and it was from a source, cable with the matching transformer so it was a good signal. The screen is lit but blank and snow for sound. I traced the 33v source right to the tuning knob or whatever it's called it also has the 45v source. I changed the caps on the video-IF board but nothing helped at all. I just thought there may be a common thing with these sets. It makes no sense , sound then nothing after putting new battery's in and I also took the battery's back out to double check that I didn't hit something in there but it was all good and intact. One end of the 33v is at the IF then to an IC then out as 45v but the 33v does branch off also but there are no bad traces, checked them all.

Do you mean greasy type stuff under a piece of tape that is original because there is just that and this would kill the signal in the IF ?

I'm talking about the stuff that will kill any hi impedance circuit, like mos or c-mos logic or analog circuits. I have seen way to many boards tossed when all they needed was a good cleaning. Showed young bucks what was needed to keep things going, but most didn't want to take the time to find out what the real trouble was. Not saying that is your problem but something to look into. All the best, Tom.J

Well I may have a clue what's wrong with this little set after reading an old thread from someone in London as he says that the tuner is dead because the varicaps are no good which oscillate the tuner so what does this type of varicap diode look like and does it make a difference what the voltage is which is supplied with 33 volts. I know these varicaps tune the tuner but this guy said there are no marks on these diodes only colors. The thread was short so couldn't get much more info.

Did the "hint of a pix" look something like this?

https://farm5.staticflickr.com/4583/...6aaac7_z_d.jpg

There is a face there, and sound is good, but the set needs more work... any ideas?

jr

yes it did look like that and had sound but now it's gone.
Can you give a closer pic of where the 2 antenna wires go on the board. For all I know at this point you could have the same problem as I have only yours didn't get to the point mine is at now. There are a lot of these sets out there with the same thing going on with them, after all they are 39 years old not like the old sets resistors and caps and tubes and they work. I checked caps and as amazing as it is all caps I checked were above MF which is good. If you give up on yours and want to sell it to me maybe I could use parts for either one and just maybe I could get one working.

As requested... pix of antenna connection to board.

https://farm5.staticflickr.com/4506/...92279e_z_d.jpg

Made some progress... installed my one good speaker, resoldered the antenna connection, tuned my DTV converter box to ch 3, swapped out Video board and was rewarded with a decent pix and sound.

https://farm5.staticflickr.com/4486/...703b8f_z_d.jpg

Still much work ahead, tuning voltage is low. Although I have about -34 volts to the hot end of the tuning pot, I can't get full range. (that's why I moved the converter box to ch 3 as the tv tuner was maxed out and not quite tuning the signal on ch 4.) See pix from last night.
I still have not got supply voltage from either of the rear jacks to operate the set, I supply 5 volts to the "bat" terminal on the sound board to power the set. Likely more corroded open runs will be found... the tuner boards are plagued with corrosion. Fun project!

jr

Did you say you changed the video-IF board ? Yes there is lots of corrosion on the little brass connectors. Thanks for the pic I wanted to make sure I put the wires back in the right place although I cannot get it to work, if I had another one of those boards maybe that's the problem with this one, who knows.

Yes, but the main problem last night was solved by moving the converter box to ch 3... I need to figure out what is pulling down the output of the tuner pot, as it will only go to about -14 or -15 volts and will not quite tune in ch 4.

jr

Yes I noticed that same thing on mine tuned close to 3 the voltage is at 33 v but move it the other way and it will drop to 15 or so volts . Maybe this is how it was designed I don't know but I wish I could get a pic on mine by finding what's wrong. The thread I read from England said the same thing and he thought that the varicaps were bad, I have no clue even where they are , possibly in the tuner it's self being the 33 v goes directly to the tuner fed by the video- IF board and dropped down from 45volts via a 120k resistor and the end pin2 next to the -33 v output should show - 0-33 v negative as the tuning is moved , it's marked variable in the half ass schematic which don't give any electrical info at all.

Indeed the varicap diodes are inside the two tuner modules ... they are shown on the schematic as VC-601, 602, 603 and 606 in the VHF tuner module and VC-501, 502 and 503 in the UHF tuner module. I have yet to open either tuner module, so have never seen them.
If your set is no longer making sound or even a hint of a picture anymore, I would suggest that something went wrong in re-assembly after the batteries were installed... it is very easy to mis-align the Berg pin connectors, usually with disastrous results. :sigh:

jr

These pins fit perfect and I used a Dvom in diode test to indicate each pin is infact in but there were some that were green so I took care of that. I went over the tuner board several times and all is good. Well I guess it's possible that when I first turn this set on for the first time in maybe 30 years the varicaps or a varicap went bad, just don't know. So I understand to check them it's the same as a regular diode. What kind of schematic do you have is it the one online where you can bearly make out numbers and symbols because mine is near impossible to find these varicaps on this print. Well if I take these diodes out and test and find that they are leaky or shorted or open how on earth would I find the right value of them to replace .

Yes, I am using the on-line schematic... somewhat hard to read, but the 2 tuner schematics (pages 15 & 16) are pretty clear. I expand the picture as necessary on this iPad. The varicap diodes are fairly easy to locate, just follow the lines from the "tuning" input(s) of each tuner down to the 33k isolation resistors to the VC diodes.

I have no idea how to check them or how to find replacements. :no:

jr

Ok I got them I just took 2 of them out its like brain surgery . I read to test them like a regular diode so I checked the 2 out of 4 and I get a reading one way but the other way I get a reading but only momentary then nothing. I'm wondering if one is bad will I shut down the oscillator which is where they are along with transistors. I don't know if getting a momentary reading may mean they are leaking. Oh and they have numbers, mpn3404. Do you know which is the vhf tuner , would it be the smaller case ?

The tall slender tuner standing up on the board is connected to the UHF antenna, the fatter shorter tuner under the copper shielding is connected to the VHF monopole antenna, so I think that pretty much shows which is which.

Sure you can test for diode action, but a more complete test would necessarily involve some method of applying voltage to the varicap diode and measuring its change in capacitance as you change the voltage. :scratch2:

I still think that you have a more basic problem than tuning, since you have no raster.

jr

edit add: I believe the MPN 3404 is a switching diode, not a varactor... are you sure you found the correct part?

Yes I found them in the right tuner thanks and the mpn3404 I looked up and it's app is for the vhf tuning. I have a dead raster bright on but no snow which does indicate the tuner is waiting for an antenna. The sound is snow but no real sound. I'm just not sure why I would get a momentary reading in the opposite direction and if these diodes are said to be checked like any other diode then maybe they are leaking or maybe normal so until I get the rest out maybe I will find one open. Otherwise there is nothing on the battery board that I found wrong. I wish it were that easy to find a simple problem.

"the mpn3404 I looked up and it's app is for the vhf tuning"

I don't think so, I believe the 4 MPN3404s that you located are used for band switching. They are VHF switching diodes, not varactor tuning diodes.

jr

Well yes they are and the others in there are 3 leg transistors there is nothing else. These varicaps were explained in the thread I read from England so I hope I find a bad one . I seen listings calling then varicaps as well as varactors also.

I don't see how PIN switching diodes can be used as varactor (or varicap) diodes ... a real head scratcher... I assume MPN3404 is indeed the correct number for the diodes that you found :scratch2:

jr

Well all I can hope for is get the rest out and test and maybe find a bad one. Any clues why this tuner stopped oscillating. I don't think the IF- video board would have anything to do with the tuner oscillator, well I hope not as I don't have another. There is nothing more I can check on the tuner board relating to changing the battery's.

You have certainly gone where I have not gone before. Could you perhaps post a pix of the inside of the tuner box? I have not had the guts to take one apart yet.

jr

Question: Are you working on the UHF tuner module box or the VHF tuner (under the low shield cover).

Yes I was working on the vhf the short box and yes it's like brain surgery I would not recommend anyone do this. I hooked up a DVD player and got a picture and was able to see what it was so while I had the chance I used a very thin flat screw driver to adjust the focus, and attempted to lower the line amp because of horiz lines and it turned out that where the pot is , is almost impossible to get between without touching the other and the worst happened as careful as I was it touched the other pot next to the line amp pot and the pic flickered and now I have bearly and brightness, I can see the pic but with the light off. So this project is probably going on the shelf and maybe if I find another I could use this for parts. Well live and learn I'll know better with another one.

Ouch! sorry to hear.
These sets are a real pain in the rump to work on, and apparently somewhat fragile.
Is the -45 volt supply still working ok?

jr

For grins, I took the cover off of the VHF tuner... YIKES! brain surgery is correct! Here is what mine looks like:

https://farm5.staticflickr.com/4517/...ef42e1_z_d.jpg

I have identified the 4 varactor tuning diodes and painted the tops with red paint... the 5 band switching PIN diodes were factory painted yellow (#5 is barely visible under coil next to blue and red top semiconductors)... the RF transistor (barely visible under tan wire and coil) was factory painted white and the oscillator and mixer transistors are painted blue on the tops. Do we agree?

jr

As a result of me trying to be careful the 45v supply is now down to 26v .

yes it looks right although I did see the other diodes and didn't know what they were but you had said there were varactors and pin diodes so what I took out may have been good and it could have even one of the transistors. The tan wire on mine was green corroded and making no contact so I fixed it but made no difference , befor I lost brightness. Well I'll tell you if someone knew these little sets and told me things to change and it would be successful I would probably do it since the only thing I know is the tuner is dead but the brightness problem if it didn't damage the IC next to the line amp pot then it's more likely a ksp44 transistor not a problem but the IC , there is no info on that to change it.

Can anyone identify this part?
 

This is what I believe is one of the varactor tuning diodes in the Sinclair MTV-1 VHF tuner section (pictured above)... it is in a TO-92 style case with no markings on the flat side, just 3 color bands on the back. (I painted the red on the top for identification in my previous post)

https://farm5.staticflickr.com/4583/...55883c_z_d.jpg

Apparently the tuning voltage in the set can go as high as 31 volts.

Identification and/or substitution recommendations needed.

Thanks,
jr

.

Yes your right according to the fellow in England working on this set said in the posts that he too identified those varicaps and could not find replacements and did note that they had 3 colors just as you said. If we had a real schematic that may show values of sorts it may help but it's looking like a dead end for me. Looking at your pic there is green on the components and if that's moved it will probably break off as one of the pin diodes I took out one leg broke but it believe it tested leaking and they are available so that's not a problem . Would you know by the schematic what I possibly burned that killed the brightness and brought down the 45v to 26 volts.

I can only guess, but is the 3 volt supply still working? (pin 7 on the ic just behind the pots) Is the HV at the ends of the large mylar caps (standing up behind the position pot board) close to 1900 Volts?

jr

I have 3 volts on pin 7 but the hv just bearly gets of the start line of the hv probe. Actually aside from a dark screen the pic was still visible so maybe it's the hv I knocked out.

Grasping at straws, I would check/replace the 3 electrolytic cans between the ic and the HV transformer... the surge might have injured them... the diode standing up between two of the cans could be suspect as well as the 4 transistors in the area. A very real possibility is that the regulator circuit on the ic has been damaged. I really can't guess.

jr

Well I'll look into those caps and I have these transistors if they are bad so I'll pull one at a time and check . However if the IC is bad then it's game over because there is no info on the web about this chip. What I actually did was touched the wiper of the line amp and the 45v adjust wiper both did this. Thanks.

Slaps forehead!
After checking all of the corroded traces on the tuner board which ohmed out ok, I discovered that the center pin of both the 6 and 12 volt rear connectors is NEGATIVE!
Duhh!

jr

Yes lol the neg is in the middle not like all those plugs today are positive.