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

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