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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 .
Last edited by timmy; 11-07-2017 at 04:57 PM. |
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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. jr |
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Last edited by timmy; 11-07-2017 at 06:38 PM. |
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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. 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? Last edited by jr_tech; 11-07-2017 at 07:20 PM. Reason: add 3404 question |
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Audiokarma |
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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
jr |
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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.
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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). Last edited by jr_tech; 11-08-2017 at 02:25 PM. Reason: add question |
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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 |
Audiokarma |
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For grins, I took the cover off of the VHF tuner... YIKES! brain surgery is correct! Here is what mine looks like:
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 |
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Last edited by timmy; 11-09-2017 at 05:22 AM. |
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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)
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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jr |
Audiokarma |
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