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 |
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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. :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? |
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"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 |
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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 :scratch2:
jr |
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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). |
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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 |
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 |
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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)
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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