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| Tektronix 2235 repair thread |
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| bd139:
Had a few minutes. I think we have a smoking gun! Capacitance was fine but I noticed some browning on the bottom of the tape seal on the outside of C907. So I peeled it back about 20 turns and found some heat damage. Not sure if this was the cause or an outcome yet. |
| bd139:
And another smoking gun. Someone has been at this. I noticed this, perhaps later than I should have. Some numpty has changed two of the 510K HV resistors for what looks like some 470K ones they had lying around. In the process of doing this, they left shit all over the board and managed to burn the power switch shaft. The human race disappoints me sometimes. This wasn't hard to not fuck up. |
| bd139:
Ok quick update. I have replaced capacitor, the driver transistor, MOSFET and the switching diode from the list above. I added the 39R resistor in series with the gate as per the service note. Turned it on, blew the fuse straight away. Hmm what did I cock up? Turns out I put CR907 in the wrong way. :palm: Idiot! Swapped it round, brought it up on 46V DC this time across the preregulator filter cap and bingo, it powered up fine. Thank goodness that didn't break anything. Or did it? Plugged mains in, boom fuse went again. Checked voltage at schaffner filter and all is good there, thus there is something wrong along the line. I reckon it's one of the diodes gone short. I will debug that later. Good progress at least. |
| bd139:
Ok fuse is my fault. I'm using fast blow where I should have slow blow in there :palm: Inrush current to the main filter cap is blowing it before it gets to do anything. I'd be dangerous if I knew what I was doing :) |
| bd139:
Ok so after 20 minutes on 46V DC straight into the filter board it's running nicely. I have noticed another issue though which I will add to the list at the top. IRF820 runs entirely cold at 46V. Lets see how it does on 350V or so later! |
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