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Old 08-26-2015, 12:31 AM
JamesDean's Avatar
Electrical Engineer
Join Date: Apr 2005
Location: NE Ohio
Posts: 5,009
Inside the box: OM617 Tachometer Amplifier

Hi All,

Its been a while since I've post any of these photos series, figured I'd do one.

A few weeks ago a customer of mine asked about repairing a tachometer amplifier for his 617. In the past I've shied away from these because they are somewhat of a pain to do.

The whole unit is filled with a liberal amount of silicon. The only way to get to the solder side of the circuit board is to mechanically remove the silicon!

I was able to remove much of the silicon and expose the capacitor leads. I found an enormous amount of cold solder joints, a number of them had big holes around them too. I replaced the capacitor and touched up what I could.

I tested the unit in my 300SD (stationary) and sent it out to my customer.

Unfortunately it only worked for a few miles. It turned out there were more cold solder joints lurking under the silicon that I didn't remove.

I had him send it back and this time I cleared out everything and re-flowed everything. Nearly every join on the circuit board was bad!

Anyway here are the photos! Not a huge amount but enough to give you an idea of whats going on!

Filled in with home high quality hot glue to keep everything in place!

Mercedes Diesel Tachometer Amplifier - Album on Imgur
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