VW / Porsche VDO Clock repair

This clock came out of Johns mid 70's Westfailia bus.  When I had received it it was non operational though I was advised that an attempt was made to replace the two electrolytic capacitors as recommended on a website.  This is where the adventure begins.

This is not a mechanical demonstration but an electronic one.


     Given the capacitor replacement I first wanted to see if there were issues with the original ones, so I opted for the Genrad 1650 to obtain a value from both of them. The Heathkit IT-11 was also used to ensure that there weren't any DC leakage issues. Note that a rebuild of each cap was required before testing could actually begin.

    The next challenge was the clock disassembly, its important not to break anything or the whole job is for nothing.  During disassembly it was found that circuit board sections were cracked and warranted repair though no traces were shown to be broken. A current limiting power supply was setup for 20ma @ 12v for testing.    Also the coil was tested for continuity and shown to be good @ 290 ohms.

   At this point the broken piece of circuit board was reattached with super glue and the traces repaired.  Tested was able to be resumed. However the issue was not solved.  Further tests show that the IC SCL5419AE was not providing any useful output pulse when power was applied. Furthermore, the fact that the same voltage was coming out of several of the same pins from the IC does not mean good news.

     John was able to source a SAJ300R CMOS, which is a drop in replacement for this IC, as well as the data-sheet.  Given this I was able to confirm the broken IC.


Part 1 of 2

     With the arrival of the new IC work gets underway, solder is added to the existing connections so that the vacuum can remove them easier.  The old IC removed and the new one installed.  The final repairs of the traces are conducted before continuing.  With that the Heathkit IP-20 is setup for current limiting as well as current monitoring for test events.

No load current is at ~4ma
Test pin on IC now shows a stable frequency @ 1.06mhz or 1/4 the oscillator frequency

     At this point reassembly begins enough to load the coil and test the clock.


Part 2 of 2