HP 98xx series cassette EOT-sensor fix : war story

Lately I acquired some HP 9865 cassette units and a HP 9821A three cassette drives suffered from 'slow running' I found out the LDR's used in the EOT sensor are causing this behavior, the used LDR's are from the NSL314 type which is practically unobtainable. The NSL314 is a IR-sensitive CDSe type LDR, replacing it with a standard low resistance LDR doesn't do the trick, the LDR needs to have a resistance lower then ~5k at low light. The most of the cassette drives also suffer from bad sensor bulb, replacing the bulb by a white LED doesn't give enough light to do the trick. The combination white LED and new LDR isn't working because the LDR resistance isn't getting low enough. The sensor amplifier circuit needs to be modified to work correct, I did this by creating Wheatstone's bridge by adding a grounding resistor to the LDR input. Adding a series resistor of 10k to LDR input and changing the feedback resistor from 18k2 to 100k made the amplifier work correct with the combination white LED and 'normal' CDS type LDR. I made some pictures from this fix which can be found at my flickr page : http://www.flickr.com/photos/hp-fix/sets/72157630819381098/ -Rik