Looking for Silent 700 Model 765 lower board

[James B DiGriz]

On Sat, 7 May 2022 19:05:49 -0700
Cameron Kaiser via cctalk <cctalk at classiccmp.org> wrote:

> These things must be cursed by G-d. I've landed three. Two had bad
> lower boards (the ones with the bubble memory and the main 9980 MCU)
> already. The third had a bad print mechanism. I replaced the printer
> mechanism, but the printhead on that was bad, so I powered it down
> and replaced the print head. I power it back up and its lower board
> suddenly stops responding too. Then, shortly after that, the upper
> board! (What is that circular metal can bolted down with wires
> exiting near the power switch? It squeals like a stuck pig.)

According to the manual on bitsavers, it's a power supply transformer.
http://bitsavers.org/pdf/ti/terminal, page 6-57, pdf page 197, item
234. Relevant board diagrams starting at 6-38. Power supply description
and diagrams beginning on 4-29 (pdf p. 107). Interesting that there are
ps sections on both the upper and lower boards, which may be consistent
with your symptoms.   

> 
> I have plenty of bubble memory boards from these damn units, but what
> I need is a working lower board itself. Unless someone is experienced
> with board level repair and knows the typical faults on these.
> 

Never had one to work on, so I can't help you with typical faults, but
I'd start by looking at the supplies, in particular the caps and diodes
on the lower board. Section 4.7.1 seems potentially relevant.  Good luck
with it, though. Be nice if you get all 3 working.

jbdigriz