Re: How to troubleshoot a faulty 7B50 timebase
Q784 and Q794 form an emitter coupled differential amplifier. Put a signal in on the base of Q784 and you should see a signal on the collector of Q784 and the inverted signal on Q794.toggle quoted messageShow quoted text
It looks as if Q794 B-E junction is open.
It would be interesting to see what the collectors of Q794 and Q784 are doing. I'd think (offhand) that Q794's collector would be very high (close to -15 volts), and likely Q784's got too much current flowing through it.
I"d be suspicious of Q794, and I'd want to check Q784 as well.
Not sure of the replacement transistor, but if you can find a decent replacement for cheap, I'd be tempted to replace both. I'd certainly think that swapping the two ought to make the bad readings symmetric. VBE on any transistor turned on should be 0.6 volts or so, and Q794 seems to be, oh, 5 volts too high.
On 3/8/2021 4:43 PM, Thierry Delaitre via groups.io wrote: