Re: How to troubleshoot a faulty 7B50 timebase

Harvey White

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.

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 wrote:
thank you.

I can find a sweep at the base of the Q784 (output driver) and nothing after.

I have correct voltages for the line voltages -15V and 50V

Other voltages are as follows (with a time dev of 1ms):
- Collector of Q774: 5.6V instead of 6.41V
- Collector of Q764: 6.6V instead of 6.42V
- Emitter of Q794: 11.5 instead of 7.22V
- Emitter of Q784: 6V instead of 7.33V
- 13.1V across C799

I've also tried to disconnect C799 just in case if it was leaky but it is still the same

I've also checked the resistors (in-circuit) around Q774 and Q784/Q794 and none have exceeded their values.

Does it mean that one of the output driver Q784 and/or Q794 is leaky?

Should I replace them with a 2N4122 or are there any other equivalents that are easier to source?



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