A couple of years ago, I had triggering issues with my 7B70. It turned out to be a bad tunnel diode (VR304). At that time, I was finding suitable tunnel diode replacements on eBay (1S2199); however, I'm not seeing those at the moment. Hopefully yours will turn out to be something else but, if not, then the 1S2199 diodes may resurface.
Good luck with it,
Barry - N4BUQ
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Hi all,
I just acquired a 7704A with plugins (7A26, 7CT1N, and two 7B70s). Everything
more-or-less works, although I'll have some mainframe questions eventually.
For the moment, I'm trying to figure out why one of the 7B70s won't trigger on
anything other than a square wave/pulse. Note that I've cleaned the cam switch
contacts with alcohol/paper, reseated all transistors on the trigger board,
relays check good, and the trigger amplifier transistors aren't shorted or open
(using a DMM diode test).
Using FG502 as input source:
With 7A26 on 0.5V/div, EXT trigger displays a stable waveform with a 1.25 Vpp
square wave. INT trigger displays a stable waveform with 2Vpp from the FG502.
With lower output voltage from the FG502 on either trigger, the TRIG light is
on, but no waveform is displayed, so I assume it's triggering on noise? If I
adjust the trigger pot until the TRIG light goes off, I can see the envelope of
the waveform, so it's not like the signal vanished.
With the 7A26 on 1V/div, EXT works with 1.25Vpp, but INT requires 4Vpp to
display the waveform. I do not understand this interaction with the vertical
amplifier, but the INT trigger only gives a stable waveform if the signal is 4x
the vertical V/div setting. EXT only requires 1.25 Vpp.
The other oddity is that it will not trigger on a sine or triangle wave,
although it will display them if I use the external trigger output from the
FG502. I'd assumed it was a dead transistor in the trigger amplifier until I
realized this, and now I'm wondering about a bad tunnel diode? No extender, so
I'm kind of limited on tracing for the moment.
thanks,
Adam
