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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

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.

VR304 is the one I was wondering about, so I figured I may as well test it on my
new 7CT1N. It looked okay on the curve tracer, and since it's socketed I
swapped it in to my working 7B70. The good 7B70 continued to work fine, so I
guess I can cross that TD off the list for now.

Maybe I can pull a side panel off the 7704A and probe the trigger board through
the side.

thanks,
Adam

VR322 and VR358 both look OK on the curve tracer, or at least what I think a 4.7 mA TD should look like. I started tracing things out with the side panel off, and things looked reasonable until the Trigger Generator <4> of the manual from tekwiki.

A waveform was present at Q309 and Q317, but not at the base of Q326, so I'll look at that area more closely and take better notes. The triggered holdoff stage description in the manual is over my head, as usual.

thanks,
Adam

A waveform was present at Q309 and Q317, but not at the base of Q326, so I'll
look at that area more closely and take better notes. The triggered holdoff
stage description in the manual is over my head, as usual.

thanks,
Adam

On Apr 1, 2024, at 00:14 , Ozan via groups.io <ozan_g@...>

wrote:

Base of Q326 will only show a signal if sweeps are running.

If you set 7B70 as described in test conditions, and with a 1V/1kHz sine

wave at External input:

Yes, and I verified 1Vpp 1kHz sine on my 2465A.

- Do you see any sweeps?

No. My assumption has been that it's sweeping too fast to see.

- Do you see sharp pulses (not a sine wave) at base of Q309? What amplitude?

I think the shape is correct, and amplitude is a bit over 200 mVpp. Photos
here; GND reference set at centerline.





The manual shows ~10x lower, I think due to the note about 10x probe? All
waveforms I measured through the trigger amplifier were 10x larger than shown
on the schematic. The PDF scan is missing some of the detail, though.

If signal at Q309 base doesn't have sharp edges, and because you ruled out a

bad VR304, bias point of VR304 could be wrong.

- Is +15V correct?
- What voltages do you see at the bases of Q309 and Q317?
- What voltages do you see left sides of R300 and R306?

+15 is 15.01 at the test point.

Base of Q309 and Q317 is the same, +3.8 from chassis ground.
Emitter of Q317 is 4.46, collector is 0.

Left side of R300 is 3.54; other is 3.96.

R306 I measure 3.49 at one end (physically the top), and 3.54 at the other
end. I can't see the foil trace, so not sure which is left vs. right in
schematic.

thanks,
Adam

Thanks to Ozan for helping off-list with some additional measurement advice and
insight on expected values. I continue to be amazed at what some of you can
understand from a schematic.

He suggested replacing Q309/Q317 in the trigger pulse generator with the pair
from my working 7B70, and that did the trick: stable waveform with
sine/triangle/square, and controls work as expected.

I had previously tried replacing them individually with a 2N3906 (one of the few
PNP transistors I have on hand), with no luck. Replacing both Q309 and Q317
with 2N3906s actually shows a waveform instead of a blank screen, but it
stutters so I've ordered some 151-0220-00 replacements.

Adam

I'll second Ozan's helpfulness with this sort of thing. He has helped me a
few times isolate a problem like that whcih I'm pretty sure I would not have
found on my own. Glad you have something going and hope the correct
transistors will fix the rest of the issues with this.

Barry - N4BUQ

Thanks to Ozan for helping off-list with some additional measurement advice

and

insight on expected values. I continue to be amazed at what some of you can
understand from a schematic.

He suggested replacing Q309/Q317 in the trigger pulse generator with the

pair

from my working 7B70, and that did the trick: stable waveform with
sine/triangle/square, and controls work as expected.

I had previously tried replacing them individually with a 2N3906 (one of the

few

PNP transistors I have on hand), with no luck. Replacing both Q309 and Q317
with 2N3906s actually shows a waveform instead of a blank screen, but it
stutters so I've ordered some 151-0220-00 replacements.

Adam

On Fri, Apr 5, 2024 at 11:14 AM, Mark Vincent wrote:

The high speed transistors could be replaced with KSP10BU. The pinout is BEC.

I need a PNP transistor, though. Pretty sure KSP10BU is NPN, as I have some and
checked everything in my limited stash as a potential replacement. Finding a PNP
over 500 MHz looks unlikely, especially in the 2 GHz range of the 151-0271.