Repair and Re-build of SE Labs SM 112 Scope

[Geaney]

Thanks Steve and Paula. I hope to get another shot at it over the weekend - see if I can narrow it down a bit more.

Cheers, John

[Geaney]

I spent a few hours on this over the weekend without much luck. The trigger control is just a pot across two dc rails so provides a simple DC bias to a long tailed pair - one end of which is fed with the trigger signal. When set to EXT there is a signal there but not when set to CH1 or CH2. When I followed the wiring back to the Y amplifier section I had great difficult finding any signals - though all the dc applies are there for sure. However the signal must be getting through as it displays on the CRT!

At this stage I am somewhat embarrassingly reduced to RTFM. I'll spend some more time reading before I try again.

I was making video clips along the way but it seems pointless to post until I've made a breakthrough - which might be a while yet!

Cheers, John

[wme_bill]

I have just seen your plea for the manual for the EMI/SE Labs SM112, but happily you seem to have found a copy. The SM111 and EM102 are very different, and will be very little help all for the SM112 / SM599 / SM602 (100Mhz,dualtrace, double TimeBase). cf SM111 -20Mhz,dual trace single TB (also NATO CT570/3), and the EM102D / EM530 - 30Mhz,double beam,delayTB.
If you are still stuck for the manuals, let me know.
The mains plug/socket is the small Bulgin P429. See my recent posting upon the SM111 for more details. Very safe and reputable, but overtaken by the fashion for moulded plugs as "safer".
The trigger pot you talk about is the Trigger Level, which changes the DC level of the signal to the Trigger Multivibrator TR49,50,51, so that you can trigger on different parts of the signal. Happily it doesn't use any tunnel diodes, unlike Tek and HP at that time. (1970 to 1977). Trigger feed failure - clean all the switches, and check the feed to the TB on pins 8 and 1 on the Y plugin, and then follow through the Trigger Source and Trigger Coupling switches on the TB. I assume the Auto Trigger setting produces a trace, but not synchronised.
The SM112 is fairly highly stressed to get the performance, and the output transistors for both the Y and X amplifiers tend to go. The Y uses 2N5109 and BFW30 which are very scarce. 2N3866 is nearest easily available. The X Output is easier, with TV driver types BF178 n & V765 p.
Watch the zener diodes as they fail frequently, and that pulls the rest of the amplifier out. I find my SM112 has to be watched very carefully. Whereas my SM111 just keeps going, when all other have failed. wme_bill m0wpn