Tek 468 Oscilloscope B Trig circuit in always reset?


Staffan
 

Hello,

I just got hold of a Tek 468. The B Trig seems not to be OK, possibly a broken IC?

I'm checking U240 and U445 on schematic sheet 6. These are the trigger circuits for A and B Trig with inputs for trigger level, polarity, input signal and reset etc. The ICs seem to be Tektronix custom chips 155-0196-00. Datasheet can be found at https://w140.com/tek_made_sm.pdf (P5-125).

First checking signal levels to U445 for A Trig (working), it is fairly clear how it works. Input signal should cross the trigger level and when it does, the circuit outputs a trigger signal. This happens only when input reset signal is low. The reset signal to U445 (A Trig) goes as a square wave between 2.2 V and 4.5 V. The 2.2 V is apparently sufficiently low to pull the circuit out of reset as this chip indeed generates a trigger signal. This reset level is also confirmed by the datasheet stating that logical Zero is at 3.25 V.

Looking at U240 for B Trig (not working), input signals are very similar to the signals into U445. Trigger level is set at approximately 10 mV DC, input signal is a square wave from -10 mV to +40 mV (very similar to A Trig). The reset signal to this chip varies between 2.3 V and 4.5 V, but there is never a trigger signal generated. Output (pin 14, GATE) is fixed at 4.0 V. Noticeably, pin 15 GATE' is also fixed at 4.0 V. On U445, GATE and GATE' are complementary; one being around 3 V and the other being around 4 V.

The AC Waveforms in the service manual (signal 45 and 47 for schematic sheet 6) show that the reset input should vary between almost 0 and +5 V, i.e., TTL-like. This sounds reasonable, but when looking at the driver circuits for the reset signals on schematic sheet 7, I don't see how this works out.
The circuit is a common base NPN transistor (Q467 for A Trig and Q266 for B Trig) where the collector can sink 3.3 mA (-8 V/ 2.4 kOhms) when disabling reset. The collector is connected to a 1 kOhms pull-up resistor to 5 V so low voltage should really be 1.7 V (similar to what I'm seeing) or am I missing something?

Questions:

* Does anyone have experience with these circuits (155-0196-00) ? Is it likely that one chip is broken? If so, any suggestions where I could find a replacement?

* My guess is that at least some of the AC Waveforms shown in the service manual are not quite correct. Is this common or very rare? When trouble shooting, the graphs are very helpful, but they do need to be correct.


Any comments appreciated!

Regards,
Staffan


Ozan
 

This trigger IC (155-0151-00) looks very similar, you might be able to use its schematic to guess what is inside 155-0196-00.
https://w140.com/tekwiki/images/3/36/Tek_155-0151-00.pdf

Ozan

On Sun, Aug 8, 2021 at 07:20 AM, Staffan wrote:


Hello,

I just got hold of a Tek 468. The B Trig seems not to be OK, possibly a broken
IC?

I'm checking U240 and U445 on schematic sheet 6. These are the trigger
circuits for A and B Trig with inputs for trigger level, polarity, input
signal and reset etc. The ICs seem to be Tektronix custom chips 155-0196-00.
Datasheet can be found at https://w140.com/tek_made_sm.pdf (P5-125).

First checking signal levels to U445 for A Trig (working), it is fairly clear
how it works. Input signal should cross the trigger level and when it does,
the circuit outputs a trigger signal. This happens only when input reset
signal is low. The reset signal to U445 (A Trig) goes as a square wave between
2.2 V and 4.5 V. The 2.2 V is apparently sufficiently low to pull the circuit
out of reset as this chip indeed generates a trigger signal. This reset level
is also confirmed by the datasheet stating that logical Zero is at 3.25 V.

Looking at U240 for B Trig (not working), input signals are very similar to
the signals into U445. Trigger level is set at approximately 10 mV DC, input
signal is a square wave from -10 mV to +40 mV (very similar to A Trig). The
reset signal to this chip varies between 2.3 V and 4.5 V, but there is never a
trigger signal generated. Output (pin 14, GATE) is fixed at 4.0 V. Noticeably,
pin 15 GATE' is also fixed at 4.0 V. On U445, GATE and GATE' are
complementary; one being around 3 V and the other being around 4 V.

The AC Waveforms in the service manual (signal 45 and 47 for schematic sheet
6) show that the reset input should vary between almost 0 and +5 V, i.e.,
TTL-like. This sounds reasonable, but when looking at the driver circuits for
the reset signals on schematic sheet 7, I don't see how this works out.
The circuit is a common base NPN transistor (Q467 for A Trig and Q266 for B
Trig) where the collector can sink 3.3 mA (-8 V/ 2.4 kOhms) when disabling
reset. The collector is connected to a 1 kOhms pull-up resistor to 5 V so low
voltage should really be 1.7 V (similar to what I'm seeing) or am I missing
something?

Questions:

* Does anyone have experience with these circuits (155-0196-00) ? Is it likely
that one chip is broken? If so, any suggestions where I could find a
replacement?

* My guess is that at least some of the AC Waveforms shown in the service
manual are not quite correct. Is this common or very rare? When trouble
shooting, the graphs are very helpful, but they do need to be correct.


Any comments appreciated!

Regards,
Staffan


Mark Vincent
 

Staffan,

The fastest way to determine if the IC is bad is to swap U240 and U445. If the problem tracks, the IC is bad. If it does not, the problem is earlier in the circuit.

Mark


Staffan
 

Hello,

Ozan: thanks for the link. Taking a look at it, it does look similar. I found someone selling the IC on Ebay, but unclear whether they would be prepared to ship to Sweden or not.

Mark: your suggestion was definitely on my mind, but the ICs are soldered so I'm a bit hesitant so far to just swap. Also, it would be good to know if these chips have a habit to break or if it is worthwhile to replace - I _can_ live without B Trig, but it would be nice to have of course...

Regards,
Staffan


Mark Vincent
 

Staffan,

I did not realize these were soldered in. My apologies. I looked at the schematic to attempt to help you. I do not have a 468. I am so used to seeing the active devices in sockets. Go ahead and get a replacement IC. The IC sounds like it is bad from your description. The "worst" thing that could happen is you get a spare IC because the problem is elsewhere. Having a spare is good because you may need it in the future. If you want to also get a socket, fine. If you choose to get a socket, Augat would be a good choice.

Mark


Staffan
 

Mark,

Thanks, appreciated. Should probably try and get the replacement. Given the development of ICs, I was sort of hoping there would be a nicer, easier to get, substitute, but this function is probably too specific.
I have a few spare sockets already, but should I need a new one, I’ll check out Augat - seems like they are available from TE Connectivity.

Regards,
Staffan


Staffan
 

Hello,

Short update. The Tek custom trigger IC 155-0196-00 for the B sweep was broken. With a new one, B sweep was still dead. Turned out to be the B sweep driver transistor (A15Q967) that was totally open circuit. With it replaced, the machine works - happy end!

Regards,
Staffan