Taktronix 2215 No trigger

[DaveMX5mk1]

Hi, first post so please be gentle with me.
I aquired a Tektronix 2215 oscillascope from a friend who han't used it for years. Apart from being a bit dirty it looked in good condition.
I turned it on and it came to life, good trace and seemed to be working fine. However after about 20 mins it started to smoke and the smell of melting capacitor wafted to my nostrils. The trace was still on the screen but of course I hit the off button.
On ivestigation this was C901 ( input filtering ) had popped it's clogs. I replaced this and the unit nows comes on fine again.
However, I can't get the scope to trigger. Both channels are the same, ie wont trigger. In auto mode you can see the trace but running freely and it wont lock. In normal mode, not unsurprisingly, there is no trace.
I've serched the forum but couldn't see anything relating to this. It seems a shame to bin it as it's much lighter and more portable than my other scope.
Just wondered if anyone on here had come across this before or could give me any fault finding advise.
Many thanks in advance.
Dave.

[orbanp]

Hi Dave,
Welcome to the forum!
There is absolutely no reason to ditch that scope!

Do get the service manual, the TekWiki site has several versions:

http://w140.com/tekwiki/wiki/2215

The service manual has troubleshooting charts in it, do follow that.
PS. For whatever reason, this site messes up the formatting!

Regards, Peter

[jdragoset]

Check to see if it can be triggered with the A channel also connected to the external trigger input and set for "external".

[CaptDon]

That vintage of Tek products is notorious for front panel buttons/switches
malfunctioning. The modular 500/5000 series was the worst!!! You may
just try punching in and out of all the trigger selections. Leave it in auto
and observe if it suddenly sync's. My DMM 500 series stuff is a nightmare
unless I use it daily. Leave it sit while on vacation or dispatched to sea duty
and presto.... next time I use the stuff the buttons/switches all need cleaned
or at least exercised about 100 times each!!!

[DaveMX5mk1]

Thanks for the speedy replies all.
In reply to jdragoset,  good call,  if I connect the external trigger to the same source as the A channel the scope triggers fine.
As the scope has been stored for years without use, given the comments above I think I'll take the unit apart and check / clean all the switches associated with the trigger circuitry.
Many thanks again.
Dave.

[jdragoset]

The front panel paddle switches can be fed a drop of Deoxit fader lube with a long hollow needle, if you position the slide switch assembly with an end facing up, gravity will allow the drop to go in where the contacts and slide are, then work the switch several times.
I had one that the problem was fixed by changing U460, which is a CA 3102E.

[DaveMX5mk1]

Here is a bit of an update.   I think the switch is working ok as I can see the voltages to the trigger source switching diodes changing when I move the switch.  It looks like they should change between +8.6 volts when the source is not selected and -0.8 volts when selected.  What I seem to be getting is when I select "internal" the control line is going to -3.5 volts not -0.8. It looks as though the switch dissconects the selected line so it is not connected to anything in the switch, the other lines areconnected to the 8.6 volt supply.
I'll try to explane.
The switch connect to the main board by soldered links  numbered  36  37  38  below shows what I am getting on these connections for the different switch possitions.

Switch possition           link 36    link 37    link 38
Internal                           8.6         8.6        -3.5
LINE                                8.6         -0.3        8.6
External                          -0.3        8.6         8.6

The fact that when internal is selected I'm getting -3.5 volts on the control line makes me think that maybe this isn't switching the diodes correctly.
Does that make sense?   Any thoughts?
Thanks again.
Dave

[David Hess]

That indicates a problem with the trigger signal coming out of the emitter of U421B.

Does adjusting the position control for the selected trigger channel make any difference for the -3.5 volt reading?

Check both the channel 1 and channel 2 trigger sources.

If you want to probe voltages, then the bases and collectors of U421B and U421A will be of interest.  Maybe R433 is open.

[DaveMX5mk1]

Hi David et al,  here is my latest update.  Adjusting the position control makes no difference to the -3.5 volt reading for either channel. I checked all the resistors in the trigger select circuit in situ and they were all pretty close except for R430. I removed R430 and it was 8.3k which is close to what it should be.
I measured all the voltages on U421.  Most were as shown on the diagram except 3, 4, 12, 15, and 16.
Below is showing all the pin voltages in internal and external switch position.
PIN   Internal      External   says should be with switch in Internal
1       +8           +8
2       +8           +8
3       -3.6          +0.9      -0.8
4       -2.5  *             -2.5  *            0
5       -8          -8
6       +2.2          +2.2
7       0                   0
8       +0.6   *       +0.6  *          Those marked * looked to have very small pulses on.
9       -1                     -1
10       -1.9          -1.9
11       +0.6   *       +0.6  *
12       -1.8   *        -1.5  *    +0.8
13       -1.9          -1.9
14       -1                     -1
15       -1.8   *       -1.5  *            +0.8
16       -2.6   *       -2.5  *              0

(Sorry the spacing seems to have got messed up when posting)

This morning I removed links W421,  W422, and W444 also removed R436 and R701.  This effectively isolated the trigger select circuit but the voltage readings on links 36,37,38 were the same, so were the voltages on pins 12/15, and 16 of U421.

Any guidance  or words of wisdom much appreciated.
Thanks again.
Dave.

[jdragoset]

U 421, 156-1294-00 here:
https://www.sphere.bc.ca/test/tek-parts/tekparts4.html
I had checked the individual transistors with a BK 510 transistor tester and one of the five would not test.
I substituted a 2N3904 for that one and it began internal triggering.
I then replaced U 421 with a CA3127E, which latest report was still working last weekend.
This was done about nine months ago for a Tek 2215 used by a music amplifier repair guy who powers it up every day.

[David Hess]

I hate to say it but it looks like the problem might be from where the internal trigger signal originates with U170 shown on schematic 2.  The internal trigger is not going to work until the emitter of U421B is at the proper level.

[DaveMX5mk1]

Hi all, many many thanks again. I decided to remove U421 and test it as all the problems seem to be around this chip and jdragoset had had a problem with this before.  I only have a DVM, not a transistor tested ( note to self, add to Christmas list ), so used that set to diode mode.  It showed the U421A and U421B are both open circuit between base and emitter.  Hopefully this is my problem.
I've ordered a couple of CA3127EX chips to try.  I'm hopeing the EX as against just an E wont make any difference.
Interestingly the part description on the website jdragoset posted a link to is Plessy SL3127C  and the part number on the chip I removed was CA3127E.  Made me wonder if it had been replaced before, but the pcb looked untouched.
Anyway I will report back when I've fitted a new chip. Not sure how long that will be due to the situation we find ourselves in at the moment, and how it will effect shipping times.
Thanks again and stay safe.
Dave.

[David Hess]

While not ideal, if necessary U421 can be replaced with discrete transistors of which only some are matched.

[DaveMX5mk1]

The new CA3127 arrived and has been fitted.
Happy days, the scope is now  triggering fine in all modes.
Many thanks to everyone and hope this thread may be of some help to someone who has a similar problem in the future.
Cheers all.
Dave.