Trigger Level malfunction on Tek 475A

[ChrisDeep]

Hi,

I am trying to fix a scope. Generally working fine. Switching on shows a line on screen and DC levels measure fine. However there seems to be a problem with the trigger level on A-Trigger.

Symptoms:-

1. Input test signal at 50kHz shows the usual blurred sine wave you get when the trigger level goes beyond the peak amplitude of the sine wave. Tried turning the A-Trig trigger level knob fully CCW then full CW but no change.

The pot itself and +/- slope switch seem to be working fine. Measuring the voltage on the cable connecting from the pot to the main-board shows voltage varies from -8V to +8V in relatively smooth fashion when pot is CW or CCW respectively.

2. Pushing Trig-View button does not show a clear line on the screen either with fully CCW or CW. The line seems to move way up to the top out of view. Turning the Trig-Level pot seems to brighten or darken the top of the screen (with Trig-View pushed). Tried setting coupling to AC, DC etc.. no luck. It seems to me like something is pulling trigger level voltage to an extreme level.

I have access to basic lab equipment (Multi-meter, Soldering-Iron, Backup Scope at 100MHz, tools etc...). The service manuals don't seem to have specific info on how the trigger level circuit works.

Has anyone seen this type of thing before? Any advice on where to start looking for the problem?

[MrBungle]

Every time I get a problem with a 465/475 it's always a tantalum capacitor somewhere. Triggering, blanking, weirdness, anything is the damn things. I've had some epic battles and considered it was something else and it always ends up being the wretched tants. I got to the point I spent about £20 on the things from CPC each time and shotgunned the entire scopes. Other than a bust horizontal position pot, I've had no other problems with them.

Go round with a DMM on ohms and look for any in the area the trigger is in. If it's the same as the normal 475, this is on the A4 board near the front at the right and the right hand board. Check for any shorted ones. Snip them out, then desolder the stubs. If the behaviour changes, stick an electrolytic cap in of roughly twice the capacitance and it'll probably do the job fine.

Sometimes they go short and due to the R-C decoupling network they burn out the resistor so look around for any smoked resistors near the caps.

If it's not that, go through the performance checks section of the service manual, then trace the trigger. Worth attempting to see if it's specific to the vertical amp trigger pick off as well (vertical amp board towards the rear). More tants there!

Like I said I've fixed a few of these so will be around to help

[RogerEvans]

Have you tried input to channel 2 and trigger on channel 2? Also try external trigger and line trigger if you have a suitable input signal?

One of the other regular problems with this generation of Tektronix kit is that the cam operated switches need cleaning. This is an easy job if they happen to be accessible but a nightmare otherwise. Remove the aluminium cover and set the switch so the pair of contacts you want to clean are open. Cut a strip of paper about 3mm wide and dip the end in IPA, insert between the switch contacts, turn the cam so the contacts close, pull the paper out, repeata few times. There are suggestions about particular types of paper which are less abrasive, shiny paper and old business cards. I found a set of schematics at w140.com/tekwiki which suggest the trigger source selector is one of these cam switches.

Regards,

Roger

[cmjones01]

Originally Posted by RogerEvans

One of the other regular problems with this generation of Tektronix kit is that the cam operated switches need cleaning. This is an easy job if they happen to be accessible but a nightmare otherwise. Remove the aluminium cover and set the switch so the pair of contacts you want to clean are open. Cut a strip of paper about 3mm wide and dip the end in IPA, insert between the switch contacts, turn the cam so the contacts close, pull the paper out, repeata few times.

Hear hear, both for this and the tantalum capacitor plague. I have quite a collection of Tektronix 7000-series scope plugins and I use them frequently. Almost all of them have had faults when I got them, and 99% of those faults were short-circuit tantalum capacitors (usually after a couple of minutes of operation after being switched on for the first time in years) and dirty cam switches. I reckon I could strip and rebuild a 7A26 attenuator in the dark...come to that, for some of the less accessible contacts, light doesn't make a lot of difference...

Chris

[ChrisDeep]

Originally Posted by RogerEvans

Have you tried input to channel 2 and trigger on channel 2? Also try external trigger and line trigger if you have a suitable input signal?

Many thanks for all the suggestions. I did try the channel 2 setting you mentioned along with lots of other combinations but with the same result. I did get it partially trigger (trigger light came on) after playing with the precision trigger level offset pot on the main board. This seemed to put it right on the edge of the trigger point so even a large sine wave input would still be blurry but much less than a low input sine wave.

Thanks also for the tantalum cap suggestion. Funny enough this was the first thing I tried. All the caps around the trigger level circuit seem to be holding a voltage drop but havent tried every single one on the board.

Wondered if anyone had any experience with debugging this part of the main board and could maybe suggest a good starting point. Something like testing a voltage on specific caps or resistors. The 475A is not that different from the B or the 475 or other scopes around this series.
I have 2 service manuals for the 475A. Both don't seem to mention anything about the trigger level but lots of detail on the triggering waveform generator (which seems to be working fine in my case).

[RogerEvans]

Chris,

You have another scope so that is a huge advantage. You say the low voltage supplies are Ok but you also need to check them for ripple, there will be a spec in the manual. The power supply is just from a mains transformer so ripple should be mostly at 100Hz.

If you set the Ch1 input coupling to GND and the vertical position to roughly its centre is the trace roughly in the middle of the screen? Does the vertical position control work symmetrically to move the trace to top and bottom? If yes and your sine wave input looks like a sine wave then the Ch1 preamp is probably OK.

I don't have a 475A or a manual but I am working from the schematics obtained from Tekwiki. Diagram <5> A trigger generator shows waveforms at the junction of R518 and R519 and also at TP526 after a buffer and amplifier. These look as if they are from the built in calibrator but the full manual will have the details. See if you can replicate the traces in the diagram. There are also three voltage test points shown in this part of the schematic, 0V, 0V and +0.7V, check these as well.

The test condition is AC coupled input but if you also try DC coupled you should be able to move these traces smoothly up and down using the vertical position control. TP526 is the pickoff for 'trig view' so you may find that the DC level is already wrong at TP526.

The actual comparison of the signal and the trigger level is done in U520 with the trigger level control altering the input to pin 14 via Q532. The manual shows +0.7V at pin 14, you should be able to replicate this by adjusting A trigger level. This should probably be with the trigger level around mid point but again the manual should detail the setup needed to get the traces and voltages shown on schematic <5>.

Let us know how you get on and you can work forward (U520 and the tunnel diodes) or backward (channel switch) according to what you find.

Roger