Tektronix 2246 Oscilloscope Problem.

oscilloscope problem

If the 7.5 Volt adjust is at exactly 7.5V, the "dashes" produced by the readout scans across the trace. If you bring the 7.5V adjust lower, it scans faster and faster. If you bring the 7.5V up, the "dashes" scan faster and faster disappearing right around 8 volts. All other voltages check out ok. The 15v supply is almost 16v when 7.5v is exactly 7.5. All other voltages are right on.

Added after 20 minutes:

One more thing: if there are two readout displays on (CH1 and time) there are two sets of dashes that scan across the trace. If you add other things to the readout (CH1 CH2, time, etc.) another set of dashes scans across the trace for everything you add to the readout.

Added after 1 hours 1 minutes:

Also, the speed of these dashes can be controlled by the trigger holdoff. When there is no signal, the dashes go away when the holdoff is at MIN, when there is a triggered signal applied, it is opposite, when holdoff is at MIN the dashes scan through the waveform. When it is turned up to about 10-o-clock, the dashes go away as the wave flickers.

Have you fixed the 10 MHz clock?

/Ram

Yes.

Hmm, I'm running out of suggestions...
It feels like some sort of timing problem, as it gets worse when the scope has more things
to draw on the screen.

If it is free-running, i.e. not triggered, it also has "less" to do.

Maybe there is some interference (beat frequency) with the mains frequency?
And the beat changes when you adjust 7.5 V because of this... Dunno...

How old is this scope? Maybe you should check the electrolytes in the power supply to see
that they haven't gone dry and lost some capacitance.

/Ram

First of all, thank you for all your help. Secondly, I think it may be a power supply or trigger issue. Far fetched, but what about a bad AC Line filter? If you play with the timing and the trigger holdoff, with the right combinations, the problem disappears completely. It is like this on all trigger modes except LINE. If it's in the LINE trigger mode, you get the same dashes in the trace that disappear when the readout is turned off.

In LINE mode - do the dashes stand still or do they move very slowly?

/Ram

They move. And there are more. In other modes, there are only 2 sets of dashes (there will be one set for every different readout item on the display) in LINE mode the multiply and are all over the place.

You could measure the power supplies with your DMM in AC mode to check if there is excessive ripple.
Let's say it should be less than 50 mV, just to have a figure.

/Ram

So you want me to check all the DC power supplies in AC mode? And if I get a reading less than 50mv, I have a lot of ripple?

No, if you have more than 50 mV you might have too much ripple.

I'm still thinking that there might be some dried out electrolytes in the power supply.
If their capacitance is low they will not smoothen the DC voltage well enough.

With another oscilloscope it would be easier to measure... :wink:

/Ram

Ok, I checked the voltages for ripple...there is nothing over .006V AC on those DC supply lines. I have discovered something new that I do not think is right. There is a pot adjustment for "Readout Jitter" It is right by the HV part of the main board. If it is turned anywhere but fully clockwise, the readout and traces go crazy. I'm no expert, but I don't think Tektronix would design it to be fully clockwise. I'll attach the schematic containing this adjustment.

Here is what little the manual says about that particular part of the circuit...

On the 55V and 15v lines, the meter won't register anything in AC mode, it blinks back and forth between 4 and 9V AC on the 15 volt line and 113V and 17V on the 55V line. It does this very quickly as the frequency bar along the bottom goes up and down. I'll attach a schematic of the power supply.

Also try asking your question in Usenet newsgroup sci.electronics.repair or sci.electronics.equipment. I've seen lots of test equipment discussions there.

XNOX_Rambo said:

In LINE mode - do the dashes stand still or do they move very slowly?

/Ram

Click to expand...

Today they stood still unless the holdoff was turned up...

I've gotta hand it to Tek - they (used to?) make great documentation.
Come to think of it - I worked for them for six months in '98 until their stock plummeted
and we got laid-off. Ah well... :wink:

The Readout Jitter pot being fully turned does sound odd.
If the scope has been "repaired" it could be an obviously failed attempt to fix the real problem...

If you measure the 15V and 55V supplies in VDC mode with you DMM, will they be more or less okay?
The only adjustable voltage +7.5V will also set -7.5V accurately but the rest of the voltages are
unregulated and will change according to their load conditions.
They should be within ±10% of their nominal value though.

If the values are low I would suspect the capacitors C2229, C2230 and/or C2233 to be bad.

The dashes standing still in LINE mode "today" is probably due to the fact that the mains frequency 50/60Hz
varies over a day depending on the load. It will be lower during the day and higher at night when the
power companies rev their generators up to "catch-up". The goal is to keep the long time average at
precisely 50/60Hz.
Therefore I would expect the dashes to move either slower or faster during a 24 hour period as the internal
oscillator frequency is fixed, relative to LINE.

/Ram

The voltages are all well within the limits listed in the manual. As for the readout jitter pot, the only thing I could see affect that is the U701 Vert. Out amp. The pot itself checked exactly 5K and that 1k resistor connecting the wiper to U701 was perfect. I got about 1 volt on pin 23 when the jitter pot was turned all the way CW (to ground) and I got about 3 volts when turned CCW (to the 5V supply). One more thing to note. I don't know if this would have anything to o with it or not, but on those lines coming out of DL 21 to U701, I get voltages a little over a volt...schematic says they should be +90mv and +72mv.

Yeah, that seems high. Are all Delay-Line Driver voltages okay - including +15VB, -5VE and -5VC?

I dropped the scope at the shop today. I'm sick of messing with it. I am waiting for an estimate. In the meantime, I've found a Tektronix TDS 320 Scope locally. What do you all think about this scope? How does it compare to my 2246?

Well, it depends on the type of measurements you do most.
I.e. what is your field of work - analogue, digital etc?

/Ram