Tektronix 2246 Mod A Trace problem

[chevymn1964]

Hello all, I have a Tektronix 2246 that has a problem showing traces... When you first power the unit up you can see a trace as long as you have the time base set at .5s/div or more, as the scope heats up the trace slowly gets dimmer to the point that you cant see it at all... You can see all of the other
good stuff on the CRT such as voltage/time readings (if set up to display), so I woulden't think that the problem is with the CRT... When you push beam find you can see the trace(s) sweeping & scrunched up with everything else on center of the CRT... I have a service manual and am seeking help to aim me
in the general area I should be looking at... If I would have to guess I am thinking their is a problem with Z-axis???

 

[BradtheRad]

My Tektronix RM504 (with CRT) will show a faint trace at certain trigger modes when there's no signal. Particular when the sweep speed is fast.

The trace brightens immediately when I switch the trigger to free run.

I think Trigger Mode waits momentarily for a trigger signal. Resulting in fewer sweeps per second, therefore a dimmer trace.

Guess that's not the problem you're describing.

Could be a case of the hi-voltage circuit losing power as a component overheats. Especially a component which has deteriorated over the years?

Do you hear a snapping sound (arcing) inside the scope at any time? Maybe once, after which you get the problem?

Can you access test points on the circuit boards? Do you have a meter that can read 1000 V or so? It would tell you a lot if you take readings as the scope warms up.

Have you checked calibration of the sweep circuit? If it were to get really far out (at the fast range settings), then it might extend way far out to either side of the screen. Making the trace travel so fast that it's nearly invisible.

 

[chevymn1964]

Thanks for replying so fast Brad, I don't think I have a HV problem because even after the trace(s) dim out you can still bring up menus on the crt with no problems... No HV arcing is heard, the problem happens when you power the unit on the trace will slowly get dimmer and dimmer within 1-2min the trace would be gone... I will check into the sweep circuit makes since on what youre saying. -Ryan

 

[chevymn1964]

Checked sweep circuit, all voltages seem to be okay, I performed a self diagnose and the test halts for a fail at SR 3- Wrote: 5555 Read: FFFF, I found in the service manual the SR3 is testing the 2 shift-registers that are on the Front Panel Switch Board. The 2 registers are Parallel in, serial out registers, but all of the buttons & switches seem to work fine.... humm -Ryan

 

[BradtheRad]

Don't know whether shift registers have anything to do with the trace. Digital components usually stay bad when they go bad. They don't generally get better or worse with temperature.

Did you perform the diagnosis both when the scope is cold and when it's warmed up? Because you say it's better when first powered up.

By the way, does the intensity adjustment affect brightness of the menus as well as the beam? Does the intensity affect brightness in Beam Find mode? Or are Beam Find and menu display independent of the intensity control circuit?

Because suppose the intensity only affects the beam. Then the disappearing could be due to deteriorating components in the intensity control circuit. They might become worse as the components heat up. It would account for what you observe in your scope.

A poster had a similar problem a few months back. He tracked it down to bad components in the intensity control circuit.

See:

https://www.edaboard.com/threads/210797/

 

[chevymn1964]

Finally had a chance to get the scope fixed, thank you for all youre help/ hints BradtheRad... What it ended up being is a comparator IC, U1001 -( LM324 ), it is part of the intensity circut that goes in-between the A INTEN, B INTEN, READOUT pot's & the rest of the Z-Axis Driver's... Thanks again -Ryan

 

[BradtheRad]

Very positive. A little work. Major rewards.

 

[priya942]

The voltage readout on my Tek 2246 is far off for cursor, Peak and Peak to Peak, ie, Type 1, type 2, type 3 and type 4 all show gross errors. For example, it reads 246 volts when it should be 7 volts. This is true with with cursor measurements as well. The actual scope display appears to be well within calibration (checked with a frequency counter and DMM) and everything else (including time measurements) work correctly.

Would appreciate any pointers as to where to begin diagnosing the issue.

 

[BradtheRad]

The voltage readout on my Tek 2246 is far off for cursor, Peak and Peak to Peak, ie, Type 1, type 2, type 3 and type 4 all show gross errors. For example, it reads 246 volts when it should be 7 volts. This is true with with cursor measurements as well. The actual scope display appears to be well within calibration (checked with a frequency counter and DMM) and everything else (including time measurements) work correctly.

Would appreciate any pointers as to where to begin diagnosing the issue.

This sounds like it has to do with the analog-to-digital conversion.

Can you confirm whether your signal gets to the ADC unit? Whether it is amplified?

Could there be a bad ground somewhere?

An unstable supply level to the ADC section?

 

[priya942]

You hit the nail on the head, except that it means some bad news for me I think. A bit more information - my serial number is B110324 and I have an 2246A (the front panel says 2246, however, the back plastic cover says 2246...A (the lettering is faded). So that explains why the processor board did not match up with the 2246 service manual. Can someone help me confirm that from the serial number or some other way to know for sure?

So when I downloaded the 2246A service manual, it turns out that there is a separate DAC board (separate from the ADC and DAC system on the A16 processor board. This board is supposed to be next to the CRT on the side according to the service manual, but is entirely missing from my scope. I bought it "AS IS" on eBay - Oh well!

Can this explain why the volts readings are so far off, where the time readings are correct on this display? Why would they have two separate ADC and DAC on the 2246A when the 2246 had just one?

Appreciate your help with this.

- - - Updated - - -

As a further update, it appears that I have a mix of the 2246 and 2246A. There is no connector for a separate DAC/ADC card like the 2246A processor board shows, however, the processor board layout and components otherwise exactly matches the 2246A processor diagram (other than the jumper for the separate DAC/ADC card), not the 2246 processor diagram. I hope this is not confusing, but now I still have the issue of fixing my problem with the wrong voltage calculations and display. If the measurement and processor and ADC/DAC system were not working, how would I get accurate time and frequency measurements displayed? Also, I would like to know what model of scope I actually have ....

Help - this is confusing ....

 

[priya942]

Another update: It turns out that I have a 2246 Mod A which has the updated processor from the 2246A, but the rest of the hardware is very close to the 2246. I finally downloaded the correct manual ...

It points me to the Trigger B area where the vertical measurements are made for DC, -Peak and +Peak (that's the only part that's not working in addition to the wrong volts cursor measurement which is obviously related to the Peak measurement issue). I am thinking about starting with the cursor pots with the volts measurement on and working my way into the Trigger B area until I find something.

Would really appreciate if anyone has some insight as to where to zero in.