tektronix 2246 fault needing help

[joe90uk]

Hi All.
Im new to this so please be patient. My trusty scope has packed up, It may be my age or I could be an idiot cos I just can't seem to put my finger in the starting blocks of the problem. The unit starts up but only displays a single dot on the display. There is no vertical or horizontal scan. None of the buttons seem to work and the only working knobs seem to be the focus and me. The only other life seems to be a single flashing light on the front and the fan spinning. All of the power lines seem to be spot on apart from the 130v AC line which is fluttering on and off and yes, my meter is set to measure AC. Is there anyone out there who would like to give me some pointers.:-D

 

[E-design]

For starters try to find the service manual for this online. They often have a fault-finding-flow-diagram which could narrow it down to a section or board which you can concentrate your efforts around.

 

[BradtheRad]

Did you open it up and check the main fuse?

Perhaps the high voltage beam generator is a separate circuit, therefore it works.

And perhaps the fan works because it is designed to continue cooling after you shut off power?

 

[betwixt]

Be careful, there are also 2246A and 2246B versions which are electrically quite diifferent. Sounds like a power supply failure to me.

Brian.

 

[joe90uk]

Thank you for the reply. I have the service manual. It did have a fault section but did not have any help for the symptoms I have.

The only fuse is on the back and that's fine. The fan operates as it should. Thanks for the help anyway.

Mine does not say A or B model but the information I gathered from the net tells me it is an (A) model. The PSU was my first choice too. I can confirm without any doubt that the PSU is not the fault as this is my area of expertise and I carried out every test applicable. Thanks anyway.

 

[betwixt]

The 'A' and 'B' are different from the one with no letter at all.

If the PSU is working, it suggests a total logic failure. Is it possible to put it into self-test mode? It would be interesting to know if the dot moves if you use the horizontal or vertical position controls. They are read by an ADC so no movement or vertical deflection would indicate the front panel wasn't being read by the software.

Do you have another oscilloscope you can use for debugging?

Brian.

 

[E-design]

Can you use the diagnostic routines to aid troubleshooting as suggested in section 6-5 to 6-33 of the service manual?

 

[D.A.(Tony)Stewart]

Check all the high voltage DC regulators for Hsweep and V sweep, neither which are working.

 

[joe90uk]

Hi Brian
The unit can't be put into self test mode because none of the buttons work and the H and V controls also have no effect. However, even without any controls, shouldn't the unit display a horizontal line? Everything points to a PSU fault because there are multiple symptoms and the PSU seems to be the only common factor but I am positive that the PSU is fine.

I don't have a second scope available, only digital volt meter.
Thanks
Joe

 

[betwixt]

The horizontal line depends on two things, the timebase oscillator is running and the HV to the deflection plates is present. As none of the controls work, I suspect the timebase isn't running but that alone wouldn't stop the buttons/indicators working. I'm guessing (that is all I can do!) the problem is either the logic power supply or the main processor isn't running properly. Look at the main board and see if you can find any 74 or 74LS series ICs and measure the voltage between it's power pins. It is possible the 5V is wrong or tripping so the logic circuits can't initialize.

If that doesn't reveal any clues, look for the microprocessor, it is usually a 40-pin device and tell me it's manufacturer and type number. If I remember correctly, two versions of logic board were used, and they used different microprocessors so knowing which you have will help.

Brian.

 

[E-design]

Have you confirmed all the voltages to be in spec as per table 6-4?

What about the status on the trigger mode led's, which aids in power-up diagnostics?

 

[D.A.(Tony)Stewart]

Check all the high voltage DC regulators for Hsweep and V sweep, neither which are working.

Since this is a CRT and not an LCD, in order to get a horizontal line, the flyback transformer circuit, incl wires, capacitor to get a high voltage sawtooth sweep ...must all be working...possible a HV defect .....

or the 100~200V (or so) supply for the timebase sawtooth sweep which drives the flyback transformer...

or anything in between.


.. same for vertical, start with vertical PSU.

 

[betwixt]

I'm leaning toward this being a logic or logic PSU fault. The HV must be present for the CRT to operate but the front panel controls are also dead apart from one flashing LED. If there was a deflection fault the controls would still work. I think it has failed a self test during start-up, finding the cause is the issue. It would be fairly easy with another scope but very difficult without one.

Brian.

 

[D.A.(Tony)Stewart]

MY point was if the flyback HV has no sawtooth, it will be vertical line. If it has no vertical power, it will be a dot.

Obviously the common single point failure to V & H is the PSU.



Check your HV supplies 100~200V range.

 

[joe90uk]

Thanks for the reply. This gives me some ideas to try. The processor is a 8088.

- - - Updated - - -

Thanks for the reply. It is the auto level light which is flashing. All of the PSU voltages are within spec but the chart does not show the 130v AC supply which is very unstable. It is not the PSU which has the fault so I am going to take a closer look at the possibility of something on another board dragging it down.

 

[betwixt]

The flashing AUTO LED indicates a ROM failure during self-test. It would cycle through all addresses except those with holding the self test code itself, totalling the value of the bytes as it does so. At the end if the total is incorrect (either checked against a stored correct value or by 'fudging' a dummy byte to make the result zero) it will refuse to execute the normal start-up procedure. Most likely, one of the ROMs has corrupted. If it has a checksum printed on it's label, you can read it with an EPROM programmer and confirm if it is good or not, otherwise you need to test by substitution. If one has failed, you should be able to replace it or even reprogram the same IC as they get amnesia as they get older. The place to ask for the data file to reprogram them is the Tek scopes forum on Yahoo Groups. It's a very active group specializing in older Tek models and someone there is bound to have copies of the ROM contents for you.

Brian.

 

[D.A.(Tony)Stewart]

User Manual says on p1-6
"TRIGGER MODE LEDs may be flashing to indicate the circuit location of a start–up error; "

But I agree with @betwixt about locating the source from Yahoo groups


Service manual p6-10 indicates LED status as below

**broken link removed**

Now the question , is . which LED?

According to Maint. Manual link above which LED determines if ROM error or RAM error.
( 95 yr copyright lifetime if not renewed in USA prevent me from sharing )

 

[joe90uk]

Thanks Brian
Looks like a good lead to me. Would this stop a single line being present though ?
Joe

 

[D.A.(Tony)Stewart]

A bad RAM or ROM bit can be considered the same as a bad pixel on an LCD. ROM errors from aging can sometimes it can be refreshed after aging just as dead pixels can be restored with exercising a pixel ( DPT.exe) but not always.

 

[aminoacid]

My first thought would have been the X-Y Mode ( a dot showing )