Tektronix 2246 : only a solid dot (beam). Processor board problems? ...

[vintagesynthlab]

Hello,

I have a Tektronix 2246 Oscilloscope that worked great for about 3 years and started performing intermittent on start-up... and is now only producing a solid beam of light.

Please note that this particular symptom / problem is not[/i] the same as the "Tektronix 2246 - No Beam!" post found in this forum. Rather, a different issue!

The Problem: After about 3 years of using this scope, it started developing a strange problem. Sometimes it would not start up successfully. I would occasionally get no LEDs lit up on the front panel. When this happened, I would just see a solid mean of light concentrated (at one random point on the CRT). Occasionally, I would receive an error message on the front panel screen: "Part Number 160-3493-00" (see photos).

However, it would start up about 80% of the time. But as the months go by, it would fail more often. Before it failed completely, I ran the self-diagnostics. They all passed. Every function on the scope seemed to work great when it started up successfully.

Steps I took once it failed completely:
1. Checked all power supply voltages: good (within specified DCV and AC ripple/noise)

2. Referencing the Troubleshooting Hints By Diagram in the Maintenance section of the Service Manual:
 a) For Measurement Processor:
   1. U2501 for RESET condition: good
   2. ?SYS_RESET? at U2504E pin 10: good
   3. RESET for U2521: good
   4. CLKs at U2501: good
   5. INTR line: bad when unit won't power up (50% duty cycle); or good when unit powers up
   6. ?INTA?: unclear (must check again)
   7. KERNAL test: TBD (waiting on new digital scope to arrive!)
 b) I replaced the following ICs:
   1. U2400 (8-bit microcontroller)... this is the "Part Number 160-3593-00" that occasionally displayed when faulty start-up.
   2. U2501 (8-bit microcontroller)... was thinking that this might have been a problem. I carefully removed this one and socketed it. I preserved the old IC without damaging it.

Where I am now:
I sold my old Tek TDS220 scope a couple weeks ago and am about to receive a Rigol MSO (4-ch, with 16-digital ch bus for logic analyzing).

Any experience or suggestions would be greatly appreciated, as I really like this scope.

[vintagesynthlab]

just to clarify... when I say it has failed completely, what I mean is that the solid beam of light (dot) on the CRT is the only thing I get. Nice and bright, but no other LEDs, no function. A complete disconnect from the processor board of some sort.

I suspect a faulty component in the start-up process that is causing something to hang in the early stages (before self-diagnostics begins). While all power supply voltages are present, and all ribbon cable connections are good (no tears) and reseated (carefully), it would seem this should be obvious. But not so much to me at this point.

Any advice is greatly appreciated.

[Tom45]

Since there isn't much response here try the Yahoo TekScopes forum. Those people will help you get it going again.

Tom

[Vgkid]

If/when you get it fixed. postr up a link. I own a 2246.

[vintagesynthlab]

@Tom45: Thanks, I am now joined up and waiting for that hopeful advice.

@Vgkid: I will definitely do that. I always appreciate finding concrete / useful information when searching forums for answers, so I shall report back. It may take some time--but I will.