tek 2445b repair problem

[jsblues81]

In the process of testing and repairing this scope...according to the service manual in figure 5-1...I am to place my DMM on pin 13 of j119 they want you to turn the delta knob first ccw to get a negative voltage.(about -1.25v) and then cw to get a small positive voltage (about +1.25v). when I do this I get about .225v and the voltage never is negative.  No matter which way I turn the delta pot...the voltage I get is positive. Has anyone out there have any ideas on this...what could be wrong? I will continue to investagate...

[jonpaul]

Hello: Test is  is for the DAC REF range, and must be approx _1.25V>>0>>+1.25V.

Your result is typical of damaged A5 board due to leaky caps.

Have not fixed 2445, but seems similar to 2465/7B:

See long threasd on "2465B teardown" 79 pages.

Check for A5 control board leaky SMP electrolytics.

Also see groups.io>>tekscopes and tekscopes2 groups.

Bon chance, Jon

[jsblues81]

Thanks for your quick response. Changed all electrolytics and have found upon further investagating that the analog multiplexer switches need replacing...am changing all 5 4051's and all 4 TLO74 buffers...leave no prisinors..have been working on and off for almost a year on this...bought it non-working from ebay for $100. Wish me luck.

[EggertEnjoyer123]

I had a similar scope with the same problem and it turned out to be a bad SMD resistor.

The electrolyte is able to eat away at the resistors too. Might save you time and money to identify the broken parts instead of doing a shotgun approach.