Tektronix 2445B input shows 50Ω OVERLOAD and a Vertical error

[BriansBrain]

Hello 
Firstly, I did search but could not find an equivalent problem.

I have a Tektronix Oscilloscope 2445B Serial # B063339
(the baby brother of the 2465B)

History:-
I replaced four leaking capacitors on A5 Control board over 2 years ago,
all tracks around the area were cleaned and found to be intact and
connecting fine with everything working perfect since then.

Now:-
Powers up OK, everything looks normal.
Run self tests = ALL 00 PASS
All Power Supply Voltages and ripples at J119 check out OK.
Errors:-
Control Board A5...
The  -1.25V reads  -1.01V
The +1.36V reads +1.103

Using the ''Vertical Position'' controls for any channel it's impossible to set the trace in the centre of the screen.
(however with an input signal applied the Vertical amplifiers seem to be working as normal)

Horizontal system will not trigger to any input signal.

CH1 or 2 with no input signal, if I select 50Ω DC (50 Ohm DC) the 50Ω OVERLOAD error appears at t he top of the crt,
relays click and the input resets to GND, if I up select either to DC GND or AC the 50Ω OVERLOAD error MSG disappears.

I've looked through the manual but I can't seem to work out what circuit sets the -1.25V and +1.36V references.

I would appreciate any help or advice on the problem or what to check.

Thanks in advance for looking 

[BriansBrain]

 
On the facebook Old Tek Scopes page a member replied with this...

These reference voltages are derived pretty much directly from the DAC. It's possible that you have trouble with the digital circuits or the MUXes and S&H downstream. However, my bet is that the reference current to the DAC is off. If you had leakage on the A5 board and you didn't clean it up, the electrolyte will continue to eat the tracks and the resistors that feed the reference current to the DAC. Also, while those reference voltages are off, you can't reason about anything else going on, so that needs fixing one way or another.

This area 

[BriansBrain]

OK I found some bad components 

On the A5 board I replaced 2x10K R2013 and R2012 with 0.1% Low Temperature Coefficient SMDs.
I also replaced C2010 the trimmer R2010 a new vref compensation cap on pin 16 100PF C2110.
All other resistors in the DAC circuit were measured before the new ones where put in and they all were well within tolerance.

I checked all the joints with a meter and with a digital microscope and all seem to check out OK.
I have also given a good cleaning of the area with with Propanol Alcohol.

I can't get a DAC range of 2.500V.
The maximum DAC range I can obtain is (SUM of CCW and CW) = 2.375V.
 
Any suggestions on what to look for next?

[Testtech]

You still have some corrosion damaged parts, and/or, open traces. Did you remove and clean under the DAC? You will likely need to. When I restore one of these damaged boards, I remove and replace all of the SMD parts, and carefully continuity test all the traces before installing the new parts.

[BriansBrain]

You still have some corrosion damaged parts, and/or, open traces. Did you remove and clean under the DAC? You will likely need to. When I restore one of these damaged boards, I remove and replace all of the SMD parts, and carefully continuity test all the traces before installing the new parts.

Thanks for the reply 

No... I have not removed the DAC... yet 
I was trying to avoid that as I don't really have the proper equipment to do so.

I have done a continuity test on all traces and they are OK, all other resistors measure correct.

[fant]

Good morning;

looks very strange that on the right picture the voltage on pin 14 and 15 are both 0 V.
Should be somewhere more than 0 Vdc.

Mandi

[BriansBrain]

looks very strange that on the right picture the voltage on pin 14 and 15 are both 0 V.
Should be somewhere more than 0 Vdc.

Please note the position of the trimmer R2010 to obtain those voltages

[BriansBrain]

Firstly I replaced all 9 resistors in the DAC circuit including the 2 x 681Ω in the output.
Same error.   

I waited two months for delivery of a new DAC IC from China.

Fitted it today and all is good and I have a DAC range of 2.500V.