2246 Mod A voltage measurement error

[priya942]

I had posted this before in a different thread which is not related to the issue at hand. My apologies.

Time and frequency measurements work very well using the cursors, however, volts measurements are far off, perhaps 150 times larger than they should be. For example, a 0.5V PP square wave reads back 46 V. Cursor movements and displayed voltage are very smooth, just off by a large factor.

I set up the unit to measure DC with a variable power supply. I traced the output of the DC filter that is used by the comparator in the B trace section and found that it tracks the power supply changes very well. I also was able to verify that the measurement processor is able to put out a DAC voltage exactly the same as the input DC and it appears that the comparator output is zero with the ZERO HYST signal high. I know that the entire DAC, ADC and comparator systems are working, else the displayed voltage would not track the cursor movements so well.

It almost is as if the measurement processor thinks it is putting out a much larger voltage than it actually is and this much larger voltage is also displayed on the screen, ie, some sort of software error or bad EPROM where the voltage codes are stored or some such. I thought at first that this is a readout error (ie, displayed value error), but if that were the case, it would not go to OVERV after a displayed level of several hundred volts.

Everything else on the scope works very well. This is a tough error since nothing is pointing to a hardware issue. Perhaps I have a wrong version EPROM? Can someone help me understand the workings of the system or point me where I could diagnose further.

Thanks in advance.

 

[priya942]

Update: Ch1 and Ch2 work a bit differently. Ch1 and Ch2 vertical deflection, position, time and frequency measurements etc work fine, however, Ch2 shows only OVERV when trying to measure voltage (DC, Peak and with the cursors), where Ch1 is able to measure voltage multiplied by 375 or some such and that also only on 10mv, 50mv, .1V, .5V, 1V and 5V. The 20mv, .2v and 2v deflection settings all read OVERV. Also, the multiplication factor changes with time, sometimes it is 375, sometimes 180, so I can't really tell what part if failing.

This would suggest some gain related issue specific to a channel, however, the displayed waveforms work flawlessly at all deflection settings on both channels. ALL internal voltage are correct also.

Would appreciate anyone's insight as to what may be happening here.

Priya.

 

[priya942]

Update: Found it. A blown 3082 input selection transistor array on Ch1. It is the GND relay stuck on Ch1 stuck and drawing too much current from the 3082 that caused the 3082 to blow or the other way around - not sure. I have ordered the 3082 but would prefer someone with more experience than me repair the scope including replacing the 3082, verifying/replacing/bypassing the GND relay and verifying the unit's power supply. Anyone interested in taking on this type of repair please email me with quote. I am in Cincinnati, OH.

Thanks in advance.

 

[priya942]

I removed the burnt part, but the unit won't power up. I assume the next step is to look at the power supply. Interested in knowing if anyone would repair such a unit.

Thanks,