THS720A Tektronix Scope Power fail

[TekTHS720A]

Have had THS720A scope since new and it has worked without incident until recently.  Believe a portion of the power supply has been compromised but please give other opinions.  Some details:  Power from 110VAC charger is 14VDC,  voltage at NiCad battery terminal is 12VDC.  Battery was in poor condition and I'd operated the scope without the battery installed using only the power supply.  Channel 1 shows no response but appears on screen as open channel, Channel 2 shows waveform but voltage magnitude is wrong--too low in amplitude.  Frequency and scaling (both axes) is correct and can be changed with associated switches.  A 9VDC battery attached to DMM terminals returns 4.5VDC when this mode is activated.  Battery voltage was confirmed using a separate handheld meter.  Tested scope probes on another instrument without issue.

There are 4 electrolytic caps mounted in main board power supply region.  Have not replaced these but there is no evidence of deformation or leakage.  No components appear damaged and no evidence or arc tracks.  Was using scope on 10VDC signal, channel 1 only when failure occurred.

Don't quite understand how this power system should work.  Voltage to battery seems very high and shouldn't a negative supply be generated?  Thanks in advance for your thoughts on troubleshooting.

[HighVoltage]

May be the PCB is contaminated from a leaking battery?
How bright is the LCD?

Follow the power rails and see if it drops somewhere.


 

[TekTHS720A]

No battery leakage.  Can't see any cracks on PWB either.  Going to start probing Vcc's on nearest neighbor IC's.  I've seen related posts about screen illumination but I've not had the problem--my screen is bright, remarkably easy to read.  Thanks.

[z01z]

The schematic diagram is available, as well as the component BOM. Google for THS7XXAPDiag.pdf and THS700_Supplement.pdf.
There's more to the power supply than the section you've shown. Seems like there's a main supply and isolated ones for the 2 channel and the DMM. For start I'd check the voltages for these.

[Zucca]

Here you can find the schematics as well

https://www.eevblog.com/forum/testgear/tek-ths720a-portable-scope-teardowndiscussion/

[TekTHS720A]

Thanks for the details.  Been distracted with another problem--Will update this blog when I find problem or develop strategy.