Topics

2445 jumpy readout


Ondrej Pavelka
 

Sorry for post duplication, i thought the post got lost but it was only waiting for admin approval. I have worded my questions slightly differently so maybe it will be easier to understand since I'm not native English speaker.


Ondrej Pavelka
 

Hello to the group,
I am a one man repair shop and I fix all various Recording studio equipment and musical instruments such as synthesizers. 2445B is my daily workhorse and it's been very reliable over last 7 years or so up until now.

There were suddenly two issues I noticed simultaneously but I can't tell for sure if they didn't appear one by one.
I noticed the delta V readout using the cursors is suddenly lot lower and it's consistent for all channels so it is unlikely it's the input amplifiers. It's still "in calibration" so to say but every level is too low. I noticed when I measured some TTL level signals looking for dodgy levels causing odd Z80 behaviour I'm measuring only 2.8V instead of 5 instead of cursors. I tried the other channel and different probe but all the same.

At the same time I noticed the readout is not stable on the screen. It's jumping up and down when I move the trace to the top of the screen it's more noticeable. It's as if the readout is dodging the trace and moves out of the screen slightly.

I went through the low voltage PSU and checked the capacitors, they all have very low ESR and low leakage currents. I haven't found one showing any signs of trouble. There are 3 small 33uF surface mount capacitors on the A board which leaked their guts out and started eroding the PCB. I will sort that out but I don't think that's the problem area.
Dallas chip is still Dallas and probably original from 92 so the battery could be on the verge. How do I transfer the contents to new Dallas chip?

I would appreciate any pointers where to look in the extensive service manual to fix the issues described above.