Elektronika MK-61 - today I ordered one NIB

[voltaage]

(09-19-2014 08:29 PM)Francois Lanciault Wrote:  I bought a MK-52 a few month ago. It works for about 2 days then died.

I am now a bit reluctant to order an other one...

François

Hello François.

Don't panic!. It is a common problem that these work for a bit, and then refuse to power on or to work properly. In my experience, when I ordered one NIB from e**y, it worked for a month before refusing to turn on; when it did, no matter what key you pressed but it returned to that transe. I started by replacing the only eletrolythic in the machine: a "K50" 20uF 25V capacitor. No change at all unfortunately. Then I took my soldering iron and reflowed all the solder joints in the board: all of them. I spotted some cold soldered points here and there, and corrosion of course (very little). Still no improvement. Lastly I took apart the keyboard assembly, removed all the left sponge debris, brushed the contact points on the motherboard (don't brush the ones on the plastic mat since they are VERY brittle and prone to falling apart) and replaced the keyboard sponge with a 0.5cm thick card size one found at a small portait. Oddly enough, I reassembled the moudule, placed the black protective case of the power supply board, and stored the calculator for 2 weeks (After doing the keyboard replacement, anything worked. I got so frustrated that I decieded to continue later and refresh my thoughts in the mean time). After that it worked perfectly. It has been working since (almost a year now) with the keypress feedback as it would originally have been; no repeated keypresses and no jingling candy inside the case (the keys, since the sponge wasn't printing the right force to keep them from playing in their spot, made a jingle noise when moved slightly because of the clearance. One of my friends made that joke when I presented the calculator to him before doing the repairs.)

In conclusion, consider these steps:

• Change capacitor in the power supply board
• Look for solder bad solder joints (corroded, cold, cracked).
• Remove debris between the contact layers of the keyboard assembly to ensure connectivity.
• Replace sponge of the keyboard assembly with original hole pattern to ensure keys aren't pressed twice (yes, the weight of a key can cause that).
  

In general terms, it is good to take a look at these too:

• Look for broken components.
• Look for smoke marks, water damage, battery acid corrosion, etc.
• Check for any weird objects inside the display (another MK61 I have has some of the epoxy used to glue down the control grid stuck underneath it, but that is a story for another post).
• Are your batteries ok? (1.5 volts each).
• Are the battery contacts ok? (Sometimes they have a thin layer of corrosion due to storage. Use the back eraser of a pencil to remove that).
• Is the back connector shorting correctly? (There is a lead touching the left pin of the connector at the back of the machine which supplies Vbatt to the machine; it is disengaged when the plug of the AC adapter is inserted, to avoid recharging non-rechargeable batteries).
• Is the power switch ok? (It should glide with ease, making a solid "clack" when toggled).
  

Finally, remember that this devices are 30y+ (yes, they might be NOS, but they haven't been used for a long time). Components get old, and sometimes need a time to "come back to life"; so give them that time as well.

PD: Sorry for replying after this long, but I just found this forum acquaintanced with it.