Hameg HM705 Repair

[Nicholas1]

Greetings,

I am in the process of repairing a Hameg HM705 oscilloscope. I repainted the case as the old paintwork was chipped and had rust underneath. After blowing out the insides with high-pressure air, I realised that that wasn't such a good idea . One of the wipers on the potentiometer was blown off. I did fix it, though, by repurposing a wiper from another potentiometer of different value. The high-pressure air didn't remove much grease or dust at all, so I bought some electronic cleaning solvent to try instead.

Now I want to replace fragile and cracked polyester film capacitors on the circuit boards (after not applying much force with a pair of pliers, I accidentally pulled the capacitor body from its legs while desoldering it. Oops.). I saw a video Mr. Dave Jones posted on YouTube a while ago, about film capacitors. After finding out that moisture ingression can cause issues, and with the scope's age in mind, I decided that I want to try replace every film capacitor possible.

I have a few questions regarding capacitors and the repair process in general:

• On the boards, there are some Phillips "MKT" capacitors. I found out that they are polyester film capacitors. However, to the right of their printed values are some extra markings, whose meanings I don't know. E.g.:
  •     0.1  M
  •     0.1  /20
  • 0.022   K
  Are these some sort of tolerances?
• Should I also replace the other capacitors while I'm replacing the poly film capacitors (e.g. ceramic and electrolytic)? I envisage that this will be a big job as there is a lot of disassembling I must do to first before I can replace the capacitors, so I might as well do as much as I can in one go.
• Are there any disadvantages of replacing capacitors with ones that have higher voltage ratings while everything else remains the same? E.g. 0.022uF 400V vs. 0.022uF 630V.
• What else do you recommend I do during this repair process?

Before I start desoldering and putting new capacitors in, I will first wait patiently for some advice.


Thank you for reading,

Nicholas.