Hartke 3500 Mosfet

[hellsgate]

Hey guys,

I've just bought a non-functional Hartke 3500 Mosfet from Ebay. I did this for two reasons:
1. It'd give me the chance to get familiar with the workings of my amp
2. It was the only way I was going to be able to get a 350w amplifier for 70 quid (GBP that is)

When it arrived I noticed a rattling noise - not a good sign.I opened it up and found a capacitor had come loose and was bouncing around inside. I can see where it has come off of the board and there is another one still on there. My question is should I replace the loose one onto the board, or should I buy two new capacitors and replace them both? If its the second option, where would I find replacements? The ones currently in it have "Samhwa (RG)105 C (M)9107" and there is a degree's symbol between 105 and C. They are also marked 250v and 100uf (not really a "u", but thats the closest on the keyboard). Should I try to find the same ones or can I get suitable others?

When it comes to fitting them does it matter which way round I solder them to the board (ie there are two pinholes in the board for each capacitor. Does it matter which way round the capacitor goes into the holes?) There is also the remains of some glue on the board where the capacitor originally sat, so what type of glue should I use to hold the caps onto the board and what should I use to clean the old glue off the board?

Thanks for reading folks.

[BOD2]

Not sure how to answer this post !

There are simple answers to your specific questions but I'm just a little worried that it seems you don't know too much about electronics and yet seem to want to poke around inside an amplifier - no offense meant !

Firstly if you're going to attempt to fix this then you need to work safely. Amplifiers often contain large capacitors which can deliver electric shocks to the unwary even when the amp is no longer plugged in !

Your broken capacitor is specified as 100uF (read as "micro Farads" where "u" means "micro") and the power rating of the capactor is 250Volts (which is the voltage it can handle). You need to replace it with one of the same 100 uF value and a voltage rating of 250V or greater - but 250V should be common as this is the UK mains voltage (more or less).

Without seeing it I can't be sure but capacitors are often polarised - they'll have a "+" or "-" on them somewhere, and if they are marked as such they'll need to be inserted the correct way. There will often be writing on the circuit board to indicate which connection is which. Have a look at the other one and see if there are markings on the board.

The glue is to stop the capacitor vibrating and will probably be some kind of silicon compound. It doesn't hold the capacitor in place - the solder does that - it just stops it from vibrating and weakening the solder joins, which sounds like it is exactly what has happened. You can get suitable silicon from electronics suppliers but, if the worst comes to the worst, then a "bath sealant" type silicon will do the job.

If the other capacitor is ok and the solder joins look ok (i.e. nice and shiny with no cracks as opposed to dull and grey) then there is no reason to replace it.

Capacitors can be purchased through Maplin, or CPC Farnell (Google these names) or any electrical component supplier. If you have a tv repair shop nearby they might even have a suitable part in stock - take the old one on. It'll be easier if you can get one the same size as it will make fitting it easier.

Now.... please BE CAREFUL if you are working inside an amplifier. Better still, enlist the help of someone who has done this type of thing before.

[hellsgate]

[quote name='"BOD2"']There are simple answers to your specific questions but I'm just a little worried that it seems you don't know too much about electronics and yet seem to want to poke around inside an amplifier - no offense meant ![/quote]

No offence taken as that is exactly the situation.

Anyway, I emailed Samson (they distribute Hartke gear these days) and they were able to email a schematic of the board to me. I was able to use that as well as the placement of the remaining capacitor to work out which way to put the loose capacitor in. I soldered the loose cap back on and repaired a few bad solder joints already on the board. When I then powered up the amp, it worked! I still need to put in a new passive input jack and get new pot knobs as they are all missing just now. The only other issue is that when I hit the button on the amp to bypass the 10-band EQ i then get no sound, but that isn't a big issue as I'll be using the EQ anyway.

Thanks for your advice and warnings BOD.