Hartke 3500 Mosfet - Powering on, but not much more

[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.

[Steve Conner]

Just because the capacitor fell off the board, doesn't mean that it's faulty. If all faulty components ejected themselves from the board, troubleshooting would be a piece of cake!

The fault may simply have been that the capacitor was badly soldered at the factory and came loose. So solder it back in and see if the amp works. Yes it does matter which way round! Put it in backwards, and it may well explode messily, possibly taking out other more expensive parts with it.

Check for any other poor solder joints while you're at it. The loose capacitor is a canary telling you that there may be more bad joints in there: maybe the original fault was somewhere else and that capacitor simply got smashed off in transit. Wouldn't hurt to reflow them all, or at least on any other big heavy components.

And check those power MOSFETs too. They have nothing to do with the capacitor, but just check them anyway. Maybe the original fault was blown MOSFETs and the fact that the amp got brutalised in transit is just a coincidence.

If you don't know what a bad solder joint or a power MOSFET looks like, or how to tell which way round a capacitor ought to be installed in a circuit board, time to get googling.

By the way, the codes tell you that it's a 100uF ("100 microfarad", the funny u is the Greek letter mu) 250 volt electrolytic capacitor. Any other 100uF 250V electrolytic that physically fits will do fine. The rest of the stuff means that it was made by the Samwha capacitor factory in the 7th week of 1991 and is rated for a maximum temperature of 105'C.

[hellsgate]

Thanks for the reply Steve.

If you don't know what a bad solder joint or a power MOSFET looks like, time to get googling.

Sounds like a bloody good idea. I'll post back and let you know how i get on after I do it (tomorrow night probably). Thanks again for the info, its much appreciated.

[Enzo]

And yes it certainly DOES matter which way you install the cap. it is as important as which way you install a battery in your car - you wouldn;t want to get that backwards.

[jrfrond]

I used to work for Samson Technologies back in the late-90's, the Hartke distributor (in fact, I remember our own Enzo from those days!). There were MAJOR issues with the MOSFET amp design in the 3500. You are lucky that yours hasn't gone up in flames, and I mean that quite literally. Inter-M (the company that builds the amps in Korea) eventually came up with a retrofit module using bipolar transistor topology, and any amp that came through service got swapped-out with these automatically. If you want to keep that amp for a long time, you may want to call Samson and get the replacement bipolar module. It's not expensive.

[52 Bill]

...Inter-M (the company that builds the amps in Korea) eventually came up with a retrofit module using bipolar transistor topology, and any amp that came through service got swapped-out with these automatically. If you want to keep that amp for a long time, you may want to call Samson and get the replacement bipolar module. It's not expensive.

John:
Can you describe the differences, maybe his amp already has the retrofit in it.

[Enzo]

Differences? The MOSFET power module has MOSFET output transistors, while the bipolar version has... bipolar transistors.

[jrfrond]

Differences? The MOSFET power module has MOSFET output transistors, while the bipolar version has... bipolar transistors.

Dude, you are my hero!

[52 Bill]

Sorry for asking.

I'll go back to my room now.

[jrfrond]

Sorry for asking.

I'll go back to my room now.

Seriously, don't be offended. Enzo and I are old warhorses. It's just a little shop humor. C'mon, it IS funny if you think about it! No offense intended here. Even us warhorses say goofy shit.....MANY times!!!

[hellsgate]

Thanks for all the advice guys! Much appreciated.

I emailed the Samson support dept and they sent me the schematic for the board. I was then able to solder the loose cap back onto the board and I re-soldered some bad solder joints which were already on there. The amp now works! 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.

Differences? The MOSFET power module has MOSFET output transistors, while the bipolar version has... bipolar transistors.

I'm afraid I don't really know the difference. If I post an image of the inside of the amp, would someone be able to tell me? I don't want the thing to go up in flames now that I have it working!

[Enzo]

Your output transistors are in a row screwed to the heat sink. What numbers are on them?

if the amp works, then stop fixing it.

[clintronics]

HA5500

does Samson sell replacement amp modules for HA5500? I checked their website and their idea of service and support is allowing you to download a users manual or a hi res image.

[Enzo]

They have a phone there, ask them.

[hellsgate]

does Samson sell replacement amp modules for HA5500? I checked their website and their idea of service and support is allowing you to download a users manual or a hi res image.

They have a phone there, ask them.

Or send them an email through the contact page on their website. I did that to ask for the schematic for the 3500 and they replied the same day.