RandomBass Posted March 5, 2013 Share Posted March 5, 2013 (edited) 1989 vintage Trace Elliot AH200 GP12 Amp Repair My first contribution in the Repair & Tech forum, but thought I'd share this experience. I recently bought this on BC for what I considered a good price, from a nice chap. Unfortunately it very soon developed a problem which obviously was quite a disappointment, but which enabled me to reawaken my dormant electronics skills. I asked the seller what had been done during a recent service - apparently a few caps needed replacing. He also had one of the 1/4" jacks replaced with a speakon (which is a good mod actually). So what was the problem? Simple really - the output sound was heavily distorted, and the output level was fading in and out. The headphone output seemed fine, which I assumed came from the preamp, which kinda isolated the problem to the output stage. The DI was fine too. So, an evening on Google came up with the schematics together with a wealth of other information, out came my DMM and I reinstalled xoscope onto my trusty Linux lappy to monitor various points around the amp, after having removed the lid of course lol. Signal and supply readings throughout the circuit seemed fine, and I couldn't detect any crosssover distortion at the output. Now here's the odd thing - having the schematic in my hands, I was clearly wrong about the headphone output: it's taken from the output of the power transistors (through a wirewound resistor), although the initial symptoms would suggest otherwise. And something else: when I used the headphones without a speaker load connected, the sound through the phones was fine: as soon as the speaker load was connected the distortion came back through both speaker and phones. Remove the speaker load, and all is well again. My 8 Ohm dummy load had the same effect. This is all pointing to the heavy current side of things. The output MOSFETS were not open or shorted, they didn't seem to exhibit any unusual readings when tested in situ, although best tested out of circuit of course. The power supply had a solid voltage, and wasn't dipping under load (well, nothing that I would consider worrying), but clearly as soon as proper current flow was required, the amp had problems. A little more time spent on Google came up with an identical set of symptoms with an AH300 which basically has the same output stage, supplanted by a beefier PSU, cooling fan etc. Apparently replacing the MOSFETS solved the problem, so lets take a chance..... The original Hitachi 2SK135/2SJ50 output MOSFET pairs were still in place. These are now obsolete so I decided to go for Semelab BUZ901/906 alternatives, which may or may not require a little rebiasing. Farnell delivered in a timely fashion, complete with new mica washers, spacers, heatsink compound etc. A couple of hours of careful component replacement and comprehensive testing last night, I now have a fully functioning AH200. I didn't even need to touch the bias trim pot, which was nice, not a Trace (lol) of crossover distortion. Tomorrow is rehearsal night, so that'll be the real test - proper volume levels! So far, I'm jolly pleased, but with fingers crossed for tomorrow... Cheers Geoff Edited March 5, 2013 by RandomBass Quote Link to comment Share on other sites More sharing options...
Guest Posted March 5, 2013 Share Posted March 5, 2013 Thanks for posting Bud. I have an AH200 and a 1995 350 smx so it's good to know what modern parts will fit. Good luck with it. Quote Link to comment Share on other sites More sharing options...
RandomBass Posted March 5, 2013 Author Share Posted March 5, 2013 Yeah thanks for that, thought I'd share my experience to maybe help others on here. Having studied these Clive Button designs, they are pretty straightforward to fix and keep chugging along. Quote Link to comment Share on other sites More sharing options...
BassBod Posted March 6, 2013 Share Posted March 6, 2013 Thats useful - I've got an AH150 that makes some horrible noises (not all caused by me) one day I'll get it in for repair. Quote Link to comment Share on other sites More sharing options...
RandomBass Posted March 6, 2013 Author Share Posted March 6, 2013 (edited) IIRC the AH150 is also very very similar design-wise. A half-decent tech should be able to diagnose and repair it, no worries. If the problem is the output stage, although the original power MOSFETs are no longer in production, there are several alternatives available which will do the job perfectly - not just the BUZ901/6 that I chose. Make sure they also renew the insulating washers and heatsink compound - it's a pennies job worth doing while the trannies are off. Edited March 6, 2013 by RandomBass Quote Link to comment Share on other sites More sharing options...
RandomBass Posted March 6, 2013 Author Share Posted March 6, 2013 As a slight aside, the guy who sold me the amp has offered to pay the cost of the replacement parts, which is a kind thought. As a gesture of Basschat goodwill I thought going 50/50 would be ideal. Cheers Geoff Quote Link to comment Share on other sites More sharing options...
RandomBass Posted March 6, 2013 Author Share Posted March 6, 2013 One last bit from me...I promise lol. Just back from band practice and the AH200 was perfect. Loud, clear and punchy with a slight peak around 100Hz on the graphic and dropping the low end, through an ABM410. Heatsink got pretty hot a couple of times, but not unbearable to touch. I'm happy. Well worth the £50 in parts and a couple of evenings investigating/discovering/work/fun. Quote Link to comment Share on other sites More sharing options...
MoonBassAlpha Posted March 9, 2013 Share Posted March 9, 2013 Thanks for sharing. Someone somewhere sometime will find this invaluable. Good man! Quote Link to comment Share on other sites More sharing options...
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