timmyi23 Posted May 16, 2013 Share Posted May 16, 2013 Hi Have had a tech(ish) guy look at this for me and he has replaced some transistors that had blown on the power amp. Thing is, when turning the head on, and without any cabs or guitar connected, the heat sink is getting incredibly hot ( too hot to actually touch ). The fan is running ok but why would the heat sink get so hot when the amp is basically not doing any work?? Cheers Tim Quote Link to comment Share on other sites More sharing options...
dincz Posted May 16, 2013 Share Posted May 16, 2013 There could be more faulty components still in there, or the new transistors have different characteristics meaning that the bias might need adjusting. Either way it's probably a job for a real tech. Quote Link to comment Share on other sites More sharing options...
Phil Starr Posted May 16, 2013 Share Posted May 16, 2013 Something is still wrong, mine just blows cold air when there is no signal. I don't think bias is adjustable so probably the transistors in this part of the circuit have blown. This is probably what blew the original output devices. Or he may have caused another problem in fixing the original. Don't use it, it will only make a more expensive repair. Quote Link to comment Share on other sites More sharing options...
icastle Posted May 16, 2013 Share Posted May 16, 2013 This is generally what happens if you just replace 'broken' parts without ascertaining what made them break in the first place. The original fault is still there and your new power trannies will go the same way as the original ones unless you get it sorted out by a competent amp tech. Quote Link to comment Share on other sites More sharing options...
MarkBassChat Posted May 16, 2013 Share Posted May 16, 2013 (edited) This usually happens when you replace output transistors without replacing driver transistors, or bias circuit. But a tech should know it Mark Edited May 16, 2013 by MarkBassChat Quote Link to comment Share on other sites More sharing options...
BILL POSTERS Posted May 16, 2013 Share Posted May 16, 2013 (edited) Something not right there. Is it just a 3500 head, or a combo ? and did the tech have you spekers as well? As has been said, drivers, bias cct etc usuall fail along with the O/P transistors, O/P transitors are 3 pairs wired in parralell with a 3500 iirc, so could be that not all of them are switching on. Dont try and use it, Either take it back to him or find a competent tech. Just wondering whether you have a shorted lead or speaker, although the amps protection cct should sense it if you have. Edited May 16, 2013 by BILL POSTERS Quote Link to comment Share on other sites More sharing options...
timmyi23 Posted May 16, 2013 Author Share Posted May 16, 2013 [sub]Hi guys. Many thanks for the responses. The head is still in the workshop, the guy working on it although not expert with amps per se is pretty skilled with internal electrics and circuit boards, etc. I know he mentioned that the transistors attached to the heat sink had failed and he has now replaced all these, prior to that when the head was switched on it would blow the fuse immediately unless the power amp was disconnected. Any further ideas would be appreciated so I can pass them on .[/sub] [sub]Tim[/sub] Quote Link to comment Share on other sites More sharing options...
BILL POSTERS Posted May 16, 2013 Share Posted May 16, 2013 Not being fnny mate, but if he needs ideas of the general sort, such as would be passed on in that manner from this type of forum. Your best bet is to get it back and see if you can find somebody who does know what he's doing and who is prepared to get involved now its been got at. "skilled with internal wiring and circuit boards" just aint enough. Quote Link to comment Share on other sites More sharing options...
.gif.9ad672bb8b84505709f76b39c8c256ec.gif)
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.