jjscreeds Posted April 2, 2014 Share Posted April 2, 2014 (edited) Hi everybody , Thought i might use my first post as a cry for help from the guru's here. I have a slightly not working GP12 SMX amp combo that i have in pieces in front of me. The problems are that the standby switch did nothing, still noise when off, even on max input gain with an active bass i could only just get the ok light to flicker, and a slight hum when on passive but silent when switched to active. Other than that it sounds great . As im not scared of taking things apart i have found a few problems, input jack joints was loose, broken wire to the main board ( think it was a power supply ) , loose speaker wire at the board and a smashed tiny tube valve ( Luna G II or IT on it) that was on the right hand side of the EQ board . I fixed the rest but would like to know what the valve type thing is and where i can get a replacement from? Please dont go too technical, i am no electrician . Thanks Edited April 2, 2014 by jjscreeds Quote Link to comment Share on other sites More sharing options...
icastle Posted April 2, 2014 Share Posted April 2, 2014 I believe it uses a single ECC83 (also known as a 12AX7). Dead easy to source them, just stick 'ECC83 UK £' into Google and it'll throw back page after page of places selling them. Quote Link to comment Share on other sites More sharing options...
jjscreeds Posted April 2, 2014 Author Share Posted April 2, 2014 its not the valve, its a tiny uv lightbulb. I found the number for EMS on this site and just got off the phone. Man said that i can just remove it and it wont affect the amp so i am hoping that the other bits ive fixed have cured the problem. Its almost back together now, 30 mins and i will let you know if ive fixed it or not. Quote Link to comment Share on other sites More sharing options...
jjscreeds Posted April 2, 2014 Author Share Posted April 2, 2014 i probably should have said that i knew it wasnt the main tube, it looked like small versions. sorry Quote Link to comment Share on other sites More sharing options...
icastle Posted April 2, 2014 Share Posted April 2, 2014 [quote name='jjscreeds' timestamp='1396452029' post='2413832'] i probably should have said that i knew it wasnt the main tube, it looked like small versions. sorry [/quote] No problem. Good luck with the repairs. Quote Link to comment Share on other sites More sharing options...
jjscreeds Posted April 2, 2014 Author Share Posted April 2, 2014 well i got it all back together and its exactly the same as before. Is using the amp the way it is going to kill it completely, as i cant switch it to standby? Also it makes a screech thud when i turn it off, is that still related to the standby not working problem? Quote Link to comment Share on other sites More sharing options...
icastle Posted April 2, 2014 Share Posted April 2, 2014 There's a service manual here: http://elektrotanya.com/trace-elliot_gp12smx_pre_amp_t0600_sm.pdf/download.html Looking at the circuit diagram of the area around the Standby switch (first diagram, RH side near the bottom), there's not much that can go wrong there. However, you could potentially get some noise if the capacitor (C107 on the diagram) is faulty. HTH Quote Link to comment Share on other sites More sharing options...
jjscreeds Posted April 2, 2014 Author Share Posted April 2, 2014 (edited) Thanks for the diagrams, reading the output stage bit about a transistor between the output and ground that is switched on by sw3 (standby switch) and the drain is the same voltage as the source and shorts that out to ground for standby. Could that be why i cant switch to standby? and if so where is it on the diagram? because the standby switch does nothing but turn the led on, the amp functions as normal, maybe its not grounding the output. maybe tr22 and tr23 and tr1 have blown , they are all JFET transistors. tr22 and tr23 look like the ones that goes to ground for the standby and tr1 is in the input to input gain circuit, could all 3 of them have died and cause both the standby and low input gain problem? I had already fixed a broken power wire to the pcb board, they had two orange and a black wire, one of the orange ones was broke at the connector. Could a dodgy power supply from this break have caused all the FETS to die from a low voltage or something? My Googled knowledge of circuits can be a little dangerous and if there is anybody that can tell me if this could have happened, i would really appreciate it. thanks Edited April 2, 2014 by jjscreeds Quote Link to comment Share on other sites More sharing options...
icastle Posted April 2, 2014 Share Posted April 2, 2014 [quote name='jjscreeds' timestamp='1396465219' post='2414063'] Thanks for the diagrams, reading the output stage bit about a transistor between the output and ground that is switched on by sw3 (standby switch) and the drain is the same voltage as the source and shorts that out to ground for standby. Could that be why i cant switch to standby? and if so where is it on the diagram? because the standby switch does nothing but turn the led on, the amp functions as normal, maybe its not grounding the output. maybe tr22 and tr23? but the parts list for the main pcb is incomplete [/quote] I didn't read the whole thing, I just got as far as the first diagram and looked for something that'd make a noise. Yes, TR22 and TR23 will both drop a signal to ground when the standby switch is off, so (provided the switch is working of course) that'd be a good place to look at. I've no idea what they are though, as you correctly state, they aren't listed. They'll be in the positions labelled TR22 and TR23 on the PCB though and the components will have a value stamped on them. Quote Link to comment Share on other sites More sharing options...
jjscreeds Posted April 2, 2014 Author Share Posted April 2, 2014 they are j112 transistors, i just edited the above post with another idea, 1 more transistor is on the pcb and thats in the input gain circuit Quote Link to comment Share on other sites More sharing options...
JapanAxe Posted April 2, 2014 Share Posted April 2, 2014 @jjscreeds - where are you based? Might be a BC-er local to you that could help (including me possibly). Quote Link to comment Share on other sites More sharing options...
icastle Posted April 2, 2014 Share Posted April 2, 2014 [quote name='jjscreeds' timestamp='1396475161' post='2414220'] they are j112 transistors, i just edited the above post with another idea, 1 more transistor is on the pcb and thats in the input gain circuit [/quote] One of the big drawbacks with FETs is that they are prone to failure from voltage spikes and you could have potentially had those when the cable broke. The direct equivalent is a BF245C and they're only pennies... http://www.conrad-electronic.co.uk/ce/en/product/157120/BF-245-C-MOSFET-BF-245-C-N-channel-TO-92-01A-30V As for replacing TR1, it's more common to test components before replacing them, but quite honestly, by the time you've got the PCB out, and for the sake of 30p it's probably easier to just replace it. Quote Link to comment Share on other sites More sharing options...
jjscreeds Posted April 2, 2014 Author Share Posted April 2, 2014 ordered some , like you say they are cheap so why not Quote Link to comment Share on other sites More sharing options...
jjscreeds Posted April 4, 2014 Author Share Posted April 4, 2014 parts arrived today, so got my tester and soldering iron out, changed tr22 and tr23 as tr22 have gone open circuit, changed tr1 because it only took 5 mins extra while it was out. Standby is now fixed fully functioning . tr1 didnt seem to need changing and made no difference to the high input gain required, but i can live with that as it may not be a real fault anyway. Quote Link to comment Share on other sites More sharing options...
icastle Posted April 4, 2014 Share Posted April 4, 2014 A good repair with enough change out of a fiver to buy a bag of chips. Excellent. Quote Link to comment Share on other sites More sharing options...
jjscreeds Posted April 4, 2014 Author Share Posted April 4, 2014 thanks for the diagrams icastle, just shows that even with almost zero knowledge of electronics, a bit of common sense and google can fix anything Quote Link to comment Share on other sites More sharing options...
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