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shaun1264 View Drop Down
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Joined: 06 September 2013
Location: Surrey
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Points: 292 		Post Options Post Options   Thanks (0) Thanks(0)   Quote shaun1264 Quote  Post ReplyReply Direct Link To This Post Posted: 07 September 2023 at 7:56pm


Another Micro-Tech 1200.  New outputs and drivers installed.  OnSemi NJW3281G/1302 drivers which have higher sustained beta at higher amperage and slightly higher SOA.  And MJ1504/5s.  All very tightly matched in VBE and HFE.  Bias readjusted set at 315mV, very stable once at temp.   ODEP  readjusted on high and low side. Load tested and distortion tested.  THD at 1Khz was coming in at around 0.0024!!  No oscillation at high frequencies.  Very clean sinewave.  Sorry, only two pics as I was pushing myself to get this done in an afternoon.
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Young Croc
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Joined: 26 June 2020
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Points: 766 		Post Options Post Options   Thanks (0) Thanks(0)   Quote EmPro Quote  Post ReplyReply Direct Link To This Post Posted: 07 September 2023 at 7:25pm
Also i didn't like the fact that StudioMaster didn't put thermal compound betwin heatsinks as are made from three different pieces....just compound on the outputs and betwin the first small heatsink and second...and betwin the second and the biggest one nothing Angry,also no thermal compound on the drivers...not even on the superdiode....this is unacceptable from my point of view...ofcourse i did the job by the book:


Edited by EmPro - 07 September 2023 at 7:26pm
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Young Croc
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Joined: 26 June 2020
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Points: 766 		Post Options Post Options   Thanks (0) Thanks(0)   Quote EmPro Quote  Post ReplyReply Direct Link To This Post Posted: 07 September 2023 at 10:18am
First time i identify it with my Fluke 115 multimeter on the board (i like this one more then my 177 because when i measure diodes and transistors and are good it makes a beeping sound,the Fluke 177 doesn't make the sound just shows the values on the screen),then i removed,and checked with TC1 tester as well.

Edited by EmPro - 07 September 2023 at 10:22am
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Young Croc
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Joined: 26 June 2020
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Points: 766 		Post Options Post Options   Thanks (0) Thanks(0)   Quote EmPro Quote  Post ReplyReply Direct Link To This Post Posted: 07 September 2023 at 9:08am
Me neither to be honest,this is first time....i meet open ones but never perfect shorted ones,and looks perfectly fine no any signs on it,the ,,resistor'' one and the good one:
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kedwardsleisure View Drop Down
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Joined: 20 January 2009
Location: Staffordshire
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Points: 4947 		Post Options Post Options   Thanks (0) Thanks(0)   Quote kedwardsleisure Quote  Post ReplyReply Direct Link To This Post Posted: 07 September 2023 at 8:47am
I've never knowingly met a transgender capacitor. How would I know?
Kevin

North Staffordshire
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Young Croc
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Joined: 26 June 2020
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Points: 766 		Post Options Post Options   Thanks (0) Thanks(0)   Quote EmPro Quote  Post ReplyReply Direct Link To This Post Posted: 06 September 2023 at 8:54pm
So yeah,the amp is up and running flawless on both ch,the fault was a transgender capacitor C4 47nf from Ch A that identifies as a 0.34r Resistor that was shorting to +-66vcc gnd the -76vcc AUX blowing R38 1w/1r resistor and the F9 100ma fuse and letting both channels dead,the amplifier sounds really nice...has a nice attack and rich sound to be honest,i like it,i give it a honest 8 out of 10 in sound reproduction and quality.


Edited by EmPro - 06 September 2023 at 10:21pm
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Young Croc
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Joined: 26 June 2020
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Points: 766 		Post Options Post Options   Thanks (0) Thanks(0)   Quote EmPro Quote  Post ReplyReply Direct Link To This Post Posted: 06 September 2023 at 3:06pm
I do have the Studiomaster Horizon 1208,powered mixer old as hell,but with such a nice and modern functions on it,also has the AMCS sistem,and is build 100% the same as this amp,same modules,same design,obviously with more outputs per ch for 600wpc,mmm it does sound good overall but seems not to be the 1200w you would normaly expect.
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jbl_man View Drop Down
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Joined: 12 January 2005
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Points: 11155 		Post Options Post Options   Thanks (0) Thanks(0)   Quote jbl_man Quote  Post ReplyReply Direct Link To This Post Posted: 06 September 2023 at 12:58pm
That generation of Studiomasters do seem to fail in a very odd ways. Is about the only vintage amp i have not owned, be interesting to get you views on the sound when you get it up and running. Complimentary MJ15003/004 set up, so should, in theory sound ok. 
Be seeing you.
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Young Croc
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Joined: 26 June 2020
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Points: 766 		Post Options Post Options   Thanks (0) Thanks(0)   Quote EmPro Quote  Post ReplyReply Direct Link To This Post Posted: 05 September 2023 at 11:01pm
Hmmm... i found F9 blown (100ma fuse) and R38 1w 1R open,for now,that's mean i have more parts faulty around Dead
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Young Croc
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Joined: 26 June 2020
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Points: 766 		Post Options Post Options   Thanks (1) Thanks(1)   Quote EmPro Quote  Post ReplyReply Direct Link To This Post Posted: 05 September 2023 at 9:25pm
Studiomaster 700D,bad shape,runs into clip on both ch even with volume at 0,i get 8vdc on outputs and sound as well,i just checked and restored both output boards and are in perfect working order,so my fault is somewhere on the psu board (also the main caps waits to explode but i still have +-66vcc supply,and the fan driver looks nicely rosted but still works,didn't have time to investigate further,here some pictures,i also replaced the broked pot.



Edited by EmPro - 05 September 2023 at 9:26pm
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Young Croc
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Joined: 26 June 2020
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Points: 766 		Post Options Post Options   Thanks (0) Thanks(0)   Quote EmPro Quote  Post ReplyReply Direct Link To This Post Posted: 22 August 2023 at 10:32pm
The most powerfull amps i ever repaired were Lab Gruppen FP10000Q and Qsc Powerlight 6.0 PFC so far Smile lab had a blown ch,sanken's,and the QSC had problems with IGBT's in PSU,i don't deal anymore with smd-s even if the amp costs 2.000.000£ i hate SMT and SMD's.

Edited by EmPro - 22 August 2023 at 10:38pm
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Pinyorouk View Drop Down
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Joined: 31 January 2011
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Points: 551 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Pinyorouk Quote  Post ReplyReply Direct Link To This Post Posted: 22 August 2023 at 10:19pm
Well done. I have repaired high powered amps. Crest 7001, 8001 and 9001,.classic amps and SMD Peavey amps amongst many more hifi amps, mixers in the past. I now repair and refurbish professional reel to reel tape decks as a hobby.
Originally posted by EmPro EmPro wrote:

Hi mate,i do repair most of them for me,but i also sell sometimes or do swaps,deals etc etc most of them comes from ebay and london from people i found on market places or i know them in person Smile this is my hobby Smile i love to save good amps from being trashed.


Edited by Pinyorouk - 22 August 2023 at 10:19pm
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