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Re: K200b-2 [message #24003 is a reply to message #23913]

Wed, 30 September 2015 22:23

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Those results look better. Now test the last transistor in the section. It's straight down from Q313, underneath the large orange drop cap between the two pots.

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Re: K200b-2 [message #24004 is a reply to message #23913]

Wed, 30 September 2015 22:49

Iowa Boy
Messages: 767
Registered: June 2014

Senior Member

collector 2.157 black probe base emitter OL; collector .703 red probe base emitter .695; collector -emitter 2.153 reversed 1.732

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Re: K200b-2 [message #24005 is a reply to message #23913]

Wed, 30 September 2015 23:34

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Well, the good news is that all of the transistors seem to be okay. The bad news is we haven't found the problem yet. The next step will be to start reading voltages and comparing them to the schematic.

All of the measurements will be to ground, so the black lead will be connected to the chassis and the red lead will connect to different points in the circuit. As long as you are careful not to short anything with the tip of the red lead you should be okay.

Keep your hands away from any of the 120 volt ac sections like the back of the power switch, the fuse and the thermal cutoff switch.

The preamp section runs on plus and minus 8 volts dc, so the maximum voltage you will be dealing with is 16 volts or so. Not enough to really hurt you, but if you short the wrong things together you might see some unwanted fireworks. Can you read the schematics drawings?

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Re: K200b-2 [message #24006 is a reply to message #23913]

Wed, 30 September 2015 23:47

Iowa Boy
Messages: 767
Registered: June 2014

Senior Member

As long as there not written in Chinese. Spoke too soon. After blowing up the schematic as large as possible, I could finally read it. I guess I'm more use to reading schematics that are broken down into segments of each function......I really don't know where to start with this. Wished there was some way to print this off so I can go thru it while your online.

[Updated on: Wed, 30 September 2015 23:59]

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Re: K200b-2 [message #24007 is a reply to message #23913]

Thu, 01 October 2015 02:33

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

I open a second window so I have one for the schematic and one for the board.

Look at the left edge of the schematic and then look at the center section. You should find the three transistors that we started testing there Q310, Q311 and Q312. If you don't know it already, the drawing of the transistor is the circle with the vertical bar in the center and three terminals coming out of it. The center lead on the left side of the center vertical bar is the Base, the plain line on the right side of the bar is the Collector and the line with the arrow head is the Emitter.

If you look at Q310 there are dc voltage numbers next to the different leads, -1.1 B, 0 C and 0 E. You'll find these numbers near most of the transistors. What you need to do is to read the voltages in your amp and compare them with the voltages listed on the schematic. They don't need to match exactly, but what you are looking for are voltages that are way off.

If the schematic reads +6 volts and you get -3 volts there, you need to try and find out why.

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Re: K200b-2 [message #24008 is a reply to message #23913]

Thu, 01 October 2015 05:55

stevem
Messages: 4733
Registered: June 2004
Location: NY

Senior Member

If you go to the music parts site you can print off the schematic.

NOTE that all the voltages per the schematic are taken with all the controls full up with no speaker load!

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Re: K200b-2 [message #24009 is a reply to message #24008]

Thu, 01 October 2015 09:54

Iowa Boy
Messages: 767
Registered: June 2014

Senior Member

Steve what do you mean the music parts site?

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Re: K200b-2 [message #24010 is a reply to message #23913]

Thu, 01 October 2015 10:35

Iowa Boy
Messages: 767
Registered: June 2014

Senior Member

Okay now see what your taking about as too whats what on the schematic. Once I know the starting point, I can follow the path. Waiting to here back from Steve as to printing a schematic.

[Updated on: Thu, 01 October 2015 10:58]

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Re: K200b-2 [message #24011 is a reply to message #23913]

Thu, 01 October 2015 12:45

chicagobill
Messages: 2005
Registered: April 2003

Senior Member

Music Parts is a site where you can buy copies of the schematics for a lot of older music gear. I've bought stuff from them before, but most everything you could want is out there free on the web.

When I was fixing my Guitorgan, I really needed a schematic to figure out where things were and they had what I needed.

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