| K200B1 overheating [message #13028] |
Wed, 17 February 2010 20:31  |
real_to_reel
Messages: 11
Registered: September 2007
Location: Los Angeles
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Junior Member |
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Does anyone have schematics for a K200B-1?
Needed are PC703, PC203, PC105
I believe the previous crackling problem is resolved. My current issue: amp is hot enough to bond the naugahyde to the speaker cabinet T&R.
readings:
Output jack 11.6 DCV
Green preamp wire –8.3 DCV
Red preamp wire +8.7 DCV
High voltage DC power supply: -38.9, +39.4
If amp is overheating and not shutting down, is the Thermal circuit breaker (black round plastic, 2 spade connections) failing?
Re. Bias diode - should I add transistor heat compound to it? (there is none on it)
Chassis mounted RCA 36892 transistors – From reading previous posts it appears that these can be easily removed for testing by removing 2 hold-down screws – is this correct?
Thanks for you assistance.
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| Re: K200B1 overheating [message #13039 is a reply to message #13028] |
Thu, 18 February 2010 15:58  |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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If there is DC on the speaker jack, you may have a shorted output or driver transistor.
The outputs have plug on connectors for the base and emitter legs. Pull the triangular black plastic connector up off of the pins and you can test it without removing it from the chassis. For reference, the blue lead is the base and the yellow the emitter.
Adding grease to the diode on the heat sink will help, there was some originally, but it has probably melted through the years.
I'm not sure, but I think that the thermal shut off is designed to open at 180 degrees F.
As for the schematics, PM your email address to me and I'll send them.
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| Re: K200B1 overheating [message #13048 is a reply to message #13043] |
Fri, 19 February 2010 19:09 |
real_to_reel
Messages: 11
Registered: September 2007
Location: Los Angeles
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Junior Member |
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Thanks to all for the schematics.
Please excuse the following questions that are probably Electronics 101. I’ve read previous postings but, can’t find the answers.
How exactly do I test Q1, Q2, Q3, Q4 transistors after I have pulled the triangular black plastic connector up off of the pins? Multimeter set at? Red probe goes to? Black probe goes to? Sequence? What readings am I looking for, what is a bad reading indicating a defective transistor?
Can Q704, Q706, Q708 be checked on the board or, are they to be removed from the board for testing?
There are no “Q” numbers on the PC boards. How do you know what Q number corresponds to the schematic and the board? (example: 36892 transistors) I know Q1 to Q4 are not on the board but, don't know how to identify them in their chassis position.
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| Re: K200B1 overheating [message #13049 is a reply to message #13048] |
Fri, 19 February 2010 22:51 |
ellum68
Messages: 71
Registered: November 2007
Location: Sioux City, IA
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Member |
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Set your multimeter to the diode test mode. Remember where all your wires goes before you remove the black plastic caps. Take your positive probe and touch it to where your blue wire went to (the "base" of the transistor) . Then take your negative lead and touch to where your red wire (collector) went to and then to where your yellow went (emitter). Your meter should show something in .6-.8 range. Then do the same test again, but swap your negative and positive probes. You should show no current flow. If you switch your meter to ohms, you should see an open circuit or a very high resistance. Q704, Q706, and Q708 are the transistors that lie in the big metal heatsinks near the bottom of the PCB. I THINK you can test them still in circuit, but don't quote me. These are NPN transistor, so the test procedure should be the same. Look here to see what I'm talking about. Use your schematic to help identify where your base, emitter, and collector are located for these transistors.
[Updated on: Fri, 19 February 2010 23:05] Report message to a moderator |
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| Re: K200B1 overheating [message #13052 is a reply to message #13049] |
Mon, 22 February 2010 20:55 |
real_to_reel
Messages: 11
Registered: September 2007
Location: Los Angeles
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Junior Member |
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I tested Q1, Q2, Q3, Q4 attached to the chassis but, all wires removed.
I designated Q1 as the transistor on the left side of the heatsink that is riveted to the chassis. When looking at the back wall of the amp from the front side, Q1 is furthest from the output jacks. Have I identified Q1 correctly? I assume Q4 is at the end of the line.
Results of Q1 test:
Base to Collector: 581
Base to Emitter: 603 on 1st test, 586 on second test
The other 4 combinations were -0-
Results of Q2 test:
Base to Collector: 602-598
Base to Emitter: 601-606 on 1st test, 590 on second test
The other 4 combinations were -0-
Results of Q3 test:
Base to Collector: 578
Base to Emitter: 577
The other 4 combinations were -0-
Results of Q4 test:
Base to Collector: 569
Base to Emitter: 571
The other 4 combinations were -0-
Possibly there should be a decimal point after the first number but, meter set on diode - no decimal appeared.
Can Q704, Q706, Q708 be checked on the board or, are they to be removed from the board for testing?
I have not been able to replicate the overheating condition at idle with the chassis out of the cabinet but, the 11 volts DC is still at the output.
Thanks.
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| Re: K200B1 overheating [message #13058 is a reply to message #13028] |
Tue, 23 February 2010 13:01 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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It seems that the four outputs are ok, so now test the drivers. These are the ones that have the square metal heatsinks.
Test them in circuit first and see if they seem ok. If you get any really odd readings, you may have to remove them to check them out of circuit before moving on.
When you check a transistor or diode with your meter, you are reading the voltage at which the junction conducts. So the numbers that you are getting actually mean 0.601 volts. This is normal for a silicon transistor, it would be lower for a germanium one.
The most important thing is that when you reverse your meter connections, that the reading is higher if not completely open. The transistor is supposed to conduct only in one direction so you should get a low reading with the meter hooked up one way and a high reading when the leads are reversed.
Good luck and let us know what you find out.
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| Re: K200B1 overheating [message #13067 is a reply to message #13066] |
Mon, 01 March 2010 12:30 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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In answer to your question, yes other components can cause an unsure reading when testing transistors. This is where you need to compare how much voltage is being read by your meter and in each direction.
Q706 and Q707 are part of the output amp protection circuit. They both have some low value resistors in circuit that can cause unclear test readings. Retest Q706 and Q707 and compare them to each other. The polarities should be reversed, but the readings should be similar. If you are getting similar low readings from BE and EB on both transistors, then it's probably the in circuit resistor that is causing the low readings.
If Q1 and Q2 test ok then leave them. Q714 and Q718 are part of the low voltage regulator circuits. Your earlier tests showed that you had both + and - 8 volts, so they are fine, leave them.
Have you tested the other transistors? How about the 5 diodes on the board?
You may have to remove the questioned transistors and test them out of circuit. I don't know what skills and equipment that you have, but you don't need too much to remove most of these. Let us know if you need soldering advice.
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| Re: K200B1 overheating [message #13068 is a reply to message #13067] |
Tue, 02 March 2010 09:58 |
C4ster
Messages: 686
Registered: June 2001
Location: Mukwonago, WI (Milwaukee...
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Senior Member |
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After you have tested the transistors and have unclear readings on the voltage scale, change to resistance and you might see the value of the resistor in question. This is typical of resistors across the emitter/base.
Conrad
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| Re: K200B1 overheating [message #13084 is a reply to message #13028] |
Wed, 03 March 2010 18:06 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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It took a little while to go through all of your numbers.
Nothing seems to stand out to me as being really wrong.
Please for now, don't worry about the regulator transistors Q710 thru Q718, as I stated earlier, the voltages are ok so they are working.
Did you reverse test the diodes?
The next tests would be to start checking voltages on the board and comparing them to the voltages listed on the schematic. To do this, you will need to work on the amp while it is plugged into the wall and turned on. If you don't feel comfortable doing this, it will be time to refer to a tech type.
If you do check the amp while it is live, work slowly and carefully, and remember that there can be enough voltage in the chassis to cause you bodily harm.
The voltage readings will not match exactly, but they will show you where something is really wrong. The voltages are all referenced to ground, so attach the black lead of your meter to the chassis and carefully probe the connections with your red lead.
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| Re: K200B1 overheating [message #13098 is a reply to message #13028] |
Fri, 05 March 2010 05:41 |
stevem
Messages: 4773
Registered: June 2004
Location: NY
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Senior Member |
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Before you re-mount the board get a black and red marker and trace out the base and collector paths onto the componet side of the board.
I find this helps me to trace things out faster when the board is bolted back up to the rear wall, as kustom never supplyed board layouts in regards to resistors and caps other than in the latter k150 and 250 amps.
Aslo make sure that non of your 1 ohm 5 watt resistors are open, as many times they may not look burnt, but are open.
Also the first time you fire it up to check volatges, check the output jack for DC agian.
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