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| Re: K150-6 pumps out DC voltage when powered on. [message #23710 is a reply to message #23707] |
Mon, 24 August 2015 17:07   |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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PC5032 is the one for the power amp. I'd start by pushing on a few things on the power amp board to see if anything is loose.
Do you feel confident in working on an amp that is plugged in and turned on? There are things inside the amp that can hurt you, so if you don't feel confident, please refer servicing to a qualified tech.
Alternatively you can pull out the board and inspect the soldering on the bottom side without having it plugged in. How are your soldering skills?
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| Re: K150-6 pumps out DC voltage when powered on. [message #23726 is a reply to message #23707] |
Mon, 31 August 2015 06:38 |
stevem
Messages: 4774
Registered: June 2004
Location: NY
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Senior Member |
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Take a good close look at the two rectangular 5 watt resistors mounted near each square metal heat sinked cased drive transistor on the output board.
You may need to unsolder one end to see the non lettered bottom, but if that bottom side shows any signs of cracking then replace it as it may be going intermitantly open on you.
If you replace any then steep up to a 7 watt version if you can.
Also note that the any other small metal cased transistor needs to be looked at close too, as back then there leads tended to snap off at the bottom of the transistor if the factory instlled them bent up.
[Updated on: Mon, 31 August 2015 06:39] Report message to a moderator |
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| Re: K150-6 pumps out DC voltage when powered on. [message #27660 is a reply to message #23707] |
Wed, 12 February 2020 14:52 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Welcome back.
Visual inspection of electronic parts is a good thing to do, but not all bad parts will show signs of burning, leaking, etc.
If I recall correctly, the amp would sometimes work but usually just hummed and the speaker cone would extend and stay there. These are signs that there is dc voltage on the speaker and that there is some intermittent connection that is causing it.
You have visually inspected all of the components and the wiring and have not found anything that looked questionable.
Did you ever try flexing, tapping or knocking on the power amp board to see if you could either cause the problem to start or to stop from a mechanical stimulus? Often doing that will point you in a direction to investigate for a cold solder joint or broken component lead, etc.
If the problem does not respond to a physical movement, you will have to move on to testing parts and wires, etc.
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| Re: K150-6 pumps out DC voltage when powered on. [message #27661 is a reply to message #23707] |
Wed, 12 February 2020 17:32 |
corybrown
Messages: 51
Registered: August 2015
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Member |
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Thank you again for all the help so far.
I'll fill you in on the progress that I made last night.
After trying the amp again and verifying that it still had 27 volts DC showing on the output jack I went back through all of the previous suggestions.
I turned the amp on and using a wooden probe I pushed around on all of the components on the poweramp board and various other wired connections throughout the chassis.
That didn't seem to affect it.
Then I unhooked and removed the two big blue filter caps and reinstalled. I tested them while they were out and they tested fine.
That didn't seem to affect it.
I removed transistors Q18 and Q19 and tested them with multimeter. They tested at 585mV when testing base to collector and base to emitter. I then reinstalled them.
That didn't seem to affect it.
I came home today and saw your post and went to check on it again.
This time when I fired it up to verify if the 27V was still there so that I could begin testing and it metered at 0V.
So I hooked the speaker back up and tried it and it worked perfectly. That lasted for a good 5 minutes and then it made a popping sound and resumed the hum.
The 27V is now back on the output jack.
I don't know if this matters or not but I tested the monitor jack that is on the back panel and it does not have 27V on it.
I tried again manipulating the poweramp board and components with the wooden probe and so far no luck.
Thanks
[Updated on: Wed, 12 February 2020 17:34] Report message to a moderator |
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| Re: K150-6 pumps out DC voltage when powered on. [message #27667 is a reply to message #23707] |
Sun, 16 February 2020 23:04 |
corybrown
Messages: 51
Registered: August 2015
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Okay was out of town this weekend and finally am getting back to this.
5 days in a row now after letting the amp sit over night it works great for 5 mins.
I swapped 18 and 19 as you suggested and I'm still getting the same behavior. I discovered why I was getting the negative voltage reading. I had the two probes reversed when I hooked them to the speaker leads to test. So swapping 18 and 19 had no impact on the voltage being positive or negative. Just changing the leads made it positive.
I think Q18 and Q19 are good. I've taken them out prior to the problem and test them with my DMM and they test fine. They I've taken them out once the DC voltage appears on the output jack and they test the same. I would love for them to be the problem as they are easy to take in and out but I'm thinking it's some other component that decides to stop working 5 mins into being powered on.
[Updated on: Sun, 16 February 2020 23:04] Report message to a moderator |
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| Re: K150-6 pumps out DC voltage when powered on. [message #27668 is a reply to message #23707] |
Mon, 17 February 2020 12:38 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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Are you using a light bulb limiter with this amp?
One way to find a thermally sensitive component is to use a freeze spray type product. Use it to carefully spray individual components until you find the one that is acting up.
Alternatively you can start reading voltages on the different parts of the circuit when the amp is working and then is not working to see where voltages go wrong when it warms up. Again this requires you to take readings with the amp plugged in and turned on, so be careful.
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| Re: K150-6 pumps out DC voltage when powered on. [message #27678 is a reply to message #23707] |
Sat, 22 February 2020 06:28 |
stevem
Messages: 4774
Registered: June 2004
Location: NY
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Senior Member |
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27 volts D.C. On the output should be making for a dam loud hum / buzz coming out of the speaker, in fact that much voltage can take out the speaker(s) depending on what they are!
Try this simple swap that should take all of 5 minutes.
Swap the 2 big + and - power supply rail filter cans and see if your D.C. Voltage on the output jack changes over to a negittive 27 volts.
[Updated on: Sat, 22 February 2020 06:30] Report message to a moderator |
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| Re: K150-6 pumps out DC voltage when powered on. [message #27681 is a reply to message #23707] |
Sun, 23 February 2020 06:30 |
stevem
Messages: 4774
Registered: June 2004
Location: NY
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Senior Member |
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Well if swapping around the main filter cans does not produce any changes , and seing as your tryied out already swapoing the 2 output transistors , then the only other place I can see you getting a + 27 volts from is if transistor Q4 is the one goingi leaky after 5 or so minutes.
Q4 is one of the ones in the heat sink on that 5033 board.
Also you should rig up a away to hook up another speaker to the amp or get a big 25 watt resistor in series with your amps speaker to stop harm coming to it!
27 volts applied to that speaker is equal hitting it with over 90 watts and if it's a original CTS driver it's not made to handle that much power!
[Updated on: Sun, 23 February 2020 06:35] Report message to a moderator |
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| Re: K150-6 pumps out DC voltage when powered on. [message #27684 is a reply to message #23707] |
Tue, 25 February 2020 07:20 |
stevem
Messages: 4774
Registered: June 2004
Location: NY
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Senior Member |
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Forgive my oversite here, but you have a early model K150, and I have been looking at the schematic for a late K150 model with the 5033 board.
The early model 150s like yours still use the K100s +8 and -8 volt regulator sections and preamp section, so besides Q5 maybe going bad as the issue, something in that + 8 volt regulator circuit could be the bottom line on your issue maybe?
To narrow this down when the amp acts up check to see if your + 8 volts ( red wire to preamp boards) is still 8 volts when you have that 27 volts on the output.
By the way you do not need to keep a speaker load hooked up once the issue starts, this will save your ears and speakers!
[Updated on: Tue, 25 February 2020 07:22] Report message to a moderator |
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| Re: K150-6 pumps out DC voltage when powered on. [message #27693 is a reply to message #23707] |
Sun, 01 March 2020 01:03 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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The left power transistor is Q18 and the right one is Q19.
Those voltages are what you would expect with -27 volts on the speaker output, but they do not point to any cause for the circuit malfunction.
If you look at the schematic, there are voltages listed for a number of different points in the circuit. Do you think that you could follow the schematic enough to test some of those posted voltages?
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| Re: K150-6 pumps out DC voltage when powered on. [message #27696 is a reply to message #23707] |
Sun, 01 March 2020 16:15 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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There is a pictorial diagram of the board on the schematic sheet that shows the basic position of the transistors. This will help you to figure out what components are where.
The schematic has voltages marked at most of the important circuit points, usually at transistor connections. Any voltage will have a plus or minus sign in front of it and most but not all will include a V for volts.
Due to the case heatsinks, the two driver transistors Q5 and Q6 do not have leads that can be seen from the top of the board. The way to get these voltage readings, is to measure at points on the board where the transistors connect to other components. For example, the bases of Q5 and Q6 connect to the ends of the three diodes that form the bias setting circuit.
Be very careful with your meter leads while probing the circuit board. It's fairly easy to short two component leads together while trying to get a reading. If you have access to clip on meter leads, you could clip on the lead with the amp turned off and then turn on the amp to get the voltage reading.
My point is that you can cause additional damage to the circuit if you accidentally short something together while you are taking voltage readings. This is where you might want to look into using a light bulb limiter to help minimize the risk.
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