| K200B power amp distortion [message #21090] |
Sat, 09 August 2014 23:58  |
RobbieNuke
Messages: 66
Registered: October 2006
Location: Baltimore
|
Member |
|
|
My K200B PC703 power amp is distorting and I also have -17 VDC on the speaker output as well as the collectors of Q1 thru Q4 and the neg side of C704.
Pre-amps are not the issue; pristine output at monitor jack (using sine wave signal generator as source and looking at waveform with oscilloscope). Everything seems good up to Q700.
All DC supplies are in spec (+/- 40VDC and +/- 8VDC) as are DC readings of Q710 thru Q718.
Odd reading of 7.5VAC at thermostat (reading 115VAC on known good amp). All DC readings of of Q701 thru Q709 are whacked.
Any suggestions as to most likely culprit(s)?
|
|
|
|
| Re: K200B power amp distortion [message #21091 is a reply to message #21090] |
Sun, 10 August 2014 02:13  |
chicagobill
Messages: 2006
Registered: April 2003
|
Senior Member |
|
|
With dc on the speaker output leave the speaker disconnected until you get it straightend out.
How are you measuring the ac on the thermoswitch? If there was a problem there, you wouldn't have the correct dc voltages on the power amp board.
As for the dc on the output, there's no one simple thing to check. You should check all of the transistors in the circuit. I don't know what whacked voltages are, but compare what you have with the schematic values and see if there are any clues in why they are whacked.
|
|
|
|
|
|
| Re: K200B power amp distortion [message #21093 is a reply to message #21092] |
Sun, 10 August 2014 08:56 |
RobbieNuke
Messages: 66
Registered: October 2006
Location: Baltimore
|
Member |
|
|
Using a DMM for both DC & AC voltage check. Same results regardless of polarity switch position.
I have two K200-B model amps: Good=working=older=1969 / Bad=non-working=newer=1970
Whacked means readings are far off of values listed on schematic. Unit set-up per engineering notes (i.e. no input/no load/all controls full on)
NOTE: both amps have original AC cords and the neutral (white) lead is wired the same way in both amps... however I did a continuity check and discovered the wires are molded 'correctly' at the plug (with the clip oriented as if a ground pin) in the older (1969) working amp and reversed in the newer (1970) non-working amp! I do not think this has anything to do with my problem, just an interesting observation. Also; AC cord is 18AWG in the older unit, 16AWG in the newer unit. All internal wiring in newer amp is of a heavier gauge as well.
Black lead on the ground side of the 'death cap' I read 7.5VAC on either lead of the thermostat on the bad PCB. On my known working board I measure 115VAC.
On the bad unit if I disconnect both leads of the thermostat I measure 115VAC on the lead coming from the front panel switch/fuse area and 7.5VAC on the lead from the transformer.
On my known good unit I measure 115VAC on the lead coming from the front panel switch/fuse area and <0.5VAC on the lead from the transformer.
Black lead on the ground side of the 'death cap' I read 17.5VDC at the speaker out of the non working amp. When speaker was attached there was no hum at all! Sine wave input did not come through until Volume knob was about 1/4 way up and the signal was very distorted sounding.
Scope tracing at speaker output only showed lower half of sine wave and it was very asymetrical looking. As stated earlier sine wave looks great up until after Q700.
[Updated on: Sun, 10 August 2014 09:53] Report message to a moderator |
|
|
|
| Re: K200B power amp distortion [message #21099 is a reply to message #21093] |
Sun, 10 August 2014 14:46 |
chicagobill
Messages: 2006
Registered: April 2003
|
Senior Member |
|
|
The thermoswitch is in series with the power transformer and will normally be closed circuit. When the amp overheats the switch opens up and turns off the amp until the heatsink cools off.
If you pull the wires off the switch and measure the ac voltage to the chassis, you will get differing voltages due to the position of the on/off switch and the condition of the power supply primary components. But, as it is in series with the power transformer primary you should expect to have the line voltage on one side and no voltage on the other side. If you are getting 7.5ac volts on the neutral side of the circuit, then that is leakage coming from the hot side somewhere.
If the top half of the waveform is missing from the power amp signal, then either there is a transistor not turning on or or it could be the bias diode string. Just like a few of the other threads that are going here, bad transistors, off value resistors and bad caps can all cause the problem that you are trying to solve.
|
|
|
|
| Re: K200B power amp distortion [message #21101 is a reply to message #21099] |
Sun, 10 August 2014 16:10 |
RobbieNuke
Messages: 66
Registered: October 2006
Location: Baltimore
|
Member |
|
|
A daunting task indeed! At least it's not intermittent.
What is best way to access/service the PC703 board?
Since it won't be 100% original anymore, should I cut wires and incorporate Molex plugs for easier service in future? Since this will be a gigging amp I may consider reworking the power supply with increased filter cap values, etc.
[Updated on: Mon, 11 August 2014 00:18] Report message to a moderator |
|
|
|
| Re: K200B power amp distortion [message #21122 is a reply to message #21101] |
Mon, 11 August 2014 12:55 |
chicagobill
Messages: 2006
Registered: April 2003
|
Senior Member |
|
|
I start by testing everything in circuit without removing the board until I have to. The less you disrupt the better.
Certain things will not test correctly in circuit and will have to be removed to be tested correctly. The first thing to watch out for is the bias diode that is clipped to the heatsink. The leads of this diode are fragile and should be handled with care. In fact I pull it out of the clip and leave it floating until all of the repairs are made to the board, so as to not stress the leads any more than necessary.
The output transistors have sockets for the base and emitter pins, and the collector leads are removed by taking off one nut. Then you only need to loosen the mounting screws that hold the board to the back panel.
I would not bother to install any plugs to the wires, they will only add to long term reliability problems. And you really won't need to remove the board that often, so just a little care in handling the board and you should have no problems.
Upgrading the power supply is a good thing and can help improve the overall performance of the amp. I would suggest that you leave those modifications out until you have fixed the original problem, so as to not confuse the repair process.
|
|
|
|
VintageKustom.com
Search
Help
Members
Register
Login
Home
]