K200b-6 [message #21343] |
Thu, 04 September 2014 14:01 |
Daytopper
Messages: 7 Registered: September 2014 Location: Wake Forest
|
Junior Member |
|
|
Hi, I'm a newbie to this Forum and look forward to being a part of what seems to be a pretty good one!
I received a K200 bass amp for repair and since its been a very long time since i've worked on Solid State amps, I'm somewhat stuck on this repair. The original complaint is noise and popping which I have not yet observed. During my initial testing I've noticed that the negative side of the output is clipping way before the plus side. I've done all the preliminary troubleshooting and all voltages both the high rails and the =- 8volts are on the money. I've tried to swap the 2 pairs of output transistors and clipped the diodes in the protection circuits and checked multiple resistor values to no avail. Am I looking at driver transistor replacement? Any ideas would be greatly appreciated.
Thanks
Kenny
|
|
|
Re: K200b-6 [message #21345 is a reply to message #21343] |
Thu, 04 September 2014 14:35 |
chicagobill
Messages: 2006 Registered: April 2003
|
Senior Member |
|
|
Welcome to the place. While not rare, a model 6 is fairly unusual.
If you know that one half of the signal is reduced, I will assume that you have a scope to look at the waveforms in the power amp. A couple of weeks back we were discussing a very similar problem that turned out to be the protection circuit turning on too early on one side of the amp.
Tell us what has been done to this one in the past. Are the drivers and outputs original? I assume that both the +40 and -40 supplies are solid and clean.
If you haven't already done so, look up the schematics for your amp in the technical section, that way we can discuss with part numbers, etc. First check would be voltages on the power amp board.
Again, welcome.
|
|
|
|
Re: K200b-6 [message #21347 is a reply to message #21343] |
Thu, 04 September 2014 16:58 |
stevem
Messages: 4772 Registered: June 2004 Location: NY
|
Senior Member |
|
|
How does R725 and R728 test at the base of Q706and Q707 respectively ? In fact how do all the 1% resistors test out?
Be sure to test out the 5 tantalum electrolytic caps in the output stage, since you have check the 8 volt regulators those can be passed over!
Noise and popping could be just a good old bad solder connection, have you pounded on that pc703 board yet, gave a good dance to all the components ?
NOTE that when you do voltage checks by the schematic that they are taken with no signal, no load and all controls full up!
[Updated on: Thu, 04 September 2014 17:05] Report message to a moderator
|
|
|
Re: K200b-6 [message #21352 is a reply to message #21347] |
Thu, 04 September 2014 20:49 |
Daytopper
Messages: 7 Registered: September 2014 Location: Wake Forest
|
Junior Member |
|
|
Bill
Right on! Found voltages off on Q703 found a bad Tantalum (c707) dead short. I owe you a beer! Just gotta pick it up and I'll let you all know how I made out. oh almost forgot, r719 did look a little funny like it was drawing a lot of current!
Thanks
PS Boy am I glad I found this Forum.
[Updated on: Thu, 04 September 2014 20:52] Report message to a moderator
|
|
|
|
|
Re: K200b-6 [message #21376 is a reply to message #21343] |
Sat, 06 September 2014 02:06 |
chicagobill
Messages: 2006 Registered: April 2003
|
Senior Member |
|
|
steve probably missed your question. My list shows that a 36735 transistor is an RCA 40408, which is crossed to an NTE190.
Q703 is a voltage amp which boosts the signal from the input transistor and sends it on to the drivers.
|
|
|
|
|
|
|
Re: K200b-6 [message #21383 is a reply to message #21343] |
Sat, 06 September 2014 12:30 |
chicagobill
Messages: 2006 Registered: April 2003
|
Senior Member |
|
|
And the newer NTE16005 has a TO-39 case like the original. I'll have to look up an old NTE book to see what case the NTE190 had in the past.
NTE transistors are guaranteed to meet the electrical specs of the original transistors. Sometimes the cases and pinouts don't match the original ones. If the case has to fit a specific physical specification, then it won't work.
|
|
|
|
|
Re: K200b-6 [message #21391 is a reply to message #21343] |
Sun, 07 September 2014 06:55 |
stevem
Messages: 4772 Registered: June 2004 Location: NY
|
Senior Member |
|
|
Yeah, if it's good keep it in there!
I love helping folks out here on the board , all I ask in return is the winning power ball numbers! Lol!
|
|
|