Repair [message #23916] |
Fri, 25 September 2015 06:42 ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
stevem
Messages: 4763 Registered: June 2004 Location: NY
|
Senior Member |
|
|
The output stage will see no load without the / a speaker hooked up.
I wonder if the amp is oscillating at full tilt power when the issue takes place, that would / could make the light glow brite and also block the amp from passing input signal!
Osscilation can take place above the frequency that the speaker you have the amp hooked up thru can reproduce and make for the ntermitant issue you have.
One way to prove this out is to keep your voltmeter that is set for AC volts hooked up across the speaker, when the brite bulb issue takes place if you read any voltage on that meter than the amp is indeed osscilating and we need to help you pin down where that is originating from in the amp.
[Updated on: Fri, 25 September 2015 06:43] Report message to a moderator
|
|
|
|
|
Re: Repair [message #23920 is a reply to message #23916] |
Fri, 25 September 2015 12:52 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
|
Senior Member |
|
|
Oscillation could be the cause, but it could be any number things.
Without the speaker connected and therefore no load on the power amp there is little current draw through the circuit. It doesn't matter if the cable is plugged into the amp or not as long as the speakers are not loading the amp.
The basic principle of any push pull circuit like the power amp is balance. The top and bottom half of the circuit move evenly together in sync. As long as that balance is kept steady, then everything works the way it should and we have a happy amp. But when the circuit becomes unstable and gets out of balance, one half of the circuit takes over and forces the other side into submission and the circuit draws too much current causing failure of transistors, resistors, etc.
You haven't mentioned seeing any burned components on the board and I don't know if the board has been repaired before or not, so we need to run a few tests to see if we can figure this out.
The first one is the loose, cold solder, broken lead test that I asked you to do in the earlier posts. Because this problem is intermittent, this is a good possibility.
The next step will be to read the dc supply voltages and the voltage if any at the speaker terminal when it is in failure mode. This could help us to find out which circuit is failing.
|
|
|
Re: Repair [message #23922 is a reply to message #23920] |
Fri, 25 September 2015 13:47 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
OldSchool1
Messages: 35 Registered: September 2015 Location: East Coast US
|
Member |
|
|
i will investigate for loose/broken/cold solder leads tonight and measure voltage at speaker terminal.
i'm confused however as to why the amp will work/play/sound when first powered on (guitar and speaker not plugged in) and then i plug in the speaker cable and guitar. i understand powering up without the speaker plugged in draws little current but wouldn't it fail (not work no sound and the limiter bulb stay bright) as soon as i start to play because now it is drawing power as evidenced by the bulb going bright then dimming when chords are hit? that's the part i don't get, if the speaker is plugged in before i turn on the amp it fails, however if i plug in the speaker after amp powers up it will work all night as long as i don't turn off and back on. and then like i'm resetting it it will work again if i unplug the speaker, power up, then plug speaker cable back in. interesting brain teaser.........
[Updated on: Fri, 25 September 2015 14:05] Report message to a moderator
|
|
|
Re: Repair [message #23923 is a reply to message #23916] |
Fri, 25 September 2015 14:17 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
|
Senior Member |
|
|
You'd think it was that simple wouldn't you. When the amp first powers up a number of things start to happen. First the two power supplies start to rise up to their rated plus and minus 40 volts. While this happens the slowly rising voltages are applied to the power amp circuit.
As the power amp circuit starts to wake up, each of its' transistors and capacitors start to see slowly rising voltage levels as the two power supplies get up to full voltage.
As I described before, the two halves of the power amp circuit are trying to keep themselves balanced against each other until the voltages all stabilize. If there is any difference between the plus and minus voltages as they rise up, it could cause one half of the power amp to power up before the other half. This startup imbalance is usually what causes the familiar turn on thump that we hear when turning on the amp.
With no load and little current draw during power up the two halves don't get a chance to get locked into that imbalance while the voltages get up to normal level. But with the speaker load connected one part of the circuit could draw so much current during startup that the other side gets shut down so hard that it can't pull itself up enough to rebalance the circuit.
Some amps don't like being limited with a light bulb and will never startup while using one. I know from personal experience that all basic Kustom amps will work with them.
Your amp was blowing fuses before we started testing. So far there has been no smoking gun discovery so the problem is still in there. We just need to find it.
|
|
|
|
|
Re: Repair [message #23928 is a reply to message #23916] |
Fri, 25 September 2015 22:34 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
|
Senior Member |
|
|
Anything is possible. If it is working then you may be right.
Have you checked the speaker cable for shorted ends or cuts in the wire?
|
|
|
|
|
|
|