Sporadic loss of power VH100R
Moderators: pompeiisneaks, Colossal
Sporadic loss of power VH100R
Hello guys, it happens rarely but my amp looses power suddenly, it effects both channels and both (HI or LOW) inputs. It resolves within seconds by me pulling the guitar jack in and out of the amp few times. The jacks are PCB mounted so they were the first thing to check, but nothing wrong. All connections are firm, good contacts, re-soldered few spots just in case, even installed wire point to point on one place where I thought the post in PCB might be a little loose... the resistance between live and ground on a HI input is 1 MegaOhm on a Low Input is 440 KOhms. I have today completely removed wires of the Low input putting it out of use, since I do not use it at all anyway. So now it's a straight path from Hi Input to the 12ax7's Will see what happens in the future but I wanted to hear few more ideas here, could it be anything in the power section? Thanks..
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Stevem
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Re: Sporadic loss of power VH100R
You can step thru the amp and check for signal drop out even with a voltmeter set for ac volts.
If you do not have a signal gen you can at least confirm that the PI section and output stage is not dropping out by feeding in one leg of the heater string in to the PI grid.
With some 3.3 volts of input the 60 hz output volume will not be too loud, but enough to hear if the signal loss is happening in those two stages or not!
If you do not have a signal gen you can at least confirm that the PI section and output stage is not dropping out by feeding in one leg of the heater string in to the PI grid.
With some 3.3 volts of input the 60 hz output volume will not be too loud, but enough to hear if the signal loss is happening in those two stages or not!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Sporadic loss of power VH100R
Is there a large-ish capacitance on the input that you are now bypassing?
Sometimes a large high voltage cap, around 0.5 - 1 microfarad, is placed on the input to give a safety factor in case of major disaster (some valves will do strange things). If that is in place, it will build up charge before signal can pass.
No problem with that, but if it subsequently leaks after using the amp for a while, you have to build up charge on it again before it will pass signal. You can do that by the plugging, strumming, unplugging, re-plugging procedure you describe.
If you have a cap in the line, it is time to change it.
edit: have to change from my tablet to read pdf files.
Now I can see the circuit, there's only that 100pF in the normal input to consider. So if the fault disappears with that normal circuit disconnected, you know what to suspect. I suspect it is leaky.
Sometimes a large high voltage cap, around 0.5 - 1 microfarad, is placed on the input to give a safety factor in case of major disaster (some valves will do strange things). If that is in place, it will build up charge before signal can pass.
No problem with that, but if it subsequently leaks after using the amp for a while, you have to build up charge on it again before it will pass signal. You can do that by the plugging, strumming, unplugging, re-plugging procedure you describe.
If you have a cap in the line, it is time to change it.
edit: have to change from my tablet to read pdf files.
Now I can see the circuit, there's only that 100pF in the normal input to consider. So if the fault disappears with that normal circuit disconnected, you know what to suspect. I suspect it is leaky.
Re: Sporadic loss of power VH100R
Stevem: I do not have a signal generator, but I could follow AC signal path and check things, but again while the amp is working fine everything will look OK I would have to wait for it to fail completely in order to notice anything right? Oh yes, just to make shore and clarify things because I am not professional:
Measurements with AC voltmeter are to be taken AFTER the coupling caps in each gain stage in order to avoid contact with DC which stops at the cap. basically I can measure at the grid input at every stage, and on the cathode in the case of cathode follower stage.. right?
Dotfret:That C1 100pF is a really tiny cap printed on the board, I don't think I can replace that ?
Measurements with AC voltmeter are to be taken AFTER the coupling caps in each gain stage in order to avoid contact with DC which stops at the cap. basically I can measure at the grid input at every stage, and on the cathode in the case of cathode follower stage.. right?
Dotfret:That C1 100pF is a really tiny cap printed on the board, I don't think I can replace that ?
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Reeltarded
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Re: Sporadic loss of power VH100R
Do you have a way to plug a computer into your input? Make that happen and google 1k test tone. Run that tone and check the whole signal path until it fails. You will find the failure is in the circuit just prior to where the signal drops.
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Stevem
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Re: Sporadic loss of power VH100R
The amp may also be going into high frequency oscillation above what the speakers can reproduce.
To prove this out when the amp starts to act up have a voltmeter set for ac hook up across the speaker and see if you read any thing over 20 MV on the meter, if so that's a intermittent oscillation problem we can help you track down ,
To prove this out when the amp starts to act up have a voltmeter set for ac hook up across the speaker and see if you read any thing over 20 MV on the meter, if so that's a intermittent oscillation problem we can help you track down ,
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Sporadic loss of power VH100R
All right guys I will wait for it to start acting up and will let you know, so far so good..