EVH 5150 III 50W Help
Posted: Wed Apr 03, 2019 1:33 am
I purchased a EVH 5150 III 50W about a month ago from Guitar Center for a good price. When I showed up, it didn't have a fuse holder so I couldn't test it. They found one for me to use for a test but I didn't pay any attention to the value of the fuse I borrowed. Either way, the thing sounded good, functioned well and I was pleased as punch.
$499 later, I contacted EVH and they sent me the fuse cap. I ordered a foot switch and a bunch of 250mA fuses and jammed one in the fuse holder. Amp would power on, then die immediately upon flipping the standby switch. Shit.
I test out some basic voltages and find one of the voltage regulators for the DC heaters to be burnt out. I then replace it and those tubes come back to life.
Same situation as before though. Still popping fuses.
I discover that I am not getting any negative bias for the power tubes and decide to build a light bulb current limiter and then go through Rob Robinette's transformer testing for dummies section to find out if there are any shorts. The answer is no. No shorts in the power transformer. That's good. My thought process here was that perhaps a power tube shorted and took out the bias supply tap in the transformer.
But...same situation and I am still popping fuses.
Still not getting any negative bias voltage, I figure out where all of the Pico fuses are on the board and test them. All are good except for F5. I order it and swap it in. I am now getting SOME negative bias voltage. But my readings are all over the place and not where they should be.
Same situation. Still popping fuses.
Here are some pics of the board for any one's reference. https://www.flickr.com/photos/150885180 ... 4088330612
Here are my test voltages. Now, I had to take all of the voltages with the current limiter on. Limter has a 60w bulb in it. I used my TP51 (negative, rectified raw bias voltage) as the mathmatics to scale up the readings. It runs with no limiter as long as the stand by is off. I got -73.1 VDC off limiter and -46.2 VDC with it on. So I used those ratios to scale the test voltages up. This kind of seemed to work. I will do some googling to figure if that is a reliable tactic when working with a current limiter.
5150 III 50W Test Voltages by Paul Abbott, on Flickr
Here is the schematic (click on the image and you can zoom in better).
Fender-EVH-50W 1 by Paul Abbott, on Flickr
Fender-EVH-50W 2 by Paul Abbott, on Flickr
Fender-EVH-50W 3 by Paul Abbott, on Flickr
Fender-EVH-50W 4 by Paul Abbott, on Flickr
So there are some clear problems with voltages on this amp. My observations are that I am not getting enough negative bias voltage/current for the power tubes. I should be getting 60mvDC and I am getting 1/10th of that. My plate voltages seem about 100 volts off. Just a few of the preamp voltages appear correct...sigh.
I guess I need to figure out if I should be replacing every transistor in this thing as there are many. Resistors and caps are easy to replace too. I still haven't figured out how to get good capacitor readings unless I pull them from the circuit and even then...capacitors have always confused me.
Any ideas?
$499 later, I contacted EVH and they sent me the fuse cap. I ordered a foot switch and a bunch of 250mA fuses and jammed one in the fuse holder. Amp would power on, then die immediately upon flipping the standby switch. Shit.
I test out some basic voltages and find one of the voltage regulators for the DC heaters to be burnt out. I then replace it and those tubes come back to life.
Same situation as before though. Still popping fuses.
I discover that I am not getting any negative bias for the power tubes and decide to build a light bulb current limiter and then go through Rob Robinette's transformer testing for dummies section to find out if there are any shorts. The answer is no. No shorts in the power transformer. That's good. My thought process here was that perhaps a power tube shorted and took out the bias supply tap in the transformer.
But...same situation and I am still popping fuses.
Still not getting any negative bias voltage, I figure out where all of the Pico fuses are on the board and test them. All are good except for F5. I order it and swap it in. I am now getting SOME negative bias voltage. But my readings are all over the place and not where they should be.
Same situation. Still popping fuses.
Here are some pics of the board for any one's reference. https://www.flickr.com/photos/150885180 ... 4088330612
Here are my test voltages. Now, I had to take all of the voltages with the current limiter on. Limter has a 60w bulb in it. I used my TP51 (negative, rectified raw bias voltage) as the mathmatics to scale up the readings. It runs with no limiter as long as the stand by is off. I got -73.1 VDC off limiter and -46.2 VDC with it on. So I used those ratios to scale the test voltages up. This kind of seemed to work. I will do some googling to figure if that is a reliable tactic when working with a current limiter.
5150 III 50W Test Voltages by Paul Abbott, on Flickr
Here is the schematic (click on the image and you can zoom in better).
Fender-EVH-50W 1 by Paul Abbott, on Flickr
Fender-EVH-50W 2 by Paul Abbott, on Flickr
Fender-EVH-50W 3 by Paul Abbott, on Flickr
Fender-EVH-50W 4 by Paul Abbott, on Flickr
So there are some clear problems with voltages on this amp. My observations are that I am not getting enough negative bias voltage/current for the power tubes. I should be getting 60mvDC and I am getting 1/10th of that. My plate voltages seem about 100 volts off. Just a few of the preamp voltages appear correct...sigh.
I guess I need to figure out if I should be replacing every transistor in this thing as there are many. Resistors and caps are easy to replace too. I still haven't figured out how to get good capacitor readings unless I pull them from the circuit and even then...capacitors have always confused me.
Any ideas?