A
alexsm11
New member
Hi Everybody,
Kinda new in this forum.
This is a follow-up for this topic:
viewtopic.php?f=59&t=53538
Well, apparently I’ve found the solution to the problem that at least one more person (dgaemc2) and I have experienced.
The problem was this: When the amp is switched to standby, the footswitch and power tubes turn on, but none of the amp’s front panel indicators light up, neither the preamp tubes. When the amp is turned on, either in 20W or 40W mode, the power tubes turn blue, as expected but no sound at all out of the speaker.
So, after checking fuses, power and preamp tubes and everything that Line6 online support recommended, the amp was still broken. It was not cost-effective to me send my amp for repairs due to shipping costs, besides at the nearest L6 service center couldn’t give me even an estimate for the repair.
After all this, and considering that I have some experience with electronics, I ventured in trying to identify the problem myself. I’ll try to cover every detail of the process.
After I had extracted the amp from the cabinet, I tried to measure some voltages using my multimeter, but the test leads of the multimeter were too thick, so, I sharpened them so they can get into the connectors without disconnect them. I show this in the pics below:
With the amp in 40w mode, the first reading I took was the voltage at the 12ax7 plates, which was solid at 400vdc.
The power supply for the preamp section basically consists of two different voltage levels: +-3.5 vdc and +-18 vdc. So, the next readings I took were those (in standby mode). The +-18vdc level was ok, BUT instead of having +-3.5vdc, the voltage was fluctuating at 1.4-1.6 vdc. And this +-3.5 vdc power supply turned out to be feeding the heater of the preamp tubes!!! So, THERE WAS A PROBLEM.
After this I cut the cables of the +-3.5 vdc power supply and took readings again, just to make sure that the power supply was the problem, and yes, the readings didn't vary.
So I disconnected and extracted the power supply section and drew a diagram of the +-3.5 vdc power supply, that turned out to be a very simple 6.3 vac to +-3.5 vdc power supply rectified with a 8A bridge rectifier and filtered with a huge 68000uF capacitor, and with a couple of resistors connected to ground. The capacitor appeared to be fine, so I suspected that the problem was in the bridge rectifier. I desoldered it and tried to find a replacement in one local shop. The closest thing I found was a 6A bridge rectifier, that was the replacement, but first I replaced the 8A slow blow fuse with a 6A slow blow fuse just to make sure that there was no more than 6 amperes feeding the new rectifier. I show this in the pic below.
After I assembled again all the boards together, I turned on the amp in standby mode and guess what…. I WAS RIGHT. The rectifier was the problem, and the 6A fuse didn’t blow up, but, the new bridge rectifier heated up a little as the preamp tubes were heating up.
Just to be sure, I visited a couple more of the local electronics supply shops and I found an 8A rated bridge rectifier very close to the original one, just with another reverse voltage rating, that shouldn’t make a difference. (In my country is somehow tricky to find some components, I am from Mexico). I tested the amp playing for 20 minutes or so at 40w mode, also I let the amp in standby mode for 2hr or so and seems to be working just fine
Below I show a pic with some pink arrows and some letters corresponding as follows:
A) In this red cable you should be reading +3.5 vdc
B) In this white cable you should be reading -3.5 vdc
C) In this white cable you should be reading +18 vdc
D) In this white cable you should be reading -18 vdc
All this readings are in standby mode and shouldn't change too much in 20w or 40w mode. Also, in the upper-right corner you will find the location of bridge rectifier KBU8J (or similar code).
The new bridge rectifier I used (and the damaged one
The power supply section whith no bridge rectifier:
I hope this documented experience may help someone who has a similar problem, since there seems to be impossible to find any service manual or similar for this amp.
Cheers.
Alex Sotelo.
Kinda new in this forum.
This is a follow-up for this topic:
viewtopic.php?f=59&t=53538
Well, apparently I’ve found the solution to the problem that at least one more person (dgaemc2) and I have experienced.
The problem was this: When the amp is switched to standby, the footswitch and power tubes turn on, but none of the amp’s front panel indicators light up, neither the preamp tubes. When the amp is turned on, either in 20W or 40W mode, the power tubes turn blue, as expected but no sound at all out of the speaker.
So, after checking fuses, power and preamp tubes and everything that Line6 online support recommended, the amp was still broken. It was not cost-effective to me send my amp for repairs due to shipping costs, besides at the nearest L6 service center couldn’t give me even an estimate for the repair.
After all this, and considering that I have some experience with electronics, I ventured in trying to identify the problem myself. I’ll try to cover every detail of the process.
After I had extracted the amp from the cabinet, I tried to measure some voltages using my multimeter, but the test leads of the multimeter were too thick, so, I sharpened them so they can get into the connectors without disconnect them. I show this in the pics below:
With the amp in 40w mode, the first reading I took was the voltage at the 12ax7 plates, which was solid at 400vdc.
The power supply for the preamp section basically consists of two different voltage levels: +-3.5 vdc and +-18 vdc. So, the next readings I took were those (in standby mode). The +-18vdc level was ok, BUT instead of having +-3.5vdc, the voltage was fluctuating at 1.4-1.6 vdc. And this +-3.5 vdc power supply turned out to be feeding the heater of the preamp tubes!!! So, THERE WAS A PROBLEM.
After this I cut the cables of the +-3.5 vdc power supply and took readings again, just to make sure that the power supply was the problem, and yes, the readings didn't vary.
So I disconnected and extracted the power supply section and drew a diagram of the +-3.5 vdc power supply, that turned out to be a very simple 6.3 vac to +-3.5 vdc power supply rectified with a 8A bridge rectifier and filtered with a huge 68000uF capacitor, and with a couple of resistors connected to ground. The capacitor appeared to be fine, so I suspected that the problem was in the bridge rectifier. I desoldered it and tried to find a replacement in one local shop. The closest thing I found was a 6A bridge rectifier, that was the replacement, but first I replaced the 8A slow blow fuse with a 6A slow blow fuse just to make sure that there was no more than 6 amperes feeding the new rectifier. I show this in the pic below.
After I assembled again all the boards together, I turned on the amp in standby mode and guess what…. I WAS RIGHT. The rectifier was the problem, and the 6A fuse didn’t blow up, but, the new bridge rectifier heated up a little as the preamp tubes were heating up.
Just to be sure, I visited a couple more of the local electronics supply shops and I found an 8A rated bridge rectifier very close to the original one, just with another reverse voltage rating, that shouldn’t make a difference. (In my country is somehow tricky to find some components, I am from Mexico). I tested the amp playing for 20 minutes or so at 40w mode, also I let the amp in standby mode for 2hr or so and seems to be working just fine
Below I show a pic with some pink arrows and some letters corresponding as follows:
A) In this red cable you should be reading +3.5 vdc
B) In this white cable you should be reading -3.5 vdc
C) In this white cable you should be reading +18 vdc
D) In this white cable you should be reading -18 vdc
All this readings are in standby mode and shouldn't change too much in 20w or 40w mode. Also, in the upper-right corner you will find the location of bridge rectifier KBU8J (or similar code).
The new bridge rectifier I used (and the damaged one
The power supply section whith no bridge rectifier:
I hope this documented experience may help someone who has a similar problem, since there seems to be impossible to find any service manual or similar for this amp.
Cheers.
Alex Sotelo.
