Hey.
I've been building tube guitar amps for a while, so when one of my students asked me if I could fix his solid state bass amp, I said I could try.
The amp is a line 6 lowdown 300pro. (I'm attaching the schematic).
The amp stays silent when I connect and try to play a bass.
If I connect an mp3 player to the mp3 input mini jack, I can get sound through the headphones, but still nothing from the regular inputs.
I seemed to get the right voltages from the psu board.
I looked over the preamp board (page 1 in the schematicl), and found a smd resistor that was fried (R82). I couldn't tell why it had fried.
I replaced it with a regular resistor of same value (5k1).
I also looked over the electrolytic caps, but couldn't find any that looked like they'd died.
I fired the amp up again, while starting to check various voltages, but after a little while, I felt something getting quite warm: the U2 voltage regulator providing 3,3V for the preamp logic and codec circuits (page 5 in the schematic).
It seem to output 3,3V just fine, so I presume the regulator itself is working alright?
But I'm a little uncertain about how best to pinpoint where the problem now is. The heatsink on the regulator gets too hot to touch after a while, so I presume something is pulling way too much current through.
Am I right?
Any help?
Regards
Anders
I've been building tube guitar amps for a while, so when one of my students asked me if I could fix his solid state bass amp, I said I could try.
The amp is a line 6 lowdown 300pro. (I'm attaching the schematic).
The amp stays silent when I connect and try to play a bass.
If I connect an mp3 player to the mp3 input mini jack, I can get sound through the headphones, but still nothing from the regular inputs.
I seemed to get the right voltages from the psu board.
I looked over the preamp board (page 1 in the schematicl), and found a smd resistor that was fried (R82). I couldn't tell why it had fried.
I replaced it with a regular resistor of same value (5k1).
I also looked over the electrolytic caps, but couldn't find any that looked like they'd died.
I fired the amp up again, while starting to check various voltages, but after a little while, I felt something getting quite warm: the U2 voltage regulator providing 3,3V for the preamp logic and codec circuits (page 5 in the schematic).
It seem to output 3,3V just fine, so I presume the regulator itself is working alright?
But I'm a little uncertain about how best to pinpoint where the problem now is. The heatsink on the regulator gets too hot to touch after a while, so I presume something is pulling way too much current through.
Am I right?
Any help?
Regards
Anders
On your schematic:
Look at headphones jack. See the contact on the sleeve that essentially grounds? Follow that line over to the left through pin 3 of the connector to see it is a control signal called HP_Sense.. Plugging in phones kills the main out, I;d wager. if that contact is dirty, your amp remains muted, check that.
I hope your power amps - all four - are OK, because they do so resemble the Mode four Marshall MF350. A double bridged arrangement. I refuse to work on those.
Any of your 3v chips getting hot?
Look at headphones jack. See the contact on the sleeve that essentially grounds? Follow that line over to the left through pin 3 of the connector to see it is a control signal called HP_Sense.. Plugging in phones kills the main out, I;d wager. if that contact is dirty, your amp remains muted, check that.
I hope your power amps - all four - are OK, because they do so resemble the Mode four Marshall MF350. A double bridged arrangement. I refuse to work on those.
Any of your 3v chips getting hot?
Thanks for your suggestion.
The contacts on the headphone jack are ok.
But there must be a problem with the preamp board, 'cause even when I use headphones, I don't get any signal through from the input jacks. (I put a jack cable in the input and touched on the tip of the other end - that should usually give some noises).
Yeah, that specific voltage regulator is the only chip that gets hot. (the other regulators stay cool).
The contacts on the headphone jack are ok.
But there must be a problem with the preamp board, 'cause even when I use headphones, I don't get any signal through from the input jacks. (I put a jack cable in the input and touched on the tip of the other end - that should usually give some noises).
Yeah, that specific voltage regulator is the only chip that gets hot. (the other regulators stay cool).
Thanks for your suggestions.
I checked the headphone jack. The contacts are clean and work.
Anyways, the problem must be elsewhere in the preamp, 'cause I don't get any sound through the headhphones, when I connect a bass to the input. (or even just a jack cable, with no instrument on the other end).
It seems that it's only the voltage regulator (u2)that gets hot. Even the other regulators stay cool.
I checked the headphone jack. The contacts are clean and work.
Anyways, the problem must be elsewhere in the preamp, 'cause I don't get any sound through the headhphones, when I connect a bass to the input. (or even just a jack cable, with no instrument on the other end).
It seems that it's only the voltage regulator (u2)that gets hot. Even the other regulators stay cool.
Did you find out the issue?
Hi, I have the exact same resistor fried. R82. Did you get to the bottom of why this happened to yours and did you find a fix to get the LD300 pro working again? Thanks in advance.
Hi, I have the exact same resistor fried. R82. Did you get to the bottom of why this happened to yours and did you find a fix to get the LD300 pro working again? Thanks in advance.
Hi,
If R82 burned it is getting loaded possible by the IC U14 or anyplace where the voltage +5VA it is feeding and it is in every place that you see the tag +5VA. You said that you replace the R82 smd resistor with a regular one. Does it get hot? If it get hot means something it loading it. You can do a test by removing the jumper R90 zero ohms resistor and see if still R82 get hot. If still get hot it is possible that U14 it is bad. If does not get hot then the loading it is coming from the left side components. I would check voltages at the Q6 and Q7 emitter pins.
If R82 burned it is getting loaded possible by the IC U14 or anyplace where the voltage +5VA it is feeding and it is in every place that you see the tag +5VA. You said that you replace the R82 smd resistor with a regular one. Does it get hot? If it get hot means something it loading it. You can do a test by removing the jumper R90 zero ohms resistor and see if still R82 get hot. If still get hot it is possible that U14 it is bad. If does not get hot then the loading it is coming from the left side components. I would check voltages at the Q6 and Q7 emitter pins.
Are you sure R82 is 5k1? Looks to me like 5R11 / 1% -- its the Vref for a bunch of +5V, aside from the 4528.
If it was > 5k there wouldn't be enough voltage in the area to kill it.
Also, electrolytics can and do fail open without spilling their guts.
Regards
If it was > 5k there wouldn't be enough voltage in the area to kill it.
Also, electrolytics can and do fail open without spilling their guts.
Regards
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And if you need a new AK4528, Digi Key has 2500 of the larger SO-28/the -VM suffix, for $6.08 each.
Best Luck
Best Luck
You have 4 things to check as one is shorted. U14, C94, C22 or C91. The 5R11 resistor is burned if one of the 4 is BAD.
Duke
Duke
Thanks for all the info everyone. Most helpful. I checked the four suggested by Audio1Man and I noticed tracing back from U14 via the vbias pin that C96 was shorted - would this have caused R82 to burnout? C96 also has damaged casing.
I am not the original poster of this thread but have the same resistor R82 burnout. I haven't replaced the part yet. The smd copper traces look scarred. I'm used to thru hole not smd but happy to have a go. Do you suggest I replace C96 and R82 if able to make good contact and then try powering on again? Thanks again.
I am not the original poster of this thread but have the same resistor R82 burnout. I haven't replaced the part yet. The smd copper traces look scarred. I'm used to thru hole not smd but happy to have a go. Do you suggest I replace C96 and R82 if able to make good contact and then try powering on again? Thanks again.
I am also experiencing the same issue as described above with R82. Did replacing the components rectify the problem?
Good news: I replaced the following components (R82, C22 & C91) and now have a working amp again!
