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Messages - oleskool
« on: August 10, 2012, 01:21:10 AM »
Thanks. I was hopeful that these measurements were more reasonable. There is a spot on the circuit board that is fried. It is a very short run. The connection between the collectors of Q8 and Q9. I used a piece of wire to bridge that gap. The first measurements when the supplies were 1v, and more important stuck + were taken after I removed Q8 and Q9. When I removed them I also removed the wire that bridged the gap. Another thing I learned along the way, if something changes and doesn't make sense, look at the work you did. Look at the schematic and imagine the entire line that connects the collectors of Q8 and Q9 erased. Well that is what my board looks like. I have placed a piece of wire there to bridge the gap, and then took new measurements. I will try to put up a few pictures. My camera is eating batteries.
« on: August 09, 2012, 04:29:49 PM »
OK I hope i didn't mess up, but earlier in this post there was a spot I needed to jump from collector to collector at Q8 and Q9. Well when I pulled Q8 and Q9 I also pulled that jumper. I replaced the jumper Q8,Q9,Q12, and Q13 are still out. Across D12= -51.3v. -51.3v. Black lead connects at end of diode with the band for both. At first these were jumping up and down. Now they are fairly stable with these two measurements. I was talking to a guy he said the zener D5 is most likely bad. He also advised to change all the transistors. He also noted which does sound correct, that if J5 was bad it would cut the speaker out. But he is getting said parts.
I'm getting 20mv at the speaker. C34=51v and C35= -51v.
« on: August 08, 2012, 05:48:21 PM »
Well the first test C34=1V, C35=-1V, speaker=1V.
Pulled Q8, Q9, Q12, Q13 They all test good with a multi meter.
Plugged back in switched on, very dim bulb for about two seconds then it fades out.
D12= -.384V, D13= 104.7V
I guess I should test Q5 & Q11?
« on: July 21, 2012, 11:20:09 AM »
Thanks Roly!!! Will do.
« on: July 21, 2012, 12:04:21 AM »
Oh well the line fuse went as soon as I hit the power. will get another tomorrow to see if it happens again.
« on: July 20, 2012, 11:22:06 PM »
Hello again guys. thank you all for assisting me.
I did make all of the measurements you all instructed me to. They were all as you all expected. +/- 51v, -54v, +15v,-15v and all of the other measurements followed suit. pin 1 on U5 did however measure 13v.
To update I decided to look at the board again. One of the first things I learned was a good visual is the first thing you do. Well I found a spot on the board that is not etched. it appears to be some kind of foil. It may even have been a thin line of solder. Anyhow it had a small amount of corrosion on it. I brushed at it with a small brush, and it was not even connected to the board anymore. I took a bad pic.
It is a straight line from the collector's of Q8 & Q9. I am about to just jump a piece of wire from the solder connections, and see if that could be my problem. I will post the results. That line of foil is only 1/4" from the heat sink.
« on: July 18, 2012, 08:44:00 AM »
Yes. I understand very well what those wires are. That is why I asked this question. I don't know what would be wrong about checking them the way i explained though? Perhaps you would care to elaborate. I think rail voltage means voltage that is present ...well it could be anywhere in a circuit. It was stated check the power supply rails. I took that to mean voltage in voltage out. Could someone please explain what was meant by that statement? If I am asking for too much I apologize. That is how I thought this fourm worked. I am here to learn.
I am also taking photographs if this works out I am hoping that it will be a help to someone else who may want to learn.
« on: July 17, 2012, 11:39:58 PM »
Ok from the power supply to the power switch two black wires and, two white wires plug into the switch. One green wire is screwed to chassis ground. If I understand correctly you are saying check each of the four wires that plug into the switch ( 2 black, 2 white ) black probe on volt meter to chassis ground red probe to each of the four wires?
« on: July 17, 2012, 09:44:13 PM »
Enzo I am a absolute beginner when it comes to amps. You will have to give me detailed instructions
I really don't want to mess this up. If that is asking to much if you can point me to something that has a detailed photo and explanation as to what I am doing and why that would help also. Thank you for taking the time to even answer at all. Maybe if i ask enough questions i can one day help others.
I guess I am trying to say I don't understand how to check the power supply rails.
« on: July 17, 2012, 10:47:39 AM »
Thanks. So with with preamp looped back into amp in I get the same intermittent bumps I hear in the headphones. Of coarse they are much louder. Bass plugged into amp in the samething.
« on: July 17, 2012, 12:45:57 AM »
Thanks , but just so I know I am understanding correctly. Take a good instrument cable place one end in pre out. The other end right around to amp in. Or are you saying to run the pre out to a different amp?
No sound from the speaker at all. Not even a hiss. I did look good and it doesn't appear that the speaker is getting a straight dc signal if that is what you mean full forward or backward. It is 1am here so i had better check in the morning.
« on: July 16, 2012, 09:55:44 PM »
Ok this guy had this sitting around and needed it gone. I purchased it from him. It is a combo amp the speaker is good. When powered on I hear one bump from the speaker, plucking strings it doesn't say a word. I plugged in headphones plucking the E or A string I can hear very faintly that there is something there. I can't find a schematic on the net. I really need help. I am doing this as a confidence builder. What does the faint output from the headphones imply, if anything? I have the head out of the cab and a visual turned up nothing. Where should I start with this?
« on: July 15, 2012, 12:14:02 AM »
Yes Roly I understand that it is best to start small. I'm starting even smaller than suggested. I am working on small DC projects on a breadboard just like school. Truth be told the field I worked in required high mechanical skills because, tax accountants, lawyers, schools, and it seems everyone else wants their copy machine working when you leave that day. I didn't do a lot of electronic repair because I was in the field. I would find the problem board, replace it, and have the clients machine up and running in 15 to 20 minutes. I would then take that board to a bench tech to repair at component level. Usually you work your way up from bench tech to field tech. I worked hard in school and when i hired in they must have thought i didn't need to go that route. It was was a blessing because, i made more money, it was a curse because, it made me look at things different. I became complacent doing it the easy way. I also feel i can just jump over the smaller things. The good news is i understand those weaknesses.
Teemuk see that is what causes me problems. If you would not have said class g/h amplifiers i would have stayed on course. I just had to find something about them. I am learning how to design a transformer-coupled class a amplifier ( to make maximum 25% efficiency better) from a book.
To make it worse i just acquired a $40 Hartke b-90 combo amp with no sound out.
« on: July 13, 2012, 09:02:48 PM »
Thanks for the answers guys. I don't know why I ask these questions that end up making me start down a path with many more questions. I came here about to try a simple chipamp. Now i'm about to try a 100w amp. I think i better stop reading and start building.
« on: July 12, 2012, 01:31:17 AM »
Thanks for taking the time to answer. I was really trying to ask what makes an amplifier class a or ab? I can recall something about when current flows. I can't pull up those old brain connections though.