| adlaic |
I just bought an old amp that has 14 toshiba 2sa1943 and 14 toshiba 2sc5200 the amp plays but it has distortion at all volumes.
Any idea what could be wrong with it? It is a two channel amp has 7 2sa1943 and 7 2sc5200 for each channel it also has fault and clip lights on the amp. The fault and clip lights are not on. |
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| darkfenriz |
| quote: | | an old amp that has 14 toshiba 2sa1943 and 14 toshiba 2sc5200 |
These are rather modern transistors.
Are you sure they are originally in this amp? Or maybe omeone simply swapped output transistors with newer ones without readjusting idle current? |
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| adlaic |
I don't guess its that old of a amp the amp is a legion by madison model no LSA-2700 does anyone know anything about this amp. The louder you turn it up the less distorition you get. it is similer to this amp
http://www.legionsound.com/legion%20web/lsa2000.htm
any ideas |
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| adlaic |
| here is a picture of the amp |
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| adlaic |
| can a bad transister cause it to have distortion or would it not work at all if it had a bad transister. I bypassed the build in crossover and vol. knobs and it has the same distortion. So I know its not the crossover or the vol. knobs. could a bad resistor cause this? Both channels are doing the same thing. what should the rail voltage be? the rail voltage is around 130v is that to high or about right? |
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| adlaic |
| it has some ceramic resistors .22 ohm 5w and the ceramic is comming off of a couple of them. What would cause this? |
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| ilimzn |
| quote: | Originally posted by adlaic
I don't guess its that old of a amp the amp is a legion by madison model no LSA-2700 does anyone know anything about this amp. The louder you turn it up the less distorition you get. it is similer to this amp
http://www.legionsound.com/legion%20web/lsa2000.htm
any ideas |
You seem to write a lot more than you read! :)
It sounds like a missing bias problem. Do you know how to measure it? |
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| Elso Kwak |
| quote: | Originally posted by adlaic
it has some ceramic resistors .22 ohm 5w and the ceramic is comming off of a couple of them. What would cause this? |
The resistors could have been overheated causing them to open. Check with Ohm-meter. In that case I would not trust drivers and output transistors either.
:cool: |
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| adlaic |
| I'm sorry that I have reading on this site for over 24 hours and havent found anything that is helping me so I am asking questions for someone that might know how to help me. |
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| adlaic |
| I have checked the resistors and they check out ok |
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| Elso Kwak |
| quote: | Originally posted by adlaic
I'm sorry that I have reading on this site for over 24 hours and havent found anything that is helping me so I am asking questions for someone that might know how to help me. |
Huh?????
:confused: |
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| Netlist |
The amp has a high and a low Z input. Does it distort on both inputs? Both channels?
/Hugo |
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| cliff |
You have to start measuring the transistors and resistors using an ohm-meter.
If you don't know how to do that or have no test equipment it is almost impossible to help you. |
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| Eva |
| That amplifier (like most others) appears to have low voltage auxiliary rail supplies, probably +-15V. Since the fault appears in form of distortion equally in both channels, one of the first things that should be checked is whether these auxiliary supplies are working properly. The capacitor board appears to have a few smaller capacitors and heatsinks where +-15V regulators are probably located. |
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| adlaic |
| I have unsoldered the transistors off of one board when I check them with a ohm meter what reading should I get. On the NPN I am going from base to emitter and then base to collector. Same with the PNP but with the leads reversed. On the NPN I get readings and when I reverse the leads I get OL same with the PNP. But each transistor does not have the same reading. |
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| Eva |
| When testing the transistors this way (B-E and B-C), the multimeter should read between 500 and 700 in one direction and OL in the other. Also, testing between E-C should produce OL in both directions. A few milivolts of difference between devices is normal. By the way, when mounting the transistors back try to place the ones with similar B-E readings together in the same bank. |
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| Netlist |
If both channels behave the same, it's most likely a power supply problem like Eva said.
I'd still like to know if you are not overloading the inputs.
/Hugo |
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| adlaic |
| I was using a cd walkman for the input and it distorts as soon as you turn it up loud enough to hear it. I am getting ready to test the power supply and transformer to make sure it is ok. |
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| Netlist |
Before you further dismantle the amp, try a normal line level input.
Tuner, CD player, tape recorder, DAT...
I presume you loaded the amp's input with the walkman's headphone output.
/Hugo |
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| adlaic |
| I think I might have found the problem. On the PSU it has 41v ac in that is all working ok and the rail voltage is 132.9 + and - on both channels it has 15v ac + and - and that test ok. Plus it has 12v dc + and - when I test the 12dc - it test -12v but when I test the 12v + it is +21.2v must be somthing wrong with the PSU. |
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| Netlist |
Nice find. Keep us posted.
/Hugo |
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| adlaic |
| I fixed the 12v side of the amp and it is not as distorted, but it is still distorted. Could a bad cap cause it to distort? I tested all of the transistors on the board that I took apart and they all tested good. |
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| Elso Kwak |
| Now figure out which parts could be damaged from the over voltage..... |
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| adlaic |
| Does anyone know where I could find some Toshiba a1937 and c4793 transistors, and when I test them should I test them like I did for the 2sa1943 and 2sc5200? |
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| adlaic |
| does anyone know how I would go about testing the bias of the amp? |
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| bobo1on1 |
| Try measuring the voltage over the bases of the driving transistors to check the bias voltage or place a multimeter in series with the output transistors and power supply to measure the idle current. |
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| adlaic |
| I hooked up my volt meter to my speaker out and it read around 500mv. so i turned the pots till it was down to about 10mv and the sound cleared up after about 5min it blew a fuse. |
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| clem_o |
| quote: | Originally posted by adlaic
I hooked up my volt meter to my speaker out and it read around 500mv. so i turned the pots till it was down to about 10mv and the sound cleared up after about 5min it blew a fuse. |
I certainly hope you took note of the original pot settings... turn them back to where they were, try putting in a new fuse and keep your fingers crossed. If the new fuse blows the instant you power up the amp, you destroyed your output transistors the last time.
Dont go around turning potentiometers, not unless you know exactly what each pot is supposed to do (and how to set each properly). From the looks of it, you managed to over-bias the amplifier into thermal runaway.
See if you can locate a schematic or service manual first. |
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| adlaic |
| If I can't get a manual how would I know what to set the bias to? There are only two pots on each channel. are they both for the bias or one for the bias and one for something else? |
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| infinia |
The other pot is most likley output voltage offset adjust
(the offset adj should be located closer to signal input.)
(the bias adj pot should be located near center of pca or closer to the heatsink) |
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| adlaic |
how would you adj. the output voltage offset?
When the amp blew the fuse it also blew a big hole in one of the output transistors. |
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| infinia |
Turn the volume control to min and adjust the Voffset pot very slowly while monitoring Vdc the + speaker out for lowest reading usually around 10-30mV or lower if lucky.
Unfortunately you have to repair the scrapnel first. |
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