Hello,
maybe someone can help me. I got a bass amplifier cheap from a store; new but broken. According to the salesman it died when he first turned it on. As I am experienced in tube amps and guitar stompboxes I thought this should be easy to repair for me. But it looks like this is much more complicated then expected.
I contacted the manufacturer and got a very poor (almost impossible to decipher) copy of the schematic and no information about the transformer. With this and the original pcb I puzzled the attached document.
The original schematic says R68 and R70 should be 680, but on the pcb are 150 Ohm.
When I opened it R69, R81, R73 and R73 were burnt through. R69 was splattered as smut all over. I desoldered all transistors and Q6, Q7, Q12 and Q15 had a base to collector failure (either conducting in both directions or not conducting at all). C35 had a short.
After replacing the above mentioned parts, R78 and R82 get very hot within seconds after truning power on, so there must be still a failure in the circuit.
My main problem is that I am not sure what the psu voltage should be. This is not quoted in the schematic. I measured 2x 33 V AC without load and +-48 V DC without power amp stage. As the highest rating for capacitors is 50 V, it looks to my like the Chinese supplied this one with the wrong transformer and this cause the failure.
The amp's specs say 100W at 8 Ohm.
1. Can anyone guess what the VCC/VSS voltage should probably be.
2. Can anyone give me a hint what is the best way to make an analytic failure search? Are there some stages which can be run and tested stand-alone. I only have a multimeter, an oscilloscope and limited know-how.
Many thanks in advance for any help.
Best wishes
maybe someone can help me. I got a bass amplifier cheap from a store; new but broken. According to the salesman it died when he first turned it on. As I am experienced in tube amps and guitar stompboxes I thought this should be easy to repair for me. But it looks like this is much more complicated then expected.
I contacted the manufacturer and got a very poor (almost impossible to decipher) copy of the schematic and no information about the transformer. With this and the original pcb I puzzled the attached document.
The original schematic says R68 and R70 should be 680, but on the pcb are 150 Ohm.
When I opened it R69, R81, R73 and R73 were burnt through. R69 was splattered as smut all over. I desoldered all transistors and Q6, Q7, Q12 and Q15 had a base to collector failure (either conducting in both directions or not conducting at all). C35 had a short.
After replacing the above mentioned parts, R78 and R82 get very hot within seconds after truning power on, so there must be still a failure in the circuit.
My main problem is that I am not sure what the psu voltage should be. This is not quoted in the schematic. I measured 2x 33 V AC without load and +-48 V DC without power amp stage. As the highest rating for capacitors is 50 V, it looks to my like the Chinese supplied this one with the wrong transformer and this cause the failure.
The amp's specs say 100W at 8 Ohm.
1. Can anyone guess what the VCC/VSS voltage should probably be.
2. Can anyone give me a hint what is the best way to make an analytic failure search? Are there some stages which can be run and tested stand-alone. I only have a multimeter, an oscilloscope and limited know-how.
Many thanks in advance for any help.
Best wishes
Attachments
If it is Chinese, you can bet that those 2SA1943/2SC5200 are fakes, and that's what blew the circuit.
I would also say that 48V rails with only 1 pair of outputs is suicide. I wouldnt go over 42V, and even then only into 8 ohms.
You will need to test every single transistor. You will probably find that the 2SA1943/2SC5200 are blown (as I mentioned, they will be fakes), and the TIP41C/TIP42C also.
One of R63/R64 should really be a trimpot to adjust bias with. also Q6 should be on the main heatsink for bias stability.
To be honest, I would ditch it and maybe keep the useful parts such as transformer, heatsink etc. For bass use, you will want something with more power handling. Perhaps take a look at some of Rod Elliot's projects at http://sound.westhost.com
I would also say that 48V rails with only 1 pair of outputs is suicide. I wouldnt go over 42V, and even then only into 8 ohms.
You will need to test every single transistor. You will probably find that the 2SA1943/2SC5200 are blown (as I mentioned, they will be fakes), and the TIP41C/TIP42C also.
One of R63/R64 should really be a trimpot to adjust bias with. also Q6 should be on the main heatsink for bias stability.
To be honest, I would ditch it and maybe keep the useful parts such as transformer, heatsink etc. For bass use, you will want something with more power handling. Perhaps take a look at some of Rod Elliot's projects at http://sound.westhost.com
hi meezger_seep,
This schematic is very, very similar to the DX Amplifier that you will find in one of the popular threads. You can compare component values.
This schematic has extra protection circuitry that is "supposed" to protect it when using rail voltage over 35 volts, that obviously didn't work. R68 and R70 are part of the protection circuitry.
Jaycee's comments are probably right.
I you have a lower voltage transformer available, I would turn it into a DX Amplifier, after checking the transistors are not fakes and are OK.
regards
This schematic is very, very similar to the DX Amplifier that you will find in one of the popular threads. You can compare component values.
This schematic has extra protection circuitry that is "supposed" to protect it when using rail voltage over 35 volts, that obviously didn't work. R68 and R70 are part of the protection circuitry.
Jaycee's comments are probably right.
I you have a lower voltage transformer available, I would turn it into a DX Amplifier, after checking the transistors are not fakes and are OK.
regards
Hello,
@jaycee:
As I wrote, I tested all transistors and 2SA1943/2SC5200 and 2 2SD600 (Q6, Q7) were blown and are now replaced with genuine Japanese😉
Q6 is on the heatsink. I will replace R64 with a trimpot.
This normally a rather expensive (400 US$) bass amp combo from a US manufacturer with a good reputation designed in USA. I want to repair it in any case. 100W is fine for me. This is only a practise or small gig amp. I already have a Fender Bassman 135.
@Greg Erskine
I want to keep the original PCB, as the preamp is very complex and there is almost no space for another power stage. I also found no other transformer to fit in the case.
Comparing with the DX may help, but 2500 posts to read –eek-
Do you think there is a chance to reduce the rail voltage by using e.g. a voltage stabilizer? I don't mind if it has 100W or 50W.
@Nordic
I am not designing this thing. I just want to find the assembling error which was put in.
Best wishes
@jaycee:
As I wrote, I tested all transistors and 2SA1943/2SC5200 and 2 2SD600 (Q6, Q7) were blown and are now replaced with genuine Japanese😉
Q6 is on the heatsink. I will replace R64 with a trimpot.
This normally a rather expensive (400 US$) bass amp combo from a US manufacturer with a good reputation designed in USA. I want to repair it in any case. 100W is fine for me. This is only a practise or small gig amp. I already have a Fender Bassman 135.
@Greg Erskine
I want to keep the original PCB, as the preamp is very complex and there is almost no space for another power stage. I also found no other transformer to fit in the case.
Comparing with the DX may help, but 2500 posts to read –eek-
Do you think there is a chance to reduce the rail voltage by using e.g. a voltage stabilizer? I don't mind if it has 100W or 50W.
@Nordic
I am not designing this thing. I just want to find the assembling error which was put in.
Best wishes
meezger_seep said:
Hi meezger_seep,
I meant to keep the original PCB, just use the DX Amplifier for a guide. Reducing the output of a transformer simply, is be a PITA. A regulator can do it but it requires more work and maybe an PCB.
There is a DX Amplifier website that has the basic information. I'll ask Carlos to have a look at this thread.
regards
Ahahaha.... Dx amplifier is now very famous...there are clones too!
Interesting.
Can you post images about this Dx Clone?
hehe
regards,
Carlos
Interesting.
Can you post images about this Dx Clone?
hehe
regards,
Carlos
Hi,
@Greg:
I found the webpage and the information where to find which voltage in the circuit is very useful. Thats why I want to switch to the same rail voltage.
PITA? I am not good in english abbrevations. My dictionary say it stands for pain in the a..😀
Having an 2SA1943/2SC5200 expert's input here would be great.
Thanks and best wishes
@Greg:
I found the webpage and the information where to find which voltage in the circuit is very useful. Thats why I want to switch to the same rail voltage.
PITA? I am not good in english abbrevations. My dictionary say it stands for pain in the a..😀
Having an 2SA1943/2SC5200 expert's input here would be great.
Thanks and best wishes
Hello,
@Carlos:
I don't want to name the brand here, but I send you the details via mail.
Any help for my problems?
@Carlos:
I don't want to name the brand here, but I send you the details via mail.
Any help for my problems?
BTW I don't think it's clone. I think it's more kind of conicidence. The original schmatic is dated 2001. And ww.waybackmachine.org showed me this amp is on the their webpage since 2002.
Remove transistors, one by one and test them outside the board.
And measure every resistance that seems burned, replace the ones have errors, obtained during measurements, bigger than 20 percent.
Lift one side of diodes and test them into the board.
This is the most fast method to fix an amplifier...the time people will lose trying to track errors by voltages is bigger than the time you spent to remove, to test and to put them back again into the board.
I am telling that:
two guys starting the same time, same equipment, same damage:
The one that will remove transistors, and test them out of the circuit, will be 3 times faster than the other.
Also, someone not experienced, cannot "detect" defects easier watching voltages...but someone not skilled can discover burned transistors measuring them.
I have made tests about, during my work time in Motorolla.... the beginer, novice guy into repairs, were faster then skilled folks.... one analising circuitry without dessolder and other removing parts.. because of that were developed VHF radios using plug in sub circuits to allow fast corrective maintenance.
But the one that remove parts will also never learn how to debug amplifiers...a matter of option, and what is more important to each life moment.
Dx voltages, the standard Dx amplifier voltage chart, will not help you too much, as you are using different supply voltage...also the voltage measurements is the analitic method... depends on you if skilled or not.
If you did not accept my suggestion about the transistors removal, parts removal, and testings.... tell me your supply voltage and i will prepare a chart of voltages to you...or a diagram with some voltages on it.
Repairs, fast service, corrective maintenance, is more adequated to "changeneers" (replacement parts engineer...non engineer... clever folks trained to be fast)... circuit analisis is to engineers... repairs is a waste of time to creative engineers... unless very well paid to do that.
If my life will depend of how fast, and good can be someone, to fix the Cardiac electro mechanical pump (heart aid machine)... i will prefere my life in the hands of the changeeneer.
To create those machines.... of course i would prefer the Engineer.
"Changeeneer is a replacement technical man...skilled to replacement of parts and skilled to measurements and adjustment...the name to do not exist in Dictionaire... creation of Brazilian Broadcasting Personnel"
They were called also "Operational Engineers" for some years... a 3 years University...a "half" EE degree they had...now not accepted as full EE anymore.
regards,
Carlos
And measure every resistance that seems burned, replace the ones have errors, obtained during measurements, bigger than 20 percent.
Lift one side of diodes and test them into the board.
This is the most fast method to fix an amplifier...the time people will lose trying to track errors by voltages is bigger than the time you spent to remove, to test and to put them back again into the board.
I am telling that:
two guys starting the same time, same equipment, same damage:
The one that will remove transistors, and test them out of the circuit, will be 3 times faster than the other.
Also, someone not experienced, cannot "detect" defects easier watching voltages...but someone not skilled can discover burned transistors measuring them.
I have made tests about, during my work time in Motorolla.... the beginer, novice guy into repairs, were faster then skilled folks.... one analising circuitry without dessolder and other removing parts.. because of that were developed VHF radios using plug in sub circuits to allow fast corrective maintenance.
But the one that remove parts will also never learn how to debug amplifiers...a matter of option, and what is more important to each life moment.
Dx voltages, the standard Dx amplifier voltage chart, will not help you too much, as you are using different supply voltage...also the voltage measurements is the analitic method... depends on you if skilled or not.
If you did not accept my suggestion about the transistors removal, parts removal, and testings.... tell me your supply voltage and i will prepare a chart of voltages to you...or a diagram with some voltages on it.
Repairs, fast service, corrective maintenance, is more adequated to "changeneers" (replacement parts engineer...non engineer... clever folks trained to be fast)... circuit analisis is to engineers... repairs is a waste of time to creative engineers... unless very well paid to do that.
If my life will depend of how fast, and good can be someone, to fix the Cardiac electro mechanical pump (heart aid machine)... i will prefere my life in the hands of the changeeneer.
To create those machines.... of course i would prefer the Engineer.
"Changeeneer is a replacement technical man...skilled to replacement of parts and skilled to measurements and adjustment...the name to do not exist in Dictionaire... creation of Brazilian Broadcasting Personnel"
They were called also "Operational Engineers" for some years... a 3 years University...a "half" EE degree they had...now not accepted as full EE anymore.
regards,
Carlos
Hello,
@Carlos
I am happy beeing "changeeneer"😉
I already removed all active parts (diodes and transitors) checked them and replaced the blown ones plus the resitors with "hot spots".
So I will also remove and check all resitors and capacitors
Do you think it is possible that my amp is equipped with a wrong transformer? I still wonder that someone uses only 50V caps if the idle rail is 49 volt.
Best wishes
@Carlos
I am happy beeing "changeeneer"😉
I already removed all active parts (diodes and transitors) checked them and replaced the blown ones plus the resitors with "hot spots".
So I will also remove and check all resitors and capacitors
Do you think it is possible that my amp is equipped with a wrong transformer? I still wonder that someone uses only 50V caps if the idle rail is 49 volt.
Best wishes
It is very common to use those condensers
Because those ones specified as 50 Volts....they do not explode if you exceed that voltage...sometimes they work, continuously into 75 volts!
They are just giving us the guarantee of 50 Volts when you buy the unit....not limited, in the reality they can hold more.
This voltage turns the output in risk... even using 8 ohms the maximum dissipation may be exceded during normal audition levels.
Output may be under risks... i am not sure, as i had not time, and care, to read all your thread in detail and with double readings...as i am not good to read in english and to understand first time.
regards,
Carlos
Because those ones specified as 50 Volts....they do not explode if you exceed that voltage...sometimes they work, continuously into 75 volts!
They are just giving us the guarantee of 50 Volts when you buy the unit....not limited, in the reality they can hold more.
This voltage turns the output in risk... even using 8 ohms the maximum dissipation may be exceded during normal audition levels.
Output may be under risks... i am not sure, as i had not time, and care, to read all your thread in detail and with double readings...as i am not good to read in english and to understand first time.
regards,
Carlos
Hello,
after changing Q10/Q11 and R69/R73 and replacing R64 with a trimpot it's working gain🙂
Thanks to all that helped for their support!!!
The only minor problem that's left is that my bias is still too high even if I turn the R63 trimpot to 0. With the 10 Ohm test resistors a suggested on the Dx page I still have 1200/1300 mV.
Does a higher rail voltage also mean a higher bias current or should I increase R62?
Best wishes
after changing Q10/Q11 and R69/R73 and replacing R64 with a trimpot it's working gain🙂
Thanks to all that helped for their support!!!
The only minor problem that's left is that my bias is still too high even if I turn the R63 trimpot to 0. With the 10 Ohm test resistors a suggested on the Dx page I still have 1200/1300 mV.
Does a higher rail voltage also mean a higher bias current or should I increase R62?
Best wishes
Do not increase R2...... keep it the same value
Increase the trimpot value you have installed.
Adjust your amplifier to low current.... 50 miliamps each rail, to avoid the protection be near to start to work.
The protection will colect voltage produced into the emitter resistances...this voltage depends from the current is flowing into the emitters....too much current means too much voltage there....because of that the protective transistors may enter in operation too much soon.
regards,
Carlos
Increase the trimpot value you have installed.
Adjust your amplifier to low current.... 50 miliamps each rail, to avoid the protection be near to start to work.
The protection will colect voltage produced into the emitter resistances...this voltage depends from the current is flowing into the emitters....too much current means too much voltage there....because of that the protective transistors may enter in operation too much soon.
regards,
Carlos
Don't increase R62! that will do the opposite, and increase the bias.
Try increasing R63. 330 or 470 ohms might work well here with a 1K trimpot being used.
I would measure bias by measuring the voltage across one of the emitter resistors, R82 for example. I'd go for about 10mV across this resistor, that would be a bias current of 45mA approx.
Try increasing R63. 330 or 470 ohms might work well here with a 1K trimpot being used.
I would measure bias by measuring the voltage across one of the emitter resistors, R82 for example. I'd go for about 10mV across this resistor, that would be a bias current of 45mA approx.
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