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meezger_seep 20th July 2007 08:48 PM

dodgy riddle - problems repairing an amp
 
2 Attachment(s)
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

jaycee 20th July 2007 10:09 PM

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

Nordic 20th July 2007 11:38 PM

http://www.toshiba.de/

Greg Erskine 20th July 2007 11:56 PM

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

meezger_seep 21st July 2007 07:32 AM

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

Greg Erskine 21st July 2007 09:55 AM

Quote:

Originally posted by meezger_seep
@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.

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

destroyer X 21st July 2007 10:33 AM

Ahahaha.... Dx amplifier is now very famous...there are clones too!
 

Interesting.

Can you post images about this Dx Clone?

hehe

regards,

Carlos

meezger_seep 21st July 2007 04:56 PM

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..:D
Having an 2SA1943/2SC5200 expert's input here would be great.

Thanks and best wishes

meezger_seep 21st July 2007 04:56 PM

Hello,

@Carlos:
I don't want to name the brand here, but I send you the details via mail.

Any help for my problems?
Quote:

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.

meezger_seep 21st July 2007 04:56 PM

2 Attachment(s)
Hello,

here is a picture of the inside. I replaced the original 4700uF/50V with bigger 63V types. As the idle rail voltage is almost 50V I don't want to risk the smell of a blown up elco in my room;)

Best wishes


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