dodgy riddle - problems repairing an amp - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 20th July 2007, 08:48 PM   #1
diyAudio Member
 
Join Date: Feb 2007
Location: Germany
Question dodgy riddle - problems repairing an amp

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
Attached Images
File Type: gif schem.gif (16.3 KB, 281 views)
  Reply With Quote
Old 20th July 2007, 10:09 PM   #2
jaycee is offline jaycee  United Kingdom
diyAudio Member
 
Join Date: Apr 2004
Location: Norwich, UK
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
  Reply With Quote
Old 20th July 2007, 11:38 PM   #3
Nordic is offline Nordic  South Africa
diyAudio Member
 
Nordic's Avatar
 
Join Date: Sep 2005
http://www.toshiba.de/
  Reply With Quote
Old 20th July 2007, 11:56 PM   #4
diyAudio Member
 
Greg Erskine's Avatar
 
Join Date: Jan 2002
Location: Sydney/Australia
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
__________________
Greg Erskine
  Reply With Quote
Old 21st July 2007, 07:32 AM   #5
diyAudio Member
 
Join Date: Feb 2007
Location: Germany
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
  Reply With Quote
Old 21st July 2007, 09:55 AM   #6
diyAudio Member
 
Greg Erskine's Avatar
 
Join Date: Jan 2002
Location: Sydney/Australia
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
__________________
Greg Erskine
  Reply With Quote
Old 21st July 2007, 10:33 AM   #7
diyAudio Member
 
destroyer X's Avatar
 
Join Date: Feb 2004
Location: Recife - Brasil Northeast
Default Ahahaha.... Dx amplifier is now very famous...there are clones too!


Interesting.

Can you post images about this Dx Clone?

hehe

regards,

Carlos
__________________
DxS, the best from the best...the cream from the cream, the best wide world ever made audio amplifier.
  Reply With Quote
Old 21st July 2007, 04:56 PM   #8
diyAudio Member
 
Join Date: Feb 2007
Location: Germany
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
  Reply With Quote
Old 21st July 2007, 04:56 PM   #9
diyAudio Member
 
Join Date: Feb 2007
Location: Germany
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
Attached Images
File Type: jpg inside.jpg (98.3 KB, 139 views)
  Reply With Quote
Old 21st July 2007, 04:56 PM   #10
diyAudio Member
 
Join Date: Feb 2007
Location: Germany
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.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Troels gravesen PMS: riddle this for me matsuru Multi-Way 3 5th January 2008 01:45 PM
2.5-way impedance riddle Klimon Multi-Way 4 21st March 2006 10:33 PM
Dodgy amp needs repair sbattersby 115 Instruments and Amps 13 29th April 2005 09:33 AM
Shunt Power Regulator Riddle gwolf Solid State 4 2nd September 2003 09:51 PM
Aleph Mini riddle makes no sense to me... MBK Pass Labs 27 2nd September 2003 05:42 PM


New To Site? Need Help?

All times are GMT. The time now is 03:54 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright ©1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2