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DIY Workshop » β22: A discrete, cascoded, fully complementary, pure class A amplifier (part 3) | wnmnkh Member Joined: Apr. 20, 2008 |  Message [#240] posted on: 01-19-2009 10:12 PM CST (US).   |  |  ....I don't find anything wrong with transistors either. OK, since I really can't accurately measure the value of resistors, I will exchange them first, then measure again when powered on, then change MOSFETs if problem still occurs. Is there other possible problems other than MOSFETs or resistors? |
amb  Headphone Council Joined: Apr. 1, 2004 | Message [#241] posted on: 01-19-2009 10:20 PM CST (US). | | wnmnkh, you are measuring the voltage across R34 and R35 with no load connected, right? Since these two resistors are in series, the same current flows through them when there is no load (except there is also a path through the global feedback resistor R4, but that's high enough in resistance not to make any difference), then if the resistances of R34 and R35 are equal, you should measure the same voltage across them. |
| wnmnkh Member Joined: Apr. 20, 2008 | Message [#242] posted on: 01-19-2009 10:25 PM CST (US). | | I found better DMM.... Measured again.... One of them are about 0.4~0.43, while other is about 0.2ish. I guess 0.2 is the one that burned.... *desoldering |
| wnmnkh Member Joined: Apr. 20, 2008 | Message [#243] posted on: 01-19-2009 10:42 PM CST (US). | | Nop, does not solve the problem. hmmm, Measuring the MOSFET again. |
| wnmnkh Member Joined: Apr. 20, 2008 | Message [#244] posted on: 01-19-2009 10:51 PM CST (US). | | ....Pair 21 and 22 are 10 volts. Pair 23 and 24 are 27 volts. They should be all same but I got different numbers from each pair. Which one is broken? |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#245] posted on: 01-20-2009 12:01 AM CST (US). | | Q21/Q22 and Q23/Q24 do not have the same voltages. See the schematic diagram to try to understand why. These MOSFETs are not in parallel. Q21 should have around 8V to 9V on its D pin relative to ground, Q22 should be -8V to -9V. Q23 D pin is connected to V+ so it should be whatever the supply voltage is. Similar to Q24 and V-. If you got +10V and -10V at Q21 and Q22's D pins, respectively, then your D9 and D10 zener voltages are probably a bit high. But this is not a problem. Aside from the mysterious unequal voltage across R34 and R35, nothing you reported so far indicates a problem, yet. Is the output DC offset ok? Are there any other abnormal symptoms? What led you to check R34 and R35 to begin with? |
| wnmnkh Member Joined: Apr. 20, 2008 | Message [#246] posted on: 01-20-2009 07:36 AM CST (US). | | Ok, here are other symptoms.... 1.) DC offset is not stable IMO. Once I set the offset by adjusting VM3, after several turn-on and off, it is off again, but not much (about 8~9mV). Even in extreme case it does not go over more than 10mV. 2.) But the real problem is that the volume of the left side (the board having problem) is just too loud, unable to adjust with volume pot. I just woke up and turn on the amp, see if I get same symptoms... [Edited by wnmnkh on 01-20-2009 at 07:37 AM.] |
| wnmnkh Member Joined: Apr. 20, 2008 | Message [#247] posted on: 01-20-2009 08:22 AM CST (US). | | Yeah, DC offset swings. It goes 0 to about 7~8mV and goes back to 0 (oscillating?) |
| Ava12 Member Joined: Jan. 19, 2009 | Message [#248] posted on: 01-20-2009 01:20 PM CST (US). | | Hi all! I've got a tiny little problem, I'm not able to solve on my own. I've built my S22 (works just fine, aprox 29,6V) and two B22 boards, now these work just fine too, but the two last boards won't work. On the first board I've got 266,4mV across R9, 4,575V across R10, 2,295V across R11 and 2,402V across R12. On the second board I've got 0mV across R9, 5,226V across R10, 4,480V across R11 and 0mV across R12. I checked the diodes, these seem to work, I don't know what to do. Could anyone of you could think of a reason to solve this. Any help would be much appreciated Greetz Ava |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#249] posted on: 01-20-2009 02:52 PM CST (US). | | wnmnkh, volume control problems like what you describe should be a problem with the pot/attenuator wiring, not the amp board. Sounds like the ground connection on the pot (for the offending channel) had lost contact. Bad or broken solder joint? Also, you should check the solder joints of R4 to make sure that the global feedback loop didn't go open circuit (which would cause a large increase in gain). As for varying DC offset, it normally shouldn't drift by more than a couple of mVs. Larger drift might indicate that one or more of the input JFETs Q1-Q4 is marginal. Ava12, check the R34 and R35 resistors to make sure they are still good, and soldered well, and check the output DC offset. Make sure you didn't forget to install the R30/R31 jumper wires. |
| wnmnkh Member Joined: Apr. 20, 2008 | Message [#250] posted on: 01-20-2009 03:40 PM CST (US). | | No! that attenuator cost me 300 bucks!  I am out of house now.... let me check whether wiring, attenuator or R4 is making the problem later. Well, and you are indicating that Q1~Q4 need to be replaced.... *sigh* But the real question I want to ask you is, can these possible problems cause the big difference in R34 and R35? |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#251] posted on: 01-20-2009 04:31 PM CST (US). | |
These shouldn't... but one other thing to check is the voltages across the D9 and D10 zener diodes. They should both be very close to 12V. |
| mrmajestic2 Member Joined: Jun. 5, 2007 | Message [#252] posted on: 01-20-2009 05:01 PM CST (US). | | Did anyone try the Hypex TR180 transformer for the B22? Its shielded, potted and pretty easy to get hold of here in Europe. Might do nicely in a single box B22. |
| wnmnkh Member Joined: Apr. 20, 2008 | Message [#253] posted on: 01-20-2009 06:38 PM CST (US). | | uh oh.... D9 is whooping 13.86V! D10 is about 12.57v So.....D9 needs replacement or? I am still checking the pot and its wiring. I really do not want to replace 400 bucks worth component...(omg cost has increased!) |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#254] posted on: 01-21-2009 12:57 AM CST (US). | | D9 is out of spec (tolerance of BZX55C12 is 11.4V to 12.7V), but not far enough to cause problems in that location. So you don't need to replace it. As for the volume pot, I doubt that it had become defective. More likely you simply have a bad solder joint or a broken wire. |
| Ava12 Member Joined: Jan. 19, 2009 | Message [#255] posted on: 01-21-2009 04:07 AM CST (US). | | Thank you for your quick response. R34 and R35 both show me between 0,4 and 0,5 Ohms, so they're probably alright. Wire jumpers are also alright. The DC offset isn't however. It is a worrying 3,296V. And whatever I do to VR1. VR2 or VR3 it stays the same. Any guess on that? Please.  [Edited by Ava12 on 01-21-2009 at 04:08 AM.] |
| masantos HeadWizer Joined: Dec. 13, 2005 | Message [#256] posted on: 01-21-2009 06:34 AM CST (US). | | jazzist any updates on the backplane you designed? |
| wnmnkh Member Joined: Apr. 20, 2008 | Message [#257] posted on: 01-21-2009 09:20 AM CST (US). | | I really did not find anything wrong with connection. And it seems the volume is fine now. But, really.... what about R34 and R35 problem? Shall I reassemble the whole board again? Or just leave as it is? |
| amb Headphone Council Joined: Apr. 1, 2004 | Message [#258] posted on: 01-21-2009 01:17 PM CST (US). | | Ava12, examine the problem boards carefully to see if you might have missed a solder joint somewhere, cold joint or solder bridge. Also do some voltage measurements and compare to the troubleshooting PDFs found at the β22 wensite's Initial check page. Since you have good, working boards to compare againt, do that too. Does any part get hot? wnmnkh, so what is the voltage across your R34 and R35 now? |
| wnmnkh Member Joined: Apr. 20, 2008 | Message [#259] posted on: 01-21-2009 02:59 PM CST (US). | | I readjusted the VM3, so now R34 is about 24mv and R35 is about 60mv. |
| Ava12 Member Joined: Jan. 19, 2009 | Message [#260] posted on: 01-21-2009 03:20 PM CST (US). | | Ok messured some voltages and first things first, the heatsinks of the working boards get about 40-50°C warm whereas the heatsinks of the other boards don't get warm at all. so hear some things that stick out, can you something out of this, I can't  messured should be q2-r6: 268,5mv ? 309.5mV q2-q6: 11,55v okish 11.48V q1-q5: 11,67v ok 11.48V q5-q6: 12,18v ok 12.15V q5-r9: 28,21v ! 24.34V q1-r5: 495,8mv ! 309.6mV r9-cr1: 28,54V ok 28.85V d1-r6: 126,4mV ! 206.2mV d2-r8: 212,7mV ! 264.0mV r14-r16:23,66V ! 25.27V d5-q13: 21,64V ! 13.33V d6-q14: 11,49V ! 13.33V Any thoughts on this? Please |
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