Matching transistors & measuring the results
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 16th December 2016, 02:15 PM #11 anatech   diyAudio Moderator     Join Date: Jun 2004 Location: Georgetown, On Hi Scott, Yes. Very, very tedious if you have to match a bunch of them - which is what I have been putting off for a bit now. -Chris __________________ "Just because you can, doesn't mean you should" © my Wife
 16th December 2016, 02:35 PM #12 cogeniac   diyAudio Member   Join Date: Feb 2015 Location: Petaluma, CA I match them one way, and then for those matches closer than 10 mV I put them in the opposite sockets. This allows me to verify the close matches, and also to remove any residual offset in the jig. I probably posted this before, but here is the logic in that: The measurement in position A (transistor 1 in socket 1, and transistor 2 in socket 2) is: Collector voltage difference=DV12=VC1-VC2+Vo Where Vo is the inherent offset in the jig. The measurement in position B (transistor 1 in socket 2, and transistor 2 in socket 1) is: Collector voltage difference=DV21=VC2-VC1+Vo So, to get rid of the Vo term we simply subtract these two results: DV12-DV21=VC1-VC2+Vo-(VC2-VC1+Vo)=2VC1-2VC2+(Vo-Vo)=2(VC1-VC2) So, the true offset is: VC1-VC2=(DV12-DV21)/2 I only do this second test for pairs that are already close, since if they are not better than 10 mV I do not consider them a pair. I do this in spreadsheet that automatically color codes the cells that pass the first (10mV) test, and then it performs the computation above and color codes good pairs (Which I defined as better than 2mV true match). For a batch of 32 devices, this requires about 400-500 measurements..VERY tedious!!! Scott
 29th December 2016, 03:06 PM #14 anatech   diyAudio Moderator     Join Date: Jun 2004 Location: Georgetown, On Hi Cogeniac, I have never before measured the current draw of these boards, but your readings don't seem to be out of whack. No, these wouldn't affect the main rail voltages at all. It is possible to have damaged bridge rectifiers that could draw enough current to reduce the supply voltages. They could also have a larger series voltage drop. Also, look at the mains input circuit for resistive connections or components. The relay isn't above suspicion either, although I've never had to replace one. Somewhere you will find a simple problem that is difficult to find. Life can be like that. -Chris __________________ "Just because you can, doesn't mean you should" © my Wife
cogeniac
diyAudio Member

Join Date: Feb 2015
Location: Petaluma, CA
Quote:
 Originally Posted by anatech Somewhere you will find a simple problem that is difficult to find. Life can be like that. -Chris

Ahhh.. wise, wise words!!!

 29th December 2016, 03:34 PM #16 cogeniac   diyAudio Member   Join Date: Feb 2015 Location: Petaluma, CA Based on the manual description of the current sources, the drop across R153 should be slightly over 11 volts, which is about right, so I think the currents on the board are about right. Time to undertake the task of pulling out the bridge.. I'll bet it has developed a high internal resistance...
 31st December 2016, 02:06 PM #17 cogeniac   diyAudio Member   Join Date: Feb 2015 Location: Petaluma, CA I found two issues with the rail voltage/output signal level issue. First, I had jury rigged the power feeds to the board so I could move it out into a panavise on my bench. The connections were just twisted and taped. I soldered those and the rail droop went away. I also changed the 1SS178 diodes, and that solved the overall output level issue. I had changed these out for a higher Vf diode when troubleshooting the board, and forgot to swap them back. The higher Vf meant that Q110 and Q109 were saturating at a lower input signal level. As soon as I changed them the output swing went to nearly 80 volts! This is much higher than usual, and it is because I have D101 and D102 out of the circuit... So there is no active soft clipping in place. I'll try putting them back now that the board is rock solid. I am also anticipating measuring the trim pots in R144/145. Once the amp was stable, I set the currents on Q111/112 using these pots. They are interdependent because the diode stack doesn't have a hard ground between the positive and negative ends (i.e. between R131/132). So you have to go back and forth numerous times. Adjust the drop across R126 and then R126 (to 0.8 volts for 16 mA quiescent current.. which seems to be the proper bias point based on my analysis). If you set one to 0.8 volts then the other will be at, say 0.7, so you adjust that one to 0.8 and then the first one will be at 0.85 or so.. So you go back and forth Increasing one and decreasing the other until both are at 0.8 volts. I'll measure the pots today and see how far they are from each other, and from the original resistor specs (499 ohms)...Probably just put the 499 ohm devices back in...The Vishay trimmers are nice for troubleshooting though! The good news is that the replacement board layout seems to work fine (at least when it doesn't have solder splatters on the traces!) My next plan is to replace the boards in all of the 565s I have (using the new CMX SMT layout), pick the best three and probably sell the others. I'll provide the original board with each amp in case the new owners want to spend their time chasing electrolyte issues... :-D
 31st December 2016, 03:24 PM #18 anatech   diyAudio Moderator     Join Date: Jun 2004 Location: Georgetown, On Hi Cogeniac, Excellent news. You did a fair amount of troubleshooting and accepted your own issues, we call TIM (Technician Induced Malfunction). I sometimes use pots in circuit to troubleshoot. Mostly when developing a new circuit. Good plan with those new boards. You should compare the performance between the two designs. -Chris __________________ "Just because you can, doesn't mean you should" © my Wife
 31st December 2016, 04:10 PM #19 cogeniac   diyAudio Member   Join Date: Feb 2015 Location: Petaluma, CA I managed to pick up an older HP distortion analyzer, so, I'll now be able to also measure these amps beyond just output power!
 31st December 2016, 05:40 PM #20 anatech   diyAudio Moderator     Join Date: Jun 2004 Location: Georgetown, On Did you get a 331A, or a 334A? The others are 332A and .. wait for it ... 333A. The 333A and 334A are nice in that they have automatic nulling. It's a better THD meter than I had back in time. -Chris __________________ "Just because you can, doesn't mean you should" © my Wife

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