| Author |
Message |
mdc
Joined: 30 Apr 2013
Posts: 50
|
| Posted: Thu May 02, 2013 5:05 pm Post subject: |
|
|
Is it still worth doing that despite the fact that all of the main transistors for that channel (Darlingtons + Bias) are currently missing?
Since they're surface mount, avoiding any screwing about with them at all would be nice  I was considering disconnecting R117SL to decouple that channel from the protection circuit, if that would be a better option?
|
|
| Back to top |
|
 |
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Thu May 02, 2013 9:32 pm Post subject: |
|
|
| mdc wrote: | Is it still worth doing that despite the fact that all of the main transistors for that channel (Darlingtons + Bias) are currently missing?
|
I guess that would be a fair question if you were looking at the schematic you have. However, I have the correct schematic which includes the placement of those diodes which is why I made the suggestion I did. So I won't scold you for questioning me just yet, LOL.
There are many ways to attempt to find where your problem is coming from and rather then start removing misc parts why not just shut down the protect circuit now at properly test for the problem, rather then guess.
Grounding pin 4 with a 100 ohm resistor of connector CP555 should shut down the protect system.
Then use your DMM on DC and test your outputs at the 101 inductors or right on the TP's if you know which pad is the output on each channel and see which ones have any voltage on.
Also note, there are a couple ICP's on that board that should be checked.
|
|
| Back to top |
|
|
mdc
Joined: 30 Apr 2013
Posts: 50
|
| Posted: Thu May 02, 2013 10:58 pm Post subject: |
|
|
Slight problem... grounding the Protect pin had no effect; unit still shuts down during power up :-\
In addition to this, I noticed a potentially "missing" component on the CNT board; C551M, just above CN555.
It doesn't look like there was ever anything there tbh; there's no remnants, no legs, no scuff or wear marks, but there are solder blobs on the back of the PCB as if it WAS connected. Very strange :-\
|
|
| Back to top |
|
|
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Thu May 02, 2013 11:13 pm Post subject: |
|
|
|
Remove that ground and use your DMM on DC and test that pin and see what it does on power up and after going into protect.
|
|
| Back to top |
|
|
mdc
Joined: 30 Apr 2013
Posts: 50
|
| Posted: Thu May 02, 2013 11:30 pm Post subject: |
|
|
|
When I press standby, it drops to -0.5V immediately then settles back to 0, then jumps to 0.5V as soon as protect engages, then settles back down to 0.
|
|
| Back to top |
|
|
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Thu May 02, 2013 11:48 pm Post subject: |
|
|
|
Try removing D135 and see what it does.
|
|
| Back to top |
|
|
mdc
Joined: 30 Apr 2013
Posts: 50
|
| Posted: Fri May 03, 2013 1:09 am Post subject: |
|
|
Success! You genius 
Does that mean we've got a problem on the CNT board, or is the issue still localised to the main PCB?
|
|
| Back to top |
|
|
mdc
Joined: 30 Apr 2013
Posts: 50
|
| Posted: Fri May 03, 2013 1:16 am Post subject: |
|
|
|
In addition, just measured all the remaining Darlington pairs and I'm getting +/- 1V between the base and emitter of each transistor.
|
|
| Back to top |
|
|
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Fri May 03, 2013 1:21 am Post subject: |
|
|
No, it just means that the protect circuit grounds the line to put it into protect mode. Normal running would be high. If you check that pin 4 now it will probably read around 5 volts.
Problem is most likely on that amp board and you have to find it. Now you can try checking for any DC on those outputs that I mentioned a couple posts ago. Check each channel at the inductor or at the TP's but if you check at the TP's you have to know which pad is the output one.
|
|
| Back to top |
|
|
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Fri May 03, 2013 1:23 am Post subject: |
|
|
| mdc wrote: | | In addition, just measured all the remaining Darlington pairs and I'm getting +/- 1V between the base and emitter of each transistor. |
My last post was before I read this. Did you also check the 2 rear surrounds on the separate board?
|
|
| Back to top |
|
|
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Fri May 03, 2013 1:27 am Post subject: |
|
|
|
Oh, and also, check for DC on that SL channel anyways, even though you removed those outputs. Check at the inductor, L101SL.
|
|
| Back to top |
|
|
mdc
Joined: 30 Apr 2013
Posts: 50
|
| Posted: Fri May 03, 2013 1:37 am Post subject: |
|
|
Yeah I checked the mini-PCB on the heatsink as well, same results as the main board.
Checking at the Inductor gives me around 20V from chassis ground, compared to 40V from chassis ground on the other inductors.
|
|
| Back to top |
|
|
mdc
Joined: 30 Apr 2013
Posts: 50
|
| Posted: Fri May 03, 2013 2:50 am Post subject: |
|
|
|
Gah, 7915 just went pop so I've ordered a replacement. Guess I'll be putting this to one side until it arrives, hopefully ALL the parts will have arrived by then and I can just replace everything in one go... Bloody frustrating but I'll be more than happy once its up and running
|
|
| Back to top |
|
|
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Fri May 03, 2013 3:37 pm Post subject: |
|
|
|
So whats your plan. Install all the new parts and see if it works?
|
|
| Back to top |
|
|
mdc
Joined: 30 Apr 2013
Posts: 50
|
| Posted: Fri May 03, 2013 4:48 pm Post subject: |
|
|
|
Well ideally I'd like to decouple the SL channel, replace C182 and R184 and see if the rest of the amp is fine first, just to rule out any other damage.
|
|
| Back to top |
|
|
mdc
Joined: 30 Apr 2013
Posts: 50
|
| Posted: Sun May 05, 2013 6:47 am Post subject: |
|
|
|
Replaced everything and on power-up, C502SL and R504SL on the preamp board went poof (with associated smoking and burning smell) Reckon those were damaged in the initial blowing of the SL channel, or failed as a result of my tinkering?
|
|
| Back to top |
|
|
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Sun May 05, 2013 4:02 pm Post subject: |
|
|
Let me know when your done burning up parts and want to get back to testing and finding a solution. 
|
|
| Back to top |
|
|
mdc
Joined: 30 Apr 2013
Posts: 50
|
| Posted: Sun May 05, 2013 7:58 pm Post subject: |
|
|
Lmao cheeky
I'm open to suggestions, just worried the next time I turn the damn thing on to get the DMM involved, something else will go up in smoke.
|
|
| Back to top |
|
|
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Sun May 05, 2013 8:45 pm Post subject: |
|
|
|
First off, what parts did you install? I need to know where you are at now.
|
|
| Back to top |
|
|
mdc
Joined: 30 Apr 2013
Posts: 50
|
| Posted: Mon May 06, 2013 3:28 am Post subject: |
|
|
Replaced everything that was missing/had blown.
Specifically, Q101SL, Q102SL, Q103SL, R110SL, R111SL, C182 and R184.
Also replaced all 3 voltage regulators (7815, 7915 and 7805), and reconnected D135 for the protection circuit. Powered up, and the two components on the preamp board started smoking. Caught the smell first and thought it might've been a poor solder joint on my repair job, but then I noticed the smoke and saw the damaged components on that board.
To be brutally honest, I MAY have missed the damage the first time around, as I just gave the preamp board a cursory glance and the cap had blown from the bottom rather than the top. Is it possible it didn't start smoking until now because the Darlingtons had failed before?
|
|
| Back to top |
|
|
|
|
