| Author |
Message |
Curt Palme
CRT Tech
Joined: 08 Mar 2006
Posts: 24396
Location: Langley, BC
TV/Projector: All of them!
|
| Posted: Mon May 06, 2013 3:55 am Post subject: |
|
|
I don't know this particular model, but the general rule of power amp repair is that anything smoking is due to a leaky transistor or a short somewhere. When working on power amps, (and Electrohome convergence boards, which are audio amps), is that I start with the power transistors on back, and remove them from the circuit, and check damn carefully for shorts and leaks. Then the drivers, then the predrivers, etc. Any partial short or even some strange diode type action that you might not catch while the transistor is in circuit can cause what you are seeing. Once I get a whole section of transistors that measure OK, then I assume I've gone back far enough in the amp to safely power up.
Another trick is to remove the output transistors, (or lift the collectors in a push/pull circuit. That way you can see if the B-E voltage is around 0.6 volts, and not something like a hard 0.7 volts, which would turn on the transistor massively, causing a short across the supply rail(s).
Before i power up, I will check all resistors around any shorted or leaky transistors. I ignore caps unless they are bulged or have a smoke show around them. A dead shorted cap in an amplifier is pretty darn rare, and a decreased value one won't cause an amp channel to smoke.
I did fix a Denon 5.1 amp recently that had a small driver PC board right at the output transistors. Almost everything had smoked on the driver board, and I subbed one transistor that even China didn't have. The sub worked, the thing powered right up, much to my surprise (sorry, not trying to rub salt in your wounds.  )
|
|
| Back to top |
|
 |
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Mon May 06, 2013 2:41 pm Post subject: |
|
|
| mdc wrote: | 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? |
What did you replace those 2 diodes with?
|
|
| Back to top |
|
|
mdc
Joined: 30 Apr 2013
Posts: 50
|
| Posted: Mon May 06, 2013 3:14 pm Post subject: |
|
|
|
Crap... knew I'd forgotten something. Don't suppose you have the values for those diodes in your copy of the service manual? Will order the parts today, with luck they should be here by Wednesday *fingers crossed*
|
|
| Back to top |
|
|
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Mon May 06, 2013 3:55 pm Post subject: |
|
|
| mdc wrote: | | Crap... knew I'd forgotten something. Don't suppose you have the values for those diodes in your copy of the service manual? Will order the parts today, with luck they should be here by Wednesday *fingers crossed* |
Do me a favor and don't order any more parts until we do some more testing. There is a service bullitin for your model that requires some parts replaced but I don't want to get into that yet.
Here is the diagram showing those diodes and there numbers. You can see why I suggested removing the companion diodes for testing. Also you should check those other 2 diodes for short. The other 2 will "HAVE" to be replaced also in case there is a value difference.
I will also be making another post shortly.
|
|
| Back to top |
|
|
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Mon May 06, 2013 3:58 pm Post subject: |
|
|
| mdc wrote: |
Checking at the Inductor gives me around 20V from chassis ground, compared to 40V from chassis ground on the other inductors. |
This post has me concerned. If you are testing this correctly you are telling me there is DC on the output lines.
|
|
| Back to top |
|
|
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Mon May 06, 2013 4:01 pm Post subject: |
|
|
| mdc wrote: | 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? |
Because of this damage you will now have to remove and test, Q501sl, Q502sl, Q503sl and Q504sl. Even if they test good DO NOT install them yet.
|
|
| Back to top |
|
|
mdc
Joined: 30 Apr 2013
Posts: 50
|
| Posted: Mon May 06, 2013 4:51 pm Post subject: |
|
|
| macgyver655 wrote: | | mdc wrote: |
Checking at the Inductor gives me around 20V from chassis ground, compared to 40V from chassis ground on the other inductors. |
This post has me concerned. If you are testing this correctly you are telling me there is DC on the output lines. |
The strange thing is, even when powered up (when I disconnected D135 as per your instructions) there was only about a volt across the Base and Emitter. I was assuming - perhaps incorrectly, mind you - that that section of the circuit operated on some sort of floating ground which didn't correlate to the chassis ground.
Gimme a minute and I'll fire up the iron and remove those caps for testing.
|
|
| Back to top |
|
|
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Mon May 06, 2013 5:10 pm Post subject: |
|
|
| mdc wrote: | | macgyver655 wrote: | | mdc wrote: |
Checking at the Inductor gives me around 20V from chassis ground, compared to 40V from chassis ground on the other inductors. |
This post has me concerned. If you are testing this correctly you are telling me there is DC on the output lines. |
The strange thing is, even when powered up (when I disconnected D135 as per your instructions) there was only about a volt across the Base and Emitter. I was assuming - perhaps incorrectly, mind you - that that section of the circuit operated on some sort of floating ground which didn't correlate to the chassis ground.
Gimme a minute and I'll fire up the iron and remove those caps diodes for testing. |
Forget about testing across emitter to base. It has no relevance at this point.
|
|
| Back to top |
|
|
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Mon May 06, 2013 5:16 pm Post subject: |
|
|
|
Also, when testing for DC at those inductors, use a speaker ground terminal for your negative test lead. Not chassis ground.
|
|
| Back to top |
|
|
mdc
Joined: 30 Apr 2013
Posts: 50
|
| Posted: Mon May 06, 2013 5:35 pm Post subject: |
|
|
| macgyver655 wrote: | | Also, when testing for DC at those inductors, use a speaker ground terminal for your negative test lead. Not chassis ground. |
From speaker ground its 52V.
Also, the transistors are of a different model than specified in the service manual. Q501XX, marked as a KTC2874B in the service manual, uses C1845s instead. Another revision?
EDIT: Scratch that, was looking at Q504SL!
|
|
| Back to top |
|
|
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Mon May 06, 2013 5:40 pm Post subject: |
|
|
|
Using the same speaker ground, then test DC at CP505, pins 11, 8 and 6 and post your readings. Have to run out for a couple hours.
|
|
| Back to top |
|
|
mdc
Joined: 30 Apr 2013
Posts: 50
|
| Posted: Mon May 06, 2013 6:56 pm Post subject: |
|
|
Did as you said; pins 11 and 8 read 54V, and when I tested the +15V line, it sparked and the unit went into an endless relay cycle (green power LED, off, green LED, off, etc etc). At this point, it's now stuck in that cycle. Voltage regulators are both reading 0 for output AND input 
I'm completely lost now; if this was anything other than an amplifier I'd probably be treating this as being in the "its buggered" category and taking a hammer to the damn thing.
One thing I did notice after this latest catastrophy - whether it's relevant or not - is that while checking the regulators, the side of my hand accidentally bridged the TOP of the two main power filter caps (not the pins, thank god) and I felt a rather disconcerting tingle. Now, I usually try to stay as far away from large capacitors as possible, so I have no idea if this is normal.
|
|
| Back to top |
|
|
Nashou66
Joined: 12 Jan 2007
Posts: 16171
Location: West Seneca NY
|
|
| Back to top |
|
|
mdc
Joined: 30 Apr 2013
Posts: 50
|
| Posted: Mon May 06, 2013 7:35 pm Post subject: |
|
|
| Nashou66 wrote: | Were you using the speaker Ground when testing the 15 volt line?
Nashou |
I was indeed 
Could just be yet another component failure of something that was on its way out already, although I'm not sure that would explain the spark :-\ The problem is now, I have NO idea what could possibly have failed to put the unit in its current state... the 7805R maybe?
|
|
| Back to top |
|
|
Nashou66
Joined: 12 Jan 2007
Posts: 16171
Location: West Seneca NY
|
|
| Back to top |
|
|
Curt Palme
CRT Tech
Joined: 08 Mar 2006
Posts: 24396
Location: Langley, BC
TV/Projector: All of them!
|
| Posted: Mon May 06, 2013 7:47 pm Post subject: |
|
|
|
As long as he wasn't in current mode, there's no way a high impedance meter can cause a spark, regardless to what ground lead it was connected to. I presume a meter probe slip. Been there,. done that!
|
|
| Back to top |
|
|
Nashou66
Joined: 12 Jan 2007
Posts: 16171
Location: West Seneca NY
|
|
| Back to top |
|
|
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Mon May 06, 2013 8:19 pm Post subject: |
|
|
| mdc wrote: | Did as you said; pins 11 and 8 read 54V, and when I tested the +15V line, it sparked and the unit went into an endless relay cycle (green power LED, off, green LED, off, etc etc). At this point, it's now stuck in that cycle. Voltage regulators are both reading 0 for output AND input
I'm completely lost now; if this was anything other than an amplifier I'd probably be treating this as being in the "its buggered" category and taking a hammer to the damn thing.
One thing I did notice after this latest catastrophy - whether it's relevant or not - is that while checking the regulators, the side of my hand accidentally bridged the TOP of the two main power filter caps (not the pins, thank god) and I felt a rather disconcerting tingle. Now, I usually try to stay as far away from large capacitors as possible, so I have no idea if this is normal. |
On pins 8 and 11, was 1 a pos voltage and 1 a neg voltage? very important. As far as the 15v issue, check you main fuses, f101 and f102. You might of lost 1. Don't fret, we'll get it. 
|
|
| Back to top |
|
|
macgyver655
Joined: 22 Aug 2007
Posts: 8508
|
| Posted: Mon May 06, 2013 8:23 pm Post subject: |
|
|
|
Also test R142 and R141, they may have gone open.
|
|
| Back to top |
|
|
mdc
Joined: 30 Apr 2013
Posts: 50
|
| Posted: Mon May 06, 2013 9:18 pm Post subject: |
|
|
| macgyver655 wrote: | | On pins 8 and 11, was 1 a pos voltage and 1 a neg voltage? very important. As far as the 15v issue, check you main fuses, f101 and f102. You might of lost 1. Don't fret, we'll get it. |
I'm FAIRLY certain, yes; although the ensuing drama wasn't exactly condusive to good memory recall haha
And spot on with the R141 suggestion; had blown open. Luckily R142 survived intact Also checked the diodes and caps around that general area and everything checks out okay.
|
|
| Back to top |
|
|
|
|
