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CR-1040, readings to noodle on.

Nuklhed

Nuklhed

Active Member
Greetings team! Got a clean 1040 on the bench, trying to save her. Here’s the scoop so far.
1. Protection will click, but have to turn the volume up to about 7 to barely hear both channels.
2. Readings into and out of the power supply look very good.
3. From my experience with 840 and 1040, the output packs seem to be good.
4. I took the pre driver readings below with the unit energized, speaker off, output packs installed.


Pin Right channel IC 602. Left channel IC 601 Schematic says

15. 50.2. 50. 50
14. 1.4. 3.2. 3.0
13. -4.4. -3.2. -2.9
12. -3.8. -2.8. -2.8
11. -1.2. 0. 0
10. -1.3. 0. 0
9. 0. 0. 0
8. -1.2. 0. 0
7. 1.2. 2.7. 2.7
6. -49.7. -49.7. -50.5
4. -48.2. -48.2. -49
3. 0.2. 0.3. 0.1
2. 0. 0. 0
1. 0.2. 0.3. 0.1

Looks to me like the left channel nearly perfect. I do have a backup predriver, but before I just shotgun it I’d like your take. Be glad to take any other readings you feel may help. Thanks in advance for any thoughts!
 
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Sorry, left out some info. Left channel idle voltage adjusts perfectly. Right channel seems like pot at end of travel and reading continuously drifts away from zero.
 
Nuklhed said:
Sorry, left out some info. Left channel idle voltage adjusts perfectly. Right channel seems like pot at end of travel and reading continuously drifts away from zero.
Click to expand...
If you haven’t so far, check the R-ch pot, preferably out of circuit, before doing anything else, maybe bad.
 
FWIW, I think Sparkplug's suggestion makes good sense. I would expect DC offset (which appears at pin 10 of the pre-driver) to be worse than that if the pot was open, but I'm having a hard time thinking of what else could account for the observations.

Well, I guess a flaky pre-driver might act like that. If you're very comfortable with soldering and have the time to spare, swapping the L & R pre-drivers could settle that question.

It sticks in my craw that there is nothing in this that could explain low volume in both channels. At least, nothing I can think of that seems realistic.
 
Nuklhed said:
Greetings team! Got a clean 1040 on the bench, trying to save her. Here’s the scoop so far.
1. Protection will click, but have to turn the volume up to about 7 to barely hear both channels.
2. Readings into and out of the power supply look very good.
3. From my experience with 840 and 1040, the output packs seem to be good.
4. I took the pre driver readings below with the unit energized, speaker off, output packs installed.


Pin Right channel IC 602. Left channel IC 601 Schematic says

15. 50.2. 50. 50
14. 1.4. 3.2. 3.0
13. -4.4. -3.2. -2.9
12. -3.8. -2.8. -2.8
11. -1.2. 0. 0
10. -1.3. 0. 0
9. 0. 0. 0
8. -1.2. 0. 0
7. 1.2. 2.7. 2.7
6. -49.7. -49.7. -50.5
4. -48.2. -48.2. -49
3. 0.2. 0.3. 0.1
2. 0. 0. 0
1. 0.2. 0.3. 0.1

Looks to me like the left channel nearly perfect. I do have a backup predriver, but before I just shotgun it I’d like your take. Be glad to take any other readings you feel may help. Thanks in advance for any thoughts!
Click to expand...
Check your switch positions. And what source are you using for music? I always use an external source (MP3 player) on the AUX input since the tuner could also be having issues.
The fact you are getting something, suggests an input or source problem.
 
Thanks all. The pot was toast. No 10k’s in my shop so waiting on Mouser. I use the aux input with music from the iPad. Will certainly recheck switch positions and try phono and fm when new pot installed and adjusted.
 
Update! Good news and bad news.
The new pot for the right channel is responsive to adjustment.
The left channel is now responding normally. Sounds great at ~ 2 volume!
Lights, meter response, fm tuner acting great!

But,

The right channel offset, although responsive will continue to drift in the last adjustment direction. Last adjusted to 000….after 10 minutes it’s at .065 and climbing slowly.

The right channel volume is very low compared to the left. At volume 2, and balance mid, the left channel watt meter reads ~ 0.1. The right channel reads ~ 0.005.

Doing the same thing with Aux and FM.

So…something squirrelly in that right channel. The chase continues. Thanks again for your help!
 
2 more nuggets from today’s session.

Sitting here watching right channel offset climb, I took a deep breath and blew into the area where tp’s and pots are….the voltage immediately dropped from 30mv to 0 and then started its slow climb again.

The low filter on/off button has an immediate impact on the dc offset when cycled. Don’t know if that’s normal, but another piece of data.
 
Nuklhed said:
took a deep breath
Click to expand...

Was that for courage? (Internet-required disclaimer: kidding!)

It could be worthwhile replacing C623 and 624, with either low-leakage electrolytic caps, or film caps if there's enough room.

I could believe that the low filter switch would produce a transient change in offset. If the offset returns to where it was within a few seconds of toggling the switch, I wouldn't worry about it. If the effect on offset is persistent, then similar suggestion as above for C613 and 614.

But to be honest, I think triage principles at this point call for tracking down the volume imbalance over tracking down tens-of-millivolts offsets. If you don't have equipment to do signal tracing, you could at least nail down whether the imbalance is in the power amp section or elsewhere by shorting together the two power amp inputs to see if the imbalance goes away. Probably the safest place to short the inputs together is at the low filter switch, at the terminals where C613 and 614 connect to that switch.
 
I appreciate good humor, even at my expense! Honestly I thought I might start getting offers to blow other folks machines….

I agree, time to get out the oscope and start the signal hunt. Thanks so much Chazix!
 
So, finally got some rain…(lab time!)
Update….input a 1khz signal into the Aux input while monitoring pin 13 on the predrivers. On the good channel got a beautiful sine wave that varied as I would expect with the volume control. The bad channel was a 500mv trash wave that was unresponsive to volume changes. So, I removed the predriver and installed the new style predriver….
Attempted idle voltage adjustment. Unresponsive. Also noted that the Overload led is constantly lit. Regrouping….any thoughts?
 
Starting from Aux input, find out up to what point your 1KHz signal is healthy. That should indicate what part(s) may be at fault.
 
After fixing some poor soldering on my part, works like a champ! Thanks all. Special thanks to you Chaz…the replacement predriver did the trick!
 
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