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techbiker 18th September 2011 04:58 AM

Oh Shoot! Did I Kill a Channel?
2 Attachment(s)
A couple weeks ago, I purchased a Crown Com-tech 400 and with DJK's help I was able to resurrect channel 1 (which had a shorted Triac).

Not wanting to stop there, I asked about a few upgrades that can be done (like swapping capacitors, etc.) and DJK told me to look at adding a parallel .1uf capacitor on c100 and a nice film capacitor on c153 (in addition to adding a 22uf capacitor in parallel with each large 6300uf capacitor).

DJK was more than helpful. I want to say here that everything following is related to some stupid mistakes that I made, not anything that he suggested.

Here is my first thread:

I did some research myself and determined that the National lme49740na should be a good drop-in replacement for the stock MC33709 opamps. Yesterday, I threw in the new opamps (dip 14 socket packaging) and began replacing capacitors.

Anyway, I was almost done replacing the remaining electrolytic capacitors on the input board (C110, C210, C100, C200, C153, C253) and adding parallel .1uf capacitors to C100 and C200 when I managed to pull the solder contact pad off of one of the ends of C153. I began to freak out, however I calmed myself down and looked at the schematic.

Notice C153 in F10. I believe that I pulled up the contact pad on the side of C153 that contacts c146, q112, r102, and r194.

At first, I simply bridged R194 and C153 (not realizing that I also needed to bridge the capacitor to c146).

I turned on the amp and I noticed that the yellow IOC light was on solidly while the ODEP and SPI indicators were flashing intermittently. I turned off the amp after a few seconds and reexamined my work. Without thinking, I believed that I needed to bridge c146 and c153 instead so I did that while removing the bridge from r194 and c153.

Obviously, the yellow ioc light turned on again and the the other two lights flashed. I was perplexed until I figured out that I needed to run bridges to both C146 and R194. After doing so, I double checked my work, turned on the amp again and saw that the yellow IOC light was off!

Here is a picture of my final soldering work:

I was thinking that everything would be great, so I ran through checks 1 and 2 on the service manual.

Surprisingly, the output bias was only 180 mv on both R31 resistors so I adjusted R26 on both channels until the output bias was 310 mv on both R31s.

Satisfied with my work for the evening, I put the case back on the amp and wired her up. I put on some Toto and realized that I wasn't getting any sound from channel 1... :bomb: I checked all of my speaker wires and saw that there weren't any problems there....

In case I shorted out the Lme opamp on channel 1, I removed it and reinstalled the old opamps. I also rechecked the output bias on both channels and saw that it had increased to 550 mv on R31 on channel 1. I then adjusted it down to 320 mv.

Finally, I turned on my amp again but still I did not hear any music from channel 1. This time however, I was getting some static from channel 1 instead. I checked my other soldering joints on the board and they appear to be fine.

Anyway, if you have any guess at all as to what my problem might be, please let me know. I hope that I haven't destroyed this awesome amp for good!

Here is another picture of the input board:


tauro0221 18th September 2011 06:21 AM

I think you should use a cleaner and clean the area were you solder the wire. Second why use a wire so heavy. Next time find a stranded wire and remove one and use it to solder the copper run. Right now I do not think that copper run is solder right. Normally I use acetone to clean the rosin that left when you solder.

techbiker 18th September 2011 06:43 AM


Thanks for the tips, but I cleaned all of the rosin off with isopropyl alcohol and then continuity checked all of those points. They all show no resistance on my volt meter. I also checked the connections on these components on the top side of the pcb and everything appears to be hooked up as I described. :(

I wonder if this wiring issue damaged something else on the pcb?

es44 18th September 2011 07:34 AM

I can\'t help with Your initial problem, but i noticed in your first picture, that you have at least two cracking solderings at R176 and R182.
May not be a problem right now, but it will eventually create more trouble.
I wish you good luck.
Best regards Ebbe

techbiker 18th September 2011 08:20 PM


Originally Posted by es44 (
I can't help with Your initial problem, but i noticed in your first picture, that you have at least two cracking solderings at R176 and R182.
May not be a problem right now, but it will eventually create more trouble.
I wish you good luck.
Best regards Ebbe

Es, thanks for the sharp vision. I got those points re-flowed so that they will no longer pose any problems. I'll have to take a high resolution photo to see if I can spot any other problems.

Also, I removed the parallel capacitors from the input board and replaced the new .1uf 100v capacitor on c153 with the older .1uf 50v capacitor. I turned on the amplifier and still I could not hear anything except for static coming from channel 1.

I also noticed that the sound indicator light flashes with the music on channel 2. It almost seems like the signal for channel 1 isn't even making it to the amplifier. I swapped the inputs from my computer to check this and I still didn't get any sound.

I wonder if something got fried closer to the inputs. I'm not good enough at reading a schematic like this to even start testing components.

techbiker 18th September 2011 09:16 PM

1 Attachment(s)
Here's an (unfortunately somewhat blurry) photo of the input board. I've circled c153. Thanks

tauro0221 18th September 2011 09:31 PM

Did you check the +15/-15 that goes to IC104C? Also can you check the voltage output pin 8. If it is possible compare it with the good one.


nigelwright7557 18th September 2011 09:38 PM

The first rule of audio is if it works leave well alone ! boring but true lol

Its easy to destroy multiple components with a mistake, then a simple job turns into a massive debugging exercise.

That pcb has some very light traces so I am not surprised you lifted a pad.
I would never have disturbed it.

techbiker 18th September 2011 10:26 PM


Originally Posted by tauro0221 (
Did you check the +15/-15 that goes to IC104C? Also can you check the voltage output pin 8. If it is possible compare it with the good one.



I checked those voltages and everything appears to be normal. I am getting +15.30 volts from both channels and -15.00 volts from both channels (using my cheap volt meter). This was measured at the point of origin for both.

I don't know about output pin 8. It seems that both opamps should be facing the same direction, however the opamp for channel 2 (IC204) is wired in 180 degrees from the opamp in channel 1 (IC104).

In other words, pin 8 for IC104 is connected to a different part of the pcb than pin 8 for IC204. It seems that pin 8 on IC204 is putting out the same voltage as pin 8 on ic104, just to another trace on the circuit.

Could it be that I flipped the opamp on channel 1 around? I lined it up with the indent indicator on the pcb and I swear that I remember both opamps facing the same direction before I upgraded them....

Do you think that I should flip the opamp for channel 1 around and try to see if I get any audio?

I don't want to damage anything else so I might hold off until I get more confirmation!


Thanks for the tip but rest assured that I will put this amp away and never touch it again if I do manage to save it! I made that decision before I even posted on here lol.

Thanks again.

techbiker 18th September 2011 11:18 PM

Sorry, please ignore my last comment on the pin 8 problem. I read note 3 on the schematic.

Pin 7 is the voltage output pin on channel 2.

Here are the voltages for pin 8/7 while playing music.

Pin 8 on channel 1: .009 volts
Pin 7 on channel 2: .070 volts

Why would pin 8 on channel 1 only be putting out 1/7th the voltage of pin 7? What do you think I check out now?


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