Crown Com-Tech 400 Dead Channel?

[techbiker]

I just purchased a used Crown Com-tech 400 amplifier and after hooking up 2 JBL HSL410's (that work), I noticed that channel 1 is pretty much dead. I am getting a small bit of noise out of the speaker on channel 1 when I play music, however it is barely noticeable and garbled.

I hooked up my equipment in this order:

Computer (Creative X-Fi Titanium)->Com-tech 400->(HSL410's/ Jamo Sub 250- in parallel)

Note that my Jamo Sub has line-level inputs for amps that do not have a sub out. I have tried disconnecting the sub and reversing the polarity on the speaker wires on channel 1 to no avail.

My regular powered computer speakers also work well so this can't be an issue with my sound card. I also double-checked all of the input and output wiring on the amp. I originally had the input wiring connected incorrectly (positive in + and negative in the ground terminal), however I fixed the wiring when I moved the negative wires over to the (-) input terminals of the amp.

I am using regular Radioshack Megacable speaker cable by the way. Please let me know if I am missing any information.

What is the most likely cause of this problem? I don't mind opening up the amp and replacing components, however I am having difficulty finding where to start.

Also, please let me know if I should replace anything (like op-amps) while working on the amp to improve sound quality on both channels. If you would like any pictures of the circuit board, please let me know.

Thanks for the help.

[techbiker]

I know that bumping threads is frowned upon in many forums, however I disassembled my amp and noticed a blown resistor...



Do you think that I should replace this resistor and power my amp back up or do you think this might be a sign of more serious problems like leaking capacitors? I checked all of the capacitors that I could find but I didn't see any leaking ones.

Also, I checked the Schematic from Crown and noticed that this resistor (R701) is on the power control board. Please see page two on the following pdf:

http://www.crownaudio.com/pdf/legacy/ct400_main.pdf

Thanks

[djk]

The optoisolator (U700) and triac (Q701) should be checked (or repaced) as well. My guess is the triac gate is shorted, damaging the driver and the resistor. Less than $10 in parts.

[techbiker]

Originally Posted by djk

The optoisolator (U700) and triac (Q701) should be checked (or repaced) as well. My guess is the triac gate is shorted, damaging the driver and the resistor. Less than $10 in parts.

DJK,

Thanks for the suggestion! I just purchased 2 new optoisolators and triacs (mac224a-4 wasn't available anymore, however I purchased an equivalent part). Considering that one triac failed, I thought that it would be good to replace both with new parts rather than risk another failure.

I also purchased new Nichicon and Panasonic capacitors to replace all of the electrolytic ones on the amp. I am having trouble finding capacitors to replace the 2 large 6300uf 200v capacitors however. This type is missing even on Mouser and Digikey! Do you think that it's worth paying around $45 for each replacement (if I can find any)?

Also, the sound coming out of the working channel wasn't the best. Should I check and adjust the ODEPs? Should I also thoroughly clean out the inside of my amp? (it's very dusty) Finally, do most people clean out POTs with contact cleaner?

I apologize for all of the questions, but I would like to restore my amp to like-new condition.

Thanks

[djk]

A 150V cap will do for 70V mode, a 75V cap will do if you only run it in 8/4Ω mode (and never run it in 70V mode). I wouldn't replace these unless they're all dried up. I would tack a 22µF in parallel with each.

C100, C153 are in the signal path. Parallel C100 with a small value film type, use a good film type for C153.

I'm not sure how good the MC33079 is, or what would be a suitable replacement. It's not horrible, I would do the caps first and then give it a listen.

I wouldn't fool with the ODEP unless its had some work done on the output stages.

I clean pots, switches, and connectors with Caig De-Oxit, it can make a huge sonic improvement.

I just bought a couple of 210s and an 810, but I haven't had time to listen to them just yet. Maybe in a couple of weeks.

[techbiker]

Originally Posted by djk

A 150V cap will do for 70V mode, a 75V cap will do if you only run it in 8/4Ω mode (and never run it in 70V mode). I wouldn't replace these unless they're all dried up. I would tack a 22µF in parallel with each.

C100, C153 are in the signal path. Parallel C100 with a small value film type, use a good film type for C153.

I'm not sure how good the MC33079 is, or what would be a suitable replacement. It's not horrible, I would do the caps first and then give it a listen.

I wouldn't fool with the ODEP unless its had some work done on the output stages.

I clean pots, switches, and connectors with Caig De-Oxit, it can make a huge sonic improvement.

I just bought a couple of 210s and an 810, but I haven't had time to listen to them just yet. Maybe in a couple of weeks.

Wow! Are you considered the wizard of amps?

This is probably a dumb question, but does it matter what value of capacitor I parallel with C100? Should I look for something around .5uf?

Also, does increasing the capacitance by paralleling capacitors tend to improve audio quality?

Would a 330 volt suppression film capacitor work well in place of c153? PHE841EB6100MR17 Kemet Suppression Film Capacitors

I've also heard good things about polypropylene capacitors.

Thanks again.

[djk]

"Wow! Are you considered the wizard of amps? "

Only on Tuesdays, what day is it?

I generally buy 0.1µF in bags of 100 for bypassing caps like C100 and replacing caps like C153, I just get any name brand polypropylene (I ordered some Panasonic the other day).

That suppression cap is quite large, it may be a fit problem. It's also expensive, the caps I ordered the other day ran $12.27 for 100pcs.

[techbiker]

Thanks again for the tips.

I actually just finished repairing my Com-tech and the results are amazing. While I cannot say that audio quality has improved at all after installing those new capacitors, both channels ARE working after replacing R701 and Q701.

I wasn't able to find MAC224a-4's anymore, however I did find an industry-standard equivalent... the Littelfuse Q2040k7. I thought that this would just be a plug and play, however the new triacs were in a much bigger package (and isolated) and didn't fit in the stock location so I did some fiddling to get them to work.

I got the wrong capacitors for C100 (I needed to get non-polarized caps and I purchased polarized ones) and I realized that I needed to replace both C153 and C253 (one .1 uf cap for each channel) so I didn't order enough.

Do you think it's worth opening my amp back up just to install caps in parallel at the locations you mentioned?

Also, I discovered that my two MC33079's are installed in sockets! I've heard that National makes a very good OPAMP that could replace mine. What do you think about the LME49740? LME49740 - Quad High Performance, High Fidelity Audio Operational Amplifier

Have you gotten a chance to check out your Crown amps? On mine at least, the audio quality is pretty darn good. I'll need to wait until tomorrow to really boost the volume.

[techbiker]

Here's another update:

I have noticed a hum from my subwoofer (a Jamo 250) at around 100hz. I have made sure that all of my speaker wires are placed a reasonable distance from my power cords and I have checked to make sure that the amp and subwoofer were plugged into power outlets that were a distance from each other.

Also, I get this hum whenever I have my amp connected to my subwoofer (regardless of whether or not the amp is on). Once I unplug all of the speaker cables to the sub, the hum stops.

I also have the sub connected in parallel to the output connectors on the amp with my 2 speakers since the Com-tech amp doesn't have a sub pass-through connector. Is there something in my Com-tech that might be causing this hum? The subwoofer did not hum when it was connected to the previous owner's receiver at his house.

Thanks

[techbiker]

Sry, I keep waiting too long to edit my messages. I wanted to add that I get this hum even when I disconnect the inputs to my amp.