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->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) to improve audio quality while I am working on the amp.

Thanks for the help.

 

[techbiker]

I know that bumping threads is frowned upon in many places, however I decided to disassemble my amp and noticed a blown resistor...

Do you think that I should just replace this resistor and try to power up my amp again or do you think that there might be more serious problems like broken capacitors? I checked for leaking capacitors and didn't see anything.

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

 

[PRR]

> bumping threads is frowned upon

Crowns are so complicated (and usually reliable) that there isn't a whole lot of repair expertise.

It is hard to see how R701 could burn unless Q701 has blown-open. Q701 is a 40 AMP part, so it didn't die casually. I suspect big-part trouble downstream. D117 and C126 are obvious. Past that, in any other amp, I'd bet on a domino-failure in the power output stage.

> purchased a used Crown Com-tech 400 ... channel 1 is pretty much dead.

If sold as perfectly-working, discuss it with the seller.

 

[techbiker]

PRR,

Thanks for the suggestions. I have been trying to contact the seller, however he hasn't been answering his phone.

I went ahead and checked Q701 and it appears to be working. There is heavy resistance between the collector and emitter. My multimeter isn't giving me good readings when I bridge the collector and base and emitter and base, however I believe this might be because I haven't removed Q701 (a mosfet transistor) from the board.

Q801 gives me nearly identical readings even though it is on a separate circuit.

I'm going to speak with a support tech from Crown because they offer free diagnostic assistance over the phone. Hopefully they will be able to help me pin-point the problem. A tech that I spoke with today said that the complex overload protection circuitry in their amps usually can isolate a failure to one part of the amp and prevent a domino scenario like your describe.

Anyway, I'll see what happens tomorrow. I'm crossing my fingers!

Thanks again.

 

[PRR]

> Q701 (a mosfet transistor)

The symbol and part-number say it is some kinda solid thyratron, specifically a Triac. If it were a gas thyratron, it would be open-circuit end-to-end, until a trigger-pulse into the grid ionizes the gas, then it conducts like crazy and can't be stopped until current goes to zero some other way. In this app, current goes zero 120 times a second, so for "on" you gotta keep zapping the gate, and for "off" you stop zapping-gate and wait 1/120th second. This being one of the new-fangled non-gas thyratrons (Triacs) it works slightly different but mostly the same. For "on" the opto-coupler shorts G (nearer MT1) to MT2 through R701. Current flows in R701 for a microsecond but then the (non-)gas between MT1 and MT2 conducts heavy, no/low voltage on R701.

So if Q701 is ionizing right, R701 will never be stressed. Which suggests Q701 is not conducting. Yet it is a very tough part. (And yet, I have seen them croak "for no reason"; transients can hole the crystal.)

> speak with a support tech from Crown

Good plan. On other amps, if you tole me R69 was lightly toasted and output was 13VDC, I'd tell you to replace Q3 Q6 C13 and that R69. But amps with part-numbers in the 700s are way above my competence.

 

[techbiker]

PRR,

You are a life-saver. I think that I am beginning to realize how much I do not know about these amps. How complicated are these Com-techs compared to other similar amps? Maybe I am in over my head!

Anyway, I will speak with tech support and get back to you when I find out more. Support recommended purchasing an oscilloscope, however I couldn't find any at Fry's for less than $400 so I have to order one online. Maybe I'll be able to find out what is wrong without one. I wouldn't mind replacing everything that could be broken.

Thanks again for the help.