CS 800x DDT issue

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rascalrick
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CS 800x DDT issue

Post by rascalrick » Mon Sep 05, 2016 3:16 pm

Hi. I am in the process of repairing a CS 800x amp which was tripping the breaker on poweron. Using a lamp limiter I determined that there was a short, most likely in the power amp section. I pulled and checked all the power transistors and respective resistors. Three of the power trannys were bad in channel A. I replaced them with trannys I ordered from Peavey. The amp tested good with the lamp limiter, so I powered it up and one channel works fine. The other is very quiet and distorted, and as I turn up the gain the DDT lamp flickers and comes on steady at large gain. I tried swapping the preamp board, but the problem persists. I have a reasonably strong electronics background, and build and repair tube amps, but not so familiar with the solid state amps of today. Can anyone guide me on what else I may be missing on the power amp board? BTW, I pulled and tested the triacs on the bad channel as well, and the tested fine.

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Enzo
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Re: CS 800x DDT issue

Post by Enzo » Mon Sep 05, 2016 10:03 pm

You replaced output transistors, so did you check ALL the associated resistors for opens? That means the power resistors - 0.33 ohm 10w, but also those 1k. Look a little to the left and see three 22 ohm resistors, and don;t leave out the 330 ohm R120 wired base to base on the drivers.

rascalrick
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Re: CS 800x DDT issue

Post by rascalrick » Tue Sep 06, 2016 12:45 pm

Thanks for the advice Enzo. I will recheck the other resistors...only checked the .33 ohm biggies. A quick question...how does one pull the flat cable from the power board to the preamp board??? That connector is SO tight that I ended up breaking a 1/4w resistor next to one the other day trying to get it apart. Every time I plug them in I dread the possibility of having to unplug them. :( The hardest connectors I have encountered in 40+ years of electronics work...

rascalrick
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Re: CS 800x DDT issue

Post by rascalrick » Tue Sep 06, 2016 7:03 pm

Ok, so I realized that the channel that was giving my DDT problems isn't the channel that I replaced the power transistors in. That channel works just fine. It is the other channel that isn't working correctly. I have pulled all the resistors in the power section and they all check out fine. I pulled the two driver transistors on that channel as well and they are fine. I pulled the triac on that channel and it is fine. Soooo, since I tried swapping the preamp boards to no avail, what is left? This amp has given me fits...

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Enzo
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Re: CS 800x DDT issue

Post by Enzo » Wed Sep 07, 2016 2:13 pm

A small screwdriver pushed under the edge of the connector near one end, then twisted to wedge the thing up a millimeter, then do the other end. Once it is started, you have room for more tool use.

One of my absolute FAVORITE tools is the OTC O-ring seal pick, I bought at an auto parts store decades ago. it is similar to the pointy thing dentists use to scrape your teeth.

https://www.amazon.com/OTC-7312-Ring-Se ... B000P70QPW

I use that a lot to get under sticky connector housings.

You may have swapped driver cards, but how about the little input circuits? Is the signal reaching that driver card clean?

rascalrick
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Re: CS 800x DDT issue

Post by rascalrick » Sun Sep 11, 2016 9:31 pm

Many thanks Enzo. After careful examination, and resoldering some suspect connections on the second channel
power amp section, all is well once again. So now I hope I have a flood of these coming in for repair, as I have
them pretty well figured out! :) BTW, love your pic!

Rockin Rob
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Re: CS 800x DDT issue

Post by Rockin Rob » Fri Mar 24, 2017 1:06 am

Rascalrick, do you repair Peavy equipment? I have an issue and you sound knowledgeable.

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Enzo
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Re: CS 800x DDT issue

Post by Enzo » Fri Mar 24, 2017 5:47 pm

Hi Rob. Just start a new thread for your amp, and ask about your issue. We will either be able to help or not.

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