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1st gen DC300 Oscillation in one channel

Early DC300

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#1 MI-9584

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Posted 15 September 2012 - 11:47 PM

Bought this as a cadaver off the "Bay" for $24 bucks without a Faceplate,level controls,power transformer.Yeah I know,what the *beep* was I thinking!Well anyway I found a medium sized trans from a Marantz 240 that gives me the +/- 60 V needed so i put it in.The monster fired right up on the variac at about 35 vac and started to work.I was in shock that apparently the drivers and outputs were fine.Ran it up to 60vac for an hour then 80 and finally all the way up.The only problem is with Ch 2 level up full . ch 2 must be oscillating as the current draw goes to 1.8-2.0 amp.If you back the level control(soldered in some 10k pots to help troubleshoot)back to about 75% the draw drops to about .45 A ac.With a non factory transformer I'm assuming this is fine.In fact everything is fine.Ch 1 is flawless!Till you crank the ch 2 level over 75%.As long as the control is at 75% or less.Ch 2 is fine.No distortion,no nastiness of any kind. Any clues where to look.Haven't checked passives on the board as I thought to check here first for a lead or two.It is the first generation.The REAL early one with the normal/hysteresis switches on the sides.Bootstrap supply is good too. I know there are alot of you guys that have hundreds of repairs under your belts,and i'm just lookin to find the best place to start.And if anyone has an origional faceplate or a transformer I'm lookin.Again,the early one with the five fuse caps along the front bottom edge.

#2 MI-9584

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Posted 18 September 2012 - 07:50 AM

Did some checking as I figured it should show some output when it acts up so i connected aVTVM to the output and ran it up to see how much output it had.There was none!This had me baffled so i made a few voltage checks and what i found was the bias would go from .330 when stable to .550 when the channel acted up so the problem is raising the bias,not an oscillation.I think I will have to get the transistor tester out.This is probably going to wind up being a leaky active device or a leaky electrolytic on the board.Still looking for the correct power trans and a faceplate.I have another of these early amps so I may just get a rack panel and use my other face as a template and make a suitable replacement.Hey CROWN,Any dead units around i could buy a trans from?If not,What secondary voltage was the origional rated.I'll just get one from Signal trans or a nice1KW  toroid from ANTEK.   MI-9584

#3 dakos

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Posted 18 September 2012 - 12:50 PM

About locating the faulty transistor... You have one good channel so why not compare the readings from the working channel to the one acting on you?

Now about the secondary voltage level, just ask one of the tech support team for the schematics and you'll have all the values you need...

As for the rest, best of luck finding the parts.

#4 MI-9584

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Posted 19 September 2012 - 07:31 PM

I had planned on that course of action but since the forum is loaded with very competent and experienced techs I thought by some slim chance someone had run up against this particular problem in the past and could offer a fix based on a repair history of an actual unit with the same prob.I should know better by now.When a tough dog finds its way to my bench its really a DOG!!! I was really just looking to get out of a troubleshooting hit n miss episode. I have a schematic but the secondary AC voltage isn't mentioned.I know it has to wind up as 60 Vdc + and - at the rails.I guess the theory books will have to be dusted off as I can't remember if this is an instance of divide by .707 or the 120 Vdc total of the two rails is 1.414 times what the secondary voltage is but i have seen the formula somewhere.Again,I was lookin for the easy way out.I'm not really worried about the trans and face as I am planning to make a face from a nice charcoal anodized rack panel i found at work and my second choice of the 1kva toroidal trans from Antek is probably the way i will go,its always a good point to ask.You never know what people have layin around!!I appreciate your time in responding and think you are right.Just make notes of the good channel and see the comparison to the evil twin channel.

#5 joust

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Posted 09 October 2012 - 10:17 AM

It may not be related, but verify the output transistors' heatsink isolators. If they are mica, I would highly suggest to replace the isolators for some of the (flexible) silcon type. With time, the mica can become resistive and cause a whole bunch of problems!

Alain

#6 MI-9584

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Posted 27 November 2012 - 08:11 PM

I have to update this as I got it sorted out.Had a similar problem on an old Crown 30-30.One channel ok the other low vol.not exactly really nice etc. etc. Tracked that down after remembering one of the power amplifier rules.If it's blown up,catches fire,makes the speakers catch fire or just works great as an arc welder,Its one of the ACTIVE components!If it just behaves badly,A passive is the main culprit.Sure enough,the large electrolytics in the feedback on the 30-30 were so dried out they were simply not there!!!Checking the small electrolytics on the driver boards of the DC 300 by substitution was like turning on a lamp in the dark!It is stable,was able to set the bias & had ample adjustment on both input and output offset potentiometers to get it to.000vdc on the outputs.Just goes to show,Always check everything ......

#7 joust

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Posted 11 December 2012 - 02:40 PM

I'll try to remember that! Thanks for the ffedback!

Alain