Help with Mark III rebuilds

for Dynaco Mark II/III/IV and DIY PP monoblocks

Re: Help with Mark III rebuilds

Postby 20to20 » Thu Jun 20, 2013 9:10 pm

geezer,

Double check the (-) ground speaker terminal for a good chassis ground there. Since you had to swap tranny leads, maybe the ground is loose.
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Re: Help with Mark III rebuilds

Postby 20to20 » Fri Jun 21, 2013 5:48 am

Hmmmm... I posted a reply last night that seems to be hanging in limbo. The index page shows I posted it but the post isn't showing? (???) (???)

Geezer,

Just wanted to suggest double checking the (-) ground speaker terminal for a solid chassis ground. And to check the feedback line again for propper component value and connection polarity. Your feedback line is part of the biasing for the driver. If that's not right you could have a bum channel. Since you had to swap tranny leads to correct the other problem, this one may be related... could also be something that was masked by the first.
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Re: Help with Mark III rebuilds

Postby Shannon Parks » Fri Jun 21, 2013 6:11 am

kygeezer wrote: I cannot see any difference in the Poseidon boards, and have checked the voltages (at least some of them) but not every pin of every tube. Where do I go from here?


Check and record the voltage of every pin on every tube (V1 thru V4) on the bad amp and post here.

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Re: Help with Mark III rebuilds

Postby kt88pppamp » Sat Jun 22, 2013 5:08 pm

It seems like something may be loaded down somewhere.

I would first check the voltages against the schematic as Shannon Parks suggests and look for any anomalies. All the voltages should be within about 20% of one another from unit to unit.

In an absolute worst case scenario, a component, or possibly the output transformer could be defective. Before you conclude that, everything else needs to be ruled out 100%.

I feel it is a simple mistake you made, like a wire strand or solder bridge somewhere. I do things like that all the time, the big dummy I am (lol).
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Re: Help with Mark III rebuilds

Postby Shannon Parks » Sun Jun 23, 2013 8:10 am

Strikes me that the LM334Z can blow under an "oscillation event", so that is my main focus. But all the voltages tell a story.

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Re: Help with Mark III rebuilds

Postby kygeezer » Sun Jun 23, 2013 9:36 am

To put this posting to rest, I discovered through a lot of trial and error that the second amp, after Roy's correction, is working fine! By coincidence, a tube blew in my preamp while I was trying these amps out. Only by reversing the preamp outputs did I discover that the poor sound followed the left preamp output. I don't know the odds of having both problems at once, but I proved that it CAN happen! All is well and they sound great. My thanks to Shannon Parks for a great design!
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Re: Help with Mark III rebuilds

Postby Shannon Parks » Tue Jun 25, 2013 4:55 am

Thanks for the update! Enjoy the music.

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Re: Help with Mark III rebuilds

Postby kygeezer » Sat Jun 29, 2013 3:39 pm

My bad! Forgot I had posted the final solution.
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