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Help--Dynaco MK3's



 
 
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  #1  
Old August 31st 05, 04:08 AM
mike m
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Default Help--Dynaco MK3's

Hello All
Help!
I'm going nuts trying to get My MK to work properly have 2 identical
Dynaco MK3's. Completely rebuilt 6 months ago and both worked
beautifully. Went on vacation for 10 days, came back Sunday night turned
on the system and heard almost nothing from the right channel(amp#2).
Switched connections first to see if it was a source problem. Problem
follows Amp #2 . swapped 6an8's, swapped 5ar4's swapped kt-88's. Problem
follows amp #2

Unit #1 works perfectly.clean sound to max. gain. Unit #2 has a very
week output and as gain is increased the sound becomes louder and more
distorted.
I have been comparing both units A to B
Both units have identical voltages at all test points within 5% of each
other.
Both units have been completely rebuilt with all new caps and resistors.
All speaker connections are now with banana plugs.
I have checked every value of caps and resistors on unit 2 and all are
correct. I have checked all solder connections at least a dozen times,
including putting a hot iron to every joint. I have checked for any
touching wires, contacts etc. I have gone over the unit 3 times now on 3
separate days.

I checked the feed back circuit and all values are correct.
What am I not seeing?
Any suggestions?
Thank you in advance
Please remove nospam from e-mail to respond back
Mike Mueller
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  #2  
Old August 31st 05, 04:26 AM
Chris Hornbeck
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On Wed, 31 Aug 2005 03:08:56 GMT, mike m
> wrote:

<snipped for bandwidth>

Just for ****s-n-grins, swap both amps physically,
leaving everything else the same. This will
elimanate loads, wiring, etc. The Sherlock
Holmes ("After you've eliminated the...")
approach.

Then measure the DC resistance of your output
transformers and other esoteric-but-possible
failure modes. These guys are getting up
there in years.

Good fortune, and good hunting,

Chris Hornbeck
  #3  
Old August 31st 05, 04:30 AM
mike m
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Chris Hornbeck wrote:

> On Wed, 31 Aug 2005 03:08:56 GMT, mike m
> > wrote:
>
> <snipped for bandwidth>
>
> Just for ****s-n-grins, swap both amps physically,
> leaving everything else the same. This will
> elimanate loads, wiring, etc. The Sherlock
> Holmes ("After you've eliminated the...")
> approach.
>
> Then measure the DC resistance of your output
> transformers and other esoteric-but-possible
> failure modes. These guys are getting up
> there in years.
>
> Good fortune, and good hunting,
>
> Chris Hornbeck

Hi Chris
Did the old swap amp-leave all else same.
do you have Dc values for out put tranny?
Thanks
Mike M
  #4  
Old August 31st 05, 04:36 AM
Chris Hornbeck
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On Wed, 31 Aug 2005 03:30:18 GMT, mike m
> wrote:

>Hi Chris
>Did the old swap amp-leave all else same.
>do you have Dc values for out put tranny?


Cool. No need; all you care about are
differences between the two.

Keep us updated; nothing better than a
good puzzle.

Thanks,

Chris Hornbeck
  #5  
Old August 31st 05, 03:14 PM
Ian Iveson
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"mike m" > wrote

> I'm going nuts trying to get My MK to work properly have 2
> identical Dynaco MK3's. Completely rebuilt 6 months ago and both
> worked beautifully. Went on vacation for 10 days, came back Sunday
> night turned on the system and heard almost nothing from the right
> channel
> (amp#2)....


> ...What am I not seeing?
> Any suggestions?


I can't remember the last time anyone asked a question about a
specific amp and got a specific reply. Not many round here work on
enough commercial stuff to know many by heart, or to be aware of
their particular foibles.

It generally helps therefore to provide a link to the circuit
diagram. Many can be found in the "schematic index" at

http://www.duncanamps.com/

We are unlikely to know where the test points are unless you tell us
or they are shown on a diagram.

So we don't know quite what you have eliminated.

Assuming *all* DC conditions are correct, then you are looking for
problems which don't effect the DC conditions.

A partly-shorted transformer is one possibility...hence Chris'
suggestion of checking the DC resistance of the OPT. Conversely it
could be a partly open-circuit capacitor but you've checked them.

Resistors in series with capacitors, or in parallel with inductors,
are also suspect, but you've checked them.

No such failures are likely to occur because of your absence, so I
wonder if it could be a dusty input connection or pot...but I guess
you've tried wriggling them.

A knackered OPT is bad news, so I hope you've missed something else.

cheers, Ian


  #6  
Old August 31st 05, 06:25 PM
Sander deWaal
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mike m > said:

>I checked the feed back circuit and all values are correct.
>What am I not seeing?
>Any suggestions?



I remember an article in some old issue of "Audio Engineering", called
"the sad tale of a half-watt resistor".
In which the author describes how a simple resistor in the feedback
loop path caused one channel to sound weak and distorted.
When measured with a Dc ohmmeter, there were no problems, but under
dynamic conditions, the resistor failed.

IIRC, this was also about a Dyna Mk. III.

Just replace them with ones known to be good and try.


I've also read in one of Menno's books about saturation of an output
transformer core in one direction. This usually happens when a large
current has flowed in one direction, e.g. when a tube shorts.
this results in weak bass reproduction and again, distortion.
The solution is to pull all the tubes from the unit, disconnect it
from everything and apply a variable AC voltage to the secundary
winding of the OT, from 0 up to ca. 20V and back.
That way, all Weiss-particles will return to a random state and the
transformer is able to transform again.

Dunno of this applies to your case, but if you decide to do this, be
very careful. If possible, connect the variac via a mains isolation
transformer for your safety.


If none of this helps, get the signal generator and the scope out.

--

"Audio as a serious hobby is going down the tubes."
- Howard Ferstler, 25/4/2005
  #7  
Old September 2nd 05, 12:01 AM
mike m
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Default

Hello All
I want to thank everyone who responded to my question.
I have decided however to throw in the preverbal towel.
Unfortunately, to me, this hobby is no longer fun. Trying to keep 45
year old equipment running has reached diminished returns.
Six months ago I installed new circuit boards, replaced all resistors
with Riken Ohm resistors, installed Hovland music caps, installed new
quad caps and replaced loose tube sockets. Was it worth it at the time?
Absolutely! But 6 months later, I need to pull everything to see if one
of the components has failed at operating voltages.
I just want to enjoy the music. I want to just listen and enjoy.
I learned 6 months ago how much better, newer equipment can sound. I
replaced a Fisher 400c and Heathkit SP2a dual mono pre with an Audio
Research SP9. What a difference 40 years makes. I sold a Mc 225 since
it was sounding so veiled, I did not even get started with that rebuild.
So over the next few weeks I will start to sell off both of my st-70's
and both Mk3's. I also will replace my FM3 amd FM1. E-mail me if interested.
I also have a stash of 100's of tubes for this equipment, plus 100's of
other's I've collected to build that future super amp.
I've decided to purchase a Primaluna Prologue 5. One hell of an amp for
the price.
So again, to all thank you for the last 8 years of advice, but I'm going
to stick building loudspeakers and listen to the music
best to all
Mike Mueller
  #8  
Old September 2nd 05, 04:15 AM
west
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Default

"mike m" > wrote in message
news
> Hello All
> I want to thank everyone who responded to my question.
> I have decided however to throw in the preverbal towel.
> Unfortunately, to me, this hobby is no longer fun. Trying to keep 45
> year old equipment running has reached diminished returns.
> Six months ago I installed new circuit boards, replaced all resistors
> with Riken Ohm resistors, installed Hovland music caps, installed new
> quad caps and replaced loose tube sockets. Was it worth it at the time?
> Absolutely! But 6 months later, I need to pull everything to see if one
> of the components has failed at operating voltages.
> I just want to enjoy the music. I want to just listen and enjoy.
> I learned 6 months ago how much better, newer equipment can sound. I
> replaced a Fisher 400c and Heathkit SP2a dual mono pre with an Audio
> Research SP9. What a difference 40 years makes. I sold a Mc 225 since
> it was sounding so veiled, I did not even get started with that rebuild.
> So over the next few weeks I will start to sell off both of my st-70's
> and both Mk3's. I also will replace my FM3 amd FM1. E-mail me if

interested.
> I also have a stash of 100's of tubes for this equipment, plus 100's of
> other's I've collected to build that future super amp.
> I've decided to purchase a Primaluna Prologue 5. One hell of an amp for
> the price.
> So again, to all thank you for the last 8 years of advice, but I'm going
> to stick building loudspeakers and listen to the music
> best to all
> Mike Mueller


Hi Mike,
It's too bad that you're giving up. You see, I believe that most Rodents
like to tinker and enjoy a technical challenge. That's the best teaching
tool available. If you do not enjoy this aspect of audio, then you are
probably correct in purchasing a ready built system and let the
factory/dealer worry about problems. In reading this thread, I think the
biggest problem is that you may not have an array of test equipment that
should make troubleshooting your problem fairly simple. One can only go so
far in swapping only, even though, especially in mono systems, there is much
redundancy. Good luck.

Cordially,
west


 




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