Dynaco PAT-5: channel imbalance and dead headphone jack? - diyAudio
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Old 3rd May 2016, 01:02 AM   #1
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Default Dynaco PAT-5: channel imbalance and output too hot for power amp

Hi all,

I recently bought a Dynaco PAT-5 (factory build version).

Took it home and noticed 2 problems but not sure how to fix them, or where to start with them, any advice would be seriously appreciated.

I'm using it as preamp into my power amp using the "Output" jacks on the back of the PAT5.

Problem #1: There is no output on headphone jack at all, regardless of settings.

Problem #2: The left side output on the preamp is much quieter than the right side. I have to put the balance knob at 3 o'clock to get equal output on both my speakers. I know the power amp is not the issue because I have 2 other preamps around and both the others do not have this issue.

I figure this is just maintenance which is necessary because the unit is so old. How can I correct these two issues?

Last edited by bankroll1972; 4th May 2016 at 02:26 AM.
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Old 3rd May 2016, 01:17 AM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by bankroll1972 View Post
The left side output on the preamp is much quieter than the right side.
Those volume controls were often poorly balanced. Try running from the Dyna's tape out
into another amplifier that has its own volume control, and see if the balance is ok then.
If so, the volume control is very likely to be the problem. You can adjust the tracking
of the control with the slot on its back, while holding the knob fixed. It's very tricky, though.

The headphone jack is not connected to the preamp's circuitry at all. Instead it is connected
to the speaker switch circuit and is driven by the power amplifier's outputs through a pair of
series resistors, but only if the amplifier's outputs are connected to the rear panel of the PAT-5.
http://www.updatemydynaco.com/docume...-5Original.pdf

Last edited by rayma; 3rd May 2016 at 01:35 AM.
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Old 3rd May 2016, 01:22 AM   #3
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Quote:
Originally Posted by rayma View Post
Those volume controls were often poorly balanced. Try running from the tape out
into another amplifier that has its own volume control, and see if it's ok then.

Thanks! I will try this out.

Can I re-balance the Dynaco's volume knob if the tape out levels are correct or is it not even worth trying? I've never had a problem like this before so I'm not exactly sure how to properly re-balance it if so.
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Old 3rd May 2016, 02:11 AM   #4
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I noticed you edited your post while I was replying. Thank you for the clarification on the headphone situation. That is properly wired and works great now.

At this point i will look into correcting the volume knob's balance as I'd sure love to use this as my main preamp.
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Old 3rd May 2016, 02:25 AM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by bankroll1972 View Post
I noticed you edited your post while I was replying. Thank you for the clarification
on the headphone situation. That is properly wired and works great now.
At this point i will look into correcting the volume knob's balance as I'd sure love
to use this as my main preamp.
Set the volume control at around the right level, then hold the volume knob fixed,
while using the slot on the back to adjust the channel balance with a screwdriver.
If you are patient, the channels can be matched pretty well, at least around
that volume level. The adjustment will be very touchy to get right, but it can be done.

Last edited by rayma; 3rd May 2016 at 02:32 AM.
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Old 3rd May 2016, 03:00 AM   #6
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Quote:
Originally Posted by rayma View Post
Set the volume control at around the right level, then hold the volume knob fixed,
while using the slot on the back to adjust the channel balance with a screwdriver.
If you are patient, the channels can be matched pretty well, at least around
that volume level. The adjustment will be very touchy to get right, but it can be done.

Thanks!!!
I figure I will use some sine wave tones to test with.
I appreciate your help.
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Old 3rd May 2016, 03:08 AM   #7
rayma is offline rayma  United States
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Quote:
Originally Posted by bankroll1972 View Post
Thanks!!! I figure I will use some sine wave tones to test with. I appreciate your help.
No need for sine waves, as any source will do. Just first set the balance control to the center,
and set the mode switch to mono. It might be best, if you are doing this by ear, to move the
speakers close together, to avoid complications from room acoustics.
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Old 4th May 2016, 02:19 AM   #8
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Rayma, I did what you suggested with the tape outs before attempting to calibrate the volume knob. Fortunately the levels are proper on the tape out. Hopefully this just means I need to re-calibrate the volume knob? Is there any more "permanent" fix than your suggestion of holding the volume knob with a screwdriver while I adjust it from the inside? I only ask because I would hate to do this again, even if it's a year or more before it's necessary again.

One more question: After decades of not a single problem with my Hafler DH500 the overheating circuit kicked on today while using it with the Dynaco. I don't doubt the Dynaco is the reason why, because when the volume is turned the slighest bit up, the output is as loud as any of my other Adcom or NAD preamps turned up to at least halfway. Is the Dynaco just really powerful or is this something else I need to look into?

Last edited by bankroll1972; 4th May 2016 at 02:24 AM.
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Old 4th May 2016, 02:41 AM   #9
rayma is offline rayma  United States
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Quote:
Originally Posted by bankroll1972 View Post
Is there any more "permanent" fix than your suggestion of holding the volume knob
with a screwdriver while I adjust it from the inside?

After decades of not a single problem with my Hafler DH500 the overheating circuit kicked on
today while using it with the Dynaco. The Dynaco just really powerful or is this something else
I need to look into?
That's the only easy fix for the volume control, other than just using the balance control to compensate
for the problem. The adjustment should stay as you leave it. A tech can replace the control for a moderate cost.

I doubt very much that the Dyna is causing problems for the Hafler. The DH500 is probably showing
its age and needing attention and repair, which likely will be expensive to properly perform.
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Old 4th May 2016, 02:45 AM   #10
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Quote:
Originally Posted by rayma View Post
That's the only easy fix for the volume control, other than just using the balance control to compensate
for the problem. The adjustment should stay as you leave it. A tech can replace the control for a moderate cost.
Thanks - I will probably just do this unless it becomes a problem again.

Quote:
Originally Posted by rayma View Post
I doubt very much that the Dyna is causing problems for the Hafler. The DH500 is probably showing
its age and needing attention and repair, which could be expensive to properly perform.
The only reason I could think this is that I had the volume turned up a bit and the balance set so the louder speaker would match the quieter one. I've never seen it happen before or since.

I thought maybe the Dynaco was just too powerful for my power amp because the slighest turn on the volume knob up from silent makes it go as loud as any of my other preamps turned at least to halfway.

Since it's never happened before on the Hafler in my life, and the Hafler manual says it shouldn't ever happen unless you're feeding it with an unusally strong signal... which is what I'm worried is happening.

I know my Hafler is due for a tune up sooner or later but I'm more worried about the Dynaco for now
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