Dynaco PAT-5: channel imbalance and dead headphone jack?

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?
 
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rayma

Member
2011-04-29 8:37 pm
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/documents/PAT-5Original.pdf
 
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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.
 
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.
 

rayma

Member
2011-04-29 8:37 pm
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.
 
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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.
 
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?
 
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rayma

Member
2011-04-29 8:37 pm
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.
 
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.

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
 

rayma

Member
2011-04-29 8:37 pm
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.

If the amplifier is heavily driven for a few minutes it would heat up a little, but it would then return
to normal soon after the volume is reduced. Is the internal fan working properly?
 
Despite all Nostalgia opinions Dynas were assembled with COST as the Prime directive.
Minimum parts/cost for max benefit (profit)
The pots used in Dynas were of Poor quality.. No ifs ands or buts on that.. all bleats to the contrary aside. In my experiences
Even cheap Chinese Clone Alps are a whole world 'better' performing.
No need to mention that Dyna ran the entire input signal through the various Lo grade pots
and even the damned headphone jack.. on it's way to the amp sections :rolleyes:
IMO.. Seriously suggest using a more Moderne (capable) design.
There are Far better quality products to fret over... even used .. for Cheap.
 
As far as I can tell all fans are working properly. Something did fall next to one of the fan vents, possibly blocking airflow, so I let it cool off for about 10 minutes and I'm running it again.

As for the Dynaco I might not have time tonight to try and fix the channel imbalance... that doesn't really explain why it gets so loud so quickly though. I fear I'd blow my speakers out if I turned it to even 9 o'clock on the PAT 5 as is!
 
Sighh.. again: the Pots are Junk, they rot out easily. Decades ago most likely
The Channel imbalance is Entirely due to crapped out pots Start with the Volume then the Balance then the treble and Bass ones.. sequentially for Sanity's sake
IMO Replace ALL of them and it Will work 'better' ...within the limits of the Dorky design.
You bought the thing for a Very few $.. for Reason.
Or at least you should have :)
 
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Sighh.. again: the Pots are Junk, they rot out easily. Decades ago most likely
Replace ALL of them and it Will work 'better' ...within the limits of the Dorky design.

Thanks for the advice. I am new to Dynaco stuff and didn't have the impression these were the nicest pots out there anyway. Might get around to modding it sooner or later.

For now i'm more worried with my Hafler shutting itself off but if it stays stable for a day or so I'll get messing with the Dynaco maybe.
 
I'd say pots that lasted 30 years were not junk. But the 30 years are over now, changing the volume pot is long overdue. I replaced the one in my 1961 built PAS2 about 1982.
I knew some students that bought kenwood & sansui receivers about 1970 and the volume pots were jumpy by 1972. I bought dynaco because dynaco SC35 were in the college library & used ~12 hours per day and had gret performance on headphones with proper maintenance. My best friend's Heathkit AR15 receiver, the volume pot was bad in 5 years.
If you can't find the pot you need at newark/mouser/digikey there is a specialty house dynacodoctor. He is one of the few sources of the 250k pots I need for the PAS2. I think a transistor pre may have 10k log pots or something. Don't forget the power supply electrolytic caps they cost less than the freight on the box for the volume pot. I've done those twice on the PAS2.
I had channel imbalance on the PAS2 at 30 years due to a deteriorated paper capacitor. It is not always the volume pot.
The builder had burned the wax case off a paper cap and hid it by putting the hole on the bottom. Surprise the air found it anyway. I replaced 10 .2 uf caps before I found that one. @#$%& now the sound is too trebly with .22 plastic film caps. Someday I'll redo it with .018 uf parallel .0022 plastic caps, but until then I've upgraded a disco mixer with 33078 op amps fo it is pretty close to the PAS2 in sound. Not quite though.
 
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