Reviving A Dynaco Stereo 120 For The Studio - Jim Kerkhoff.pdf

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Reviving A Dynaco Stereo 120 For The Studio - Jim Kerkhoff
Jim Kerkhoff
Down the Rabbit Hole
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THE PIANO PROJECT
REVIVING A DYNACO STEREO 120 FOR THE STUDIO
by Jim
May 17, 2018
Sound Recording
I am listening to a classic solid-state amp from the 70’s. Just a couple of weeks
ago, it arrived from eBay in rough and partially-working condition. I intended
to bring it back to working and reasonably-clean condition. The last two weeks
reminded me of my teenage lust for the
wunderkind
of the era, direct-coupled
solid state amps, and I was not disappointed.
Dynaco Stereo 120 just in from eBay
Back in the Day . . .
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Reviving A Dynaco Stereo 120 For The Studio - Jim Kerkhoff
Contents
[
hide
]
1 Back in the Day . . .
2 A few studio upgrades . . .
3 Let’s clean this mess up.
4 and add a few new parts.
5 So how did we do?
6 Related
I should explain. At the distance of some 40 years, my interest is supremely ironic. You see, I cut my electronic teeth on tube
equipment. It’s all I could afford. In fact, while still in High School, I built — from scratch — a complete vacuum-tube-based
stereo ampli er. It had high-quality output transformers and two 6L6’s in a push-pull con guration – Just like the tube amps
that are considered desirable and exotic today.
But it’s not what I wanted. Like all my friends at the time, I wanted a killer stereo. Transistors
were the
future and
I desperately wanted a solid-state (aka audiophile) amp. I became aware
of such things through magazines like
Popular Electronics and
they regularly featured
projects that were available as kits from Southwest Technical Products Corporation
(SWTPC). They were most famous for their early microcomputer products, but also had quite
a few high-quality audio projects. But $115 for a two-channel, 180-watt ampli er  was just
too expensive for my meager resources. So I built a tube amp instead.
Years later, while managing a recording studio, I learned
of another audio kit manufacturer, Dynaco. A few of
their amps were in service around the studio, and
Southwest Technical Products
Corp. catalog page for the
Universal Tiger ampli er
knowledgeable audio friends thought they were
excellent. Later, I bought a Dynaco Stereo 80 on eBay
and used it to power the rear channels in my home
studio. I was especially glad to have it when the front
channel amp failed earlier this year and I could
substitute it to power the front channels.
Dynaco 1972 Brochure
A few studio upgrades . . .
Fast-forward to today, and the
repair of my JBL 4408 studio monitors .
Given the current interest in all things tube, I thought
about nding a high-quality tube amp, but I wanted it be as trouble-free as possible. Studio amps tend to sit in an equipment
rack or closet and run for years with no attention. Perhaps a solid-state amp was more practical. An eBay search led me to a
second, larger, solid-state Dynaco. The Stereo 120.
The eBay vendor had taken care with the packing, but it was clear this amp had
seen better days. A greasy, sticky lm covered everything and patches of rust
had formed on both the chromed chassis and perforated metal top. A look
inside revealed a less-than-careful build with loose circuit boards indicating an
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Reviving A Dynaco Stereo 120 For The Studio - Jim Kerkhoff
abortive attempt at repair. Making sure there were no obvious problems, I
slowly powered it through an isolation transformer and variac and it came
back to life. Well, partially. There was DC power, so thankfully the power
transformer wasn’t fried, and one channel seemed to work. Just as the vendor
had promised.
There are several large
electrolytic capacitors in the
A Dynaco Stereo 120 after years of service.
Stereo 120, and I checked them
with an ESR (equivalent series
resistance) meter. One of the
output caps was open, and likely the reason for the dead channel. Now the
hard part: where to nd replacements. This style of “computer grade”
capacitors hasn’t been made in decades, and most components have gotten
much smaller since the late 60’s. It wouldn’t be a problem to nd workable
parts, but the mounting strategies would have to change due to the smaller
size of the new components.
Defective Capacitor just behind the Dynaco
connections
Let’s clean this mess up.
Fortunately, information, including a complete assembly manual, was readily available on
the Internet. In fact, there was a surprising amount of interest in these old amps, including
a vendor,
updatemydynaco.com
who specialize in parts, repairs and even upgrades for
them. They also provide
schematics, a manual and troubleshooting guide.
A quick check
revealed a replacement output capacitor kit as well as new larger caps for the power
supply. Both promised to substantially improve the sound of the amp. I ordered both kits.
Meanwhile, the amp needed a thorough
Dynaco 120 manual cover
cleaning. You could practically smell the
french fries from its former life in a
restaurant or bar. It wasn’t hard to remove
everything from the chassis, and after a quick scrub with
Simple Green,
it and
the metal top both went into a tank of
Evapo-Rust.
That stuff is magic. A few
hours later, most of the rust had dissolved. I scraped the loose paint off the top
and then sanded it with 220 and 400 grit. A few coats of
satin- nish black
paint
and it would be good as new. Don’t forget to mask the Dynaco logo
attached to the cover!
Although most of the rust was
removed, the chrome chassis
still looked pretty dingy. A
couple of applications of Mother’s
Chrome Polish
 worked
wonders. As a nal
step, I polished all the chrome with
white jeweler’s polishing compound
on a
drill-mounted
buf ng wheel.
Not perfect, but the best it would be without re-
chroming. Time for assembly.
Cover after dissolving rust
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Chassis after using Mothers Polish
and add a few new parts.
First in, the power supply. Transformer, switch and fuse holder are easy, but
according to the updatemydynaco.com
assembly instructions,
a modi cation
was also needed to the power supply board.
This means detaching it from
the heatsink, removing a small
electrolytic and replacing it
with a new part. It was tricky to
get everything reattached, but
otherwise went well. Three
new caps replace the old one,
and triple the lter capacity.
Best of all, they t into the
existing mounting ring. It’s
important to use
thermal grease
when attaching the “L” shaped heatsink to the
Don’t forget the thermal grease!
Power supply and new caps in place
chassis. These amps get warm, and every thermal advantage is needed.
The two new output capacitors
use a similar mounting strategy — tting three, smaller capacitors into the
mounting ring from the old cap — but it’s a tighter t and requires a little
bending of the mounting ring. Like the lter cap, the output caps end up having
three times the capacity. Unlike the lter cap, the output capacitors are wound
with an inductor. A portion of the old wire is used and is wired the same way as
the old inductor. The
excellent assembly instructions
from
updatemydynaco.com are easy to follow and show these details well.
Last of all, the two output
channels are mounted after an
application of thermal grease
like the power supply board.
Two wires are reconnected to
each of the new output capacitors and two power connections are replaced on
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Output capacitors installed
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Reviving A Dynaco Stereo 120 For The Studio - Jim Kerkhoff
each board — just as they were before modi cation. The remaining
connections to speaker outputs and channels inputs are completed and it’s
time for testing. I applied power through the isolation transformer and variac,
and measured supply voltages in several places. All was well.
Completed wiring.
So how did we do?
A couple of interesting items surfaced as I researched the amp. They tend to run a little hot, and the biasing on the output
stage changes as they warm. When cold, they may suffer from slightly increased distortion at low levels. As the amp warms
and the biasing changes, the low-level distortion is reduced and the amp sweetens with more 2nd than third-order distortion
— resulting in a warmer sound. Judging by the apparent longevity of this particular amp, and since I didn’t change anything
that would effect output stage biasing, I think we’re good to go. The refurbished Dynaco Stereo 120 sounded good at power-
up, and did improve slightly as it came up to temperature. I’m going to call this a win!
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This entry was posted in
Sound Recording
and tagged
amp, ampli er, classic, Dynaco, rebuild, repair, restore, solid state, stereo 120, updatemydynaco.
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