Dynaco Stereo 120 Repair Information

Some people would rather repair their Stereo 120 without updating it, maintaining the original design. That can be difficult. We hope this web page and the hints it offers could make it easier.

Power Supply Circuit Description and FAQs

If the power supply doesn't work, then the amplifier certainly won't work. This white-paper contains a circuit description for the power supply and FAQs. These might help you troubleshoot problems with your power supply. If all this information seems like a bit much, you might choose instead to buy the PSUG, or Power Supply Upgrade Kit.

If your investigations show that some of your power supply caps have lost capacitance, then the PSRC Power Supply Replacement Capacitor Kit may fit the bill.

If you're looking for the ultimate in bass, then the C12XX kits may bring a smile to your face. They increase the power supply output filter cap from 3300 uF to 20,400 uF, greatly increasing the amp's ability to deliver powerful, distortion free bass.

Original Amplfier Module Circuit Description

This white-paper contains a circuit description for the original Dynaco Stereo 120 amplifier module. An alternative is to build upgraded module kits.

Original Assembly Manual

Of course, repairs are much easier to do with the schematics and parts lists as shown in an original assembly manual. The pdf files below are high resolution scans of an original repair manual, and include the later schematics with their additional components.

Stereo 120 Revision History

The Stereo 120 was in production a relatively long time. Changes were introduced throughout that production to make it better and/or more reliable. You can track the changes by the component numbers. For example the R numbers were added sequentially as resistors were added in the course of production history. I reviewed 5 or 6 Stereo 120 Assembly manuals that I have, and have reconstructed a history of the improvements.

It may be worth looking carefully at your amp. If it has original modules and it isn't working, then it's possible it died from the lack of the above cited modifications. You could repair it, or you could consider the update kit.

Checking C7, the output Capacitor

If there's no output or small output, but the power supply voltages are normal, and there's no abnormal heat or smoke, the output capacitor, C7, may have opened up. After 40 years, this can certainly happen. Even if the cap didn't open, it may have lost capacitance. The straightforward way to tell is to disconnect the capacitor, and measure it on a capacitor bridge. If you have one, consider yourself fortunate, and you can probably stop reading. But then again, what if you could test the output capacitor without de-soldering anything? To do this test, youíll need:

Proceed at your own risk. The amplifier, when open, exposes DC voltages of up to 100 Volts, and AC voltages of 120 Volts RMS (240 Volts RMS) in some cases. Please work safely. Keep one hand in your pocket as you probe.

I wonít belabor all the steps. Instead, Iíll just give the big picture. Of course, youíll have to remove the cover to get access to the capacitor leads. For a given channel and capacitor:

  1. Connect the amplifier binding posts to an 8 Ohm load (RL) or a loudspeaker.
  2. Connect a 31.8 Hz sine source to the input, setting its level to produce 8 Volts RMS across the 8 Ohm load resistor, using your meter to set the output for 8 Volts. Call this voltage Vout.
  3. Measure the AC RMS voltage across C7ís terminals. Call that voltage V7.
  4. Use the following formula to determine C7ís value (assumes RL=8, f=31.8 Hz, and VL=8 Volts RMS):

C7(uF)=5000/V7, where V7 is in volts

If you find that C7 is a bit low, or if you just want better bass, you may find the C7X2 kit of interest.

Adding a volume control to a Stereo 120

Sometimes people like to just pair a Stereo 120 with a CD player. Other times, they'd like to change the sensitivity of the Stereo 120 to put the accompanying preamp's volume control into a better range. You can accomplish both of those by adding a volume control. I suggested installing the VC101 Kit between the I/O connectors to a customer, Richard. It worked out very nicely, and Richard was kind enough to send along pictures of the result.

Stereo 120 with a volume control. Stereo 120 with a volume control, picture 2.

You'll note that the VC101K comes with plenty of shielded cable, but it's not really needed for this application. The placement is so nice that the volume control wiring is quite short. Twisted pairs do a good job.

A Magnetic Mystery Solves A Hum Problem

A customer assembled and installed the TCK (updatemydynaco amp modules) and a PSUG into the Stereo 120. Along the way, he tried to reduce the mechanical hum of the power transformer by mounting it up off the chassis on grommets. I'll make a long story short...Don't mount your power transformer on grommets!

The Dynaco transformer has beefy E-I laminations. In the stock arrangement, those laminations rest directly on the steel of the chassis. This confines the leakage flux very nicely. If you raise the transformer off the chassis, there's a good sized air gap which lets a lot of the flux density travel. In this customer's mod, the left channel board ended up in a flux hot-spot. The right channel was in a nice null. The confusing result was that even with the recommended grounding scheme implemented, the right channel was dead quiet, and the left channel hummed.

Eventually, we figured out the problem described above, dropped the transformer back down onto the chassis, and now both channels are quiet.

A Belly Band to Reduce Stray Flux

At some point, we might even further reduce the stray (leakage) flux with a "Belly Band". A quick experiment with a bit of aluminum foil seemd to show a 3 dB improvement. As time permits, we should revisit the Belly Band in a more official way, with copper foil.

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