Dead Channel Fix — Dynaco Power Amps

by Kevin Boales — January, 2002

Applies to: Stereo-400, ST-150 and ST-410 Amps. These models (and the ST-416) each contain a pair of Dynaco PC-28 Amplifier PCBs — one board per channel.

Overview

These power amplifiers can often be found in on-line auctions or local classifieds. I've found that a surprising number of these models are offered for sale "as-is," with the seller citing that the unit has only one "good" channel. I recently saw a Stereo 410 in fine condition — except for a dead channel — for less than fifty bucks at www.eBay.com.

It would appear that many of these amplifiers developed channel failures after they had accrued some time in service. The failures are often intermittent in nature and defy logical cause-and-effect troubleshooting. When these channel failures occur, there are usually no obvious clues as to what happened. All the fuses will be intact, and there will be no smoke or death-fizzle noises. Most of the time, the circuit boards and wiring will look fine.

In fact, most of these mysterious failures have a common cause that can be corrected by anyone who is reasonably familiar with handling printed circuit boards, knows how to use a 25W-40W soldering iron, and has about five bucks for a handful of new parts.

These models are each equipped with a pair of Dynaco PC-28 amplifier circuit boards. One edge of the board has eyelets for the wires connecting it to the rest of the amp's circuitry. Near each corner at either end of this edge are a pair of 2-Watt, carbon composition resistors. On earlier boards, they are mounted vertically; on later boards they are mounted horizontally and tightly against the board.

The Problem

Each resistor pair consists of a 750-ohm (purple-green-brown-gold) and a 1k-ohm (brown-black-red-gold) 5% resistor. In retrospect, it seems that their 2-Watt power rating may have been inadequate for the job. They function as emitter bleeds for the driver transistors, which are mounted to the aluminum heat sink/mounting bracket attached to the PCB.

The four resistors (two pair) on each PC-28 board run fairly hot even at modest power levels. Over time, this takes a toll on their phenolic outer shell and the integrity of the carbon compound resistance medium inside. The thermal cycling of these resistors can result in invisible hairline cracks in the carbon compound or phenolic shell. If the resistor does not crack, its expansion from the heat can put enough pressure on the trace pads to crack them — sometimes lifting them off the board entirely.

Either way, the emitter circuit is broken and the channel stops working.

About The Replacement Parts

Resistor technology has come a long way since the mid-seventies. This progress was driven by many factors, primarily the advances in miniaturization and a quest for thermal stability. Carbon compound resistors are massive compared with today's wire-wound and metal-film replacements. As such, carbon resistors tend to retain heat to a greater degree than the newer, smaller parts.

In addition, a carbon compound resistor operated constantly near the upper end of its power rating tends to gain in resistance value due to the effects of time and elevated temperature. In the case of the resistors in question, even a slight change in their value can wreak havoc with one of the amplifier's critical operating parameters: power section bias current.

In the Procedure section that follows, you'll be replacing these original carbon compound resistors with high-temperature, silicone-coated, wire-wound power resistors. You will need a total of four 750-ohm, and another four 1k-ohm (not 1.1k) 5% resistors to upgrade both PC-28 boards in your amplifier. These parts should have a power rating of at least 3-Watts. I ordered mine from Mouser Electronics on-line. The catalog listed their rating at 3-Watts, but the parts arrived with markings indicating a 3.75-Watt power rating – nearly twice the power rating of the originals. Mouser also carries 5-Watt versions — use them if you want a little extra insurance.

Required Tools and Supplies:
The Procedure

First, the boilerplate. Your safety and the outcome of this repair are entirely your responsibility in every usage of that phrase. In preparation, you should be familiar with the proper and safe handling of solid-state electronic components. In particular, you should be aware that power amplifiers present unique dangers to anyone attempting repairs or modifications due to the presence of very large storage capacitors. These components typically retain sufficient electrical charge — even if the amplifier is unplugged — to cause a potentially-lethal shock hazard.

Note: We will describe the repair procedure in the context of the ST-400 power amplifier because it has the most mechanically complex chassis among these amps. The others should be easier to work with because of their simpler construction.

PC-28 Mounting Detail

When you're done, re-install the board with the 6-32 screws (and spacers) and repeat the process on the other PC-28 board.

Calibration and Setup

When both boards are equipped with the new resistors and resting comfortably on the heat sink, it's time to set the amplifier's bias current and input offset voltage. This is done with the two trimmers on each board, preferably before you screw the heat sink back into its normal position in the chassis. The trimmer near the center of the board is the bias trimmer, and the one mounted at the top edge of the board is the offset trimmer.
Please refer again to [Figure 1.]

Power Supply Fuse Detail for ST-400

You'll notice that there are four fuses mounted on the relay and capacitor assembly. These are power supply fuses, mounted in positive and negative pairs. Only the negative pair of fuses will be used for the adjustments. This pair of fuses is supplied by a heavy yellow wire — the positive pair is usually supplied by a heavy white wire.

Power Supply Fuse Detail for ST-410Figure 3. The ST-410 amps have their V- fuses positioned along the left-hand side of the chassis — as shown here. The V+ fuses are below the PC-30 Power Supply board.

Note also that the PC-28 boards are rotated 90-degrees from their orientation in the ST-400. In the ST-410, the offset trimpots (P1) are near the outer ends of the rear chassis plate.

Verify that all four of these fuses are Fast-blo 8AG types rated at 6 amps. 8AG fuses are about an inch long, as opposed to the 3AG speaker-load fuses used in the front panel of the ST-400 amps. 3AG fuses are 1-1/4 inches in length. Pull the two panel fuses (if your amp has them) and replace them with Fast-blo, 1-amp 3AG parts.

To set the bias for a channel, pull one of the two negative fuses, leaving the other in place. Using a multimeter set to measure DC milliamps in the range of 300mA or more, connect the leads across the empty fuse holder and turn the amp on.

There were inconsistencies in the way the channels were wired, so you must now detect which channel to adjust. Gently turn one of the bias trimmers (located near the center of each PC-28 board) and look for a change in your meter reading. If nothing happens, try the other channel. When you figure out which fuse leads to which channel, set the bias trimmer to result in a reading of 150mA. Shut the amp off and repeat for the other channel.

Now let the amp warm up for a half-hour, with all four of the fuses in place.

Shut the amp off and pull one of the negative power supply fuses, remembering which channel it is connected to. Set the meter to read DC milliamps, power up the amp and see if there has been any drift — a little is acceptable, but if it's more than 25mA off, the amp has other problems. Use the bias trimmer to re-establish a 150mA reading. Shut the amp off and repeat the procedure to set the bias for the other channel.

The last calibration step involves taking a reading (in DC millivolts) across each pair of output binding posts. Watch for negative readings and correct the polarity of the probes if necessary. Gently turn the trimmer mounted at the edge of each PC-28 board until the reading across the binding posts is as close to zero as you can make it. Actually, within 20mV is plenty good, but this adjustment has a great influence on the amount of harmonic distortion produced at lower levels of operation, so take your time setting it.

When all four readings are good, button everything up and change out the panel fuses if the amp has them. Before you set about installing such a cumbersome amplifier into your setup, try connecting a DiscMan to the inputs, and a pair of bookshelf speakers to the outputs. If your amp has gain controls, roll them completely off. If there are no gain controls, use the headset output on the DiscMan and set the volume very, very low. Power up the amp and start the CD player, giving everything a few seconds to get working. Gradually bring up the gain controls (or headset volume) until the amp is (hopefully) driving the bookshelf speakers at low volume. Let it run for a while to make sure everything is happy. When it has run successfully for an hour or two, the amp is then likely to operate without problems for the rest of your life.


Photo 3 originally appeared at eBay, item #1321035958. If it's yours and you'd like a photo credit, e-mail Kevin.