You might want to pull this full page up concerning the 416.....I have one that is like new after reading this not going use until doing this update.....very good analyst by Kervin and his write up the 416 fix.......very good...
gray .........
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:
- Flat-blade screwdriver(s) for chassis screws
- Needle-nose pliers
- Sharp side-cutters or dykes
- Small, bright flashlight
- Small magnifying glass or folding loupe
- Soldering iron (25W to 40W, small tip)
- Fresh rosin-core solder, small diameter
- De-soldering braid
- Can of rosin flux
- Spray can of electronics cleaner with extension tube
- 4 ea. Vishay/Dale 750-ohm silicone power resistor, wire-wound 3-Watt (Mouser p/n 71-CW2B-750) or equivalent
- 4 ea. Vishay/Dale 1k-ohm silicone power resistor, wire-wound 3-Watt (Mouser p/n 71-CW2B-1.0K) or equivalent
- 6 ea. Fast-blo 6-amp, 8AG fuses (1/4- by 1-inch)
- 4 ea. Fast-blo 1-amp, 3AG fuses (1/4- by 1-1/4-inch)
The Procedure
http://home.indy.net/~gregdunn/dynaco/components/ST400/chnl_fix.html