Dynaco Stereo 400 Power Amplifier - Dead Channel Fix - Dynaco Repairs For PC-28 Amplifier Boards

My Dynaco Stereo 400 Amp...

The Page Below these e-mails with Kevin Boales. Looks like the one that I found. Back in 2006, when I did the fix on my Dynaco Amp. I e-mailed back and forth with Kevin, the Author of the Site. He was very nice and said I could ask him any questions that I had. Or about any problems that I ran in to with the Fix and he would help me get my Amp Fixed. Here's the original e-mails with the info Kevin shared with me... 

Don

From: Kevin Boales [kboales@****.***]
Sent: Friday, January 20, 2006 1:39 AM
To: *******@*******.***

Subject: Your ST-400

Don,

First of all, you take great close-up photos.  Before my health caused me to retire, I had been a photojournalist for almost 30 years.  I spend many of those years on the staffs of Hot Rod Magazine, Car Craft, PHR and Truckin'.  I was a technical editor for each of those (and a few others under the same publishers). 

As far as your offer for additional photos, I would appreciate any others you may wish to offer very much, indeed.  Wherever I might use them, I'll give you credit for the images. 

Now let's talk about your amp...  

Although you explained that the amp has problems, you weren't specific about them.  I'd guess it has a dead channel, but if there are other things that don't work, please let me know and we'll get them fixed, too.

1.)  It's extraordinarily clean, to begin with.   It appears to have been beautifully wired originally, but also appears to have been apart several times during its life, based upon the twisted and tangled power lines (the yellow and white wires at the center of the chassis).

2.)  The input signal wires (the twisted pairs of red/black and green/black) should be run from their grommet in the heat sink and forward, at enough angle to keep them reasonably clear of the heavier yellow, white and blue wires.  About an inch forward of the heat sink, run them as closely to the left side of the chassis as possible toward the PC-29 board and back again from there to the PC-28 amplifier boards.  Make certain you keep them clear of the left-side grille screws, where they might be cut or shorted by the sharp edges on the screws themselves.  I use a series of small ty-wraps about an inch apart to bundle the four paired wires along their path, and a small nylon hold-down to keep them clear of the grille screw points.

3.) Your PC-28 boards are a later, double-sided design that replaced the single-sided PCBs found in the earliest ST-400s shipped by Dynaco.  The design change allowed Dynaco to eliminate all but one or two of the jumpers found on the early boards, and to rearrange the warmer-running transistors so their radiators could work more efficiently.  The same circuit was retained on the double-sided PCBs, with only a few enhancements.  One of those was the addition of R47, which is located between the signal input eyelets #1 and #2.  The resistor is there to establish a fixed maximum gain for that channel's PC-28 board, and was not used on the single-sided boards.

 4.)  R29 and R32 should be replaced with the 750-Ohm resistors (purple-green-brown-gold), and R28 and R33 with the 1K (brown-black-red-gold) parts. 

Here are my observations from the PDF images:  

I'm not quite sure if I'm looking at the boards as they appeared originally, but I'll presume the images show the existing parts, and that you will install the new resistors you bought from Mouser.  

In your photos, the 1K resistors (those big, red boys) appear to be 3W, or maybe 5W carbon replacements, but the 750s appear to be original.  The replacement resistors should not have a higher Watt-rating than 2 or 3W as a maximum, especially if they are wire-wound types.  They should be mounted slightly away from the surface of the boards for ventilation -- they get plenty hot when the amp is in use. 

Left Channel:

* There are several bad solder joints visible in the slide show images.  They're in stark contrast to the perfect connections along the eyelets and on the majority of other passives on the boards.  I appears that someone might have replaced two or more of the driver transistors (the four rectangular devices under the bracket), and one (Q8, also on the bracket, in the center) of the two transistors (the other is Q11) used for temperature compensation.

Specifically, the visibly-crummy soldering appears on the Base of Q8, the Base and Collector of Q12, and the Collector on Q14.  Ideally, those connections should be de-soldered, cleaned and re-soldered.  However, don't do it until we know if the new resistors fix the amp.

 Right Channel: 

 * The 750-Ohm resistor near the right-hand side of this board appears to be fractured under the green band, and the other 750-Ohm resistor appears to be fractured to the right of the gold tolerance band. 

* R24 appears to have a split near its center, but it might just be the angle or lighting. 

Don, you should know that many amp technicians won't even attempt to repair these things.  It may still be dead on one channel after you replace the resistors, but the problem will most likely be due to a bad solder joint somewhere.  Whatever it is, we'll find it and fix it.  I'm explaining this because I don't want you to give up if the amp isn't fixed with the new resistors.  If you've never heard one of these things at work, get ready for an epiphany.  They stay clean at any output, they never run out of gas when working hard, and after we get it back on its feet, it will probably work for the next thirty years. 

One last thing: These amps (ST-400, ST-410 and ST-416) should be driven with a solid-state preamp with a low output impedance.  Almost all tube preamps have a very high output impedance that is incompatible with the 40K (approximately) input impedance of these amplifiers.

Kevin

From: Kevin Boales [kboales@*******.***]
Sent: Thursday, January 19, 2006 11:44 AM
To: *******@*******.***
Subject: Re: Dead Channel Fix - Dynaco Power Amps

Don,

Thanks for your comments on the fix page. The extra stripe on those original resistors were originally gold, to indicate that they were close-tolerance parts within the 5% limitation Dynaco specified. I'll look the PDF over a bit later for more details -- I downloaded it without any trouble.  Sorry, but I'm a little on the run this morning.  I'll get back to you with more this evening.  We'll get that amp to work!

 
Kevin

 

Sadly, Kevin was very ill and died soon after, this e-mail. One of his relatives, sent out an e-mail letting everyone know. Also, it was before I was able to get back to him and finish the project. I knew that he was ill, but didn't know just how ill he was. It's 6 years now and I still haven't been able to buy the Multi-Meter that I need to finish my Amp Fix and do the Calibration and Setup portion of the Fix (my Multi-Meter is a 1970's Radio Shack Analog Meter and it wont work for this). My Dynaco 400, is still taken apart in the Garage!:O With the new Resistors installed and waiting to be Calibrated and Tested out. The other Page (below), is a good one with the Specs of the Dynaco Stereo 400 Amp. I also have a Dynaco Preamp. That I got from my buddy, Lee. At the same time, when he gave me the Amp. His Dad was an old Jazz Musician and after he died. My Buddy, didn't want the Amp and other Audio Gear, since he didn't know how to set them up again, after he moved. So, I got to inherit the Amp and some other Cool Stuff too. I got a TEAC 4-Track Reel To Real and a Stereo Real to Real and a TEAC 4 Channel Mixer, from Dad's Recording Setup. We use to hang out in Dad's Recording Room and listen to his old Recordings and sometimes Dad would Play his Vintage Les Paul!:) Yes, Lee kept the Les Paul;) Dad's Recording Setup. Was complete, with a Custom Made Desk and an Audio Gear Rack that Dad, made him self (I let the Recording Gear, Slip Away later). And I was planning on Remixing those old 4-Track Tapes and Digitizing them for Lee. I also, got the Dynaco 400 Watt Studio Amp (that I already mentioned, which needed some repairs, that I did, but don't have a meter to set it properly, so I can't use the amp). A Preamp (which I use ever day for my Home Audio Setup) and an Analog AM - FM Tuner (it's in my Rack, just for fun). Oh, and an RCA Patch Bay, that Dad Made too. I use it to combine all of my Home Audio and Computer Outputs into one Input, going into the Preamp and then out to 2 Different Amps. A 100 Wat, Harman Kardon hk395i. Connected to, two 12 inch Speakers with Horns and two 4 inch Speakers with Horns (for Mid Range, mostly). Also, I have a 5.1 PC Speaker System hooked up to the Preamp too. It actually sounds pretty good, to an old Live Sound Tech. I've never owned any of the Expensive Sound Gear. But I have heard and mixed sound with some pretty good PA's over the years. 

My Audio Rack with Dynaco Gear, Harmon Kardon, JVC CD Player and newer TEAC Cassette Deck...

(At the Bottom of this Post are More Pics of my Dynaco Stereo 400 Amp, new Resistors and the Bad one's that I Replaces, as I had it apart for Repairs.)  

 

Don

Here's Kevin's Original Page...

Dead Channel Fix — Dynaco Power Amps

by Kevin Boales — January, 2002

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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

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.

• Remove the cooling fan (ST-400/416 only) if so equipped.
• Remove the perforated metal cover (ST-400/416) or remove the screws that secure the cabinet to the chassis (ST-410/150).
• Tilt the chassis onto its side (ST-400/416 only) and remove the pairs of screws (if installed) securing the heat sink to the chassis. Do not attempt to remove the rubber feet.
• Remove the pair of screws at each end of the heat sink (ST-400/416 only) and the screw (if installed) securing the sheet metal power supply isolation panel to the front of the sink. — OR — (ST-410/150) For better access to the PC-28 boards, remove any chassis screws that will prevent the rear (heat sink) panel from being tilted rearward.
• Gently rotate the heat sink assembly (ST-400/416 only) 90-degrees rearward to position the amplifier boards up where you can see what you're doing. Things might be a little less convenient with the ST-410/150 chassis, but the idea is to get maximum access to the boards. Be very careful not to pull on the wiring — especially the wires connected at the bottom of the circuit boards. One of Dynaco's signature touches for their solid-state amplifiers was a fold-out architecture to facilitate service and repairs. There will be sufficient wire length to allow complete access without straining the connections.
• Please refer to [Figure 1.] (ST-400), or [Figure 3.] (ST-410).
   Each PC-28 board is secured to its heat sink with a pair of 6-32 screws that pass though 1-inch aluminum spacers. These screws are immediately next to another pair, used to secure the heavy aluminum mounting bracket/driver heat sink to the circuit board. Be very careful not to disturb this second pair of screws.
• With the 6-32 screws and spacers removed from one board, carefully rotate the PC-28 away from the heat sink to gain access to the trace side of the board. The less you work the wires along the board's edge, the better. If possible, position it so that you can see both sides — you'll want to locate the resistors on one side, and then find the trace pads where their leads are soldered on the other side. Unfortunately, the resistors are connected at points very close to the aluminum driver bracket/heat sink. It makes for cramped work, but your patience will be rewarded.
• Do your best to stay away from the white silicone heat-transfer grease on the mating surfaces between the board's mounting bracket and the heat sink. If it migrates onto the circuit board, it can interfere with any soldering you do. On the other hand, this repair probably cannot be completed without getting some of this stuff on your hands, and likely on your clothes. It washes off with soap and warm water, and I've never known anyone who had a bad reaction to short exposure.
• Use the magnifying glass and flashlight to examine EVERY solder point on the trace side of the board. Dynaco did most of their wave soldering in-house to maintain quality and cost controls, using outside vendors only when necessary. Nevertheless, there were occasional glitches that sometimes resulted in boards that were completely functional, but cosmetically, well – impaired.
• If you see a solder point that is obviously cold, dry or (worse) cracked, consider cleaning and re-soldering it. Actually, you might want to complete the resistor replacements first, then do any re-soldering after you've verified that the amp works. If there are many connections that you'd like to tune up, you should invest in a solder designed for PCB repair work. Mouser has a complete selection of special-purpose solders, including a couple with quick-acting fluxes that assure you of a clean, solid connection with a minimum of applied heat. Incidentally, always heat the trace, not the component lead. This is especially important for small-signal transistors, of which there are nine on each PC-28 board.
• To remove the original 2-Watt resistors, you'll need to first de-solder their connections to the circuit board. There are two popular ways to do this: (1) de-soldering braid or (2) vacuum de-soldering pen. You might be tempted to simply heat each connection point while pulling on the resistor. If either resistor lead was trimmed with dull cutters (a distinct possibility), the lead will have a burr at its end sufficient to destroy the trace pad when you pull it through. Fixing wrecked traces is tedious work, to be avoided whenever possible.
• Use your choice of techniques to remove the solder from the connection points. The resistor should now be fairly loose, but avoid the urge to wiggle it, lest you damage the traces. If the ends of the leads are flattened or burred, use a sharp pair of cutters to trim them clean. At this point, it is safe to remove the resistor one lead at a time. If necessary, touch the iron to the trace as you gently pull each lead through its hole.
• If any of the traces are broken, you may be able to create a suitable connection using the excess lead wire on the new resistor. Bringing a lead through the board and putting a 90-degree bend in it may allow you to overlay the original trace, creating a bridge from the lead's hole to the surviving trace path. The caveat here is that any contact with a resistor installed in this fashion will likely break the repaired trace by lifting it off the board. However, you can use certain epoxy compounds (Mouser, again) to reinforce a repair made this way, improving its survivability.
• Prepare each new resistor's leads by forming them (avoid sharp bends — use the original parts as a guide), trimming to length and tinning. Working with one part at a time, install each new resistor's leads into their proper holes. Touch the tip of the iron into the can of flux and immediately move it to the connection point. Heat the trace and feed enough solder under the iron's tip to create the connection. This process, when done properly, should require only a second or two of contact with the iron. Any more usually means you've roasted the component or the board.

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.]

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.

Figure 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.

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Go there...
http://home.indy.net/%7Egregdunn/dynaco/components/ST400/chnl_fix.html

Dynaco Stereo 400 Power Amplifier

View the
schematic,
parts list,
pictorial diagram(very large),
MBI-400 pg. 1,
MBI-400 pg. 2,
MBI-400 pg. 3,
MBI-400 pg. 4,
MBI-400 pg. 5,
MBI-400 pg. 6,
MBI-400 pg. 7,
MBI-400 pg. 8
for the ST-400. The MBI-400 circuitry converts a ST-400, 410 or 416 to bridged monophonic operation at 600W. You may want to right-click (Mac users click/hold) to save these, as they will slow your browser down significantly while loading.
Kevin Boales has provided an excellent article on fixing an ailing ST400; give it a look! Go there...
http://home.indy.net/%7Egregdunn/dynaco/components/ST400/chnl_fix.html

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Description:
The Dynaco Stereo 400 is a basic two channel power amplifier employing all silicon solid state devices. They include 46 transistors, 47 diodes, 2 silicon controlled rectifiers and 4 integrated circuits. The series-connected output circuitry is of full complementary symmetry design. The entire main amplifier is direct coupled. Its measured distortion levels approach the threshold of the most exacting laboratory test equipment, not only at the commonly specified full power ratings, but more importantly at very low power outputs as well. This is the result of circuit techniques which eliminate any discernible crossover notch at low levels, and contributes to the Stereo 400's freedom from listening fatigue. Its lucid, utterly uncolored neutrality reveals delicate nuances and musical shadings in the best program material - one result of achieving unconditional stability with varying loudspeaker loads.
Implicit in the design of a superb power amplifier is the responsibility to include superior techniques of protection for both the amplifier and the speaker load, if long-term trouble free operation is to be achieved. To this end, Dynaco has included more separate protective systems and features than have ever been employed in such a product.
Skipping one down...
Specifications:
Power Output:
20 Hz to 20 KHz, both channels driven:

200 watts continuous average per channel @ 8 ohms;
300 watts continuous average per channel @ 4 ohms;
100 watts continuous average per channel @ 16 ohms.

Monophonically:

600 watts rms @ 8 ohms.

Intermodulation Distortion: Less than 0.1% at any power level up to 200 watts rms per channel into 8 ohms with any combination of test frequencies. Distortion reduces at lower power levels. Typically less than 0.03%.
Harmonic Distortion: Less than 0.25% at any power level up to 200 watts rms per channel into 8 ohms at any frequency between 20 Hz and 20 KHz with both channels driven. Distortion reduces at lower power levels. Typically less than 0.05%.
Power at Clipping, single channel, 2500 Hz, less than 1% distortion:

235 watts @ 8 ohms;
350 watts @ 4 ohms;
135 watts @ 16 ohms.

Power bandwidth: 5 Hz to 35 KHz at less than 0.25% total harmonic distortion - half power output, 8 ohm load.
Frequency Response: +0, -1 dB, 8 Hz - 50 KHz @ 1 watt into 8 ohms; ±0.5 dB, 20 Hz - 20 KHz @ 200 watts.
Hum and Noise: Better than 100 dB below rated output, full spectrum.
Input: 50,000 ohm load; 1.6 volts for 200 watts @ 8 ohms.
Slewing Rate: 8 volts per microsecond.
Damping Factor:

Greater than 80 to 1 KHz into 8 ohms;
Greater than 30 to 10 KHz into 8 ohms.

Channel Separation: Greater than 60 dB by IHF standards.
Connectors: Inputs: phono jacks. Outputs: Color coded 3-way binding posts with standard 3/4" spacing.
Dimensions: 17" wide;, 14" deep;, 7" high.
Weight: Shipping weight 65 lbs; Net weight 54 lbs.
Power Consumption: 120 VA quiescent; 11 amps maximum; 50/60 Hz, 120/240 VAC.
Designed by:
Wade Burns (Dynaguard™), Jim Bongiorno, Erno Borbely, Harry Klaus, Hans Frank
Year Introduced:
1972
Read More...
http://home.indy.net/~gregdunn/dynaco/components/ST400/

Here are More Pics of my Dynaco Stereo 400 Amp. With the new Resistors and the Bad one's that I Replaced, as I had it apart for Repairs. 

Don

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