Axiom Home Page Forums General Discussion Stereo Dynaco Stero 70 tube amp

Well, my first post on tube amps.

I just got a hold on a Dynaco Stereo 70 tube amp. Very old, looks dirty and I dont know the operational state of the amp. I am researching internet to get manuals, or topology, or kits for repairing (in case it needs it)

I will try to get it working as soon as I can, as I have the chance to buy it from the owner, but I want to make sure that is recoverable.

What is the things I need to look for to be sure I can repair it? I know that I should not connect it without a speaker load, but I dont want to connect my M22's before knowing I am not going to fry anything.

If any hint on what to look for, let me know please.

Thanks

First thoughts, take 'em with a grain of salt.

1. As long as you have a source for tubes, you're probably OK. Transformers are next hardest to replace, anything else is a no-brainer by comparison. Complex rotary switches are sometimes hard to replace but it's unlikely they are beyond repair.

2. Having said all that, if the tubes all light up it probably works

Hello
Unless the amp has been in use, don't power it up without a varac. I'd get a nice multimeter and check the values for the capacitors and resistors. I'd replace the capacitors in the output section.

WARNING! If you are not experienced working with electronics do not touch anything in a tube amp as they can retain lethal voltages. More than one tweaker has been turned into a smoking raisin.

WARNING! DO NOT POWER UP A TUBE AMP WITHOUT SPEAKERS OR PREFERRABLY A RESISTIVE LOAD CONNECTED! IT WILL DESTROY THE TUBES.

Well, as least I was right on the load thing!

What value of resistive load should I use? 8ohm for example?

I am familiar with working with electronics. I am not familiar with tubes, though.

Checking the values of capacitors and resistors... I will need an schematic to know the actual "book" value, i guess. I will look harder on internet.

To check the transformers, any hint please... I dont know the specs of this transformes, hence I dont know how to check them..

This is going to be my "check and repair by wire" post. I will ask a lot in order to get it working.

Thanks

Series I or II?

Here ya go

http://www.curcioaudio.com/st7_mnl.pdf

http://www.mcmlv.org/Archive/HiFi/DynSt70ser2.pdf

It is going to be tough trying to check wired in components with a multimeter. Unless you want to do a lot of unsoldering and resoldering, I would recommend against measuring parts that are wired in a circuit. I would visually inspect everything to see if anything looks burned or charred. If it does, I would replace it.

It probably wouldn't hurt to replace all the filter capacitors in the power supply. Might as well replace all the electrolytic capacitors while you are at it.

I like the idea of using a variac to gradually raise the input voltage to insure that everything is operating properly before applying full AC input.

Thanks to Bob Schneider:

CHECKING OUT A COMPONENT
Any 30 year old electronic component is likely to have some problems. That doesn't mean you shouldn't buy one, because most such problems can be easily fixed (some can't, and you should stay away from such units). But you should have a good idea of the condition of any unit you're considering, so you know what you'll have to do before you can use it. And considering that all tube audio components operate on several hundred volts, you need to be extra cautious before operating an unknown component, due to the shock and fire hazards from defects and malfunctions.
There are also a couple of specialized concerns with vintage audio equipment. Often the seller has had it stored for many years before putting it on sale. Tube audio equipment can deteriorate while not used, so that a component which worked fine when it was put in the attic in 1978 may have serious problems when next plugged in in 1994. Also, many vintage audio sales are conducted by mail, either with vintage dealers or between private parties. While vintage Dynaco gear is much more solidly built than most modern equipment, it is quite old, and may not be treated as well by the shipping company as you would like. This may cause parts to become loose or fail which were fine when the unit was packed for shipping. Hence, the inspection procedures below should be followed even when buying a unit from a reputable dealer.

The first step is a power off visual inspection. ALL COVER OFF VISUAL INSPECTION MUST BE DONE WITH THE POWER OFF AND UNIT UNPLUGGED FOR AT LEAST FIVE MINUTES!! UNTIL THE CAPACITORS DISCHARGE, THERE ARE LETHAL VOLTAGES ON BOTH THE TOP AND BOTTOM OF TUBE DYNA COMPONENTS. Look closely at the unit with the tube cage or top cover off. Generally, the cosmetic condition is a matter of taste (and the nickel plated chassis of Dyna power amps clean up quite nicely with automotive chrome cleaner). But beware of significant rust (small rust spots on power amp chassis are normal, though), or other signs of water damage. An amp which has been waterlogged in its life is a potential time bomb. Also look to make sure all tubes and other components are present, and not broken or burned (some of the larger resistors may show a little charring, which is normal). Inspect the power supply capacitor (the large silver can on the top of the chassis) for any signs of chemical leakage (which means it needs to be replaced).

Then remove the bottom cover and inspect the underside of the component. Look for the same things you did on the top. Closely inspect the bottom of the power supply capacitor for signs of leakage. Also look for burned wires (color fading is normal), other burned components (some charring of circuit boards under large resistors is normal in old Dyna gear), and any wires which aren't connected. With power amps, also check the line fuse. A blown line fuse is a sign of trouble with old tube amps. (Dyna preamps and tuners didn't have line fuses.)

Also, try to test all tubes with a good tube tester (not at the corner drugstore anymore, I know--I bought one), and replace any which don't test good before going to power up.

Assuming the unit passes visual inspection (or any problems found have been corrected), the next step is a power on test. Put the bottom cover back on, but leave the top cover off. With power amps, connect a disposable speaker to the speaker outputs (an old car speaker is ideal), a shorting plug to the input, and a multimeter to the bias test point. For tuners, just connect a cheap antenna. For preamps, don't make any connections. Ideally, you would power up through a Variac, which is a large transformer designed to let you slowly raise the voltage from 0 to 120VAC. A homemade substitute can be made by putting an ordinary incandescent light bulb socket in series with an extension cord, and using a 40 watt or so bulb in the socket (THIS ALSO HAS LETHAL VOLTAGE ON SOME CONNECTIONS, SO INSULATE IT CAREFULLY). This will reduce the voltage fed to the amp. Position the component so you can see the tubes while standing next to a power outlet at least 6 feet away. Turn the power switch to on, and then plug it into your Variac or lightbulb/extension cord assembly. Only then plug the Variac (set to 0) or extension cord into the wall outlet, but be sure to be able to unplug it immediately at the first sign of smoke, fire, sparks, explosion or hissing from the amp itself (a cap going), or tubes glowing red hot. (Do this by unplugging the amp, not by the power switch, since a defect may put lethal voltages on the chassis.) If you're using a Variac, slowly start turning it up, but be prepared to drop to 0 VAC at the first sign of trouble. With power amps, also watch the volt meter, and cut power if the bias voltage goes above 2.0 volts.

Assuming no problems (the first power up test is the scary part, but I haven't had any explosions yet), next make sure all the tubes are glowing. One or two out may mean a bad tube or socket, but all of them out may mean a bad power transformer (expensive and difficult to replace). With a power amp, next set the bias voltage by adjusting the screwdriver adjustment near the output tubes. All Dyna amps are supposed to be set to 1.56 Volts, but I usually use about 1.4 volts to preserve tube life. Listen for any sounds from the speaker. Some hum and/or crackling sounds can be easily fixed, but lots of hum may mean serious power supply problems (bad), or bias supply problems (easy to fix). With all components, use your multimeter to make sure there is no significant DC voltage at the output. If there's more than 50-100 mV, replace the output coupling caps in preamps and tuners. In power amps, DC on the outputs is very rare, but most likely means a bad output transformer (very bad).

Assuming you've gotten this far, now you're ready to hook up your new component and listen to some music. You'll also need to listen for areas where the component needs improvement. Noisy controls can usually be fixed by spraying De-Ox-It (from electronic supply stores) or tuner cleaner from Radio Shack into the controls. Low hum usually means bad electrolytic capacitors in the power or bias supply. Hiss is usually caused by a noisy tube. Crackling noises are usually a bad resistor. And generally "blah" sound may be a weak tube, or may be coupling caps that need to be upgraded.


RESTORATION AND MODIFICATION
In using old Dyna equipment (or any other vintage audio equipment), the audiophile must first determine how much he or she wants to change the amplifier from its present condition. There are three basic approaches which can be taken. The most straightforward is simply to repair the unit. This would involve merely replacing nonfunctioning or out of spec parts with exact replacements (or the closest available equivalents). Under this approach the original circuit design and properly functioning parts would be retained. For example, worn out tubes, leaky capacitors, and out of tolerance resistors would be replaced, and the rest of the original parts retained. This approach will give the original sound of the unit, and will best preserve it's collector's value. It is also usually the simplest and cheapest to implement. Repairing the unit is the minimum work necessary to use the amplifier.
A more ambitious approach is to retain the original circuit design, but implement it with improved modern parts. Under this approach, the original parts are replaced with improved modern ones of the same values. Replacing the original carbon composition resistors with 1% metal film equivalents, and the original capacitors with audiophile grade polystyrene and polypropylene versions, can improve the sound quality while retaining its original character. This approach is somewhat more complex and expensive than a simple repair, but still should be within the capabilities of most audiophiles.

The most elaborate approach is to change the actual circuitry for more sophisticated modern designs. This can range anywhere from increasing the capacitance of the power supply, to totally replacing the driver circuitry with an improved modern design and regulating the power supplies. This approach is the most costly and difficult, but can result in an amp which is equal to the best modern tube designs at a fraction of the cost. It will often totally eliminate the vintage character of the amp (which may or may not be an improvement to the user, depending upon taste), and may destroy the collector's value of the unit. I recommend that anyone taking this approach retain the original parts, and not do anything to the amp which would prevent it from being restored to stock condition. Even if you don't like the stock sound, a potential buyer may want that.

In practice, most Dynaco users follow some combination of these approaches. A typical power amp mod might involve replacing all worn out parts, installing modern audiophile coupling capacitors and RCA jacks, replacing the bias circuit with a modern 1N4007 diode (or fast recovery equivalent) and larger low ESR capacitors, and installing polypropylene bypass caps across the power supply capacitors. This would result in an amplifier with the basic characteristics of the stock amp, but with lower noise and improved transparency.

Simple modifications worth considering include:


Replace the stock input and output jacks with good quality modern ones
Replace the coupling caps with good quality modern polystyrene or polypropylene ones of the same voltage rating (or higher) and same capacitance as the stock caps
Replace the power amp bias selenium diode with a modern 1N4007 from Radio Shack (or better yet a fast recovery 1n4007 equivalent), and the stock bias capacitors (the two typically brown paper cylinders under the chassis) with modern low ESR electrolytics with the same voltage (or higher) and higher capacitance. Be sure to observe polarity with both the diode and the caps. The same mod can be made to the heater supplies in Dyna preamps and tuners.
Replace the stock carbon composition resistors with modern 1% metal film ones of the same resistance and wattage rating.
Put Zenner diode clamps on the Bias supply (45V total for ST-70s and Mark IVs, 60V for Mark IIIs).
Convert the output tubes to triode mode operation, by placing a 100 ohm 1/2 watt resistor between pins 3 and 4 of each output tube, and disconnecting the ultralinear taps of the output transformers from pin 4 of each tube socket. This cuts the output power in half, but gives a very smooth clean sound.
Bypass the power supply caps with poly caps-1uF/630V Solens work well in this application.
Many more elaborate modifications exist, from making the bias of the output tubes separately adjustable for each tube to totally replacing the stock driver circuitry. Back issues of Audio Amateur and Glass Audio will give you many possibilities. For Power Amps, see GA 1:1989 (Joe Curcio's Stereo 70 with solid state power supply regulation) GA 2:1989 (Paul Becker's simple Mark IV mods), and GA 1:1992 (Norman Koren's Stereo 70 triode mode mod). For PAS preamps, see GA 2:1994 (Koren's Cathode Follower and other mods). For FM-3s, see GA 1:1991 (James Lin's FM-3 mods). Back issue ordering information for Glass Audio is below ...

You gotta love the Internet. I googled "variac troubleshooting tubes" and the first hit was "Adams Dynaco Stereo 70 Troubleshooting Tips" :

http://www.electronixandmore.com/adam/dynaresto.html

"If you have acquired a Stereo 70 in unknown condition, there are several things you should do before trying it out. Unless it was working when you acquired it, you shouldn’t assume that it is working, and there are several parts that should be replaced even if it you think it works. Do not just plug it in and turn it on, or there could be catastrophic results. WARNING: some parts of this amplifier use voltages in excess of 400V. If you do not feel comfortable working on a device which employs such high voltages, have someone who is repair it for you. Do not work on the amplifier while it is plugged in. Even while turned off, high voltages can be found near the power cord. Before doing work near the filter capacitors, make sure they are discharged by shorting them with an insulated wire or screwdriver. Also, make sure that the solder joints you make are good. If not, they could cause problems. If you do not know how to solder, take the amplifier to someone who does."

etc....

I am more than a little concerned about someone working with high voltages who is not COMPLETELY familar with what he is doing. I am beginning to wonder about this thread. I have worked with high voltages before and you must know what you are doing and be extremely careful. The risk of inexperience just isn't worth it.

Have you worked with high voltage equipment before?

If not, I would think real hard about working on it. High voltage CAN KILL YOU very easily.

If you haven't worked with high voltages before, the BEST ADVICE that I can give you is don't start now.

Be Safe and Be Careful!!!!

I definitely agree that being safe and careful are very important.

but...

What are the chances that anything inside this very old tube amp is still holding a significant charge?