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Instructions for triode-connecting the output tubes can be found at the further down this page.
This Guide can used on most Dynaco ST70, Mk4, Mk3 and Mk2 amplifiers generally, altho some sections assume you have either our board or the original board with new parts. Some of this info is general enough to be used with other amplifiers.
Trying to troubleshoot the original board with the original parts is generally a waste of time since after 30 or 40 years, nearly everything is either going to be out of whack or on the way there.Many of the parts on the original board can go bad and create mysterious noises, and trying to track down the culprit is going to be difficult, especially for casual hobbyists.We can't do diagnosis of original PC boards with original parts by email or phone.
You may take into consideration that we sell new boards when you read our opinion on this, still, we think that printed circuit boards in Dynaco amplifiers should be completely replaced as part of routine maintenance, rather than attempts being made to repair it, for the following reasons:
Triode operation of 6CA7,EL34, 6L6-GC , and tubes with same pinouts (ST70 or Mk3 or Mk2 with our boards or original boards ONLY)
Find the green (not green/white) wire (one from each output
transformer) hooked to pin 4 on an EL34 on each side (or on each
amplifier in the case of Mk2/Mk3) of the amplifier, there is also
a wire leading from that pin to pad 11 (on one side) or pad 14
(other side) in an ST70, pad 4 on a Mk2 or Mk3.
Disconnect the green wires from their respective sockets\par and run directly to pad 11 (on one side) and pad 14 (on the other).
Remove the wires that previously ran from pad 11 & 14 (pad 4 on Mk2 & Mk3) to the EL34/6550 sockets.
Disconnect the screen tap leads from pin 4 of each of the other two output tubes, then insulate the ends of the wires so that the bare leads are not exposed.
Install a 100 ohm (1/2 watt minimum) resistor between pin 3 and pin 4 of each of the output tube sockets.
Reinstall the output tubes, and recheck bias (BIASET on front panel). This will result in a 10 to 20 watt per channel (about 30 watts with a Mk3) triode connected amplifier, power varying with output and rectifier tubes chosen, and AC line voltage input. Triode connection may be utilized with tubes with a different pinout, simply disconnect/reconnect the screen leads as above and insulate the two loose screen leads, then connect a 100 ohm resistor (usually at least 1/2 watt) between the plate and screen.
Here's some common problems with ST70's, and possible solutions:
1.Low B+ (HT) voltage from rectifier diodes or tube
cathode. (Turn amplifier off before performing tests.)
This indicates either a restriction in the power supply (open or high resistance windings in transformer or choke, bad rectifier tube) or excessive current draw (caused by leaky filter capacitors, or shorts in tubes or output transformers) pulling down the voltage.
2. One set of tubes draws excessive bias current
(biaset too high).
Swap the offending pair of tubes with the tubes from the other side.
If the problem follows the tubes to the other side indicates probable bad tube.
If the problem remains on the side where it was originally, possible bad tube socket or bad connection in grid bias circuit (often open bias control) on that side.
3.All tubes (both banks) draw excessive current (biaset too high). Check voltages in bias circuit. If they appear normal (usually will create negative voltage of 30 to 40 volts at eyelets # 6 and 21 in an ST70), then check for excessive voltage drop (use a high impedance voltmeter to check this such as a VTVM) across the tube grid resistors (100K on our board or in Mk3/Mk2, 270K on most others). If a drop of more than a few volts is found on only one, or on one of each pair, indicates severe tube unbalance or possible defective tubes. If found on all tubes indicates that amplifier must be modified for lower DC grid circuit resistance to accommodate the tubes you have chosen. If no excessive voltage drops found across these resistors, and other bias circuit voltages appear normal, indicates type or brand of tube used requires modification to bias circuit to be used in this amplifier.
4.One tube glows red or orange. Swap the offender with another tube on either side or other amp (keep track of which one you swapped!) If condition follows tube, this indicates a probable defective tube or possibly bad solder joint inside the tube pins (which may be fixed by heating the pins with soldering iron). If condition appears in the same socket with a different tube indicates either a bad or wrong connection to that socket or a bad socket.
5.Biaset wont go high enough.Generally either
means output or rectifier tubes are worn out, or the 10K or 18K
(Mk3) resistor from the bias controls to ground is either
defective, not connected or out of tolerance (value too high).
This could be caused by a bad or internally cracked/damaged bias
Occasionally even new tubes do not draw sufficient current, in this case, either replace the 10K bias controls with a higher value, or reduce the value of the 10K resistor going to ground from the bias controls (try a 5K or similar, or a 10K to replace the 18K in a Mk3 or Mk2).
6.Transformer hot or vibrating excessively. Indicates
either a component in the amplifier is drawing excessive current
due to leakage (see section 1 above for how to track that type of
problem down), tube are biased to draw an excessive amount of
current, or transformer has internal short. Note that some Dynaco
transformers as shipped from the factory vibrated excessively and
physically isolating the transformer with rubber grommets or
other methods may be necessary to solve the problem. All power
transformers will dissipate some heat during operation, even with
no load, however getting too hot to touch without burning skin
indicates a problem.
Try operating the amplifier with no tubes plugged in, the transformer should only get slightly warm.
If the first two possibilities are ruled out then turn the amplifier off and unplug, disconnect the ground (chassis) lead,remove the rectifier tube (5AR4) or disconnect from rectifier diodes, and pull all other tubes.
Check the resistance between the ground lead of the transformer and each of the leads going to the 5AR4 socket. The resistance on each side of the ground wire will not exactly the same but similar (say within 30%).
Check the resistance between the wires connected to the 5AR4 and chassis or transformer case.
The resistance should be very high (usually 1 meg or more). Check between the leads going to the 5AR4 and the different filament wires (pins 2 and 7 of the 5AR4 itself, also pins 2 or 7 of the 6CA7/ EL34's).
Again the resistance should be very high. If any of these do not check out, a defective transformer is indicated, however other possible leakage paths should be checked, including where insulation where wires run close to the chassis, where wires enter the transformer, and under transformer bottom cover.
In some cases it will be necessary to replace the 10 ohm resistors on the board with wire jumpers or zerohm resistors to stop ground loop or transformer hum.
7.Capacitors bubbling, crackling, venting or smoking. Indicates that the capacitor was exposed to an overvoltage (or in the case of an electrolytic capacitor, reverse voltage), or is simply defective. Replace the capacitor, however before doing so, check to make sure the capacitor is of sufficient voltage rating, that the cause of any possible overvoltage condition (wrong size of voltage dropping resistor installed, etc.) is corrected, that the voltages in the circuit where the capacitor is installed are correct, and make sure the capacitor, if it is an electrolytic, is installed with the proper polarity- note that in the bias circuit where there are negative voltages present the capacitors are connected with the positive side to ground (chassis), for example.
8. Resistors overheating or smoking. Indicates either a
resistor of the wrong wattage rating was chosen or that some
element in the circuit (usually a defective tube or capacitor) is
short and drawing a high amount of current through the resistor.
To determine resistor wattage rating, use Ohm's Law thusly:
Divide the voltage drop across the resistor by the resistance. This will give you the current in amperes drawn through the resistor. Then multiply the current in amperes by the voltage drop to get the wattage dissipation.
Then double the wattage to find the minimum necessary rating. For example if voltage drop is 200 volts, size of resistor is 330,000, current is .0006 amps. Take .0006 times 200 equals .12 watts dissipation. The minimum rating for the resistor would be .24 watt, ergo a quarter-watt (1/4 watt) resistor would do. Note that for resistors that effectively do not have a DC current drawn across them, principally grid resistors and phase or frequency compensation networks, the wattage rating can effectively be ignored, and the smallest convenient rating used. While AC currents exist in this amplifier, except for the resistors included in the feedback loop , these currents are negligible and may be safely ignored for wattage dissipation calculations. If the resistor is indeed of the proper wattage, then the other defective elements that are making the resistor overheat must be located and replaced prior to reinstalling a new resistor. In the case of a plate or screen resistor, test the tube for shorts or substitute with a known good tube and see if the problem recurs. In the case of possible defective capacitors, follows the procedures in the previous section for testing them.
9. Crackling, hissing, or noises through one or both
channels. If this only occurs in one channel, first eliminate
tubes as the source of the problem by swapping tubes between the
sides (EL34s or 6550's in pairs), and turning on the
amplifier to see if the problem follows the tube (s) between the
If, after substituting all tubes the problem continues, a little detective work will be necessary. First, pull the tubes from the PC board, then turn on the amplifier again. If the sound disappears, then the problem is probably on the board or power supplied to the board. If not, and the sound appears in both channels, the problem is probably in the power supply (power transformer, choke, filter capacitor (s), or resistors attached to the filter capacitor (s). If not and the noise only appears in one channel, the source is most likely in the output stage, often the tube sockets, occasionally the biaset resistor or negative bias supply circuit, more rarely the output transformer. Bad bias control pots are often the source of output stage noise, try moving the controls slightly back and forth to see if the noise changes or goes away. Try wiggling carefully (without burning yourself or breaking the tube!) the output tubes on the affected channel, this will often indicate tube socket that are dirty, corroded, or have lost contact tension and must be replaced.
Most of the noise problems you may encounter derive from the following sources (from most to least common)
10.Excessive audio hum. This is usually caused by one of the following:
11. Voltages on board are way off or amplifier distorts excessively. Check all of following:
12.Amplifier oscillates (tone comes from speakers with
no input).(ST70) Check wiring from PC board pads 1,2,22
& 23 (on our board or original board) to their respective
EL34 sockets. Pads 1 & 22 should be connected to EL34 sockets
that have striped (green/white & blue/white) wires connected
to them (from the output transformer)
Pads 2 and 23 should be connected to EL34 sockets that have solid (blue & green) wires connected to them (from the output transformer) See schematic for details.
On Mk3's & Mk2's, pad 3 goes to the 6550/EL34 socket with the striped wires from the transformer, pad 1 to the socket with the solid color wires.
Make sure the common/ 0 ohm lead on the output transformer is connected to chassis/ground.
13.No sound in one channel: First eliminate tubes as
the source of the problem by swapping tubes between the sides
(EL34s in pairs), and turning on the amplifier to see if
the problem follows the tube (s) between the channels. (The
12AU7/12BH7 on our board is common between both channels, try
Next, check all connections: swap speaker & input connectors (avoid doing this while amplifier is powered up), between the two sides, see if the problem changes to opposite side (in which case youve got a problem with a cable, speaker or preamp).
Look at wires connecting between the board & tubes, make sure no leads going to the output terminals are disconnected o r touching the chassis (except the 0/common lead on each side). If you stuck a lead in a PC board pad or eyelet and either forgot to solder it, that can have the same effect.
Check all voltages on the board where the diagram indicates, voltages way off indicate a resistor is open/wrong value.
Check voltages on pins 3 & 4 of all EL34/6550 sockets (where leads from output transformers connect). They should have between 375 to 475 volts in an ST70, 425 to 475 in a Mk4 and 450 to 525V in a Mk3 or Mk2 depending on rectifier tubes used, biaset being set correctly, and your line voltage, and power transformer tolerances, but ALL of those pins should have similar voltages. Missing voltage on one indicates an broken wire or open in transformer.
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