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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:

The PC boards were supplied by Dynaco as an completed assembly, not
as parts to be installed by the end-user, and Dynaco intended for the
board to be installed and replaced as a complete assembly, rather than
repaired.

The traces on the the original PC are extremely thin , and lift off
very easily, which is no surprise as they weren't intended to be
repaired anyway. The number of Dynaco boards with patched traces we've
seen attests to this.

The base board itself was of very low quality to begin with.The
original ST70 sold retail for $99, and in order to meet this price point (even
accounting for inflation, $99 was an incredibly low price!), some
compromises had to be made.

The PC board wasn't meant to last 40 years! The original Dynaco
folks couldn't have imagined in 1957 or 1959 that thousands of their
amplifiers would have survived and still be in daily use in the year
2008!

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.

A. Test or substitute rectifier tube or check continuity of diodes.
Replace if necessary.

B. Check continuity between the rectifier plate leads (pins 4 and
6) and between these leads and ground.If open at any point, check for
broken connections in the leads. If high resistance found on one sideof
the winding, check for cold solder joints, inside the transformer as
well if necessary. If high resistance condition exists after repairs,
probable defective power transformer.

C. Discharge all filter capacitors, by turning off amplifier, allowing the
several minutes for the capacitors to discharge. Check voltage on each
capacitor lug terminal, if a sufficiently low value (not over 100V)
short the terminals to ground (you may get a little spark), then
test with ohmmeter, observing polarity. Meter should first indicate
close to dead short then rise slowly to a high value. If one or more
sections will not rise or is much lower than the others indicates
defective capacitor. Unfortunately some capacitors will pass a low
voltage ohmmeter test but leak enough to cause trouble at high
voltages. Use of a high voltage capacitor tester would be preferable,
or a milliammeter in series with a high voltage power supply. Any
current over a few ma after charging up would be suspect. Filter
capacitors becoming excessively warm or hot during operation indicates
a leaky condition also.

D. Pull output tubes, and disconnect the leads running from the filter
capacitors to the output transformers. Check with ohmmeter resistance
between transformer leads and ground (chassis). Resistance should be
very high. If not indicates possible defective transformer. If
suspected, try reconnecting the good transformer, installing the output
tubes on that side, then operating the amplifier. If condition does not
recur, but recurs when other transformer is hooked up (try without
output tubes, so a short tube will not give you a false
indication), then that transformer is the culprit.

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 won’t 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
control.
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 (EL34’s or 6550's in pairs), and turning on
the amplifier to see if the problem follows the tube (s) between the channels.

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)

Loose, dirty, or corroded sockets, pins or connectors, in this case
specifically tube sockets or input jacks or plugs. Tighten and clean or
replace to correct.

NOTE: If you see arcing on EL34 or 6550 sockets when the
amplifier is running this generally means the socket must be replaced.
Tubes with intermittent shorts can cause this to happen as well, but
usually the arcing will damage the tube socket. If in doubt, replace
it.

Outside electromagnetic interference. Too much can be said about
possible sources and solutions to go into heavy detail here
(fortunately much has been written elsewhere on the subject), however
the general idea is to either shield the signal from picking up the
interference, eliminate the source of the noise, or shield the
source to prevent it from emanating the offending interference.

Tubes. Substitute possible suspect tubes starting from the input.
Replace defective tubes.

Bad solder joints. Reheat suspect connections to reflow solder. If solder
does not seem to stick to a wire, connector, or lead, desolder the
suspect unit, clean thoroughly, and resolder.

Broken or loose wires or leads. Inspect wires carefully and
reconnect any potential offenders.

Capacitors (particularly electrolytic ones) with intermittent
shorts or leaks. This is common if some of the old original capacitors
are still present and not replaced. Also often caused by installing the
bias supply electrolytic capacitors backwards. Note in the bias
supply (where negative voltage is present) capacitors are installed
with positive end at ground (chassis) potential.

Cracked or overheated resistors. Often found in power supply (by
the main filter capacitor(s).More common if the original carbon
resistors are still installed.

Intermittent arcing inside power or output transformer. Best indication of
this is  that other possible causes have been eliminated, and the
noise emanates from speakers even though EL34 tubes have been removed.
If an audio transformer is the culprit the noise will only be
heard from the affected channel. Usually power transformer arcing will
make noise equally in both channels. Note that in the case of internal
arcing noises will often be heard from the transformer itself.

10.Excessive audio hum. This is usually caused by one of the
following:

Ground loops, multiple ground paths, and bad ground connections.
The audio system should be connected to earth ground only at one point
(normally the preamp) and ground connection must be good. Having a
component installed with poor transformer isolation (leaky
insulation) will induce hum as well as having components with
unpolarized (one prong is slightly wider than the other ) line plugs.Try
turning over unpolarized plugs first.

Tubes with cathode-heater shorts or leakage. Hum is bleeding from
the AC filament to the cathode.Test or substitute possible offending
tube (s)  and replace if necessary.

Any audio connections (RCA plugs, tube grid pins, etc.) open or not
making good contact.

Transformer vibration (thus inducing grid vibration in the tubes).
This can be considerably exaggerated by tube microphony.

A bad capacitor(s) in the power supply (the big aluminum can on top
of the chassis).If what you have is the original part, these often need
replacing.

470K resistors on input jack missing, broken, or out of tolerance,
or bad mono/stereo switch

10 ohm resistors burnt out on the board. This can happen if the AC
ground reference of another piece of gear connected to the amplifier is
backwards from the amplifier itself, (can be caused by polarized or 3-prong
AC cords or wall sockets being installed improperly) thus causing a
large AC current to pass thru this resistor. Check between the center
of the input jack and the chassis, it should measure 470K ohms, if the
10 ohm resistor is open, it'll measure open (infinite).

Negative feedback (wires going from 16 ohm output taps back to pads
12 & 13 on PC board) disconnected,or feedback resistor(s) too high
of a value.

In some cases it may be necessary to replace the 10 ohm resistors
with wire jumpers or zerohmresistors to stop ground loop hum.

11. Voltages on board are way off or amplifier distorts excessively.
Check all of following:

The tubes you installed are the correct tubes, and they are
installed in the correct sockets.

If voltages are only off on one side or one amp, swap the tubes one at a
time on that side/amp with the one on the other side.If the wrong
voltages follow to the other side, that tube is bad. Replace it.

If voltages are off on both channels, check resistor values. Also
check resistors connected to the can capacitor/power suppy, and test
power supply rectifier (5AR4) itself.

Check output transformer connections. Having any of the output
transformer primary (connected to the output tubes) wires connected
wrong (occasionally, these mistakes were made when the amplifier was
built, so should *not* assume that they’re connected correctly),
or secondary wires either attached wrong, shorted to the chassis
(except for the black wires), or shorted speaker wires/voice coil
(don’t assume a speaker is good until you’ve checked it on another
amplifier), can do this.

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 (EL34’s 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 substituting
it).
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 you’ve 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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