ok, i added the missing 10k resistor and rewired the tones as on the diagram.
not sure what the correct setting on the ohmeter, but all 3 preamp tubes measure 6.2 with the standby off, exactly the same when standby on.
the power tubes give no reading whatsoever. incidentally, it biases as normal with my bias king
1st timer board install help
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Flames1950
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Flames1950
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With the meter set to the 1000V DC range, one probe on pin 3 of an output tube socket, and the other probe on a grounding point, you show no voltage at all? There should be enough DC present here to throw you across the room and into an early grave. 
If you really get no voltage at the tube, do you show any voltage with one probe at the wire going to the HT fuse from the four diodes, and the other at a grounding point? We gotta get you some B+ goin' here if you're getting nothing for voltage at these points.
If you really get no voltage at the tube, do you show any voltage with one probe at the wire going to the HT fuse from the four diodes, and the other at a grounding point? We gotta get you some B+ goin' here if you're getting nothing for voltage at these points.
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Flames1950
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Well, showing DC at the power tube pin 3's is a good start. Now we're talking. (What year Park was it? Late-70's from the ~390VDC at the power tubes? About what my '78 Marshall 50 watt runs.)
Next up let's check the DC voltage around those 10K resistors. Where the three wires join together (Black and two Yellows, right?) check for DC voltage with one probe at the wires and one at a grounding point. Then move the hot probe to the other side of the two resistors, where the blue wire takes off and the white wire starts across the board diagonally.
What are we showing at those points?
Sorry this seems tedious, troubleshooting over the Internet with pictures is a tricky business!! Keep your good cheer, we'll get ya running.
Next up let's check the DC voltage around those 10K resistors. Where the three wires join together (Black and two Yellows, right?) check for DC voltage with one probe at the wires and one at a grounding point. Then move the hot probe to the other side of the two resistors, where the blue wire takes off and the white wire starts across the board diagonally.
What are we showing at those points?
Sorry this seems tedious, troubleshooting over the Internet with pictures is a tricky business!! Keep your good cheer, we'll get ya running.
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Flames1950
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194VDC may be a bit low, and may be why you started out with only one 10K resistor there. But we can worry about that later, let's finish going through the DC voltages first and see why you aren't making noise.
Next stop is the phase inverter. The white wire that goes from the 10K resistor diagonally over to an 82K/100K resistor junction should show the 194 VDC at that junction. On the other side of the 82K and 100K resistors are blue wires going to the plates of your phase inverter 12AX7 tube. Re-check for your 194VDC at the junction, then see what you get on the other side of each of those resistors. Assuming you're showing voltage on the blue wire side of the resistors, double check to be sure the blue wires are attached to pins 1 and 6 of the phase inverter 12AX7 (remember that pins 4&5 are joined together for part of your heater voltage, with pin 9 as the other part if the numbers are hard to read on the socket.) You should show voltage at pins 1 and 6 and your phase inverter section should work.
Assuming good voltage at the phase inverter, it may be a good time to try the somewhat scary sounding "pop" test. With an insulated small screwdriver, touch the wire junctions coming off the 1meg resistors [to the left of the phase inverter plate resistors (the 82K and 100K we just checked) in your picture.] The wires are green, like all Marshall input grid wires, and go to pins 2 and 7. With a correct speaker load hooked up, you should hear a nice big POP when you tap these junctions. It still makes me jump when I try it so be careful not to let the screwdriver hop over where it doesn't belong!!
If you hear a pop, you can also continue to try this test on the other two 12AX7 tubes at pins 2 and 7 also, it may help us isolate where the problem is as well.
If you hear no pop at all with good voltages, I'd have to assume either the input grids on the power tubes are on the wrong pins (but they look right in the pics!), or your output tranny secondary may have gone south for the winter.......we'd rather hear a big POP though.....
Next stop is the phase inverter. The white wire that goes from the 10K resistor diagonally over to an 82K/100K resistor junction should show the 194 VDC at that junction. On the other side of the 82K and 100K resistors are blue wires going to the plates of your phase inverter 12AX7 tube. Re-check for your 194VDC at the junction, then see what you get on the other side of each of those resistors. Assuming you're showing voltage on the blue wire side of the resistors, double check to be sure the blue wires are attached to pins 1 and 6 of the phase inverter 12AX7 (remember that pins 4&5 are joined together for part of your heater voltage, with pin 9 as the other part if the numbers are hard to read on the socket.) You should show voltage at pins 1 and 6 and your phase inverter section should work.
Assuming good voltage at the phase inverter, it may be a good time to try the somewhat scary sounding "pop" test. With an insulated small screwdriver, touch the wire junctions coming off the 1meg resistors [to the left of the phase inverter plate resistors (the 82K and 100K we just checked) in your picture.] The wires are green, like all Marshall input grid wires, and go to pins 2 and 7. With a correct speaker load hooked up, you should hear a nice big POP when you tap these junctions. It still makes me jump when I try it so be careful not to let the screwdriver hop over where it doesn't belong!!
If you hear a pop, you can also continue to try this test on the other two 12AX7 tubes at pins 2 and 7 also, it may help us isolate where the problem is as well.
If you hear no pop at all with good voltages, I'd have to assume either the input grids on the power tubes are on the wrong pins (but they look right in the pics!), or your output tranny secondary may have gone south for the winter.......we'd rather hear a big POP though.....
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white room
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Flames,
This may be a non-relavant question,
I can't tell for sure because of the glare in the pics but does it look like all 4 diodes in the bridge rectifier are going in the same direction?
Forget it, I think I see what's happening there.
This may be a non-relavant question,
I can't tell for sure because of the glare in the pics but does it look like all 4 diodes in the bridge rectifier are going in the same direction?
Forget it, I think I see what's happening there.
Last edited by white room on Tue Jan 25, 2005 1:27 pm, edited 1 time in total.
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Flames1950
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We'll hope that the popping is just pretty faint at this point. If we can get the first preamp stages figured out trying the pop test on the first preamp tube should be pretty loud, as it gets amplified by more stages.
We've got plate voltages in the power and phase inverter sections then. Let's trace the B+ line back into the next stage.
The white wire that cut diagonally across the board continues on to another 10K resistor at about the halfway point across the board. Again, you should be able to read your 194VDC at this point. Go ahead and take a reading at the other end of the resistor as well, which should have a blue wire going to pin 6 of the second 12AX7, as well as a jumper across to another 10K resistor. Read at the end of the 10K, then follow the blue wire and read for voltage at that pin 6, which should be exactly the same. There should be a resistor almost in the way of taking that reading, going from pin 6 to pin 1 across the tube socket (a 100K.) Pin 1 is then jumpered to pin 7 with a short bare wire under the resistor. Verify that all this stuff is present and accounted for (your pics run out at about this point!) and get the voltages at pin 6, pin 1 and pin 7. Some of those are a little redundant but it gives you a chance to follow the circuit and check solder joints as you go too.
We've got plate voltages in the power and phase inverter sections then. Let's trace the B+ line back into the next stage.
The white wire that cut diagonally across the board continues on to another 10K resistor at about the halfway point across the board. Again, you should be able to read your 194VDC at this point. Go ahead and take a reading at the other end of the resistor as well, which should have a blue wire going to pin 6 of the second 12AX7, as well as a jumper across to another 10K resistor. Read at the end of the 10K, then follow the blue wire and read for voltage at that pin 6, which should be exactly the same. There should be a resistor almost in the way of taking that reading, going from pin 6 to pin 1 across the tube socket (a 100K.) Pin 1 is then jumpered to pin 7 with a short bare wire under the resistor. Verify that all this stuff is present and accounted for (your pics run out at about this point!) and get the voltages at pin 6, pin 1 and pin 7. Some of those are a little redundant but it gives you a chance to follow the circuit and check solder joints as you go too.