Originally posted by mac dillard
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Marshall JCM 25/50 Low Volume & Distorted
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yes those will tell a lot more about what's going on.
if your tubes idle current are both close then ,
another possibility, return jack might need closer inspection check for worn contacts
if you didn't already try to clean it with some deoxit and run a plug in several times.
they seem to be notorious in these amps. just a thought.
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Aerostoon, you still around?
here is your schematic with the DC voltages on them, don't worry about preamp grid voltages, they are not important and are usually erroneous due to voltmeter loading of the hi Z node, this you can see on your inverter grid voltages which are off by about 10 volts,
the health of an amplifier is determined by the bias voltage, we need the bias voltage across that 470 ohm resistor hanging off the inverter,
all the DC voltages look good,
do you have a signal generator? run about 25 millivolts into the input jack at 1000 hz and take down the AC voltages for the test points circled in red,
if no generator, just strum the guitar nice and easy with all the controls set at 50% and take some readings, you will need to use some alligator clips ahead of time to free up your hands, if you can do this then finding the trouble will be a walk in the park,
Last edited by cjenrick; 07-21-2017, 08:37 AM.
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Originally posted by cjenrick View PostAerostoon, you still around?
here is your schematic with the DC voltages on them, don't worry about preamp grid voltages, they are not important and are usually erroneous due to voltmeter loading of the hi Z node, this you can see on your inverter grid voltages which are off by about 10 volts,
the health of an amplifier is determined by the bias voltage, we need the bias voltage across that 470 ohm resistor hanging off the inverter,
all the DC voltages look good,
do you have a signal generator? run about 25 millivolts into the input jack at 1000 hz and take down the AC voltages for the test points circled in red,
if no generator, just strum the guitar nice and easy with all the controls set at 50% and take some readings, you will need to use some alligator clips ahead of time to free up your hands, if you can do this then finding the trouble will be a walk in the park,
[ATTACH=CONFIG]44154[/ATTACH]
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Originally posted by cjenrick View PostAerostoon, you still around?
here is your schematic with the DC voltages on them, don't worry about preamp grid voltages, they are not important and are usually erroneous due to voltmeter loading of the hi Z node, this you can see on your inverter grid voltages which are off by about 10 volts,
the health of an amplifier is determined by the bias voltage, we need the bias voltage across that 470 ohm resistor hanging off the inverter,
all the DC voltages look good,
do you have a signal generator? run about 25 millivolts into the input jack at 1000 hz and take down the AC voltages for the test points circled in red,
if no generator, just strum the guitar nice and easy with all the controls set at 50% and take some readings, you will need to use some alligator clips ahead of time to free up your hands, if you can do this then finding the trouble will be a walk in the park,
[ATTACH=CONFIG]44154[/ATTACH]
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I injected a 25MV 1000Hz signal in. Set all controls half way and recorded the AC MV readings in red. I checked the DC volts across R40 with no signal and the volume controls at 0. It reads 0.70VDC. The high/low selector was on high.
Definitely different readings on each output transformer lead. What do you think of the results?
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plug a patch cable from send to return and see what happens, you are losing signal voltage in that area, check vol control grounds,
try a patch cord again from send to return and take AC measurements from there (V2b)onward to the phase inverter, (V3 pins 1 and 6)
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Originally posted by cjenrick View Postplug a patch cable from send to return and see what happens, you are losing signal voltage in that area, check vol control grounds,
try a patch cord again from send to return and take AC measurements from there (V2b)onward to the phase inverter, (V3 pins 1 and 6)
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Well I feel pretty silly. I tried jumpering the effects loop and measuring the voltage. No change. Then I thought maybe I should swap the power tube positions and see what AC measurement I get. The imbalance followed the tubes. One of my power tubes was shot. I put some new EL84's in and it works just fine. I tried the new tubes early on but I have done quite a bit of work and must have corrected the issue.
Oh well. Now to check the bias and then I'll have a look at the output sine wave on my scope. I sure chased my tail on this repair...
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The amp is definately louder than before but is probably a third as loud as it should be. With my Eurotube Bias probe I am getting 40MA plate current at 440 volts on pin 3.
With the scope connected to the dummy load leads I can turn the clean channel up to max volume and I'm at 50MV per division on my volts/div control. The sine wave takes up most of the screen but no clipping.
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well 70% is 39.7 , 40ma close enough really , it does depend on how you want it to sound and how often you want to change tubes.
I'll just add that 70% usually works but nothing wrong will less, i usually bias my marshall between 60 and 65%. but thats just me.Last edited by dstrat; 07-25-2017, 04:10 AM.
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"'im at 50MV per division on my volts/div control. The sine wave takes up most of the screen but no clipping..."
probably 8 squares on your scope? 8 x 50 mv = 400 mv P-P, still very low, s/b at least 12 V-rms into 8 ohms, with lots of clipping at full volume,
jumper send/return with patch cord, record ac voltages with all controls on 11 and record voltages on chart after send/return portion of schematic, use volts instead of mv for this far down in the signal chain,
bias current don't matter that much once you crank max volume, by then the AC signal is dominating the DC grid voltage, to see this inject a 100 mv signal with controls maxed and turn pot as you are watching output voltage into dummy load,
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Originally posted by cjenrick View Post"'im at 50MV per division on my volts/div control. The sine wave takes up most of the screen but no clipping..."
probably 8 squares on your scope? 8 x 50 mv = 400 mv P-P, still very low, s/b at least 12 V-rms into 8 ohms, with lots of clipping at full volume,
jumper send/return with patch cord, record ac voltages with all controls on 11 and record voltages on chart after send/return portion of schematic, use volts instead of mv for this far down in the signal chain,
bias current don't matter that much once you crank max volume, by then the AC signal is dominating the DC grid voltage, to see this inject a 100 mv signal with controls maxed and turn pot as you are watching output voltage into dummy load,
I am just setting up to read the AC voltages at max volume.
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