Hey folks,
First post here, and also my first venture into fixing up an amp. Please treat me like a complete beginner as I am still learning. I have built some FX pedal kits, but that's about it. My soldering skills are solid. I bought this amp second hand knowing it needed some attention. The tools I have at my disposal are a Tenma DMM, cap discharge tool and a soldering iron.
So far I have done the following:
-Checked over board for cold/broken solder joints, re-flowed any that looked suspect
-All pots replaced with new like for like values PCB mounted pots
-All input jacks replaced with new like for like replacements
-Fitted external speaker out
-All electrolytic caps replaced including power filter caps
-Power stage resistor replaced at r21
-Socketed the IC and replaced with a new MC1458
-Sorted out ground connections to chassis that were loose
The amp now works and sounds pretty good. However, there's one niggling issue that's driving me nuts. With a driven tone, i.e. the gain cranked a bit, sustained chords seem to fizzle out (kind of like a voltage starved fuzz) as they fade out. I made a quick recording to demonstrate this (you can hear this fizzle at 0.15 when I start letting chords sustain).
At this point I am struggling to fault find the issue. I have scoured most of the Lead 12 threads here and have taken the below readings with my DMM at test points on the schematic. At this point I suspect it may be one of the transistors, or perhaps the zenner diode is on its way out. A friend suggested what I am hearing might be cross-over distortion? Or that the bias of the power stage transistors is wrong.
Below are the results of the test suggested in other threads, I tried to hit most of the voltage test points on the schematic. I THINK the combo version (5005) circuit differs ever so slightly from the head schematic 3005 which seems to be the only schematic I can find online, so that's what I'm using. I have the combo (5005) version. For clarity, my process was: amp on, speaker attached, volume down. My DMM set to measure Voltage at AC range, black lead clipped the chassis for ground, red lead / probe to the appropriate test points for reading. Left hand in pocket at all times! I haven't fried myself yet.
DMM across the speaker and set to DC Volts to verify there is no DC across the speaker.
No DC voltage across speaker, my DMM wavers a bit, but it's all in the 0.001 DC range
Next set the meter to AC volts. Is the reading almost zero ? You are looking for an ultrasonic oscillation that may read several volts depending on your meter.
AC reading 0.034 V
The Emitters of Tr1 & Tr2 should be -.7Volts, Collector of Tr1 should read +18Volts.
TR1 emitter: -0.7v
TR2 emitter: -0.7v
TR3 emitter: 18.87v
Junction of R15 & R16 should read -9Volts.
Wasn't sure what the 'junction' is, so measured each resistor.
R15: -9v
R16: -070.5v
Zenner diode ZD1: Neg side -9.12v / pos side -000.8mv
Power section Filter caps (replaced)
C17: 18v
C18: 0v (measured at + side)
Check that pins 1 and 7 of the IC are near zero Volts. If not, that indicates a bad IC, component or broken connection problem.
IC Pin 1: 0 volts
IC Pin 7: 0.046 mv
IC Pin 4: -17.34v
IC Pin 8: 17.30v
Lift (unsolder) the input side of C8 (it's the input to power amp) now use your walkman or any audio device that has line/headphone output.You should hear a reasonable medium level CLEAN sound. If the sound is severely distorted or very low volume then it's a fair bet the power transistors are shot or TR3 is dead.
Not done
Measure the DC Voltage across R26 and R27, the two 0.33 Ohm resistors. The Voltages should be close to the same.
R26: -068.3mv
R27: 068.1mv
Check R 11 & R 6 check they have aprox 19 volts on one side - I think I should take these readings again!
R6 -000.1mv on both sides, reading jumps around a lot. Hard to get exact reading.
R11 4.5v and 3.2v
Check they have approx 19 volts on one side and 16v on the other or another way measuring across the resistor should give a reading of 3 volts the amount "dropped" by the resistor.
R21 -4.9mv (should be -10V?) - replaced with new resistor
Does any of that strand out as 'not right'? Not really sure what to do now, as far as diagnosis goes.
Gut shot as it currently stands.
Any help gratefully received. Keen to learn and work out what is the cause of this issue.
First post here, and also my first venture into fixing up an amp. Please treat me like a complete beginner as I am still learning. I have built some FX pedal kits, but that's about it. My soldering skills are solid. I bought this amp second hand knowing it needed some attention. The tools I have at my disposal are a Tenma DMM, cap discharge tool and a soldering iron.
So far I have done the following:
-Checked over board for cold/broken solder joints, re-flowed any that looked suspect
-All pots replaced with new like for like values PCB mounted pots
-All input jacks replaced with new like for like replacements
-Fitted external speaker out
-All electrolytic caps replaced including power filter caps
-Power stage resistor replaced at r21
-Socketed the IC and replaced with a new MC1458
-Sorted out ground connections to chassis that were loose
The amp now works and sounds pretty good. However, there's one niggling issue that's driving me nuts. With a driven tone, i.e. the gain cranked a bit, sustained chords seem to fizzle out (kind of like a voltage starved fuzz) as they fade out. I made a quick recording to demonstrate this (you can hear this fizzle at 0.15 when I start letting chords sustain).
At this point I am struggling to fault find the issue. I have scoured most of the Lead 12 threads here and have taken the below readings with my DMM at test points on the schematic. At this point I suspect it may be one of the transistors, or perhaps the zenner diode is on its way out. A friend suggested what I am hearing might be cross-over distortion? Or that the bias of the power stage transistors is wrong.
Below are the results of the test suggested in other threads, I tried to hit most of the voltage test points on the schematic. I THINK the combo version (5005) circuit differs ever so slightly from the head schematic 3005 which seems to be the only schematic I can find online, so that's what I'm using. I have the combo (5005) version. For clarity, my process was: amp on, speaker attached, volume down. My DMM set to measure Voltage at AC range, black lead clipped the chassis for ground, red lead / probe to the appropriate test points for reading. Left hand in pocket at all times! I haven't fried myself yet.
DMM across the speaker and set to DC Volts to verify there is no DC across the speaker.
No DC voltage across speaker, my DMM wavers a bit, but it's all in the 0.001 DC range
Next set the meter to AC volts. Is the reading almost zero ? You are looking for an ultrasonic oscillation that may read several volts depending on your meter.
AC reading 0.034 V
The Emitters of Tr1 & Tr2 should be -.7Volts, Collector of Tr1 should read +18Volts.
TR1 emitter: -0.7v
TR2 emitter: -0.7v
TR3 emitter: 18.87v
Junction of R15 & R16 should read -9Volts.
Wasn't sure what the 'junction' is, so measured each resistor.
R15: -9v
R16: -070.5v
Zenner diode ZD1: Neg side -9.12v / pos side -000.8mv
Power section Filter caps (replaced)
C17: 18v
C18: 0v (measured at + side)
Check that pins 1 and 7 of the IC are near zero Volts. If not, that indicates a bad IC, component or broken connection problem.
IC Pin 1: 0 volts
IC Pin 7: 0.046 mv
IC Pin 4: -17.34v
IC Pin 8: 17.30v
Lift (unsolder) the input side of C8 (it's the input to power amp) now use your walkman or any audio device that has line/headphone output.You should hear a reasonable medium level CLEAN sound. If the sound is severely distorted or very low volume then it's a fair bet the power transistors are shot or TR3 is dead.
Not done
Measure the DC Voltage across R26 and R27, the two 0.33 Ohm resistors. The Voltages should be close to the same.
R26: -068.3mv
R27: 068.1mv
Check R 11 & R 6 check they have aprox 19 volts on one side - I think I should take these readings again!
R6 -000.1mv on both sides, reading jumps around a lot. Hard to get exact reading.
R11 4.5v and 3.2v
Check they have approx 19 volts on one side and 16v on the other or another way measuring across the resistor should give a reading of 3 volts the amount "dropped" by the resistor.
R21 -4.9mv (should be -10V?) - replaced with new resistor
Does any of that strand out as 'not right'? Not really sure what to do now, as far as diagnosis goes.
Gut shot as it currently stands.
Any help gratefully received. Keen to learn and work out what is the cause of this issue.