Cheap Marshall JCM900 4102 "AS IS" - blow fuses, bad power tube and pots - low output

[spunko]

Hello again friends. Let me start saying I love this forum, it is really fun to do all this stuff, and I really appreciate all the help you guys give.

I got this marshall amp really cheap. The wooden cabinet is junk, it is infested with moth larvae. It also came with the main fuse and the fuse for power tubes 2 and 3 both blown. Gain and volume pots for channel B are broken.

So first step, I replaced the fuses and used my current limiter to power on, and determinated that power tube 3 was bad. I don`t currently have 5881 tubes, so I used the 6L6GC from my old twin silver face, these are the original tubes from the 70's. Powered the amp again, and it worked, no blown fuse, and the bulb in the current limiter stayed off. Then I noticed that channel B was at 10 volume all the time, no matter if you turned the knob, so I replaced those bad pots too.

Now the "stranger things":
- the amp has a protection in the power amp section, when there is a bad power tube, the amp works half power, and a red led next to the blown fuse lights up. This was the case of my amp, it was working only with tubes 1-4 (before I put the 6l6GC) and the clean channel indeed was at half power, but when switched to the gain channel, it sounded really really loud, like it was at full power. I know the volume pot for the CH-B was bad, but even if I turned the clean to 10, it didn't get to the level of the CH-B.

- When replaced the power tubes, fuses, and gain/volume pots for the CH-B, the amp worked nicely. But the clean channel stayed with the same output level, like it was working with only 2 Power tubes, and also the CH-B leveled with the clean channel. So now both channels sound with the same level of output, but definatelly the CH-B was louder before changing anything.

- Changed the Volume pot for the clean channel just in case, with the same result.

I have checked the voltages really quick, and all seems normal.
I comapred the output level with the Twin Reverb 2012 you helped me with in the other thread, and the twin sounds much louder that the marshall. Before changing anything, CH-B had same level output than the twin, really really loud, and clean channel like half power.

- Checked again the fuses and they are good. Also the Fail leds are off, so I think there ir no power tube failure.

Why would CH-B sound louder with the bad components?
Why the channels leveled each other but with half power even with the 4 power tubes working?

what voltages do I need to post?

Thank you.

[vintagekiki]

Check control voltage at pin1 IC1, IC4, IC6 (M2501) when changing channels.
The voltage to change the channel is obtained from TR1 point C.
If no change voltage at pin1 IC1, IC4, IC6 (M2501) when changing channels, check diodes D1, D2, D5 (1N914)


https://drtube.com/schematics/marshall/cd0189-iss7-0349.pdf

[The Dude]

If the volume pot for channel B was open, you had no attenuation at all.

[Pedro Vecino]

Difference in volume between both channels is normal. Clean channel with a gain adjustment of 5 or 6 (to keep it clean since it has a circuit that generates overdrive above those settings) requires a high volume adjustment to develop power.
Lead channel with a gain adjustment between 8 and 10 (the most common) requires a much smaller volume adjustment.

[g1]

Are you just going by ear, or measuring power output?
I agree with Pedro, the clean channel is going to sound quieter. It is possible that it's still not putting out full power as the difference between 50 and 100W is not huge. You would not notice a problem as much when using the dirt channel.
Did you get good voltage at the screens (pin4) of all the power tubes?

[vintagekiki]

Measuring by ear gives a subjective sense of power, true power is obtained by measuring power output.
Remove all power tubes (EL34) or remove fuse FS1, FS2 (T500mA) to switch off power tubes (EL34).
Connect AC Voltmeter to JS4 (Direct out)
Gain and volume pots for both channels set to max, and bring a signal (10-15 mV/ 1kHz) from the tone generator to JS1 (Input).
With panel switch or foot switch change channels and compare whether and how much the voltage changes at the power amp input (C7/ 22n).

[spunko]

Hello, I was out of town because of my work. This happens quite often. Sometimes I'm out for 2 o 3 weeks.
Now i'm home, and want to start with this amp again.

Measuring by ear gives a subjective sense of power, true power is obtained by measuring power output.
Remove all power tubes (EL34) or remove fuse FS1, FS2 (T500mA) to switch off power tubes (EL34).
Connect AC Voltmeter to JS4 (Direct out)
Gain and volume pots for both channels set to max, and bring a signal (10-15 mV/ 1kHz) from the tone generator to JS1 (Input).
With panel switch or foot switch change channels and compare whether and how much the voltage changes at the power amp input (C7/ 22n).

This amp uses 5881 tubes. I'm using 6L6 for the moment.
I took out both fuses, pots at max as suggested, fed a 1 Khz / 11.5 mV signal into the amp, AC voltmeter to JS4 and got 5.39 Vac for the clean channel, and 5.2 Vac for the drive. How do I read the voltage at C7?
What it is interesting, is that the amp still had output through the speakers with both fuses out. Is this normal?

Is it necessary to check voltages at the ICs? I'm afraid I can short one

Check control voltage at pin1 IC1, IC4, IC6 (M2501) when changing channels.
The voltage to change the channel is obtained from TR1 point C.
If no change voltage at pin1 IC1, IC4, IC6 (M2501) when changing channels, check diodes D1, D2, D5 (1N914)

The amp has no problems switching channels, they sound even. What surprised me was that CH-B sounded really really loud with the broken pot, and when I replaced it, the CH-B leveled in output with the clean channel. CH-B was louder with the broken pot. Maybe that is why it sounded so loud.

If the volume pot for channel B was open, you had no attenuation at all.

Here are some voltages:

All Power tubes
PIN3: 3.5 Vac / 490 Vdc Is it normal that Pin3 read boths AC and DC?
PIN4: Standby OFF -90 Vdc / Standby ON -53.18 Vdc Is it normal for the voltage to drop?
PIN2: 490 Vdc
PIN1: 3.94 Vac
PIN7: 3.5 Vac

The Fender 65 TRR (85 watts) sounds much lourder with the same 2x12 cab than this JCM900 (100 watts). The marshall sounds loud, but is not melting my ears
I plugged my guitar and tested it with all four power tubes, then just 1-4, and then just 2-3, and I think it didn't make much of a difference. Four tubes definately sound louder than two tubes, but not that much in this amp. And the Twin reverb wins

[J M Fahey]

Melting ears is a very poor power indicator, you´ll have to build a load resistor same impedance as speakers, get a generator or an MP3 tone to drive amp, and a scope to check clipping and waveforms.

At least a PC Scope and a homemade attenuator.

Otherwise you are playing Piñata with blindfolded eyes.

[vintagekiki]

Originally Posted by spunko
I took out both fuses, pots at max as suggested, fed a 1 Khz / 11.5 mV signal into the amp, AC voltmeter to JS4 and got 5.39 Vac for the clean channel, and 5.2 Vac for the drive. How do I read the voltage at C7?

JS4 is connected to C7 (22n) via R16 (10k). The voltages you get for clean and drive channel are OK

Originally Posted by spunko
What it is interesting, is that the amp still had output through the speakers with both fuses out. Is this normal?

You should not have output through the speakers if you took out fuses (FS1 and FS2)
If FS1 and FS2 (in cathode the power tubes) are faulty, then you will not have output through the speakers and the LED1 and LED2 lights.

Originally Posted by spunko
Is it necessary to check voltages at the ICs?

Since channel switching works, it is not necessary to check voltages at the ICs.

Originally Posted by spunko
What surprised me was that CH-B sounded really really loud with the broken pot, and when I replaced it, the CH-B leveled in output with the clean channel. CH-B was louder with the broken pot. Maybe that is why it sounded so loud.

When at volume pots was defective (open), pin which goes to ground is broken, the amlifier sounds very loud, because there is no signal attenuation.
After replaced the defective pots, the CH-A and CH-B sound should by sound fairly balanced (up to 10%)

[spunko]

Unfortunately I don't have an oscilloscope. I know "by ear" you can't measure power, but definitely the Twin sounds louder than the marshall.

With the twin volume at 5 it hurts because it is so loud. With the marhshall I can go to 10.

You should not have output through the speakers if you took out fuses (FS1 and FS2)
If FS1 and FS2 (in cathode the power tubes) are faulty, then you will not have output through the speakers and the LED1 and LED2 lights.

So, it definitely has a problem because the amp gives output even with the fuses out. The both leds light up, but still drives the speakers.

What do you thnik about the voltages?

All Power tubes
PIN3: 3.5 Vac / 490 Vdc Is it normal that Pin3 read both AC and DC?
PIN4: Standby OFF -90 Vdc / Standby ON -53.18 Vdc Is it normal for the voltage to drop?
PIN2: 490 Vdc
PIN1: 3.94 Vac
PIN7: 3.5 Vac

[vintagekiki]

Originally Posted by spunko
So, it definitely has a problem because the amp gives output even with the fuses out. The both leds light up, but still drives the speakers.

Check that LED1 and LED2 go through the R8 and R9 resistors (100k) to the power tubes cathodes.
If the leds go directly to the cathodes of the power tubes without R8 and R9, it lights up when the fuses out.
This is not good because the power tubes protection does not work in this case.
Check circuit FS1, R8, LED1 and FS2, R9, LED2.

https://drtube.com/schematics/marshall/4100-60-02-iss14.pdf

Originally Posted by spunko
What do you thnik about the voltages?

All Power tubes
PIN3: 3.5 Vac / 490 Vdc Is it normal that Pin3 read both AC and DC?
PIN4: Standby OFF -90 Vdc / Standby ON -53.18 Vdc Is it normal for the voltage to drop?
PIN2: 490 Vdc
PIN1: 3.94 Vac
PIN7: 3.5 Vac

I'm afraid you're not the best count pins on power tubes.
When you look at the power tubes from below, the pins are counted from the nose (pin 1) clockwise (from left to right)

[spunko]

I was trying to read the numbers in the socket, I guess I'm not the best I should have definitely check tube by tube and write down every value.

I get continuity between FS1 - R8 - LED1 and FS2 - R9 - LED2, both fuses out.

This are the voltages I get with all fuses IN (voltages in the picture)


Because in my country you won't find slow blow fuses, I replaced the FS2 with a fast blow fuse, and FS1 has its original slow blow fuse.

FS1 500mA Slow blow / FS2 500mA Fast blow
Tubes 1 - 4 at pin8: 39.5 mV
Tubes 2 - 3 at pin8: 9.3 mV

Now, if I switch the fuses:
FS1 500mA Fast blow / FS2 500mA Slow blow
Tubes 1 - 4 at pin8: 9.5 mV
Tubes 2 - 3 at pin8: 39.3 mV

So, I took some fuses to test, all of them fast blow. If the fuse is to high, like 4A, the voltage drop even more (like 5.2 mV), if I swap it with a 250mA the voltage rises to 44.5 mV

Also, with the 250mA fuse the amp sounded really really loud as I was expecting. If I put back the 500mA fuse Fast blow, or higher value, the amp sounds quieter, or less output.

I am waiting for the slow blow fuses to come, I got them from eBay, but they will get to my hands like in 35 days.

[vintagekiki]

I get continuity between FS1 - R8 - LED1 and FS2 - R9 - LED2, both fuses out.

I have a question.
When the mains are disconnected from the wall and the FS1 / FS2 / T500mA fuses removed, when measured by an ohmmeter how much is value resistance between the cathode power tubes (pin8) and the ground (except value R8/R9/100k)

So, I took some fuses to test, all of them fast blow. If the fuse is to high, like 4A, the voltage drop even more (like 5.2 mV), if I swap it with a 250mA the voltage rises to 44.5 mV

Fuses FS1/FS2/T500mA are located between the cathode power tubes (pin8) and ground.
The different voltage drop on fuses of different values is due to the different resistance of the of the wire contained in the fuse tube.
It doesn't have to burden you.

Also, with the 250mA fuse the amp sounded really really loud as I was expecting. If I put back the 500mA fuse Fast blow, or higher value, the amp sounds quieter, or less output.

How you tested the volume of the amplifier?
Tested the volume the amplifier with a guitar is irrelevant, only any signal source that has a constant output amplitude.

EDIT

[vintagekiki]

Online Tone Generator



In the absence of a signal source with constant output amplitude, the Online Tone Generator can serve.
Enter the desired frequency in the Hertz field.
The tone generator can play four different waveforms: Sine, Square, Sawtooth and Triangle. Click on the buttons to select which waveform you would like to generate. The tone will continue until the stop button is pushed.
You can make a whole range of frequencies for the test amplifier.

To be you independent of http it is enough to download the desired signal on HD in duration 1 min and then play it through Winamp.
To allow the test signal to run indefinitely, enable Toggle Repeat in Winamp.

Sound from sound card output jack, connect via the shield cable to the amplifier.
Please always make sure amplifier volume potentiometer are set to a low volume to avoid damage to amp.

http://onlinetonegenerator.com/
Online Tone Generator



EDIT (190825)

https://www.szynalski.com/tone-generator/
Online Tone Generator

[g1]

Any progress on this spunko?
Especially this question:

When the mains are disconnected from the wall and the FS1 / FS2 / T500mA fuses removed, when measured by an ohmmeter how much is value resistance between the cathode power tubes (pin8) and the ground (except value R8/R9/100k)