Diagnosis of Marshall 8100 Valvestate head problems

Thread Starter

wolf1419

Joined Nov 1, 2008
117
Hello Ron,

In how much detail do you want the preamplifier part explained?

If you want every opamp can be described.

Greetings,
Bertus
Thanks for asking....you're so helpful....:D
I'm going to do my homework with the site you sent regarding amplifier basics. Since this preamplifier is currently operational, I'd like to really focus heavily on the power amplifier section and really get a handle on it, then work to replace parts as needed.....and get it operational, especially since my other active project involves a power amplifier(Crown CE1000) that has the "fault light flashing" on channel 2, which disables the output on that side to protect the amp......so I'm hoping that some of your teaching here will be useful and crossover to that project as well.
I will be very interested in the preamp stage after I've absorbed this other information and gotten past these 2 repairs......so any further explanations of power stage circuits will be greatly appreciated....:)
Sincerely, Ron
 

bertus

Joined Apr 5, 2008
21,939
Hello Ron,

Wich version of the poweramp do you have?
The schematic is for 80 and 100 Watts.

This is of influence of the current limitting of the powertransistors.

Greetings,
Bertus
 

Thread Starter

wolf1419

Joined Nov 1, 2008
117
Hello Ron,

Wich version of the poweramp do you have?
The schematic is for 80 and 100 Watts.

This is of influence of the current limitting of the powertransistors.

Greetings,
Bertus
Hi,
The amp in question is the 100 watt version(model 8100)...shared schematic.
 

bertus

Joined Apr 5, 2008
21,939
Hello,

As I already told you there are parts directly related to the current protection of the TR8 and TR10.
For TR8 the resistors R116, 117, 118 and transistor TR12 and D5 make a current limiter.
For TR10 the resistors R96, 97, 115 and transistor TR11 and D4 make a current limitter.

The calculations for the current are similar to the calculations at TR9.
It will result in 1.92 volts accross R118 , this will give a limit of 1.92 / 0.33 = 5.82 A
For R96 it will result in 2.14 Volts , this will give a limit of 2.14 / 0.33 = 6.48 A.

Greetings,
Bertus
 

Thread Starter

wolf1419

Joined Nov 1, 2008
117
Hello,

As I already told you there are parts directly related to the current protection of the TR8 and TR10.
For TR8 the resistors R116, 117, 118 and transistor TR12 and D5 make a current limiter.
For TR10 the resistors R96, 97, 115 and transistor TR11 and D4 make a current limitter.

The calculations for the current are similar to the calculations at TR9.
It will result in 1.92 volts accross R118 , this will give a limit of 1.92 / 0.33 = 5.82 A
For R96 it will result in 2.14 Volts , this will give a limit of 2.14 / 0.33 = 6.48 A.

Greetings,
Bertus

Wow, I finally got through the reading of the different amplifier circuit types and have a clue about all the different methods available for amplifying a signal, from the site you suggested. It was very informative and the equations we are discussing will be helpful once the amplifier is up and running, with TR8 & TR10 aboard, but only if the fuse holds.

Prior to that, the best thing for me now would be to go back to the beginning(C18) of the power section(input stage seems fine) and re-test each component in the power section, in some type of logical order to follow each subcircuit, which I've now mostly completed, and found most everything in spec, verifying contact from point to point(thru PCB), component to component, and found R61 measuring only 1.8K vs 33k design(appears to be same as another 33k that I saw in there for band coding), but reads way off....not shorted, just a low resistance.....what would be the impact on the circuit if not replaced......??
Can we discuss the design purposes of the various output stages, from C18 on out to spkr, without power, prior to installing TR8 & TR10, walking me slowly through the circuit design elements and flow characteristics.
I'm hoping that checking continuity, R values and capacitance(I have the capacitance meter from our other project) of individual components, then a few in circuit, calculating and discussing total/integrated values along the way, which might be helpful in my learning process.
I'm also wondering if any voltage or current measurements(point to point vs. referencing to gnd) in various parts of the circuit when powered up, would be useful in testing, still prior to the TR8 & TR10 install, any sections of subcircuits(inline) that may be measured that are not directly affected by TR8 & TR10, even if only on a relative basis......or is none of this possible or adviseable at that stage, even if only for academics......:eek:

I'm applying your most recent calculations, following the logic to the schematic, and have one question..... how you determined the 0.7V used in our calculations. How do I come up with that in other situations....?.....

Oh yeah, will we be using "conventional" or electron flow direction when discussing circuits(affecting my view and symbols regarding diodes, transistors, etc.....)
 
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bertus

Joined Apr 5, 2008
21,939
Hello Ron,

Here is my try to "walk" through the schematic.
I will load the schematic to have things more clear.
Others may of course also give comments.




I will start at the output of the opamp IC8A.

I will follow the positive side first:
When voltage at the output of the opamp rises, the voltage accross R81 and R82 will drop.
The current in R82 will be lower, so the current in R84 (at the collector of TR6) will be lower.
This will result is a lower voltage accross R84.
When the voltage accross R84 is lower the voltage accross R85 will also be lower.
The current in R85 will be lower, this will result in a higher voltage accross the C-E of TR7.
The voltage on the Collector of TR7 will go more to the negative side.
The transistor TR8 will follow this voltage and will have the output going more to the negative side.

Now for the nagative side:
When voltage at the output of the opamp rises, the voltage accross R81 and R79 will rise.
The current in R79 will be higher, so the current in R77 (at the collector of TR5) will be higher.
This will result is a higher voltage accross R79.
When the voltage accross R79 is higher the voltage accross R75 will also be higher.
The current in R79 will be higher, this will result in a lower voltage accross the C-E of TR4.
The voltage on the Collector of TR4 will go more to the negative side.
The transistor TR10 will follow this voltage and will have the output going more to the negative side.

Things will be opposite when the voltage at the output of the opamp will go down.

I hope this will give some more info in the relation between the components.

Greetings,
Bertus
 

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Thread Starter

wolf1419

Joined Nov 1, 2008
117
Thanks for putting that last explanation together....I'm chewing on that and have 2 questions to start.
1)The positive side voltage drops at R81....and....
The negative side voltage rises at R81....and...
This is hard to understand voltage dropping and rising at the same point..what am I missing...?

2)I have a bad resistor R61, which may be another part of the amp's problem, which reads 1.8k ohms vs. 33K ohms by design....(I just picked up some 33K 1/2 watt resistors).....please try to explain what would make the original read so low, and how this component affects the feedback circuit, especially with an improper resistance being so far off.
 
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bertus

Joined Apr 5, 2008
21,939
Hello Ron,

There are two circuits connected at the output of the opamp.

For the positive side we have the following components:
R81, R82, Base-Emittor TR6 and R83.
When the voltage at the output of the opamp rises, the voltage difference on this string will become less.
The current will go down in this case.

For the negative side we have the following components:
R81 (again), R79, Base-Collector TR5 and R78.
When the voltage at the output of the opamp rises, the voltage difference on this string will become more.
The current will go up in this case.

The ouput of the opamp will "shift" the current balance between positive and negative.

Greetings,
Bertus
 

bertus

Joined Apr 5, 2008
21,939
Hello Ron,

When you look in the schematic the R61 is parallel to the two resistors R62 and R59, both 1K.
When you calculate the repacement value you will get about :

1 / ((1 / 33K)+(1 / 2K))=1.886 K

These R62 and R59 with C20 probably have to do with some DC feedback.

Greetings,
Bertus
 

Thread Starter

wolf1419

Joined Nov 1, 2008
117
Thanks for clarification on the previous post....
Is my reading of 1.8K ohms(amp "off") what is to be expected because it is still in the circuit and affected by the other resistors that are in parallel, or should it read 33K with no power, and only affected when power is applied, which is what I was expecting.....?
 

bertus

Joined Apr 5, 2008
21,939
Hello Ron,

When R61 is still mounted, and also R62 and R59, you will measure the 1.88 K I calculated.
This is when the amplifier is switched off.
When it is active you can not measure resitance as a voltage accross it influences the reading.
(and can damage the meter in some cases).

Greetings,
Bertus
 

Thread Starter

wolf1419

Joined Nov 1, 2008
117
Hello Ron,

When R61 is still mounted, and also R62 and R59, you will measure the 1.88 K I calculated.
This is when the amplifier is switched off.
When it is active you can not measure resitance as a voltage accross it influences the reading.
(and can damage the meter in some cases).

Greetings,
Bertus
Well I'm glad you have the saying about asking questions because I would be stupid to just go and replace R61, thinking it was bad....the value didn't make sense anyway, but I didn't include the other parts of the circuit. If a resister is in series and tested individually it should be close to it's design value, but in parallel, I will get the calculated value of the circuit...correct...? And if I read 2 series resistors as one part of the circuit, I should read R1 + R2 total ohms...correct...?
 

Thread Starter

wolf1419

Joined Nov 1, 2008
117
Yippie...!!! I feel special now....still not a technician, but I guess if you ask enough questions your count goes up.....and you are helping me to get a little smarter with every post......I really appreciate it.

I'll study more of that section, but I think I see the situation, which is that these are the only resistors between the center(signal) and the feedback loop, and are parallel, as a group, while R62 & R59 are series, making a series/parallel circuit for feedback to the opamp.

You offered to explain opamps in the preamp, but if you can just explain IC8A & IC8B(both contained in one TL072 IC), that would help in this section.....
 

bertus

Joined Apr 5, 2008
21,939
Hello Ron,

IC8A is used as a mixer for the signal from the reverb and the effect channel.
IC8B is used for amplifying the signal (using R25, R58, and R26).
(R26 is probably used to have a ground reference at the input).
IC8B is also used to look at the output with the feedback using R61,R62 and R59 with C20.

Greetings,
Bertus
 

Thread Starter

wolf1419

Joined Nov 1, 2008
117
I can picture that......and now think it may be time to install new TR8 & TR10 to the board........maybe some final checks for shorts, some calcs and checks before I attempt to turn it on.....still getting more hands on, testing and understanding before applying power. Sound reasonable...?.:cool:
 

bertus

Joined Apr 5, 2008
21,939
Hello Ron,

How old is the amplifier?
It still could be that C20 is old and not functioning well anymore.
(it is still part of the feedback loop).

Greetings,
Bertus
 

bertus

Joined Apr 5, 2008
21,939
Hello,

If you can test capacitors, also check C55 then too.
(also part of the feedbackloop through R60 and R120).

Greetings,
Bertus
 

Thread Starter

wolf1419

Joined Nov 1, 2008
117
Hello,

If you can test capacitors, also check C55 then too.
(also part of the feedbackloop through R60 and R120).

Greetings,
Bertus
Actually, I need to give you a break from my problems, and also realized I was going to check all capacitors as part of my diagnosis procedure...I last did resistors and traces, I haven't done all transistors, capacitors, and diodes................that will keep me busy for a while...
 
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