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Re: Could use some assistance on a Marshall amp [message #12875 is a reply to message #12803] Thu, 04 February 2010 15:24 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
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If you're still working on this, I see a problem with the readings at C62. The signal leaves the preamp at C22, goes to the FX loop jacks and then returns to the output section via C62.

The signal passes through a couple of resistors and the jacks, but it should be at nearly the same voltage level at both points.

What happens if you plug the new signal source into the FX return jack? Remember that the Master Volume will still control the volume here.

A question regarding your voltage readings, what do you mean by + and - lead readings?

I don't get your resistance readings for the master pot. Are you still keeping one lead attached to the chassis? What I asked for was the reading of the resistance from the right to the left leg of the pot.

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Re: Could use some assistance on a Marshall amp [message #12876 is a reply to message #12875] Thu, 04 February 2010 15:57 Go to previous messageGo to next message
RickBlacker is currently offline  RickBlacker
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chicagobill wrote on Thu, 04 February 2010 15:24

If you're still working on this,
ABSOLUTLY still working on this.


chicagobill


I see a problem with the readings at C62. The signal leaves the preamp at C22, goes to the FX loop jacks and then returns to the output section via C62.

The signal passes through a couple of resistors and the jacks, but it should be at nearly the same voltage level at both points.

What happens if you plug the new signal source into the FX return jack? Remember that the Master Volume will still control the volume here.

For measuring or for hearing something? I can take some measurments when I get home. But for audio, I definilty got audio output when I used plugged my guitar into my my FX processor unit, then plugged the fx processor unit into the FX return on the amp.


chicagobill

A question regarding your voltage readings, what do you mean by + and - lead readings?


Meaning the lead wires on the various components.


chicagobill

I don't get your resistance readings for the master pot. Are you still keeping one lead attached to the chassis? What I asked for was the reading of the resistance from the right to the left leg of the pot.


Yes, I have the neg lead from the multimeter touching ground on the chassis. As far as reading the resistace across the POT itself, I must have misread that. I'll go back tonight and retake the resistance readings. Put one multimeter lead on the right lug, put the other multimeter lead on the left nug.

Sorry, I just didn't understand what you were asking.



My amp : K100C-8

[Updated on: Thu, 04 February 2010 15:58]

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Re: Could use some assistance on a Marshall amp [message #12879 is a reply to message #12803] Thu, 04 February 2010 22:58 Go to previous messageGo to next message
RickBlacker is currently offline  RickBlacker
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chicagobill wrote on Thu, 04 February 2010 15:24

What happens if you plug the new signal source into the FX return jack? Remember that the Master Volume will still control the volume here.
Seems to work just fine.



chicagobill wrote on Thu, 04 February 2010 15:24

What I asked for was the reading of the resistance from the right to the left leg of the pot.

With multimeter on 20k ohms, I get a reading of 4.79


For shits and grins, I retook the readings on C22 and C62. I ensured that the ouput volume of the stereo that I'm feeding into the input on the amp was at the same volume. Not sure if that would make a differenc or not. The readings are the same basically.

With the multimeter set at 200AC (sorry, that's as low as it goes on AC)

C22 bounces between 3.3 and 1.5ish
C62 bounces between 0.1 and 0.2


So, are you thinking R20 or R62 is bad?

For grins, I took a RESISTANCE reading of them.
R20 reads 4.67 ohms with meter set at 20k
R62 reads 1.0 ohms with meter set at 2K.


AC voltage going INTO
R20 reads average of 1.6
R62 reads average of 0.1

AC voltage OUT of
R20 reads 0.1
R62 reads 0.1

Tried plugging in a jumper cable between FX in and FX out just to be sure, and that made no difference.



Also, not sure if this means anything or not. But, LED3 and LED4 never glow. I have a distortion pedal that I modded, and when I strum my guitar, it flutters with the addition of a LED to the circuit. Not sure if either of these are supposed to flutter or not. I tired with both clean and dirty channels.

My amp : K100C-8

[Updated on: Thu, 04 February 2010 23:28]

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Re: Could use some assistance on a Marshall amp [message #12890 is a reply to message #12879] Fri, 05 February 2010 12:17 Go to previous messageGo to next message
chicagobill
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Registered: April 2003
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RickBlacker wrote on Thu, 04 February 2010 21:58

C22 bounces between 3.3 and 1.5ish
C62 bounces between 0.1 and 0.2

Can you jumper a wire from the output of C22 to the input of C62?

RickBlacker wrote on Thu, 04 February 2010 21:58

Also, not sure if this means anything or not. But, LED3 and LED4 never glow.

They should flicker when in the distortion channel with the gain turned up.

Have you taken DC voltage readings on the preamp chips? Start with IC1. There should be -15vdc at pin 4 and +15vdc at pin 8 (referenced to ground). There should not be any real voltage on any of the other pins. Maybe a few millivolts. Be careful not to short any pins together with your meter lead when checking the voltages. Only check IC1-IC4.

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Re: Could use some assistance on a Marshall amp [message #12891 is a reply to message #12890] Fri, 05 February 2010 13:02 Go to previous messageGo to next message
RickBlacker is currently offline  RickBlacker
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chicagobill wrote on Fri, 05 February 2010 12:17

Can you jumper a wire from the output of C22 to the input of C62?

I took a little wire from some CAT5 cable that I have. Stripped two ends, put one end in the hole where the negative wires go for each cap.

When i did this, the volume reduced.


chicagobill wrote on Fri, 05 February 2010 12:17

They should flicker when in the distortion channel with the gain turned up.

I double checked again. Nothing from them at all.


chicagobill wrote on Fri, 05 February 2010 12:17

Have you taken DC voltage readings on the preamp chips? Start with IC1. There should be -15vdc at pin 4 and +15vdc at pin 8 (referenced to ground). There should not be any real voltage on any of the other pins.

I checked them all. They all look good and read to your specs there.

My amp : K100C-8

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Re: Could use some assistance on a Marshall amp [message #12892 is a reply to message #12891] Fri, 05 February 2010 13:45 Go to previous messageGo to next message
RickBlacker is currently offline  RickBlacker
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Set multimeter to 200vac. Took readings on IC1 and IC3

Pin1 = op-ampA
Pin7 = op-ampB

Readings are :

IC1A : 0.0
IC1B : 0.0

IC3A : 0.2
IC3B : 0.0

My amp : K100C-8

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Re: Could use some assistance on a Marshall amp [message #12893 is a reply to message #12892] Fri, 05 February 2010 14:44 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
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Are these AC readings with the signal applied to the input?

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Re: Could use some assistance on a Marshall amp [message #12894 is a reply to message #12893] Fri, 05 February 2010 14:50 Go to previous messageGo to next message
RickBlacker is currently offline  RickBlacker
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Yes, sorry... Those IC readings are AC signal readings.

Had my signal coming in through the input. Put multimeter on 200vac, black lead on ground, red lead on the IC pins.

My amp : K100C-8

[Updated on: Fri, 05 February 2010 14:52]

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Re: Could use some assistance on a Marshall amp [message #12895 is a reply to message #12894] Fri, 05 February 2010 15:28 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
OK, Go back to the beginning here.

How much AC signal is the pedal that you are using as a signal source putting out?
What AC signal is at the input cap C3 (either side)?
What AC signal is at pin 3 and pin 5 of IC1?

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Re: Could use some assistance on a Marshall amp [message #12896 is a reply to message #12895] Fri, 05 February 2010 15:39 Go to previous messageGo to next message
RickBlacker is currently offline  RickBlacker
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chicagobill wrote on Fri, 05 February 2010 15:28

How much AC signal is the pedal that you are using as a signal source putting out?

Output from stereo headphone jack is bouncing between 1.0 and 0.7 vac. I measured on the other tip of the cable. Just as a reference, I'm running the output of my stereo at a volume of 20 on the dial. Almost but not totally maxed out.


chicagobill wrote


What AC signal is at the input cap C3 (either side)?


0.0vac on both sides of the cap

chicagobill wrote


What AC signal is at pin 3 and pin 5 of IC1?

Pin 3 : 0.0
Pin 5 : 0.0

One thing I noticed is that when I touch pin 3 with the red lead, both red and green diodes will brighten up a bit.

ALSO

When driven hard enough, I did notice that they are flickering. So, I guess I was wrong before when I said they were not lighting up at all... They are.

My amp : K100C-8

[Updated on: Fri, 05 February 2010 15:43]

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Re: Could use some assistance on a Marshall amp [message #12897 is a reply to message #12896] Fri, 05 February 2010 15:42 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
Either you are not getting a good reading, or there is a problem with the input jack.

Try jumpering the input signal directly to C3. You need to keep a plug in the input jack, as it has a muting function switch.

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Re: Could use some assistance on a Marshall amp [message #12898 is a reply to message #12897] Fri, 05 February 2010 16:02 Go to previous messageGo to next message
RickBlacker is currently offline  RickBlacker
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Did my best... I would definitly get noise. Tried hard to not touch other components. Sometimes it would increase the audio, sometimes it would drain the cap.

What exactly am I trying to do here? If I'm hovering over the amp, the jack's input in pointing at me. WHich pin do I want to connect to? Top Left / Top Right / Bottom Left / Bottom right?

Does it matter which pin on the cap I'm trying to jump to?

Or am I supposed to somehow grab the signal directly off the cable itself? WHich I'm not sure how i could considering the jack is enclosed..

My amp : K100C-8

[Updated on: Fri, 05 February 2010 16:03]

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Re: Could use some assistance on a Marshall amp [message #12899 is a reply to message #12803] Fri, 05 February 2010 16:07 Go to previous messageGo to next message
stevem is currently offline  stevem
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You can inject the signal into eitheir side of that cap.
I would recommend trimming that test signal back to no more than .300 volts. other wise .700 to 1 volt of signal is way more than the first gain stage wants to see. No less you may blast your ears out.

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Re: Could use some assistance on a Marshall amp [message #12900 is a reply to message #12899] Fri, 05 February 2010 16:10 Go to previous messageGo to next message
RickBlacker is currently offline  RickBlacker
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stevem wrote on Fri, 05 February 2010 16:07

You can inject the signal into eitheir side of that cap.
I would recommend trimming that test signal back to no more than .300 volts. other wise .700 to 1 volt of signal is way more than the first gain stage wants to see. No less you may blast your ears out.


I've been keeping the master volume down. But, I get what you're saying about the too much signal... I'll reduce the volume on the input.

Any suggestions on which pin on the jack to tap into?

My amp : K100C-8

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Re: Could use some assistance on a Marshall amp [message #12901 is a reply to message #12900] Fri, 05 February 2010 16:17 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
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The hot on the jack is pin #1. You could try touching a wire from pin 1 to C3.

What I was suggesting was to take the signal directly from the source plug and bypass the input jack entirely. You will need to use two wires one from the tip to C3 and one from the shield to ground.

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Re: Could use some assistance on a Marshall amp [message #12902 is a reply to message #12803] Fri, 05 February 2010 16:17 Go to previous messageGo to next message
stevem is currently offline  stevem
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Bypass the input jack altogther and if need be solder a lenght of wire to the C3 side of R1 to feed you input signal into.
You will as bill posted have to have a cord plugged into the input jack to keep the shorting switch in the jack open.

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Re: Could use some assistance on a Marshall amp [message #12903 is a reply to message #12803] Fri, 05 February 2010 16:45 Go to previous messageGo to next message
RickBlacker is currently offline  RickBlacker
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Right on... It works when by passing the input jack... Both clean and gain channels fire up nice!!!

My amp : K100C-8

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Re: Could use some assistance on a Marshall amp [message #12904 is a reply to message #12803] Fri, 05 February 2010 16:50 Go to previous messageGo to next message
stevem is currently offline  stevem
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In that case eitheir you have a shorted C1 OR C2 cap.
An open L7 choke .
or like I posted way back in this string, a bad input jack or its connections to the circuit board.

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Re: Could use some assistance on a Marshall amp [message #12905 is a reply to message #12904] Fri, 05 February 2010 16:55 Go to previous messageGo to next message
chicagobill
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Tell us how you bypassed the jack.

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Re: Could use some assistance on a Marshall amp [message #12906 is a reply to message #12905] Fri, 05 February 2010 16:57 Go to previous messageGo to next message
RickBlacker is currently offline  RickBlacker
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Put a dummy jack into the input

wrapped a wire around the ground on the plug, tapped it to the chassie

Wrapped a wire around the positive part of the jack (tip) taped it on. Then probed C3. Got a nice output from the amp.

My amp : K100C-8

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Re: Could use some assistance on a Marshall amp [message #12907 is a reply to message #12906] Fri, 05 February 2010 17:03 Go to previous messageGo to next message
chicagobill
Messages: 2005
Registered: April 2003
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OK You're on the right track now.

As Steve said, check the input jack connections and all of the components from the jack to C3 and you'll be all set.

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Re: Could use some assistance on a Marshall amp [message #12908 is a reply to message #12907] Fri, 05 February 2010 17:13 Go to previous messageGo to next message
RickBlacker is currently offline  RickBlacker
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I'm going to assume that it's safe to do the same bypass probing... Yes?

My amp : K100C-8

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Re: Could use some assistance on a Marshall amp [message #12910 is a reply to message #12908] Fri, 05 February 2010 18:03 Go to previous messageGo to next message
chicagobill
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Registered: April 2003
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Yes, try injecting the signal at the input of C1, then at the input of L7, etc.

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Re: Could use some assistance on a Marshall amp [message #12914 is a reply to message #12910] Fri, 05 February 2010 19:09 Go to previous messageGo to next message
RickBlacker is currently offline  RickBlacker
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YOU GUYS ARE AWESOME!!!!!!

Thank you so much for all your help. I was able to pin point the issue and I've fixed it.

One of the legs on the output jack had a bad solder joint on it. Resoldered it and WALA...

I wasn't going to get a chance to work on it this weekend and now I don't have to.

I learned a LOT from this experiance. It's been VERY helpful for me. Now in a week or so, I get to start building my tube amp.

I can't believe I got this marshall for free. Razz

My amp : K100C-8

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Re: Could use some assistance on a Marshall amp [message #12916 is a reply to message #12914] Fri, 05 February 2010 22:37 Go to previous messageGo to next message
Jc is currently offline  Jc
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I have to admit to not understanding about half of this thread - But man, it was fun watching you guys work this thru - Nice job and really interesting - jc

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Re: Could use some assistance on a Marshall amp [message #12917 is a reply to message #12803] Sat, 06 February 2010 01:00 Go to previous messageGo to next message
RickBlacker is currently offline  RickBlacker
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I have been reading up on electronics the last few weeks. But if it wasn't for these guys It would have taken me a couple ofmonths to figure out. Like I said. I learned a LOT by this little project. Smile

My amp : K100C-8

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Re: Could use some assistance on a Marshall amp [message #12918 is a reply to message #12803] Mon, 08 February 2010 06:07 Go to previous messageGo to next message
stevem is currently offline  stevem
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Good deal!
Besides knowledge there are two other things that get you thru you first repairs like this.
#1 is you have go t to sick with it and not get bent out of shape.
#2 you have to know when to take a break of even sometimes 24 hours so you can step back and review what you have done and or covered, and then pick up from there.
Sometimes even simple distractions will kill you!
For example last fall I was working on a amps circuit installing new parts when my cell phone rag while I had a hot iron in my hand.
I put tham in standby and then took the call. 10 minutes latter got back to work, flipped the amp off of standby and the fuse for the tube heaters blew.And I had done nothing be take the cell call.
Well 10 minutes latter I tracked down the cause of the blown fuse.
When I moved to take the call I had a blob of hot solder that dropped off of my iron and onto the circuit board ubder some wires that bridged the heater trace to make the short.
So you gotta stay on your toe`s but when you get to point of stressing out, take a good long break!

Once again, congrats on the repair!

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Re: Could use some assistance on a Marshall amp [message #12920 is a reply to message #12803] Mon, 08 February 2010 11:31 Go to previous messageGo to next message
RickBlacker is currently offline  RickBlacker
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Thanks.
Funny thing is, I got all that work into it and I'm not even all that fond of it. However, when I put some pedals in front, I like it. But the amp by itself is rather bleh sounding. To my ears anyway.

I understand what you mean about taking a break if you need to and concentration. I'm a software engineer and there have been many times I've struggled with some code, walk away for a while, come back and the light bulb turns on.

Well, I greatly appreciate everything you guys did for me. Like I said, I really learned a lot. But, I still have so much to learn.

My amp : K100C-8

[Updated on: Mon, 08 February 2010 11:34]

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Re: Could use some assistance on a Marshall amp [message #12921 is a reply to message #12803] Mon, 08 February 2010 11:39 Go to previous messageGo to next message
stevem is currently offline  stevem
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Good tone rule #1.
The less circuit crap/bells and whistles that the signal has to be pushed thru,the worse it will sound!!!
A amp consisting of just two gain stages, a phase inverter/driver and output stage will 98% of the time sound killer even if its SS.

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Re: Could use some assistance on a Marshall amp [message #12922 is a reply to message #12803] Mon, 08 February 2010 12:31 Go to previous message
chicagobill
Messages: 2005
Registered: April 2003
Senior Member
Glad to hear it's up and running.

The real trick to repairing any amp, Kustom or not, is simple logic. The circuit is supposed to do something, if it doesn't do it, you just need to look at the clues and figure out why it isn't working.

If an experienced tech had that amp on the bench, it probably would have taken ten minutes to find the problem. But that's because a tech would have been able to hear the amp and see the amp and get a bunch of clues from the way that the amp responded to different signals, etc.

It's always tough to fix an amp on line, because the information we get to work with is filtered by the person giving us the information. Sometimes it's also difficult to judge how technically able someone is on line.

Have fun with it, and just remember when you do build the tube amp, that unlike most solid state amps, there are voltages in a tube amp that can hurt you.

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