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Messages - jjonas

#1
Hi,

I have a DIY fuzz pedal, which is way too bright. I thought it'd make sense to build a fixed low pass filter into it, so that it will sound ok with my regular setup, without having to change the rest of the setup just for this fuzz. So, if you think this is a sound idea:

I know something about LPF's in theory (how to calculate Xc etc.), but I'm not sure where I should place it in the schematic (input or output side), and what other factors I should take into account.

Here's the schematic:

http://www.deviever.com/fx/wp-content/uploads/2012/09/hyperion.jpg

Where should I place the LFP there..?
#2
Quote from: Roly on May 31, 2014, 02:53:39 AM
That colour rendition can't be too hot 'coz I would read C22 as;

Brown = 1
Black = 0
Orange = 3 = 000
White (9) "tolerance band".  Since resistors don't use a white tolerance marking this is a clue that it's a cap not a resistor. {another clue is that it would test as open circuit rather than 10k ohms}

You're right that one of the bands is black and not green, I don't know why I wrote green, because I did see it was black as well. Sorry for that.

As to the tolerance band, probably it's white as you say, that perhaps makes the most sense, but personally I can't really tell for sure whether it's yellow or white just by looking at it.
#3
Wow, another thing learned. I've never seen caps like that before. I've attached a pic I took just for the benefit of others who haven't seen caps like that.

BTW in the end I did solder the 51k resistor in series with the "42ohm resistor that was a cap" instead of replacing it, just to make sure. Glad to get confirmation.
#4
The colour bands are yellow-red-green-brown. Red could be orange too, it's hard to tell for sure, but I'd say it's red. The component body is light green. No text or markings can be seen. Posting pics is no problem.
#5
What a great forum!!

@J M Fahey: Thanks for clarification, it was helpful! Exactly the kind of stuff I wanted to know. Also thanks for the implementation tip, probably I'll do just that.

@Roly: Yes I meant millivolts. Also thanks for further explanations!

Edit: A few more questions. I noticed when I was making the modifications for IC1a, that the 10nF C1 that I'm supposed to "float" the other leg of, has been replaced (not by me) with a 42ohm resistor. I remember that once when this amp was being serviced several years ago the repair guy said that he fixed a few "typical Marshall faults" or something.

Other caps (all 10nF) that have been replaced with 42 ohm resistors are

- C8 (in the schematic, top segment, 5th upward wire off the 0V rail from the left)
- C22 (top segment, middle part, above TR2)
- C32 (middle segment, to the right of the reverb unit connection)

Any idea what significance these modifications have?

Anyway I replaced the 42 ohm resistor that was in place of C1 with a 51k resistor (plus added the diodes), I hope that's ok.
#6
Thanks for insights!

Quote from: J M Fahey on May 29, 2014, 12:59:13 PM
As to what fries it: loop out/in and pream in are often fried by strong external signals, usually some speaker out there and blows it to pieces.

Can you elaborate what you mean with "external signals, usually some speaker out there"..?

Quote from: J M Fahey on May 29, 2014, 12:59:13 PM
It´s advisable to add a series resistor and a couple protective diodes to send dangerous voltages away. In your case Rs may be 4K7 and diodes plain 1N4002 .

Do you advise that this be done to both IC4 and IC5 (with series resistors for both sides of the opamps), or only IC5..? So far IC4 hasn't caused any problems, but in principle is it in the danger zone in the same way as IC5..?

One way to implement this that occurs to me is making a small daughterboard with the added components and the chip, which is then pressed into a DIP8 socket on the main board.
#7
Quote from: Roly on May 29, 2014, 07:30:39 AM
You are getting a bit ahead of yourself here.  Before we start replacing stuff we do a bit of diagnostics first, that way we hopefully identifiy exactly what is wrong and fix that, rather than the scattergun approach of randomly replacing stuff in the vain hope we will get lucky (which you almost never do).

Hi, thanks for a quick reply!

(UPDATE: I think the problem is solved in its acute form, please read to the end! I found out the problem was solved while I was going through with the tests you recommended. Nevertheless, I decided to let the reply to reflect that.)

I must confess that I've already made an audio probe and checked where I can hear a signal. I could hear it at R38 (the send output resistor), but at R42 and R43 the signal was crackling. I checked the voltages in IC5 pins 4 and 8, and they checked out (-16,6 and +16,6VDC). When I found an MP1458 in one of my boxes I did replace the BA4558 (with an added DIP8 socket). Now I can hear the signal also at R42 and R43.

As to the signal from the speaker, it is not breaking up anymore, but it's not amplified to a level it should.

Quote from: Roly on May 29, 2014, 07:30:39 AM
1. The very first test is to unplug the speaker(s) and measure the DC voltage across the output.  This should be less than half a volt, but post what you find.  If it is more than half a volt do not reconnect the speakers.

It's -300mA. I'm not sure whether it matters that it's negative, but I'm sure I have the multimeter heads the right way.

Quote from: Roly on May 29, 2014, 07:30:39 AM
2. The amp has an Fx Send and Return, so the next test is to plug a known good lead into both and see if that makes a difference.

Assuming you mean that the Send/Return should be connected together using a single lead: done. The reading stays the same.

Quote from: Roly on May 29, 2014, 07:30:39 AM
3. Next we take a signal from Fx Send to another amp and see if that sounds okay or has the fault.

That seems to work, i.e. with the other amp I get a good clear signal, even when I turn the Marshall's volume up.

Quote from: Roly on May 29, 2014, 07:30:39 AM
4. Similarly you can try plugging your guitar directly into Fx Return and see if that is clean or has the fault. In this way we can tell of the problem is in the pre or main amp.

I'm using a looper pedal for the signal, I connected it to Return only now, and the amp seems to be working now..! This is actually the first time I notice that it's working properly after replacing the op-amp, I must have had something wrong with my test setup.

Quote from: Roly on May 29, 2014, 07:30:39 AM
5. If you have it out of its case you can also check all the power supply rails are within about 10%, +/-15V, +/-34V and +/-49V.

Measured values (at R44-47):
-49,6 / +49,7
-37,0 / +36,8
-16,6 / +16,5

Anyway, the amp seems to be working now. The follow-up question is, though, what could cause the same chip to fry repeatedly? The Vcc and Vee voltages were within tolerance, though it must be said that when the absolute maximum ratings in the datasheets for both BA4558 and MC1458 are +-18VDC, the measured +-16,5 or so sounds pretty close. On the other hand, there's another MC1458 in there with the same voltages, and it's doing fine.

Any ideas?
#8
Hi,

I've had a Marshall Mosfet Lead 100 for some years, it's needed repair twice, and now it's the third time there's something wrong with the sound. I don't feel like handing out another 100€ for a repair, so I thought I'll try to do it myself (rather than give up the whole thing). I'm familiar with electronics in general, I have built a dozen guitar effects and some synthesizer stuff, but I'm not particularly familiar with amplifiers. I have a rudimentary ability to read schematics, know e.g. the basics about voltage divers etc., but I can't really read complex schematics like for amps without someone holding my hand.

So I do have intermediate skills in electronics, in a way that I believe allows me to understand most advice I will get here, but I won't manage without help.

Here's a schematic for the amp:
http://www.drtube.com/schematics/marshall/3210-iss7.gif

The problem: When I plug in, I can hear the guitar only faintly (the faint signal's kind of clean). When I strum harder, the volume jumps up a lot, and the sound is very distorted, almost in an "8-bit" way or something. When I switch the amp off, there's a loud swoosh, plus some guitar sound for about a second, if the strings are ringing at the time of switch-off.

When I plug into the return jack, it's basically the same, only without the faint clean sound, so I suspect it makes sense to start by checking the stuff between the return jack and output. Please correct me if this doesn't sound like a good assessment.

If I remember correctly (and I'm pretty sure I do), last time the amp was in a repair shop, IC5 was replaced. At any rate it's not the chip specified in the schematic: according to the schematic, IC4 and IC5 are MC1458, but IC5 is in fact BA4558.

So I could just desolder IC5, put a DIP8 socket in it and replace the chip with some suitable op-amp, and see if it starts working, but I'd like to complicate things a bit and ask for your help to improve my amp repair skills as well, if you dont mind :-)  Now I know that many op-amps are kind of interchangable, but I'm not sure which ones are compatible, and what are the crucial characteristics to look for in a datasheet.

What I have available in the local electronics shop are at least RC4558 (0,50€) and MC1458P. The pinout is the same, but do you think it matters which one I choose? Here are the datasheet for them, in case it's useful:

RC4558: http://pdf.datasheetcatalog.com/datasheet/texasinstruments/rc4558.pdf
MC1458P: http://pdf.datasheetcatalog.com/datasheet2/3/07j9c8li1155f0estliki76oc2cy.pdf

In case it's relevant, I've used the amp with a Boss GT-10 multifx so that the mfx connects to directly to the return jack. My impression is that this is ok, but if you think it's problematic, please explain. If it is problematic, it could explain why a chip behind the return jack gets broken repeatedly – if indeed that is the broken thing here. Anyway it's been 6-9 months since it was in repair, and the amp has been used every second week for two hours at a time.

If you are able and have the time to help, but need to know something further, please ask! I dont' have an audio probe, but I can build one if that's necessary for your troubleshooting tips.