Background noise on guitar amp TD35 - diyAudio
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Old 28th February 2013, 04:24 PM   #1
diyAudio Member
Join Date: Oct 2012
Default Background noise on guitar amp TD35

I am trying to restore an old Crate TD-35 based on a 12AX7 preamp and a transistor stage powered by TIP120/125 (see my old thread on the restoration Crate TD35 fixing)

I think the amp got exposed to too much humidity and there were a few things to change.
Since I've got the amp I made the following changes :
- recap on the two main caps
- changing the output resistors
- changing all jacks connectors which seemed oxidized
- changing the two output transistors with a TIP121/126 couple

For now everything seems to work right and the sound is good but I have a terrible background noise and I really do not know where to look. Here is what I can say about that:

- the noise is only present when the cable is plugged but I have tested the cable and the guitar on another amp and they seem fine
- the noise seems amplified with the High potentiometer and the Reverb potentiometer and the High pot cracks a little
- the sound is really attenuated when the tube channel is selected and for what I understand about the schematics, the high pot is connected to the other channel but the reverb is connected to both channels
- I have tested every ground and each connection is good and even the star ground which is done near the input jack
- I have tried to replace the High pot (250k) with two resistors (100+150k) to simulate its action but the sound was still here
- the sound is not present in the line out and between that and the headphone/speaker output there is the transistor stage with a bunch a transistors that I did not all try but the most part seems good

So I am looking for an advice if someone had a similar problem or if someone can tell me what can be the cause of this noise (a bad cap, a bad resistor or transistor or something else)

thank you guys
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Old 28th February 2013, 05:58 PM   #2
Enzo is offline Enzo  United States
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Join Date: Sep 2004
Location: Lansing, Michigan
Good form would be to link a schematic so we all could see the circuit. I have a paper copy, but not everyone does.

SO sitting there, the amp does not make this noise? Plugging ther guitar in starts the noise? SO turn the volumje control on the guitar to zero. Still noise? or does that turn the noise down?

After that step, look at your schematic. Input jack. See that R symbol? That cutout contact on the input jack mutes the power amp. Look at the power amp page. The signal comes in at PA, upper left, but right near that is Q6, controlled by that R. Q6 is a mute that shunts the input signal until R turns Q6 off. SO that explains why you only hear it when something is plugged into the input.

Tube channel? There is no tube channel. Look at your schematic, there is one path through the amp, and that includes the tube. After the tube, there is a small branch selected by channel switch, and that selects the tone stack stage OR the Shape stage, but after that, the two branches come back together for feed the power amp via PA. Those two little braqnches both feed the reverb circuit which is common and rejoins the signal path at the last stage. One of your channel selections has more gain than the other, so it makes the noise louder. If the noise is present at those branches, then it stands to reason it will go through the reverb too.

Your controls are not likely making the noise, they just control it like they would any other signal. If the noise has mostly high frequencies - like hiss - then it makes sense the high control will have the most effect on it.

Isolate the problem.

DO this. Plug something in so it makes the noise, now turn each control up and down. ANY control that affects the noise in ANY way is either at the source of the noise or after the source. If a control has no efect, then it is either before the noise source or it is not in the path of the noise.

Knowing where the problem is really helps determine its cause.

When you have an noisy amp like this, it is way more likely that the problem is a noisy semiconductor than anything else. And from your description I'd wager the preamp is the problem. SO look at the schematic, the whole thing has three small dual op amp ICs and a tube. You can easily swap a sifferent 12AX7 into the socket to find out if that is the issue. And that leaves the three ICs. This amp is funny, they used a 4558 as IC1, a TL072 as IC2, and a 5532 as IC3. All three are extremely common ICs and very inexpensive. Since your highs control affects the noise, I have to think the noise comes from before that stage, so that points at the tube or IC1. If the noise is present at TP1 or TP2, I;d be changing that IC.
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Old 28th February 2013, 07:45 PM   #3
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Join Date: Oct 2012
Ouah first, thank you for a so complete answer. You really helped me to understand the schematics which I'm not used to for the moment and especially the connections named R, A, B and so one...
Thank you for your suggestion, you're right the noise is present before the high and volume potentiometer. Although the volume pot only produces some noise when it's at 10 so I'm not quite sure about it.
- I've tried with another tube, a brand new one, and the noise is still present.
- I turned each pot on the guitar and it doesn't change the noise and even a 0 it's still here
-I checked the tensions and every IC seems well powered with stable tensions.
- Unfortunatly I don't have an oscilloscope so how can I measure the noise around IC1 ? I checked the tension and I got the following
INPUT A : +17 ; -16.5 mV
OUTPUT A : 16.6mV
INPUT B : +42; +43 mV
OUTPUT B : 32.7mV
With only the guitar plugged in but no signal, could that mean anything ? Is there a way to test without reaplcing it ?
What do you think about the Caps of the tube power supply ? Since the caps from the power amp were burned...
and the poly outputs (c13 and c15 one page 2)

Thank you for your help

I've posted the schematics for those who might be interested :

Page 1

Page 2
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Old 28th February 2013, 08:09 PM   #4
Enzo is offline Enzo  United States
diyAudio Member
Join Date: Sep 2004
Location: Lansing, Michigan
You won't measure any noise with a meter. Oh if it is really high level noise, you might measure some noise signal on AC volts. But noisy ICs may have perfectly fine DC power supply and no unwanted DC offsets. They are just noisy.

The circled letters R, A, B, whatever refer to some other point in the schematic. They do that rather than draw long lines all over. And a chanel switching signal like A or B may be used in more than one place - that is the case here. So they draw A and B at the footswitch circuit, with the understanding that other places in the circuit with A and B symbols will be connected to those points.

A scope lets you see the noise. A signal tracer lets you listen to the noise. Google "signal tracer" and see this simple tool. It is in essence some other amaplifier used as a signal monitor. We connect a probe to the other amp input, then we can listen to any point in the circuit of the amp under repair. The basic audio ones are what you want. The ones that add a diode are for detecting RF, and that is a feature you don;t need here.

In any case, I would probably simply replace the IC at this point. It is possible it is something else, a noisy JFET or even a noisy zener diode, but my money os on the IC1.
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Old 1st March 2013, 07:49 PM   #5
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Join Date: Oct 2012
Thank you Enzo for taking the time to explain all of this ! I'm going to try that ! I'm leaving for Tokyo for a week, I'm gonna get back with a bunch of 4558 and 5532 (4$ for 8 !!) so I'm gonna try each one!
I didn't know those signal tracer but this seems a good thing for those who can't afford an oscilloscope ! I'm gonne look if I found one there !
I will try that when I get back and keep you posted ! Thank you very much for your help
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Old 13th March 2013, 07:01 PM   #6
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Join Date: Oct 2012
Came back from Japan this Monday ! With a lot of treasures and a bunch of 4558 and 5532 and begin by changing the 4558 and still no change !!!

But after that I think the sound and the noise by the same occasion was enhanced and I was able to amplify the noise with each potentiometer and especially the Drive one which I did not detect a first glance. So I test every component before the tube and also changed the film capacitors of the tube and I detected a faulty JFET (Q2 and the second page). I compared with Q1 and Q2 give open circuit between Drain and Gate so I think that it's responsible for this noise!! I am gonna try to change both to see if it helps!
But nice idea on the 4558 since I think it was also defective and without it I would not have been able to detect the Jfet !
Do you think it is possible that it was the only cause as regard as the overall problem described before ??

Thank you again Enzo
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Old 27th April 2013, 10:16 PM   #7
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Join Date: Apr 2013
Location: cincinnati, oh.
hello i also have a crate td 35 with the exact same issues, the high control or treble does make for some real hissing sounds on the clean channel, i was looking for a fix as well, you said q2 if im not mistaken could be the culprit, did you find out if thats what the problem was? also i was thinking about changing some stuff around in the tube stage ala marshall with the usual 2.7k and .68 cap on the cathode and swap out the 47uf coupling caps to .022 on both plates.. any ideas. one last thing, i think it would be cool to change the values in the tone stack, like marshall wonder if that would work, or make it hiss more, thanks bruce
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