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Old 11-28-2012, 12:21 PM   #21
GABarrie
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Quote:
Originally Posted by whoomit
You mean you were hoping my values would be wrong, but they were correct?

Or you were hoping your dignosis would be wrong, but it was correct, meaning the values were incorrect?

No, I don't, no ICs anywhere I'm afraid. All I've got lying around are caps and resistors.

the bold one, means that wasn't the issue and we can't fix it yet
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Old 11-28-2012, 12:21 PM   #22
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Quote:
Originally Posted by GABarrie
the bold one, means that wasn't the issue and we can't fix it yet

Balls.
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Old 11-28-2012, 12:30 PM   #23
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I'm gonna go ahead and guess you don't have a signal generator lying round?
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Old 11-28-2012, 12:39 PM   #24
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Quote:
Originally Posted by GABarrie
I'm gonna go ahead and guess you don't have a signal generator lying round?

Ehm, I have a stupid 'noise generator' kit or something from Maplin. No 1/4" out or anything though.
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Old 11-28-2012, 12:43 PM   #25
GABarrie
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PM your address, ill send you a tl072 and we can pick this up on friday when it arrives
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Old 11-28-2012, 12:45 PM   #26
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Originally Posted by GABarrie
PM your address, ill send you a tl072 and we can pick this up on friday when it arrives

Okay, but you're giving me your address too. I'm send you some chocolate or something
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Old 11-29-2012, 01:17 PM   #27
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What you really need is a...


ehh, screw it.
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Old 11-29-2012, 07:25 PM   #28
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I did not make it to the post office so it'll be Saturday!
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Old 11-30-2012, 12:03 AM   #29
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Hey whoomit.

One easy thing to check - see if there is any bad connection at the effects send/receive jack sockets.

Usually, if the amp is giving you sound but very low level, then something in the signal path is dodgy.
Sometimes this is because of a component - could be an electrolytic coupling capacitor, for example, or sometimes something with contacts, like a plug or switch.

You can measure with your meter, or just short them with a piece of wire or screwdriver tip and see if the the sound comes back on. Also, check all the jack sockets for hairline solder cracks. They do happen and are sometimes hard to spot.

Usually, I would use a tone generater and a signal tracer. The 'ghetto' version is to take an old radio, cut the signal from the receiver to the audio amp and use the receiver as your generator (white noise or radio station), and the audio amp becomes your tracer.

Anyways, if you don't want to bother with that, you can run the amp and touch a screwdriver to several points in the circuit to inject noise.

I would start at the last amplification stage and work back to the input.

Put the volume about half - if you hit a stage that is working, you may get startled by loud noises Increase it if you hear something bit not very loud.

Basically, you only have to touch a screwdriver onto the low level stages to inject lots of mains noise and hum, just like you hear on those cheap, unshielded guitars (expensive ones too..).
You need to be touching the screwdriver shaft (metal part) to make it work, otherwise you might just get a few little clicks on contact.
Don't worry - these stage are only running on +-15V, so they won't hurt you. Just be sure to stay away from any mains level voltage, which should be out of your way anyway.

Go:
Pin6 on IC3
Pin2 on IC3
Pin3 on IC2
Pin3 on IC1

You should get noise on those first 2, as Channel B takes the same path, and we know that's working.

Hrm, I'm really suspecting those jack sockets/solder joints.
I usually go ahead and solder that stuff as a matter of course when I service anything, as the manufacturing process is not always perfect and wear and tear is pretty common.

If you're at all worried about shocks, don't worry, but be sensible anyway -pay attention.
I worked with a guy who taught me to put my left hand in my pocket when working on live gear.
It's supposed to prevent any voltage passing across your chest (IF you happen to touch high voltage, it will take the shortest path through the right side of your body. Your heart is on your left)

So far so good

Last edited by sethasaurus : 11-30-2012 at 12:07 AM.
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Old 11-30-2012, 12:43 AM   #30
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On further inspection of the schematic, the footswitch circuitry could possibly be at fault. This part mutes the normal/boost/reverb sections. It could be that part of it is muting your signal, due to a faulty transistor, for example.

Definitely check and resolder all those jack sockets anyway and see if you can inject any noise into the circuit.
This will at least narrow down the fault.

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Old 11-30-2012, 07:59 AM   #31
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Quote:
Originally Posted by GABarrie
I did not make it to the post office so it'll be Saturday!

Thanks man

sethasaurus, that's a lot for me to take in I'll try to get to it either this afternoon or tomorrow though, thanks!
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Old 12-01-2012, 11:57 AM   #32
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Right, I'm going to go over all the jacks in a second.

My footswitch arrived today, and when plugged in, Boost is constantly on, and Reverb is constantly off. Could this have something to do with it?

Edit: Just checked over all the soldering, re-done anything that looked remotely dodgy. My back is absolutely killing now It hasn't worked.

If I touch the Channel B treble pot I get a loud pop, bass, middle and volume give a hum and crackle sound.

Pin 3 on ICs 1 & 2 don't make the same sound as 2 & 6 on IC3. They barely make a noise when I touch them.
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Old 12-01-2012, 07:48 PM   #33
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OK, that's good progress! You haven't wasted your time anyway. At least you'll know everything is solid once you put it back together.

Looks like the problem is in the normal/boost/reverb switching.

Can you measure the following DC voltages? (Without the footswitch plugged in):
TR1 Base
TR1 Collector
TR2 Base
TR2 Collector
TR3 Base
TR3 Collector
TR4 Base
TR4 Collector
TR5 Base
TR5 Collector
TR6 Base
TR6 Collector

Transistor (BC184) pins are:
(Looking at front face)
___
|__|
| | |
| | |
CBE

That should show up any faults.

Seth

Last edited by sethasaurus : 12-01-2012 at 07:54 PM.
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Old 12-02-2012, 10:55 AM   #34
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^ What exactly am I measuring?

TR1 B > TR1 E
and
TR1 C > TR1 E

or something else?
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Old 12-02-2012, 11:51 AM   #35
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Just put your negative meter lead to ground and positive lead to each point for measurements (base and collector).
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Old 12-02-2012, 01:29 PM   #36
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Tr1
B - 0.61
C - 0

Tr2
B - 0.63
C - 0

Tr3
B - 0.63
C - 0

Tr4
B - 0.1
C - 0

Tr5
B - 0.73
C - 0

Tr6
B - 0
C - 0
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Old 12-02-2012, 04:12 PM   #37
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OK, some weird readings on Tr4 and 6.
It's a little confusing as Tr5 base is connected to Tr6 collector, so they should read the same voltage.

It looks pretty likely that one of these transistors is faulty. Since we've narrowed it down, easiest thing is to take them out and test them. My money would be on Tr5 or 6.

This page gives some info on testing a transistor:
http://www.kpsec.freeuk.com/components/tran.htm

If you find a dud, BC184s are cheap to replace.

Does your footswitch have a mono or stereo plug?
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Old 12-02-2012, 06:10 PM   #38
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Well once I'm back home tomorrow night I'll retest them to make sure I got the values right. Then I'll check the transistors themselves.

I couldn't tell you about my footswitch. I assume it's stereo. There's no jack input, it goes straight to a lead. I'm assuming it's stereo though as there's three wires into the cable.
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Old 12-04-2012, 03:43 PM   #39
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If the plug's got two "bands" on it, it's stereo. If it's a single band like a guitar cable, it's mono.
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Old 12-04-2012, 04:01 PM   #40
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Quote:
Originally Posted by stratman_13
If the plug's got two "bands" on it, it's stereo. If it's a single band like a guitar cable, it's mono.

Oh yeah, it's stereo then.
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