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Old 4th February 2013, 07:09 PM   #171
Enzo is offline Enzo  United States
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In terms of signal flow, input and output make some sense as terms, but as for transistors, that is not a good way to think of them. The majority of current flows between colletor and emitter. Doesn;t really matter which direction, that is a philosophical discussion.

The base is what controls the flow between them. More or less, the more current you draw through the base, the more can flow from E to C.

Try looking at an amp like this: You have a speaker. Put a positive voltage on it, and it moves forward. Put a negative voltage on it and it moves backwards. The amp has a positive and a negative power supply. We have a transistor from positive to the speaker, and we have one from negative to the speaker. By turning one or the other on, we can control the changing voltage at the speaker. And that moves the speaker to make sound.

So for signal, the base is the input and the emitter is output in your final stage. But for speaker current, the collectors are the inputs and the emitters the outputs.

Look at Q20. The collector is at +42, and the emitter is to the speaker. The more we draw through its base, the harder Q20 conducts, pulling the speaker terminals closer and closer to +42. If you scope the parts of Q20, the collector will sit there at +42. You will see your signal at the base, and also at the emitter. You will find a fairly steady .6v between base and emitter, but not from base to ground. Q18, Q20 are current amplifiers, not voltage amplifiers.
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Old 4th February 2013, 07:30 PM   #172
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You really need to start small to start understanding amplifiers.

Look up single stage amplifiers first and understand how they work.

You will also need to understand long tailed pairs LTP, constant current sources CCS, voltage amplifier stages VAS and class ab bias circuits.

Then you need to understand class ab feedback.
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Old 5th February 2013, 01:42 AM   #173
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Does anyone have a line on the data sheet for the 2N4001? I keep doing searches so I can get the pinout and all I seem to come up with is:
2N4001 - Bipolar NPN Device in a Hermetically sealed TO39 Metal Package. - Seme LAB

I don't have any metal canister transistors on my board.

I'm trying to get the pinout on these transistors leading up to Q18,19, 20 and 21.


Thanks!


BR
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Old 5th February 2013, 09:05 AM   #174
Enzo is offline Enzo  United States
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Pick one. Follow the trace from each pin over to some other component on the board, and check the schematic to see which transistor element that component connects to. You can thus identify the base, emitter, and collector of the part. All the 2N400x in your amp will have the same pinout. PNP and NP won't change the pinout.

Look at Q23, The base conects only to R132, and the emitter only to D48. That should be simple enough to find on the board. And once you know which pin is base and which emitter on Q23, you will know the pinout for the bunch of them.

Do you have reason to believe they are defective?


Contact Fender and ask them. Ask them if the part is available through their dealers, and if so, what is the part number. Ask them if there is a domestically available substitute part if not.

Q16,17 are just limiters. You have several of these 4000 series parts in the footswitch circuits. Do all the switching functions happen? If you suspect Q16,17 and do not have the new parts, just swap Q16,17 for Q22,23. It is unlikely both sets are defective. If moving them makes a difference, then they are involved in your problem. If not, they aren't.

2N4001 LOOKS like a JEDEC number, and back 30-40 years ago there were 2N4001, 4003 types, in the metal TO39 even. That is the old data sheet you are finding. I suspect that the parts in this Fender ar not JEDEC 2N4001, but someone is making transistors and has recycled that number. Fender has absolutely no need to invoke 40 year old transistor types for simple switching circuits, and I am sure they have not.
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Old 5th February 2013, 09:25 PM   #175
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Default Transistors

Enzo

I am thinking a transistor must be bad or like you offered, may be turning one of the last ones on too hard. I've tested all the resistors, made corrections after you showed me why I was getting weird readings. I replaced the couple of resistors that were out of spec. I have been through all the diodes and haven't found any out of spec. I can't find any broken tracks on the underside.
The couple of connections I suspected, I desoldered and resoldered, just to make sure.

Unless I am missing something, transistors are what's left. Unless of course you or someone else here has a different idea. I haven't actually asked anyone if they know what's wrong or think they know what's wrong. Is there someone here that does? I'm not pridefully, if someone wants to shed some light I won't complain... lol

I'm reading everything I find on SS guitar amps and how transistors are made up, wired up, adjusted, biased, manipulated etc... Apparently transistors are generally used in a voltage divider circuit, the transistor being the bottom variable resistor. Which is frickin' cool that someone figured out how to do that, I might add. I can not seem to get my head wrapped around biasing. I read it over and over in every article I read. But I just don't get it. I understand that two supplies are being compared and the circuit is self adjusting based upon the balancing of this arrangement. But I don't get how its doing it. Its like there is a missing piece or something. And until I get the understanding of how this is working I don't think I will ever figure this amp problem out unless I accidentally change the right part. Which I hate to imagine but I fully admit would be just fine with me at this point.

That probably sounds like a cop out but I'm stumped man. Getting frustrated and tired of messing with this thing. But I don't want to let it beat me either.

Maybe I'll just take a couple days off and give my brain a rest. Its not like I haven't being trying. Maybe I just need to step back.

BR
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Old 5th February 2013, 09:34 PM   #176
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The bias circuit is also called a Vbe multiplier.
It basically takes the transistor BE voltage and multiplies it to across CE.

This means the voltage always stays the same unless the temperature of the transistor changes and this will make the CE voltage go up ore down slightly. If temp goes up the voltage drops and vice versa.

If the bias setting is wrong and has too many volts on it then the output transistors can pass to much current and get hot.
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Old 6th February 2013, 03:22 AM   #177
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Default Bias current

Quote:
Originally Posted by nigelwright7557 View Post
The bias circuit is also called a Vbe multiplier.
It basically takes the transistor BE voltage and multiplies it to across CE.

This means the voltage always stays the same unless the temperature of the transistor changes and this will make the CE voltage go up ore down slightly. If temp goes up the voltage drops and vice versa.

If the bias setting is wrong and has too many volts on it then the output transistors can pass to much current and get hot.
Ok, so how do I tell if a given transistor bias is too high?
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Old 6th February 2013, 10:00 AM   #178
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Quote:
Originally Posted by badraven View Post
Ok, so how do I tell if a given transistor bias is too high?
There's actually two Vbe multipliers in the circuit (Q14 and Q15), it's a really weird and over-complicated circuit.

As I recall it was suggested much earlier to disable the bias network, standard practice on transistor amp repairs. Sort the bases of Q18 and Q19 together, this sets the bias to zero.
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Old 6th February 2013, 01:53 PM   #179
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Default Nigel

Quote:
Originally Posted by Nigel Goodwin View Post
There's actually two Vbe multipliers in the circuit (Q14 and Q15), it's a really weird and over-complicated circuit.

As I recall it was suggested much earlier to disable the bias network, standard practice on transistor amp repairs. Sort the bases of Q18 and Q19 together, this sets the bias to zero.
When I short the base of 18 to 19 what am I looking to find?
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Old 7th February 2013, 12:37 AM   #180
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I shorted the base terminals of Q18 and 19 together. All it appears to have accomplished is to make the amp hum.
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