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Author Topic: Fender guitar amp started humming, can't figure out why  (Read 14368 times)

DrGonz78

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Re: Fender guitar amp started humming, can't figure out why
« Reply #15 on: May 08, 2015, 06:27:09 PM »
When you say "Voltage across:" what does that exactly mean? The transistors have 3 legs so is it base to emitter, base to collector or emitter to collector? Roly asked for voltages in relation to a chassis ground reference. How are you measuring those transistors?

Also if we are not measuring any voltage across the R87 then that really implies that the resistor is open. UPDATED: Oh I see how those are low resistance fusible resistors. So yeah not much of a voltage drop to measure there... Try grounding your black probe and measure either side for the voltage with the red probe.

Foremost please let us know exactly how you are testing voltage with your meter using a ground reference. Just got to clarify for safety reasons IMHO.
« Last Edit: May 08, 2015, 06:32:23 PM by DrGonz78 »
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Enzo

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Re: Fender guitar amp started humming, can't figure out why
« Reply #16 on: May 08, 2015, 09:45:30 PM »
look at your schematic.  The power rails are 42v each, adding up to 84v.  You report you have 84v across Q11.  The only way that possibly makes sense is if you mean emitter to collector, written E-C.   84v ther tells me the transistor is not conducting AT ALL.  And that means zero current through it.  Since R87 current can only come through the resistor, that mean there is zero current through it as well.  Ohm's Law tells us that zero current through any resistor will mean there is zero voltage drop across the resistor.  So with 84v across the transistor E-C I would expect to see zero volts across R87.

Why would ther be zero conduction in Q11?  The Q11 could be open.  Or it could be not turned on at its base.  That base has to be more negative than the emitter by half a volt or so for the thing to conduct, so is it?  If that base is indeed more negative, then Q11 is likely bad.  If the base is not, then we would look to Q10 for answers.

g1

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Re: Fender guitar amp started humming, can't figure out why
« Reply #17 on: May 09, 2015, 01:22:28 PM »
An open R87 or R89 could also shut off current through Q11.

Enzo

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Re: Fender guitar amp started humming, can't figure out why
« Reply #18 on: May 09, 2015, 08:29:05 PM »
Yes, either of those would remove current from Q11, however, if one of those were open, you would not get 84v E-C on Q11.

brodie1600

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Re: Fender guitar amp started humming, can't figure out why
« Reply #19 on: May 10, 2015, 10:39:18 PM »
Sorry, misunderstood when Roly asked me to measure the transistors.

Q9 from collector to chassis ground: 440 mV
Q10 from collector to chassis ground: sits at 7 mV for the most part, but fluctuates intermittently to 0 mV and 24 mV
Q11 from collector to chassis ground: 42 V

Also measured R87 as DrGonz78 suggested. Multimeter read 42 V, measuring both sides of the resistor to chassis ground.

Enzo

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Re: Fender guitar amp started humming, can't figure out why
« Reply #20 on: May 11, 2015, 12:11:14 AM »
In general, when we are measuring DC voltages in a power amp, a few millivolts don't matter.

I'd be trying a new Q11 at this point.

Roly

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Re: Fender guitar amp started humming, can't figure out why
« Reply #21 on: May 11, 2015, 07:46:45 AM »
Quote from: brodie1600
Q11 from collector to chassis ground: 42 V

Quote from: Enzo
I'd be trying a new Q11 at this point.

Me too.

In the same way that the collector of Q9 is sitting about one diode drop above the half rail;

(400mV + VD28 - VBEQ18 - VBEQ20)

so the collector of Q11 should be sitting about one diode drop below the half rail;

(-400mV - VD29 + VBEQ18 + VBEQ20)

The fact that it isn't suggests that it has failed shorted C-E.
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

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Re: Fender guitar amp started humming, can't figure out why
« Reply #22 on: May 11, 2015, 01:07:40 PM »
I'd guess open, as there is 84v across the C-E.  Shorted would imply zero volts C-E.  Either way - bad.

Roly

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Re: Fender guitar amp started humming, can't figure out why
« Reply #23 on: May 11, 2015, 06:10:38 PM »
Quote from: brodie1600
Q11 from collector to chassis ground: 42 V

Quote from: Enzo
as there is 84v across the C-E

These are in contradiction.  He can't have the 42V +ve rail on the collector of the VAS and have 84V across the VAS C-E.  I'm uncertain how the "84V" measurement was made, there was some confusion about "across", but I suspect it was VAS collector to -ve rail, which would be consistent with 42V to ground, and a shorted VAS.

Whatever, Q11 looks SNAFU.  The question now is what do we replace a 2SA1016K with? (150V/50mA/400mW)  An MJE350 should do.  {In Oz most retailers don't carry 2Sx series, you have to go to the equipment manufacturer for a "genuine spare part" which costs $40 for a $4 part, or substitute.}

I guess the mists will clear.
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

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Re: Fender guitar amp started humming, can't figure out why
« Reply #24 on: May 11, 2015, 09:06:47 PM »
apologies to all, I had Q11 upside down in my mind, forgetting a moment it was PNP.  I pictured +42 on the top lead and -42 on the bottm.  I now see the E and C are reversed of my thinking.

I believe we earlier clarified the 84v he was reading was E-C.  But if V+ is on the collector, well now it gets confusing...

If no 2SA series are available, suitable subs might be 2N5401 or MPSA92 o r93.  Those are common enough in the USA,  Just be aware the legs are in different order.

Suburban electronics has 2SA1016 in stock.

brodie1600

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Re: Fender guitar amp started humming, can't figure out why
« Reply #25 on: May 21, 2015, 02:50:10 PM »
New 2SA1016 came in the mail and I soldered it to the board and reconnected the speakers. Still getting buzzing. Still measuring 42 V from collector of the new Q11 to ground.

DrGonz78

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Re: Fender guitar amp started humming, can't figure out why
« Reply #26 on: May 21, 2015, 08:06:17 PM »
Please measure EBC voltages to ground on Q11, all three points. One thing that bothers me is that the collectors of Q9 and Q10 should be around 39v if I am understanding this correctly. It would be beneficial to do voltage checks on Q9 and Q10 as well. What voltage drop is occuring across each D21-24.

Also note that testing a transistor after pulling it out for replacement is easy. You could have tested the old one compared to the new one to see if there was any gross differences. Perhaps that is not necessary but it is something I have done as way to learn bettter.

Don't need to hook speaker up until you measure only millivolts on the output.
“A person who never made a mistake never tried anything new.” -Albert Einstein

brodie1600

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Re: Fender guitar amp started humming, can't figure out why
« Reply #27 on: May 22, 2015, 05:59:43 PM »
Transistor readings are each point to chassis ground:

Q9
E: 1.47 V; C: 42 V; B: 3 mV (essentially nothing)
Q10
E: 1.9 V; C: 42 V; B: 5 mV (essentially nothing)
Q11
E: 42 V; C: -43 V; B: 42 V

Diode measurements:

D21: 1.12 V
D22: 1.16 V
D23: .29 V
D24: 1.47 V
« Last Edit: May 22, 2015, 06:39:07 PM by brodie1600 »

DrGonz78

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Re: Fender guitar amp started humming, can't figure out why
« Reply #28 on: May 22, 2015, 06:18:14 PM »
Okay now I think I can see, possibly where we are making mistakes. I think you might be confusing the transistor base legs for the collector legs. The -43v on the base of Q11 makes no sense to me and perhaps that is really the collector. Now you have written 42v on the emitter and collector of Q11 and -43v on the base of Q11 would also mean there is -43v on the collector of Q10. That is wrong.

What I think is going on is that Q11 is really E: 42V B: 42V C: -43V. Same with Q9 and Q10 that you have the bases and collectors flipped flopped. Lets start there and find the datasheets for those transistors. Once you figure this out the whole thing should be making way more sense.

Also that reading across D23 might be suspect.

EDIT: I attached the pinout for those transistors. Please check to make sure you are reading them correctly.
« Last Edit: May 22, 2015, 06:31:13 PM by DrGonz78 »
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DrGonz78

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Re: Fender guitar amp started humming, can't figure out why
« Reply #29 on: May 22, 2015, 06:34:01 PM »
Just as I thought look at my edited post for the correct pinout of those transistors. Here is the actual data sheet. We can explain how pin outs are different in the US, Europe and Asia later.

Edit: Now type up the correct voltage chart on your next post and lets start there.
“A person who never made a mistake never tried anything new.” -Albert Einstein