fender bassman 60 humming

[maxs]

i know there is slight normal hum in these but this isnt normal without a guitar hooked up and all pots set to 0 it hums badly turn vol up no change is sound it belongs to a buddy of mine he told me that he got it free because it quit working in a church that he goes to and being the good person i am i thought id fix it for them or at least try to he said that he hooked it up and the speaker coil started smoking so i guess its fried now...

here is what ive tried so far i chkd the vcc going to the speakers and found it is 40vdc (not suppose to be) so i change the output transistors and the prob still exists so i chked the power to the pcb and its correct moved on to the 2200uf caps one has 80vac (to high) suppose to be +40vac the other is suppose to be -40vac and has nothing i changed the caps nothing still the same i tested the bridge it showed ok but 1 side is suppose to be +31-32 vac and the other is -31-32vac but one is 80vac the other 0 so could the bridge be bad or is ther something else in the circuit causing vcc feedback or something..

here is the schematic im using its newer than the 1 i need but its pretty close actually this 1 was made in 1999 the 1 i need for this amp should be from 1991... hope someone can help....thanks

 

[Nigel Goodwin]

You can't have AC on the 2200uF unless it's completely O/C - however, unless you're measuring it with a scope then it's meaningless - most multimeters read DC on the AC ranges as well.

So measure the DC voltage on each capacitor, if there's only a low voltage on one, then the most likely problem is the capacitor O/C, and you can simply solder one across it to prove it.

Check the voltages on the other end of the capacitor as well, it should be zero as it goes to chassis, but there may be a broken connection somewhere.

 

[maxs]

i misprinted im sorry the vcc i is dc voltage sorry

tested the output off the speaker wires again its 58vac suppose to be arround 15 to 16vac i also chkd power to the 3 transistors mounted to heat synk in back (out put trannys) the two larger 1s d1046 i swapped for c3263 and a1294 both get correct readings well close to the schematic i have anyway the schematic says on q19 +41vdc on mine i get +35 on q18 i got -36vdc ...
other tests

on this amp p3 and p5 run through a fuse wire then through the bridge to the caps ect ...i chkd the vcc from p4 and p5 ( p4 being gnd) and had 31-32 vac at fuse the same for p3 at other fuse so everything is ok here....i also forgot to mention r142 and r143 resistors have overheated the pcb is slightly burnd i chkd them they read ok but they have been hot i also chkd the 2 diodes at d54 and d55 they are also ok ...

all my tests on the components on this pcb board were (except for the resistors) with the parts removed.

just for the heck of it i dbl chkd my tests and foud a problem with my meter (now fixed) so i chkd all those above mentiond test points again and they all chkd out ok and had the correct dc voltages even chkd at resistors wher the vcc is at 16.5 and its ok also guess just go on from here to find the problem...

 

[bountyhunter]

I still remember this from a class in home entertainment electronic systems I took 35 years ago:

Always assume the problem is in the power supply since most problems are. When you find it's in the power supply, assume it is an electrolytic cap since it usually is.

Electrolytic caps have very short lives compared to all the other components around them with the possible exception of vacuum tubes.....

 

[maxs]

i just cant figure how to get 58vac from 15vac without a power transformer problem unless there is another transistor or opamp that may be shorted ah well ill get it sooner or later i guess

 

[Nigel Goodwin]

i just cant figure how to get 58vac from 15vac without a power transformer problem unless there is another transistor or opamp that may be shorted ah well ill get it sooner or later i guess

As I said previously, WHY are you measuring AC volts? - it's only useful under occasional and very limited circumstances, you're just confusing yourself. There's no reason whatsoever to ever switch your meter to AC on this amplifier.

As for bountyhunter, his generalisation isn't true - by FAR the most common fault is failure of the output stages, PSU may come second, but a long way behind.

 

[maxs]

As I said previously, WHY are you measuring AC volts? - it's only useful under occasional and very limited circumstances, you're just confusing yourself. There's no reason whatsoever to ever switch your meter to AC on this amplifier.

As for bountyhunter, his generalisation isn't true - by FAR the most common fault is failure of the output stages, PSU may come second, but a long way behind.

look at the schematic tp41 - tp37 says it should be 15.5vac not dc some lines in this circuit are dc tp37 is speaker output wire that has 58vac

 

[Nigel Goodwin]

look at the schematic tp41 - tp37 says it should be 15.5vac not dc some lines in this circuit are dc tp37 is speaker output wire that has 58vac

They aren't AC voltages, they are signal levels - measured with a scope or an AC millivoltmeter with a specified frequency input and volume level.

Blindly sticking a multimeter set to AC on them isn't proving anything, and is completely meaningless - as is sticking a scope on without the correct input and the amplifier working correctly.

If all you have is a multimeter, then keep it on DC volts - that should be all you need to repair it.

Collector of Q17/19 should be -41V or so, collector of Q16/18 should be +41V or so, and the speaker output should be zero volts, plus or minus a couple of hundred millivolts (no speaker connected). Those are the three critical things to measure.

 

[maxs]

thank you now i get it sorry for lack of understanding

ran the tests as you directed and are as follows

digital multimeter set to dc

speaker wire = 28.1 vdc

note: these transistors are npn not pnp

pins
1 = base 2 = collector 3 = emitter

collector from q19 +27.5
base from q19 -36-37
emitter from q19 - 36-37

collector from q18 +36-37
base from q18 + 36-37
emitter from q18 +27-28

 

[Chippie]

thank you now i get it sorry for lack of understanding

ran the tests as you directed and are as follows

digital multimeter set to dc

speaker wire = 28.1 vdc

note: these transistors are npn not pnp

pins
1 = base 2 = collector 3 = emitter

collector from q19 +27.5
base from q19 -36-37
emitter from q19 - 36-37

collector from q18 +36-37
base from q18 + 36-37
emitter from q18 +27-28

There should be no discernable measurment at the speaker output wrt grorund...( a few millivilts offset maybe)

Given that you have 28 v or so at the output ,indicates that there is a problem with the output devices and/or the drivers..This is why it is humming and burning up the speaker coil...

you should remove the transistors in the ouptut stage and check them either with your meter or a proper device tester...

After repairing the power stage, fit some 10 ohm wire wound resistors in the supply lines to limit the current on power up...this will go along way to help minimise damaging any more devies.

 

[Nigel Goodwin]

Also check the emitter resistors feeding the output, as the emitter voltages are VERY different, they must be high or O/C.

However, the voltages on Q19 look like you've made a mistake measuring them - as the collector goes to the -ve supply, and you have it as +27.5v

 

[maxs]

ok so as i understand you you are saying that either q18 or q19 or both could be bad correct? the reason i ask is my amp only has 2 of the larger output transistors they were D1046 the smaller 1 between them is a d313 i swapped out the 2 1046 for c3263 and a1294 (which is the equivalent to the 3263) these (tested with my meter set to diode test) showed no apparent problem (that is if i tested them correctly) ill outline the test now on the 1046 they are removed from the circuit here it is

first test black wire from meter to base

red to collector reading was nothing
red to emitter reading was nothing

chkd between e and c also and same reading even in reverse

switch to red to base ran same test again
collector raeding was 500 - 501
emitter was same 500 - 501
on both trannys

note: the reason i said nothing for the first 2 readings was when i set my meter to diode test there is a 1 in the lcd touch the two leds together it drops to -001 so its working when i ran the first test with black to base there was no change in the lcd screen the 1 remained for both pins c and e

 

[maxs]

Also check the emitter resistors feeding the output, as the emitter voltages are VERY different, they must be high or O/C.

However, the voltages on Q19 look like you've made a mistake measuring them - as the collector goes to the -ve supply, and you have it as +27.5v

i double chked them and the readings are the same btw they are marked on the pcb board also B C E and i used the black speaker wire as my gnd and also used the chassis as gnd and used p4 as my gnd with same readings

 

[maxs]

here is the pinout of the transistors on my pcb board

 

[Nigel Goodwin]

i double chked them and the readings are the same btw they are marked on the pcb board also B C E and i used the black speaker wire as my gnd and also used the chassis as gnd and used p4 as my gnd with same readings

Well the negative voltages all come from the -41V rail at the bottom of the circuit, and the collector goes directly to that point - so it's either at -41V (or whatever the exact voltage is on your model), or it's not connecting to the -41V rail. The negative rail must be working, because you have negative voltages on the other two pins.

But, looking at the voltages, I would suggest you have the emitter and collector transposed, with the emitter at +27.5V (which would make sense, as the other emitter that it connects to is at that voltage), and the collector at -36V to -37V, as the base is also -36/37V then the transistor is O/C emitter.

 

[maxs]

this thing is dirving me insane, by the way its acting it very well shoud be the two output resistors or at least 1 of them but they are chking out ok sense both resiostors have good readings could the problem be comming from the 2 5w resistors on the output they are suppose to be (on my board) 5w 0.5 ohm the reading is 1.1 ohm but my meter only goes from 2000k to 200 ohm settings they dont over heat or anything im just trying to get this crazy thing its got me baffeled

 

[maxs]

ok i ran the traces down and the collector of q18 and q19 run through those 5w 0.5 ohm resistors i mentioned then the other side of both resistors connect together and run to the output speaker wire which is +27 - 28vdc then i removed both transistors and ran a powerup test and the speaker wire had 0 vcc when powered up so could these be bad even though the initial test with the meter should they werent?

 

[Chippie]

If you are still getting +28v or so at the speaker connection then the NPN output device is defective...probably shorted..or leaky

 

[maxs]

If you are still getting +28v or so at the speaker connection then the NPN output device is defective...probably shorted..or leaky

i agree here is what i did but it sounds wierd but bare with me i removed the transistors again and tested them again and found i must have missed this or wasnt paying close attention but the 2sc3236 i had replaced the 1046 with read as follows

base = black lead from meter
collector = red lead from meter reading was nothing
emitter = red lead from meter reading was same
reversed leads red to base then ran test again

collector reading was 499 wit red lead to base black to collector
emitter reading was same with red lead to base black lead to emitter

now the other replacement was a 2sa1294 this is where it gets wierd i ran the exact test above but the readings were reversed

base = black lead from meter
collector reading with red lead from meter was 489
emitter was same

base = red lead from meter
collector and emitter reading nothing

its reversed from the other transistor that is suppose to be a replacement for it!!!

although the 1294 is a pnp thats probably why!!!

 

[maxs]

i had 2 more but these were exact parts tested before installing them...

now +43vdc on collector on q18 emitter is - 0.1vdc base is + 0.6vdc
now -42vdc on base of q19 emitter is -42vdc and collector is - 0.1

but now have no humming when hooked up to a speaker because the speaker wire is not at 27vdc its now at - 0.1 so at least im getting somewhere

but when a guitar is hooked up plucking string no sound so there is still something wrong perhaps 1 of these trannys are bad or something else