I am working on a Fender Bassman 400 ss bass head, its is blowing voltage regulators to bits. I replaced the 7815 and 7915 regulators and it blew 'em again. I have now replaced them once more along with all the diodes nearby. Instead of lifting one leg to test each I just went ahead and replaced all D100-111. Q102 was also blown and replaced. Ready to start it up on the limiter, anything else I should check beforehand?
Let's be systematic.
You have common 7815/7915 Vregs. It takes quite a bit to blow them apart, which seems to be your description. Are they actually blowing apart? Or are you being dramatic, and they are just failing?
In any case, they are either being fed something bad, or something bad is happening on the output side. SO take them off the board and power up without them. Your schematic should say there is about 30v at the Vreg input pin, so is there? And is it clean DC?
The output pin of each should now have zero volts. But if something out in the amp is shorting a high voltage rail to them, that would cause damage. So: is there any unwanted voltage on those output pin holes?
The Vregs ought to shut themselves down when overloaded, but... SO power off, no Vreg installed, is the output pin hole shorted to ground?
Are they in the right places? The 7915 is the one right on the edge of the board, right next to the transformer wires. If you reversed them, it will cause blow ups.
Q102 failure is hard to associate with the other stuff.
Look right below Q102 on the print, that note is important. The fan circuit is not ground referenced. Make sure not to connect grounded test probes or anything to it, unless you know what you are doing.
Its literally blowing them apart. I hit the rafters in my garage with a piece of flying debris. I'll take out the regulators and get some numbers. I have them in the right spot btw. I did pull out a chunk of metal, a welded chassis nut for the case that had broken off. Who knows what damage that thing did in there.
:o Actually exploding regulators together with a failure of Q102 suggest to me wildly excessive supply voltage. Knowing the amp history would be helpful because I'm thinking things like the amp is wired for 110V and being plugged into 240V, or that somehow the main +/-70V supply has been misconnected to the +/-15V board. It's hard to see how one loose nut alone could cause damage that would cause both regs to actually explode. These are typically quite tough little critters and it takes some serious abuse to get them to fail, much less explode. Voltage measurements per Enzo should move us forward.
That's why I changed all the rectifier diodes in case it was AC coming down the path, but I'll admit I'm still learning these things. I'll get some voltages posted soon.
Yes.
To the point of being redundant, recheck that you have 23VAC from each TP5/TP6 to ground, +30VDC/-30VDC at TP7/8 to ground, and, *without* the regulators installed, :
a) you have 0 V at TP5/TP10 and
b) after that, if 0V present, turn amp off, wait for discharge, and only then measure resistance to ground from TP5/10, using both the resistance scale (say, 200 ohms and 2K) and repeat using the diode scale, with the read lead always on the "most positive" side meaning from "+15" to ground for TP5 and from ground to "-15".
Looks redundant, or a waste of time, but we are chasing ghosts here :(
And by the way, an LM7xxx with 30V on one side, 15V on the other and the typical low load that's meant by a preamp dissipates "nothing" , so we must find what's exploding them.
Voltage regs:
U101
Pin 1 - +55.9V
Pin 2 - 0
Pin 3 - 0
U102
Pin 1 - 0
Pin 2 - -56V
Pin 3 - 0
Power to voltage regs but nothing on the output side.
I will check the other test points per Mr. Fahey
Well there is your problem, you have 55v going into those poor things, no wonder they popped.
Just a hunch... Look at TP3 and TP4. They should be 70v each, but are they closer now to 30v?
Verify the transformer wiring. Is the brown wire from the transformer on post P8? And the brown/white wire on P9?
Also verify the plain red wire is on P5, and the red with yellow stripe is on P6, and the white with red stripe is on P7.
Unless you have a 120v amp plugged into 240v mains, the only other explanation for voltage that high on your power supply is mixed up transformer wires.
OUCH! xP
Since the power tranny doesn't seems to have an international primary (and unless you are plugging a 110V amp into a 240V outlet) then it has to be the secondary high and low voltage wires to the rectifiers mixed up.
I didn't remove any of the board wires, but doublechecked anyway, all wiring is correct per schematic and board labeling. I measure +70/-70 at TP4 and 5. Resistance check from TP5 to ground is continuity.
Bad transformer?
That would be a very odd transformer problem. Anything is possible, but in general, transformers fail with shorted turns, which makes them run very hot, or blows fuses or drops voltages way low. Next on the list would be open windings, on the primary side that results in a dead unit, on the secondary side it results in a missing voltage. Having a transformer produce 20v more than it should on a 30v circuit would be something I have not seen.
On the other hand I am not sure how else to explain it.
Quote from: markorock37 on May 08, 2013, 06:19:45 PMI measure +70/-70 at TP4 and 5.
Does this mean +70V at TP4 and -70V at TP5?
How can you have +70V at TP4 which is a negative point?
Please confirm.
Quote
Resistance check from TP5 to ground is continuity.
How can you have continuity (which is a low resistance value) and have -70V at the same point?
Which by the way is a positive point?
Where are you placing the negative/blackmultimeter probe?
Is R61 ok?
A couple of points I'll throw in: 1) Those 35V caps (C108 & C109) should be exploding if they're getting 50V on them, or at least bulging.
2) The two transformer windings are common to the fan circuit, maybe something weird there?
But for quick check, disconnect P8 & P9 (brown and brown/white), what AC voltage do you measure between those wires?
Quote from: EnzoHaving a transformer produce 20v more than it should on a 30v circuit would be something I have not seen.
That would be just a little bit remarkable.
Quote from: g1Those 35V caps (C108 & C109) should be exploding if they're getting 50V on them
My thought also.
@markorock37 - what is the mains voltage where you are?
We need to confirm some voltages to ground;
P5 (TP1) - 54VAC
P6 - 0V
P7 (TP2) - 54VAC
TP3 - +72VDC
TP4 - -72VDC
P8 (TP5) - 23VAC
P9 (TP6) - 23VAC
TP7 - +30VDC
TP8 - -30VDC
I'm currently at a loss to understand where +/-56VDC could be coming from.
QuoteDoes this mean +70V at TP4 and -70V at TP5?
How can you have +70V at TP4 which is a negative point?
Please confirm.
Sorry I mixed those up...-70 at TP4 and +70 at TP5 like it should be. I'll recheck what you have requested.
Quotewhat is the mains voltage where you are?
110 volts USA
QuoteWhere are you placing the negative/blackmultimeter probe?
To ground at the chassis AC ground
P5 (TP1) - 54VAC 52.9VAC
P6 - 0V 0VAC
P7 (TP2) - 54VAC 51.9VAC
TP3 - +72VDC +70VDC
TP4 - -72VDC -70VDC
P8 (TP5) - 23VAC 43.2VAC measured this 3 times
P9 (TP6) - 23VAC 22.4VAC
TP7 - +30VDC +56.2VDC
TP8 - -30VDC -55.9VDC measured also 3 times
0VDC between P8 and P9 unplugged.
R161 is 11.2 ohms
OK, P8 is twice what it should be, so pull the wire off post #8, and measure AC volts on that wire as it hangs in mid air. Leaving the black probe on chassis ground. If you still get 40-some volts,the transformer has some odd issue. If you now get 22vAC, then the transformer seems to work.
If the transformer alone is OK, then I guess we have to look at the fan circuit as involved somehow. If so, first thing would be to disconnect the fan itself.
Just a thought, idea is wondering if somehow the center tap and one end got reversed on the low voltage winding. The schematic shows only 5 wires on the secondary and only two on the primary. Does your transformer have only 7 wires? It shows the red/yellow wire as the shared center tap lead. ANy chance your transformer has separate center tap wires that are combined on one post?
The way this theory would work is if an end wire was grounded, then there still would be 22v from ground to one end (really the center tap now), but the other end would be the whole winding away, or 44v. I am trying to rationalize something reasonable that would cause the numbers you are getting. Or if the wires LOOK OK, it is still possible the tranny was made with a miss-connection inside.
Then again it is only an idea.
Which leaves us scratching our heads wondering a) what sort of single fault could cause this situation to arise, or b) how did this amp get so far from the factory with exploding regulators without anybody actually noticing? ???
There simply has to be more to this than a loose nut. :loco
Isolate the fan rectifier, D8-D11, and retest voltages, particularly P8 (TP5).
Quote from: Roly on May 09, 2013, 10:38:47 PM
Isolate the fan rectifier, D8-D11, and retest voltages, particularly P8 (TP5).
I think you mean D108-D111, also C116.
But before this, check what Enzo asked in reply #17.
Quote from: g1 on May 09, 2013, 11:07:30 PM
Quote from: Roly on May 09, 2013, 10:38:47 PM
Isolate the fan rectifier, D8-D11, and retest voltages, particularly P8 (TP5).
I think you mean D108-D111, also C116.
But before this, check what Enzo asked in reply #17.
Yup. We just had an eclipse and I'm a bit confused. :duh
So basically somebody disconnected those transformer wires for some reason and reconnected them the wrong way.
Not *all* of them, just transposing 2 may make a mess out of it.
Worst case, I'd follow (roughly) the following path:
1) disconnect all secondaries and leave them in the air.
2)with amp ***unplugged*** (turned OFF is not enough) I'd draw the transformer and colour wires, for extra safety would stick a small "flag" (maybe car painter's paper tape) with a letter to each, as in A/B/C/D/E and measure which wire has continuity with each other.
Just that. It will confirm or discard the theories about transformer internal failure or wis-winding or mis-labelling.
Post results here.
Mention each wire color and letter .
Tasks 3/4/5/etc. will follow, depending on what we find first.
EDIT: FWIW I'm sort of agreeing with Enzo's hunch ("sort of" only because it still needs confirmation through measurement) and I'm asking incredibly boring questions just to not leave it open to the slightest doubt.
Transformer wires are correct. No voltage on P8 or P9 to ground unplugged.
No doubled wires on posts.
Only 5 secondary wires.
I have to take board back out to disable fan rectifier, will try to get to later tonight. Fan does come on as soon as amp is turned on.
The floating piece of metal was found on D112 of the fan circuit just a FYI.
Quote from: markorock37 on May 10, 2013, 01:21:20 PM
Transformer wires are correct. No voltage on P8 or P9 to ground unplugged.
We need to know the AC voltage at the wires that attach to P8 and P9. Not DC voltage, and not at P8 or P9.
Disconnect the brown and brown/white wires from P8 and P9, connect your meter to the wires.
What is the AC voltage from brown to brown/white wire? AC volts from brown wire to chassis ground? AC volts from brown/white wire to chassis ground?
Voltages between P8 (brn) and P9 (brn/wh) is 0VAC. P8 to ground is 0VAC.
P9 to ground is 22.4VAC.
A = red wire
B = red/yellow
C = red/white
D = brown
E = brown/white
A has continuity with all but D
B has continuity with all but D
C has continuity with all but D
D has no continuity with all
E has continuity with all but D
And there it is.
The open winding leaving D with no continuity to the rest and zero volts output. Just how that resulted in the one pin having twice voltage I'll have to think about. But the transformer is bad.
Now sincve the transformer has to come out anyway to replace, you might as well try to fix it. It is POSSIBLE the problem is in the lead wires and not th windings. The transformer inside is wound with enamel coated wire, not the colored is=nsulated wires you see coming out. Inside will be some point where the lead wire is soldered to the winding wire. There may be solder terminals you can see from the outside, or they may be buried. We need to go inside enough to see where th brown wire connects to determine if the connection to it has broken, or if the end of the winding wire snapped off right at that point.
And we cannot yet rule out the crimped-on female push connetor on the wire end. Check that joint.
If you have to, you may need to slice open the fish paper (heavy cardboard) around the transformer middle.
Hey, what is there to lose?
Quote from: EnzoThe open winding leaving D with no continuity to the rest and zero volts output. Just how that resulted in the one pin having twice voltage I'll have to think about. But the transformer is bad.
Maybe I'm just going senile, but I can't see how just the D=brown wire being open alone could result in 50-odd volts and exploding regulators - that is, I suspect that the brown winding being open is a
result, not a
cause. :headscratch: ???
We may not have the cause and effect figured out, but it definitely is what is wrong at this point.
Well whatever shorted did cause quite a show, I am not surprised. I'll take a look at that tranny. The overvoltages with an open winding doesn't make much sense to me either.
Yes, the low voltage area should *still* have roughly +/-30V, only half wave rectified and with more hum/ripple but that's survivable .
I bet someday between Factory and you getting it in your hands and it was messed with.
That's why I asked you to identify and measure the *wires* themselves (which always are what they are) and not the "P" terminals, which may receive anything, if miswired.
You already posted what you read on Brn (0V) and Brn/Wh (22V) "relative to ground" but as a last check I ask you to please reread and post voltages with all 5 secondaries connected to nothing (not even ground) and relative to RdYel .
Seems redundant but we are chasing ghosts here.
"They should measure the same as before".
Yea, famous last words ;)
Ok, relative to the red/yellow wire (P6)
red (P5) - 53.6VAC
red/wh (P7) - 53.5 VAC
brn (P8) - 0 VAC
brn/wh (P9) - 22.4 VAC
Quote from: markorock37 on May 12, 2013, 01:43:19 PM
Ok, relative to the red/yellow wire (P6)
red (P5) - 53.6VAC
red/wh (P7) - 53.5 VAC
brn (P8) - 0 VAC
brn/wh (P9) - 22.4 VAC
Thanks :cheesy:
Only *now* I can trust
the transformer.
As I said before, your amp should work as-is , even missing the Brown wire.
So, having removed it from the ghost list, now we'll test the PCB wiring.
By the way, I asked you to measure "in the air" so momentarily we remove the PCB terminals from the equation.
So for what you posted above, the labels P5/6/7/8/9 do
not apply, since the wires are not soldered (or plugged ) into any of them.
At least, I *hope* you measured with wires unattached to any P post.
Please confirm.
Just finished my 8PM cup of coffee, back to work, will suggest some measurements during the next break, in about 2 hours.
Rest assured we´ll find the answer :tu:
Yes, wiring hanging in the air. P#'s only added for reference. Thanks for the help so far, always a pleasure here!
Yes, one open side of that center tapped brown winding OUGHT to still provide correct DC voltages. I have not yet put any mental energy into it, but it may be possible with that open, the floating fan supply off the same leads is creating an ersatz voltage doubler somehow.
So the brown wire should be at 22 volts correct? If so I'll get a look at the transformer connections inside to see if anything came disconnected. Should I still disable the fan circuit for further testing or are we at a standstill until I fix/get a transformer?
with just one half of the brown winding working, you still ought to get your 30vDC rails. You could lift D108-111 to remove the fan influence and see if your voltage comes back down.
AHA! I just looked more closely at the schematic, and finally realized what I was seeing. There is a reason the fan circuit does not reference ground. I was trying to figure how the same set of 23v wires could make +/-30vDC or 60v, and still make only 40v in the fan supply.
But it just sank in that the fan supply runs off one of the low voltage wires and one of the high voltage wires. In other words the fan is running off what amounts to a 34-0-23 "winding". Since it has no ground reference, the lopsidedness matters not.
But if one of the diodes in the fan supply shorts, or the C116 or C117, or not swure what else, it would wind up shorting the red wire to the brown wire, and that might be the reason the brown wire haqs burnt open inside the tranny.
So:
1. Lift the four fan supply diodes. Check them while there for shorts. See if the remaining low voltage wire now allows the +/-30vDC down there that would allow the Vregs to come home.
2. Open up the transformer to see if you got lucky and it might be repairable.
3. Solve the fan supply. Look for anything in it that might be shorted.
Transformer nonrepairable. I didn't have any luck yet finding a replacement either with those specs.
1) The "expensive" winding is the 52+52VAC, high current one, and that seems to work, so use it (save $$$$).
2) You'll need a small and cheap extra 22+22V AC transformer, maybe 30 to 40VA will do, because it will only feed the preamps and probably the fan, you *should* find space to mount it somewhere.
3) about the fan?
Oh !! the fan !!! :trouble
If I had it on my bench, in Argentina 8) , far away from the usual suppliers, I would delete the wonky Fan supply and controller they used (already demonstrated to be a troublemaker) , make a simple 2 diode 1 capacitor raw 30V DC supply (separate from the one feeding the regulators) and feed a 24V Fan with it, adding a small series resistor to take up the few extra volts.
Letting it run all day long is no problem, PC fans are designed to do exactly that, all day long, for *years* but if you want to get High Tech :lmao: , get a 50 to 60 ºC N.O. thermostat, attach it in series with the fan, and bolt it to the heatsink.
That way the fan won't run while in the bedroom or while you use it as a preamp straight into the PC or something light like that, but will turn on whenever the heat sink takes some temperature (garage band levels or higher).
To delete the wonky Fan supply, just disconnect it from AC1/AC2 but if it's hard wired (PCB tracks) simply remove all 4 diodes and the capacitor which sits between them .... and don't plug the Fan into the output connector.
Check Fan voltage, it should be printed somewhere.
Or just buy a new 24V rated one (recommended).
But you might learn more by finding out what was/is wrong with the fan supply, even if you replace it with something else.
So when you called Fender to ask if they still have part number 0055692000 they said no? (I forget, you did say 120v, right?)
Quote from: J M FaheyI would delete the wonky Fan supply and controller they used (already demonstrated to be a troublemaker)
:dbtu:
That sounds feasible to me. It does have a 24v fan in it, so a simple circuit may be the right answer. I will call Fender today and see if thet tranny is still available (I figured it was an outdated part). I do like the idea of planting a smaller second transformer inside as there is plenty of room.