Fender BXR60 blowing fuses

[bausin]

Hi,

My first post. I'm fixing this bass amp for a friend. It originally had a cracked volume pot that I thought would be easy to fix. After I replaced the pot, I tested the amp and it sounded very weak and distorted. As I was troubleshooting the signal chain with an injected 20 mV signal, per the schematic, the fuse blew. I checked the output transistors and the ballast resistors immediately. Both the xstr and ballast resistor on the negative side were extremely hot. I checked the speaker resistance and it intermittently measured less than 1 ohm. I discovered that the rivet that holds the card to the back of the speaker had broken, allowing the wires to the speaker cone to short together.

I replaced the output xstr (TIP 147) and the .47 ohm res, but it still blows fuses.
I have done all the checks from the Fender M80 thread, but no luck so far.

If I power up with CP3 and CP5 disconnected the fuse does not blow. With either connected it does blow. With the bias chain bypassed, the fuse blows. Diode checks of the bias chain and rectifier diodes all indicate good diodes. The output xstrs both check good, collectors not shorted to each other or to ground/heatsink.

The only thing that measured odd was both R63 and R64, in the bias chain, measure 3.5 K instead of 5.6K. I don't see a parallel path that explains that.

Any help would be appreciated.

Thanks,
Steve

[km6xz]

Do you have a schematic for this that you can post? I have them for similar models but the parts designations will be different, so CP3 and CP4, and R63, 64 mean little to me. Don't run any load on any solid state amp until you know it is stable and working as expected. Do the values of R63 and 64 change when reversing the leads of your ohm meter?
What is CP3/4? Transformer leads? Do you have a oscilloscope to monitor signals? This is a very simple amp so it should be easy to repair if a few more things are known. Does the fuse blow immediately or only after bringing up the drive signal? With the speaker load removed, and the amp is on, do transistors still get hot? Is there a DC voltage at the amplifier output? If there is, is it full rail or some lower than rail level?
The resistors you suspect(very unlikely to be the problem) have color codes on them, what are the really, the print might represent a different board version that did have 5.6k resistors. The most likely situation is that you are not seeing the circuit influences on those readings.

If there is a full rail offset at the output, a positive value would indicate a shorted TIP142. A Negative value would indicate a shorted TIP147.
One the BXR100, not too different an amp, the level shift network for drive and bias, the pull up resistors are 5.6k also so I suspect that is what you are meaning by R63/64. A charge on the divider network filter(a 22mfd 25v electrolytic on each side) would account for the difference in meter reading for the pull up resistors.
Report back about the no load output DC offset, is it there, how high a level.

[Enzo]

Or the parallel resistances inside the TIP transistors base to emitter.


Here is the schematic for the BXR60.

[bausin]

> Do you have a oscilloscope to monitor signals? Does the fuse blow immediately or only after bringing up the drive signal?

I have a scope, but the fuse blows in a second or two.

> With the speaker load removed, and the amp is on, do transistors still get hot? Is there a DC voltage at the amplifier output? If there is, is it full rail or some lower than rail level?

Again; can't tell.

> The resistors you suspect(very unlikely to be the problem) have color codes on them, what are the really, the print might represent a different board version that did have 5.6k resistors.

The resistors are color-coded as 5.6K. I could pull one end up and measure, but I don't think that's the problem.

> Report back about the no load output DC offset, is it there, how high a level.

All the tests I performed above were done with the speaker disconnected.
The fuse blows before I can measure anything. It blows with either the positive or negative
side connected, but the rectifier diodes all measure as good diodes.

Thanks,
Steve

[Amp Kat]

Fuses blow because something is shorted. You have to remove the load from the power supply and see if it will run by itself or at least after the rectifiers. If you remove the load and can't get the +/- 41 and 16 volt rails then it's something in the supply or shorted filter caps. Did you check the TIP147 emitter resistors and you have to lift one leg of them to test and the 4 diodes right before that circuit ?

[bausin]

> You have to remove the load from the power supply and see if it will run by itself or at least after the rectifiers. If you remove the load and can't get the +/- 41 and 16 volt rails then it's something in the supply or shorted filter caps.

The xformer isn't shorted. Any tips on an easy way to remove the load on the PS so I can check the rails?

> Did you check the TIP147 emitter resistors and you have to lift one leg of them to test and the 4 diodes right before that circuit ?

I checked both emitter resistors. I didn't have to lift a leg; they're only .47 Ohms. Checked the bias chain diodes too.

Thanks,
Steve

[km6xz]

Have you checked the out both TIP transistors? It would take both shorted to pull high current if the load is disconnected. To remove the return path for the output transistors lift a lead from both emitter resistors, it high current is still being pulled with the emitter resistor not passing current, look to the power supply.
If the transistors are ok, there is one condition in the power amp that would explain the short delay before blowing, if one of the 4 level shifting diodes, D10-D13 is open. In that case both power transistors would be forward biased to conduct fully.

[bausin]

I've tested all the components you've mentioned and they appear to be good. For example, the TIP xstrs look like back to back diodes.

Can you give me any tips on using a variac? It has no built-in ammeter. What voltage level should I attempt to bring it up to?
What should I check first?

Thanks,
Steve

[Enzo]

The TIPs may be OK themselves, but have you checked to see if either of them is shorted to the heat sink? That would put a dead short across the main power supply. DOn't eyeball, use a meter to check. A small tear in the mica washer is all it takes, or an inadvertant omission of a step washer.

And in related news... Check resistance to ground from +41 and -41, or whatever they are. Does either supply show shorted? And while we are at it, check from +41 to -41, are they shorted together?

The point of a variac is to monitor current while you ramp up the voltage from zero. If the current starts to climb at low voltage, back off. if the current stays low, continue to increase the voltage. it isn;t about applying any certain voltage. Think of it like slowly letting out the clutch in your car, and if it starts to stall the car, push it back down. if the car starts to move, then continue to let it out.


The variac may not have a current meter, but your voltmeter probably does. CLip it in series with the mains cord. or in place of the amp's fuse, whatever.

If you do much amp work, a mains current meter is a useful thing to have, or a metered test outlet. it really doesn;t have to be a precision thing. Put an AC current meter in series with the hot lead, put it in a box with an outlet, and trail a power cord off from the back. Maybe add a on/off switch. or get a larger box and build the variac into it with the meter and switch. it is like an extension cord with a meter on it.

[bausin]

Hi Enzo,

I was hoping I could get you to bite on this problem. :-)

I've done all those checks, which I found in your replies to the M80 amp thread.
That's what has me baffled. The +/- 41 are not shorted to ground or each other.
The TIPs are not shorted to anything and the bias chain measures good.
I haven't checked the +/- 16 rails; I guess I should try that.

Thanks for the tip on putting the ammeter in place of the fuse.
How high should I run the variac up? Is 1 amp too high?

Thanks,
Steve

[Amp Kat]

You did remove the speaker leads right. You can check the power transformer by pulling CP-3 & CP5 and measure across those for 31 VAC. If you look at the diagram Enzo sent the +/-41 & +/- 16 are going to the power section. If you can find that point where it ties into it and lift it there it will take the load off or at least the hi current part which is the 41 volts. The 16+/- runs the op-amps and IMO doesn't normally blow a fuse it just drags it down and gets hot. I would even consider pulling the TIP147's and see if it powers up and pull P1 & P2 connectors.

[Enzo]

I generally don't need to go up to 1 amp if it is drawing heavy, but again, ther is no magic number. I am watching the trend. if the ammeter is rising about as fast as the voltmeter, I stop and back off. it's like watching the meter ignoring the numbers, just watching the trends. If I turn the variac up, and by the time I am up to 10v it is drawing 2 amps, well, 3 amps wouldn;t really be any more of a problem.

I agree with KB, the 16v probably can;t blow fuses.

Any chance the 142 and 147 have swapped places?

I like KBs idea of just removing the TIPs to see if there is still a problem.

And a main filter cap can be bad without showing dead short. All it needs to be is leaky over 2 volts and your meter would never discover it. Removing the main filter caps would test that theory.

Removing the xfmr wires friom the pc board connections lets you check if the PT itself is a problem, but verify the wires are on the correct push terminals when connected up. The yellow center tap wire goes to the pin between the caps.

There is not much that will blow fuses here.

The transformer you say is OK, so that leaves the power supply or the power amp. The power supply has four diodes and two caps. The power amp has two transistors and the clamping diodes (CR14,15).

[km6xz]

If all the the logical resistance measurements are ok(power transistors, rails to ground or to each other, amplifier output terminals to each other etc) the problem is probably dynamic, meaning the high current condition is caused by all the components in the high current sections are working in that they are turning on when told to, by high base-emitter current. The current draw from the power supply will increase as a more abrupt rate after a certain threshold. A short in the high current sections would show a linear increase with voltage increase.

The variac, next to the scope is the most important amp test device. All my variacs back home in California have current meters which makes it easy. Last week I bought the only type of bench style variac I could find here in Russia had voltmeters but no current meters. I had some copper house wiring cable left over from adding outlets in the kitchen. The gauge is smaller than you would use in the US because we at 220-240 volt mains here. The heaviest I had was 18 guage. Wire has a pretty accurate resistance per unit length. In this case about 6 ohms per 1000 feet. I had one low current AC ammeter so I cut a length of copper wire such that it had a resistance that was 1/10,000 of the internal resistance of meter I had. That was just over 3 feet. I put the length of wire in series with one leg of the variac output inside the case. I added two pin jacks to the case with thing 2 wires running to length of shunt wire. I stripped off insulation to expose about 1/4 in. of copper 1 inch from either end of the shunt wire. I soldered the two small wires from the pin jacks 1 each to the exposed portion of the shunt. I mounted the large meter on a shelf right above the variac. I calibrate it by sliding one of the sense wires up and down the shunt, having to strip off more insulation in the process. It turned out that the actual final position was 1 inch from the first calculated position. So now I have an accurate calibrated AC current meter with full scale being 10 amps representing 2400 VA. A home shunt like this can be the most accurate standard reference on your bench. By measuring milliamps, at millivolts, shock damage or meter damage is almost eliminated.
Lower the variac output control to zero and plug in the amp. bring up the ac voltage just enough to see current being drawn. Note the current and and calculate the power, is it appropriate for the low voltage? If yes, the problem is probably dynamic, and expect to see current draw to be a linear increase with increasing voltage until as some threshold the current rises at a faster rate than voltage step increase. Stop.
On the other hand if the current drain at low input voltage is high in relation to the voltage applied, you have a short that you overlooked. Now you have current flowing so you can measure logical points and see where it is not present where you expect it to be. If you have a sensitive voltmeter, you can connect one lead to one of the rails at the power supply and measure voltage at various points along the same rail. The voltage drop should increase the further you get from the power supply reference point where because the copper traces has small amounts of resistance. As some point you will probably see the voltage drop lower quickly. You just passed the physical point where the current is shunting to supply return. This method works well when trying to track down a shorted bypass cap, 1 of dozens or hundreds on a logic board where the whole rail appears shorted because of one chip cap.

Are both rails, when running low mains voltage, the same, when measured at the filter caps?
I would remove the two power output transistors, just measuring forward conduction as 1 volt from your ohm meter tells you only that the junctions work at 1 volt open circuit. Remove the transistors and note if you can now run the mains voltage up without it pulling much current. Either both transistors are bad in this case or they are both being biased on. More likely this latter scenario. Measure the base voltage for both sides, with the TIP 147 and 142 removed. Report back.

[bausin]

km6xz,

Sounds like a good plan. I mentioned that when the fuse blew the first time,
I quickly touched components and found the negative TIP and ballast res were very hot.
The positive side seemed normal. Since the positive TIP measured good, I didn't replace it.
I'm starting to think that could be the problem. Good thing I ordered both.

I'll let you know what I find on Friday.

Thanks,
Steve in Concord, CA

[km6xz]

A good power transistor will get hot in such a case and a shorted one will remain cool.