Fender BXR300 Cutting Out

[bobloblaws]

Another way would be to check the voltage across every one of those .33R5W resistors. They should all measure similar to what you got for R71 when you checked the bias. (10mV).

OK, on the service diagram it says to set bias with heat sink cold. When I first measured the bias setting my meter showed 0V. When I started turning the trim pot I started to see numbers on the meter and I adjusted it to 0.01V. But within seconds the voltage started climbing. I assumed this was because the heatsink was no longer "cold", even after just a few seconds, and that it must be crucial to make the adjustment within the first few seconds after turning the unit on from cold. Does that sound right? Would I need to do the same to properly check those resistors?

[g1]

The idle current will increase a bit from cold, but it shouldn't be drastic. What does it climb to? Are they all in a similar range, or are there any that are much higher than the others?

[bobloblaws]

The idle current will increase a bit from cold, but it shouldn't be drastic. What does it climb to? Are they all in a similar range, or are there any that are much higher than the others?

I didn't wait to see what it would climb to but I will when I try it again and check the other ones as well. Should I let it settle before making the trimpot adjustment for 0.01V?

[g1]

I'm not sure it will 'settle' completely while it's still cold. But it shouldn't be rising too rapidly, so maybe after a minute see if everything is fairly stable and no outstanding readings.

[bobloblaws]

I'm not sure it will 'settle' completely while it's still cold. But it shouldn't be rising too rapidly, so maybe after a minute see if everything is fairly stable and no outstanding readings.

Ok, I was too hasty when I said it was ramping up quickly. I set it for 0.01V and left it for a minute or so and it was stable. I checked voltage across the .33 ohm resistors and they were all in the ballpark. At startup temperature they are all between 0.007V and 0.009V (positive on the R71 side and negative for the four resistors on the other side).

[The Dude]

FWIW: Being cautious is not necessarily a bad thing.

[bobloblaws]

Well then start looking. Are all the transistors getting real hot? Got a crayon? Touch it to the body of each one and see if it wants to melt the crayon.

For the record, yes, they are hot enough to easily melt a crayon, but apparently the melting point of a crayon is between 120F and 147F so I don't think that helps us.

I'm actually seeing a different behavior after adjusting the bias last night, more in keeping with what the owner said about the power LED staying on but lack of output. I'm gonna do a little digging and then post my findings.

[bobloblaws]

For the record, yes, they are hot enough to easily melt a crayon, but apparently the melting point of a crayon is between 120F and 147F so I don't think that helps us.

I'm actually seeing a different behavior after adjusting the bias last night, more in keeping with what the owner said about the power LED staying on but lack of output. I'm gonna do a little digging and then post my findings.

Ooops, that must have been a fluke or an error on my part. Same thing is still happening.

(or another intermittent issue)

[bobloblaws]

For what it's worth, I'm monitoring the bias voltage using the test points indicated on the service diagram. I'm putting in a 105mV sine wave at 1Khz and turning up the volume to about 7 on the dial which is giving me 100W of output, which is the power rating of my dummy load. The bias voltage I see goes from 0.01V at idle to -.140 under the test condition. After a few minutes the fan ramps up speed and then after another minute or so TS1 opens up and that's all she wrote.

[nevetslab]

Is there a lot of accumulated dust/debris on the heat sink fins, as well as caked-on dust on the fan blades? Invariably dust accumulates on the fan blades, which lowers the efficiency of the fan, as does accumulation of dust bunnies in the heat sink fins. Try running the amp at around 25W. It may get hotter faster at that level, since there's more voltage across the power xstrs. Average power on stage may be closer to 12-20W, if averaged over time. You could just short out the Thermal Switch across the Slow Speed fan resistor, if the acoustic noise isn't a problem. I haven't looked up the model to see how it's physically constructed to see if there's additional remedies....such as changing to a higher CFM-rated fan.

[g1]

The bias voltage I see goes from 0.01V at idle to -.140 under the test condition. After a few minutes the fan ramps up speed and then after another minute or so TS1 opens up and that's all she wrote.

Bias setting is DC idle condition with no signal applied. Double check with no signal.

To nitpick a bit about the terminology, the .01V across each emitter resistor is not 'bias voltage', it is the voltage created by the idle current through each output transistor. We then adjust the bias trimmer to set the idle current to the specified level.

[bobloblaws]

Is there a lot of accumulated dust/debris on the heat sink fins, as well as caked-on dust on the fan blades? Invariably dust accumulates on the fan blades, which lowers the efficiency of the fan, as does accumulation of dust bunnies in the heat sink fins. Try running the amp at around 25W. It may get hotter faster at that level, since there's more voltage across the power xstrs. Average power on stage may be closer to 12-20W, if averaged over time. You could just short out the Thermal Switch across the Slow Speed fan resistor, if the acoustic noise isn't a problem. I haven't looked up the model to see how it's physically constructed to see if there's additional remedies....such as changing to a higher CFM-rated fan.

No, it's clean as a whistle.

I know he is trying to run this amp turned up at least past 5 on the volume control and it is shutting down on him so I'm not sure what your point is re: running at 25W. Are you saying that now that I've adjusted bias it may be OK now under playing conditions even though it is still shutting down under test conditions? I'm not even sure the bias is any different than what it was before I touched it as the trimpot is in pretty much the exact same position as when I started..

[bobloblaws]

Bias setting is DC idle condition with no signal applied. Double check with no signal.

To nitpick a bit about the terminology, the .01V across each emitter resistor is not 'bias voltage', it is the voltage created by the idle current through each output transistor. We then adjust the bias trimmer to set the idle current to the specified level.

Yes, I followed the note on the service diagram, .01V with no signal applied. I mentioned that the voltage at the same test point showed -0.140V under test condition simply because I don't know if that's normal and I thought it might be helpful.

Out of curiosity, the service diagram also says "no load" as well as no signal for bias adjustment. What is the reason for that? I don't think I observed a difference with the load or without.

[g1]

Out of curiosity, the service diagram also says "no load" as well as no signal for bias adjustment. What is the reason for that? I don't think I observed a difference with the load or without.

I think it will usually be the same, but if there is any significant hum in the amp, it will give an unrealistically high number when loaded. Just a guess.

Do you have an IR thermometer or any way to check if the thermal cutout is triggering at the correct temp? They can go out of whack, though it is rare.

[bobloblaws]

I think it will usually be the same, but if there is any significant hum in the amp, it will give an unrealistically high number when loaded. Just a guess.

Do you have an IR thermometer or any way to check if the thermal cutout is triggering at the correct temp? They can go out of whack, though it is rare.

I have a cheap little Ryobi https://www.homedepot.ca/product/ryo...ter/1000837112 I don't know how accurate it is.

I got the highest reading by aiming at the mounting screws on each side of the switches. I find it hard to make out the temperatures indicated on the schematic bu according to Enzo they are 200F for TS2 and 248F for TS1. The parts list does not specify.

According to my meter TS2 kicked in at 160F and TS1 opened at 230F.

I just took delivery of a replacement 248F switch which I ordered just in case. I suppose it can't hurt at this point to try it out and use the Ryobi again and see if there is any difference.