Fender BXR300 Cutting Out

[bobloblaws]

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.

Damn, I ordered the wrong part. Wrong size, won't mount.

[nevetslab]

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..

Running the amp at around 1/3 power is usually worse case as far as heat sink temperature goes. 25W is fairly close to that. Running at 100W heats up the power xfmr a lot more than running at 25W, or 33W, which would be 1/3 power. 1/8 power is often the the thermal design criteria for the heat sink assembly of the amp, as that may average out to around that if measured on stage during a full set over the course of an hour's set.

We're still looking for a thermal solution to the amp shutting down. As you're still finding that at your home or shop space, no longer the same as on stage, that eliminates the possibility of the placement on stage being less than ideal from an air flow condition, or the club's ambient temperature, since we're relying on the cooling fan to keep the amp running.

The investigation with the bias adjustment has to do with finding any drift in the bias circuit that would cause the output stage to be turning on harder than normal, drawing more current thru the output stage, and raising the heat sink temperature at idle. We generally want as much margin between idle and full load condition as we can get, which relates to 'run time' between thermal cycling...which appears to be what's happening.

[bobloblaws]

Running the amp at around 1/3 power is usually worse case as far as heat sink temperature goes. 25W is fairly close to that. Running at 100W heats up the power xfmr a lot more than running at 25W, or 33W, which would be 1/3 power. 1/8 power is often the the thermal design criteria for the heat sink assembly of the amp, as that may average out to around that if measured on stage during a full set over the course of an hour's set.

We're still looking for a thermal solution to the amp shutting down. As you're still finding that at your home or shop space, no longer the same as on stage, that eliminates the possibility of the placement on stage being less than ideal from an air flow condition, or the club's ambient temperature, since we're relying on the cooling fan to keep the amp running.

The investigation with the bias adjustment has to do with finding any drift in the bias circuit that would cause the output stage to be turning on harder than normal, drawing more current thru the output stage, and raising the heat sink temperature at idle. We generally want as much margin between idle and full load condition as we can get, which relates to 'run time' between thermal cycling...which appears to be what's happening.

Sorry, I have no idea what any of that means from a practical standpoint.

At this point I can think of two options for moving forward. Try to find a replacement for the switch that is the correct physical size and cross fingers and hope that it shows the threshold of the existing one was too low. But I have a feeling at least one person will call that foolish.

OR

Take the amp somewhere where I can test it under playing conditions and see if it's still problematic.

That's all I got at the moment.

[nevetslab]

I just googled the amp, and came back with some photo images to post, so we can all have a look at this on the inside and overall view: The heat sink is a stamped and folded sheet of 3/16" aluminum, with the heat sink living upside down in the chassis. So, we're relying entirely on the AC cooling fan to get the heat out. Can you read the Fan's model number & mfgr's name? It's an 80mm size fan. About the best you can do in that size is 40CFM.

There's NOT much surface area for cooling on this amp. So, expect the heat sink to always be fairly hot. If Fender selected a fan that's not the highest CFM available, I'd select a faster fan. One could add a second fan in series, mounted on the outside of the chassis, same rotation, and add a grille to keep fingers from harms way. It won't double the CFM rating, but it will increase it. Both have to be the same model fan.to work best. There is room on the rear panel to cut in an opening for a second fan, having both on the inside.

It's not a great thermal system overall, from a design standpoint, so band-aiding is what you're left with, assuming there's not a thermal issue with the circuit itself, causing the heat sink to overheat. I'd be interested to know what the ambient heat sink temperature is at idle after half an hour or more. If it's sitting over 50 deg C at idle, either the bias needs to be turned down, or we need more cooling to drop it down....or find what is causing the heat.

[Enzo]

And this: the fan has flow between the rear fan and the front grille. If you run with the lid off, that interrupts the flow. The fan just blows ambient air, and doesn't bother to make a flow across the heat sink to the front grille. SO during test, make sure to sit the top on it while it sits there working. It will run cooler with the top on than off. Also make sure owner doesn't push amp right against the wall, blocking fan flow.

[bobloblaws]

And this: the fan has flow between the rear fan and the front grille. If you run with the lid off, that interrupts the flow. The fan just blows ambient air, and doesn't bother to make a flow across the heat sink to the front grille. SO during test, make sure to sit the top on it while it sits there working. It will run cooler with the top on than off. Also make sure owner doesn't push amp right against the wall, blocking fan flow.

Thanks, I've been testing with the top open. I'll try that.

[Enzo]

I don't know if that is your issue, but it does make a difference.

Also, if the guy runs it in one of those portable rack boxes and doesn't remove the rear cover, that can build up heat for same reasons.

[bobloblaws]

I just googled the amp, and came back with some photo images to post, so we can all have a look at this on the inside and overall view: The heat sink is a stamped and folded sheet of 3/16" aluminum, with the heat sink living upside down in the chassis. So, we're relying entirely on the AC cooling fan to get the heat out. Can you read the Fan's model number & mfgr's name? It's an 80mm size fan. About the best you can do in that size is 40CFM.

There's NOT much surface area for cooling on this amp. So, expect the heat sink to always be fairly hot. If Fender selected a fan that's not the highest CFM available, I'd select a faster fan. One could add a second fan in series, mounted on the outside of the chassis, same rotation, and add a grille to keep fingers from harms way. It won't double the CFM rating, but it will increase it. Both have to be the same model fan.to work best. There is room on the rear panel to cut in an opening for a second fan, having both on the inside.

It's not a great thermal system overall, from a design standpoint, so band-aiding is what you're left with, assuming there's not a thermal issue with the circuit itself, causing the heat sink to overheat. I'd be interested to know what the ambient heat sink temperature is at idle after half an hour or more. If it's sitting over 50 deg C at idle, either the bias needs to be turned down, or we need more cooling to drop it down....or find what is causing the heat.

The heat sink is heating up past 50C/122F at idle (abeit with the top off, see Enzo's note). I just measured 131F with my gun after close to half an hour and my gun may very well be mis-calibrated and read low per my previous measurements.

[bobloblaws]

Also, if the guy runs it in one of those portable rack boxes and doesn't remove the rear cover, that can build up heat for same reasons.

No it's a combo amp. I have mentioned to him previously to make sure good air circulation.

[Enzo]

Oh, right, I actually knew that... Hey, I'm old...

[nevetslab]

The heat sink is heating up past 50C/122F at idle (abeit with the top off, see Enzo's note). I just measured 131F with my gun after close to half an hour and my gun may very well be mis-calibrated and read low per my previous measurements.

OK. A good 10 deg C hotter than I had hoped for, maybe more. Can you read the mfgr's name and model number of the AC Cooling Fan? Also, can you measure what the bias is after this has been at idle over this half-hour period? Sounded like 10mV was the target bias voltage.

I'm starting to think finding a higher velocity fan is the next thing to try, depending upon what Fender selected. The airflow design looks like it would be pulling ambient air in from the front, and exhausting the hot air out the back. I haven't read the manual yet see if that's what is stated. Fender only states "Do NOT block air flow to the rear of the amplifier". You'll have to tell us if it's blowing out the back, or sucking air in from the back.

BXR 300C-300R.pdf

[glebert]

When you say TS1 is opening how are you determining that? Are you measuring continuity or measuring AC voltage? Also, can you take TS1 and put it on an electric iron or something out of circuit and see at what temp it opens up?

[g1]

Just a couple notes in case it runs a lot better with the top on.
a) It's normal for the thermal cut-out to cause cycling when testing with a load. Just a question of how early and how often. So you may have to do some live testing w/speaker.
b) Some folks replace the 8 ohm speaker with a 4 ohm to get full power without an ext. cab. If this has been done, you can no longer run an ext. cab.

[bobloblaws]

OK. A good 10 deg C hotter than I had hoped for, maybe more. Can you read the mfgr's name and model number of the AC Cooling Fan? Also, can you measure what the bias is after this has been at idle over this half-hour period? Sounded like 10mV was the target bias voltage.

I'm starting to think finding a higher velocity fan is the next thing to try, depending upon what Fender selected. The airflow design looks like it would be pulling ambient air in from the front, and exhausting the hot air out the back. I haven't read the manual yet see if that's what is stated. Fender only states "Do NOT block air flow to the rear of the amplifier". You'll have to tell us if it's blowing out the back, or sucking air in from the back.

[ATTACH]n912038[/ATTACH]

Having the cover on made a difference. When idle for a half hour my temp gun showed 122F. As I said I suspect it reads low and the actual number is higher but it is lower than the 131F I saw when the cover was off. The voltage across the test points was 0.013VDC (with load connected).

The fan is branded Comair Rotron Sprite model SU2A1. It pulls the air through the grill on the front and blows the air through the fan and out the back. Interestingly there is a perpendicular fin attached to the cover that dissects the two rows of fins on the heat sink.

Also, with the cover on and using the test described earlier it ran for 15 minutes without shutting off. However, I think it was right at the threshold because when I took the cover off and measured with my temp gun it showed 220F which is right around where TS1 was opening yesterday when I was testing with the cover off. And again,the real number is probably around 250F meaning TS1 likely functions as it should (the other possibility is that my cheap little gun is accurate and both TS1 and TS2 activate about 30F lower than spec).

[bobloblaws]

a) It's normal for the thermal cut-out to cause cycling when testing with a load. Just a question of how early and how often. So you may have to do some live testing w/speaker.

As I just replied to nevetslab, it is running less hot with the cover on. I'm in an apartment so I can't turn it up very loud but I just bought a Bugera PS1 100W attenuator. Do you think that would be an effective test to use the attenuator? I'm not even sure if it's advisable to use something like that on a SS amp

b) Some folks replace the 8 ohm speaker with a 4 ohm to get full power without an ext. cab. If this has been done, you can no longer run an ext. cab.

Internal speaker is 4 ohm. In fact when he first had a problem with it at his club I recommended plugging in an 8 ohm extension cab to the parallel jack see if it fixed the shut down issue.