Fender BXR300 Cutting Out

[bobloblaws]

A guy I know that jams at a club in the house band bought a Fender BXR300 recently for the back line and brought it to me to have an input jack fixed. He also said that the amp cut out mid set. On the schematic it looks like some kind of thermal protection in series with the AC mains but I don't know if it's designed to shut the amp off or if it's meant to just turn the fan on at a certain temperature. In any case he told me that on the night in question the air conditioning in the club was not working (and this was a few weeks ago during some very hot weather) and it was apparently extremely hot in the club. So at that time I assumed that the amp cutting out was a feature rather than a bug. However since then he's continued to have problems with it cutting out, typically when turned up to half volume or more. I live in an apartment so I can't crank it up here but I'm going to see if I can repro the problem with a dummy load. In the meantime I was hoping someone could help me understand the protection circuit as shown in the schematic. There is a component marked TS1 and one marked TS2 which I think could stand for "thermal switch" and another one called TH1 which I think is a thermistor. Any insight would be much appreciated.

[Enzo]

When you see hoofprints in the dirt, think horses, not zebras.

Loss of sound - the one most common cause of that is funky FX loop jacks. The cutout contact in the return jack interrupts the signal path, and if it gets dirty...

Also, speaker wiring.

Amp cuts out is not very specific. Does the sound cut out but the lights stay on? Or do the lights go out too? Does the fan stop?

DUmmy load is fine, but remember, vibration might be a part of your problem, and that won't happen with no speaker.

Thermals is straightforward. Look at your schematic. TH1 is the inrush thermistor and is in series with the mains. It is the disc near the fuse. It takes just a couple seconds to warm up at power on. If it cracks or comes loose you amp stops. TS1 is thermal switch and if it gets too hot it opens the mains line, killing the amp. Thermal Switch 2 shunts the fan resistor when it gets hot, thus speeding up the fan. It cannot cause amp to cut out. The thermal switches are bolted to the tops of a couple of the output transistors.

[bobloblaws]

Loss of sound - the one most common cause of that is funky FX loop jacks. The cutout contact in the return jack interrupts the signal path, and if it gets dirty...

Also, speaker wiring.

Amp cuts out is not very specific. Does the sound cut out but the lights stay on? Or do the lights go out too? Does the fan stop?

DUmmy load is fine, but remember, vibration might be a part of your problem, and that won't happen with no speaker.

Thermals is straightforward. Look at your schematic. TH1 is the inrush thermistor and is in series with the mains. It is the disc near the fuse. It takes just a couple seconds to warm up at power on. If it cracks or comes loose you amp stops. TS1 is thermal switch and if it gets too hot it opens the mains line, killing the amp. Thermal Switch 2 shunts the fan resistor when it gets hot, thus speeding up the fan. It cannot cause amp to cut out. The thermal switches are bolted to the tops of a couple of the output transistors.

Great info Enzo, thanks!

The owner tells me that it doesn't lose power (lamp stays on). He also tells me the longer they turn it off for before trying it again generally determines how long it will play before cutting out again. Sounds heat related to me, could be solder joint related?

[Enzo]

Of course could be. But as the whole thing comes up to temperature, even my jack contacts shift around and whatever.

The way to do it is find it in the failed condition and see where signal disappears, rather than guessing "maybe it's this" and soldering or changing parts or praying to the amp gods.

[glebert]

I think the thermal resistor in series with the AC mains is a a negative coefficient thermistor that is there to prevent turn on thump. At cold it is has more resistance and quickly warms up and goes to very low resistance. This is a pretty common failure point on Fender solid state amps (especially from this era) as the solder joints eventually fail. I would expect the whole amp to power down when this happens, but have seen it where it seems like there is still enough juice to power the power led.

[bobloblaws]

Of course could be. But as the whole thing comes up to temperature, even my jack contacts shift around and whatever.

The way to do it is find it in the failed condition and see where signal disappears, rather than guessing "maybe it's this" and soldering or changing parts or praying to the amp gods.

So far so good, I'm able to repro the problem with a dummy load and a test signal.

[Enzo]

OK, good, you can see it happen. Plug a cord from FX send to FX return. Does that stop it happening?

[bobloblaws]

OK, good, you can see it happen. Plug a cord from FX send to FX return. Does that stop it happening?

Actually, my friend was wrong about the power LED, it is going out after all.

I didn't realize it at first because the lamp on my work bench was making the LED appear illuminated.

It turns out it is in fact TS1 opening up that is causing the amp to shut down. So now I'm wondering, should I automatically consider TS1 defective or could it be that there are one or more other components causing excess heat, for example the power transistor that TS1 is mounted on top of, or any number of the rest of the power transistors heating up that big ol' heatsink higher than normal? Or when those thermal switches fail do they typically just stop working period and simply stay stuck open or closed?
The fan is working fine, starts slow and then speeds up when TS2 opens.

[Enzo]

SO if you short across TS1, the problem no longer exists?

Sure TS1 could be failing. Or it could just be doing its job. Are the transistors getting real hot? ANy chance the crimp push-on connectors on top of TS1 are loose?


Actually the fan slows down when TS2 opens. Note on the schematic that TS1 is normally closed, while TS2 is normally open. SO when TS1 trips it opens the circuit, but when TS2 trips it closes a circuit. Note also TS2 closes at 200 degrees, while TS1 opens at 248 degrees. SO it tries to cool itself when it reaches 200 degrees, but if it hits 248 degrees it just shuts off.

How is your bias setting? Note 7 covers it, and in fact the next page is the layout drawing, and upper right it points out just where those two resistor test points are. Do you have that? Oh heck, here it is, the whole file:

BXR 300C.pdf

It could be as simple as a bias adjustment.

Or when those thermal switches fail do they typically just stop working period and simply stay stuck open or closed?

Fail is fail, they can fail in those ways or others, they can be working fine but be intermittent even. it never matters how a part USUALLY fails, it only matters if it is failing here and now.

[bobloblaws]

SO if you short across TS1, the problem no longer exists?

I didn't actually short it out, but yes I can say with complete certainty that when the amp "cuts out" it is due to TS1 opening. For example at one point I used my continuity tester and observed no continuity between the TS1 terminals after it shut itself down. I left the probes in place and after 60 seconds or so I observed continuity restored and the amp resume operation. I also verified AC voltage or lack thereof on each side of the switch (always 120VAC on the mains end).

Are the transistors getting real hot?

I don't know how to answer that. That's my dilemma, how can I know if the switch opening is a feature or a bug as I said in #1. The transistors certainly get real hot, you're obviously familiar with this amp and know the transistors are mounted on a giant heat sink and it's going to dissipate a lot of heat at the best of times. That's why I wondered if it was logical to replace the switch and see if it behaves differently than the original. If it behaves the same than I can safely assume the amp is actually OVER-heating. (assuming bias adjustment does not cure it)

How is your bias setting? Note 7 covers it, and in fact the next page is the layout drawing, and upper right it points out just where those two resistor test points are. Do you have that?

I did not have that, I only had the gif, so thanks for that and I'll look into the bias setting first.

[glebert]

If TS1 open up at 248F the heatsink would be hot enough to boil water if it is working correctly.

[Enzo]

I will believe the TS1 is opening, and if the transistors get REAL hot, it is likely why.

I looked at the schematic and the layout, that is how I know how it is constructed. I was a Fender warranty station for many years, and certainly have worked on these, but I have no particular memory what is inside. besides, circuits are circuits.


Please check bias.

[bobloblaws]

Still shutting down after adjusting bias.

[Enzo]

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.

[g1]

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).