Fender BXR300 Cutting Out

[nevetslab]

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).

That fan is rated 31CFM Same brand has one with a slightly higher CFM rating of 36CFM. I looked thru a number of mfgr's for something with a higher rating, found a Sunon MA 1082-HLV.GN which has a 41CFM rating...moves considerably more air. Mouser won't have any until Oct 9, and has a better price than Newark Electronics, which DOES have stock. The link for them is:
https://www.newark.com/sunon/ma1082-...rmalManagement

Price is $31.86 plus tax/shpg.

Build quality isn't as good, but moves more air, which is the goal....keeping it from shutting down. Just a thought.

[bobloblaws]

That fan is rated 31CFM Same brand has one with a slightly higher CFM rating of 36CFM. I looked thru a number of mfgr's for something with a higher rating, found a Sunon MA 1082-HLV.GN which has a 41CFM rating...moves considerably more air. Mouser won't have any until Oct 9, and has a better price than Newark Electronics, which DOES have stock. The link for them is:
https://www.newark.com/sunon/ma1082-...rmalManagement

Price is $31.86 plus tax/shpg.

Build quality isn't as good, but moves more air, which is the goal....keeping it from shutting down. Just a thought.

Thanks, I'll keep that in mind in case I can't resolve it otherwise.

You mentioned the possibility of lowering bias. To be clear, a lower idle voltage reading would correspond with a lower bias? Seems intuitive, but I wanted to be sure.

Is it worth trying to see if it helps? Is there a possible downside?

[g1]

Yes, a lower voltage at the test points would be cooler bias. They call for 10mV across the .33R resistor, which means 30mA idle current through the output transistors. As you reduce the idle current, the voltage across the resistor will decrease.
The only downside is at some point you will introduce cross-over distortion. It will be visible on the scope, and is generally (audibly) most noticeable at low signal levels, such as the decay of a note.

[bobloblaws]

Yes, a lower voltage at the test points would be cooler bias. They call for 10mV across the .33R resistor, which means 30mA idle current through the output transistors. As you reduce the idle current, the voltage across the resistor will decrease.
The only downside is at some point you will introduce cross-over distortion. It will be visible on the scope, and is generally (audibly) most noticeable at low signal levels, such as the decay of a note.

I was wondering about this the other day when you asked me to check the voltage drop on those .33R resistors to see if it was similar to the bias check per the service diagram. The top of R62 as shown in the service diagram is the R63 end, therefore the bias check is across both R62 and R71 in series for a resistance of 1000.33 ohm. I'm rather confused about that.

[bobloblaws]

Any concerns with using a passive attenuator to simulate live playing conditions at apartment volume level?

[The Dude]

Why not just turn it down? It's not like you're gonna get tube compression/saturation out of a SS amp. They only thing you might miss is the sound of speakers being driven harder, which you're not going to get anyway at bedroom levels.

[bobloblaws]

Why not just turn it down? It's not like you're gonna get tube compression/saturation out of a SS amp. They only thing you might miss is the sound of speakers being driven harder, which you're not going to get anyway at bedroom levels.

I mean as a way to test without getting evicted from my apartment, not to get a better tone.

G1: "So you may have to do some live testing w/speaker."

[bobloblaws]

I see a lot of opinions elsewhere on whether it is safe to use an attenuator with a SS amp. I'm asking the experts.

[bobloblaws]

So you may have to do some live testing w/speaker.

I was hoping I could effectively do this test with an attenuator but I'm not sure if it is safe with a solid state amp. Any thoughts on this?

[nevetslab]

Solid state amps like this Fender 300BXR can drive most anything that's not lower than the minimum recommended load impedance. I don't see any problem placing the attenuator between the speaker and the amp,

[Helmholtz]

Solid state amps like this Fender 300BXR can drive most anything that's not lower than the minimum recommended load impedance. I don't see any problem placing the attenuator between the speaker and the amp,

Agree in principle, but if the attenuator is only rated for 100W, intense playing may fry it.

[bobloblaws]

Agree in principle, but if the attenuator is only rated for 100W, intense playing may fry it.

Good point, I'll stay on the conservative side. And I'm less worried about damaging my attenuator than I am about about someone else's amplifier.

[bobloblaws]

Solid state amps like this Fender 300BXR can drive most anything that's not lower than the minimum recommended load impedance. I don't see any problem placing the attenuator between the speaker and the amp,

Thanks, Nev.

[g1]

I was wondering about this the other day when you asked me to check the voltage drop on those .33R resistors to see if it was similar to the bias check per the service diagram. The top of R62 as shown in the service diagram is the R63 end, therefore the bias check is across both R62 and R71 in series for a resistance of 1000.33 ohm. I'm rather confused about that.

That sounds odd to me. The layout does not show the traces, so only you can check which end of R62 connects to R71. They do not say 'top', they said 'outside' referring to the outer edge of the board. Perhaps it does not matter as there should be zero current through R62 at idle. Or it could be a typo. But idle current is usually measured across the emitter resistor of an output device.
Do you get the same reading with probes directly across R71? And all other .33R emitter resistors give ballpark same reading?

As far as the attenuator, should be fine at 4 ohm or higher (up to power rating of attenuator).

[nevetslab]

In general, measuring across any of the emitter resistors should yield similar potentials. Doubtful if all are the same, as we're in the region where the transistors are not yet fully turned on. To get a better idea, measure from each pair of output xsrts' emitters (NPN side to PNP side), all four pairs, divide the readings by 0.66 ohms and average the result. More work, but that will give you a better overall view of the current sharing at this operational state in adjusting bias. The 1k resistor (R62, R67) are the sense resistors feeding the bases of the current limiter circuit, and are NOT part of the bias measurement test points.