Crate BT200 bass combo cutting out.

[Randall]

Amp was in two weeks ago with the complaint of cutting out at gigs. Cleaned and inspected the usual suspects, and ran reggae music thru it for over an hour three separate times. It never cut out. Customer played thru it at a three hour rehearsal, and it was fine. He brought to another gig later that week, and it cut out repeatedly. I am told it would cut out with power lamp on, for about 3 - 4 seconds, then come back on. He did say he was pushing it fairly hard to keep up with a full band, and also that he was using an active bass, but not employing the pad at the input.

It sounds like he was over driving the amp, and activating a protection circuit, but I don't see one in this schematic, only a limiter. Shouldn't that clamp the output anyway? In my opinion, a 200 watt bass amp is not enough power to play outside with electric guitars and drums, but I'd like to have some evidence that he needs a bigger amp.

Does anyone see a mechanism for cutting out if driven too hard in this power amp drawing? I will run more music thru it later, and check the bias measurement, but after that I'm not sure what to do.

crate_bt220_07s689_pwr_amp_sch.pdf

[The Dude]

You'll have to upload the actual file. That link is to the file's location on your own computer. We can't get to it.

[g1]

The attachment is working now.

[g1]

Sometimes there is a thermal breaker. Mechanically it is bolted to the heatsink. But electronically it would be shown in the power supply drawing.

[nosaj]

How about this. Do you have a variac? Lower the voltage to about somewhere 90-100v and see how the amp plays. If the amp is ok at some locations an not others I tend to look at power first then other things.
nosaj

[Enzo]

Did he bring you his speaker and speaker cords? Cannot assume all problems are only in the amp chassis.

ANyway, you asked about mutes, well, lower left of schematic is full of them. Q13 and OC1 are both mutes, separately.

And on Q11, see L? it trails off the board to the preamp via connector on left, I assume L for limiter.

[Jazz P Bass]

Please take note of the convoluted return path of the speaker to circuit ground.
Through the Ext Spkr jack and then through the Headphone jack.
I have attached the full schematic as we don't know where the signal is failing.
Then again you don't know if it is the amp failing.
Hmmm.
Active bass.

[Randall]

I believe I got it. I ran 30 minutes of Black Sabbath and then 30 minutes of Bob Marley at nearly full volume, and with the bass turned full up, and running 95 volts on the variac as suggested. I closed my shop doors and went inside to do a crossword puzzle. At the end of the hour it started cutting out, just like the guy said. It would cut out ,but just for a few seconds, before it would come back. I found that probing around the power supply area, I could make it come on and off depending which way I pushed. That side of the board was very sensitive, but most of all was one of the 10 watt cement resistors R13 or R23. I pulled the board, and found no visible bad solder joints, so I just soldered the whole power supply. Threw it back together, took it off the variac, and cranked more reggae thru it for another two hours, and it did not cut out, nor did probing have any effect. I am going to call this one fixed. I hit the resistors with a bead of hot glue, buttoned everything back up, and I'm outta here!

[nosaj]

My reasoning behind using the variac. Is because some venues just suck power wise. How many musicians check the voltage at the outlet they use. Some are strung together using extension cords and with some extension cords you get voltage drops. But either way I'm glad it helped you get to where you needed to be .
nosaj

[J M Fahey]

IC1 LM35 which must be in contact to heatsink or very close to it is a 3 leg temperature sensor.

Voltage at pin 2 is: 0V+10mV per Degree Centigrade/Celsius

So at, say, 75C it should have +750mV DC present.

That voltage is amplified 8X so 6V DC by IC2c and fed through 10k into +IN pin of IC2d which is used open loop, as a comparator.
Reference voltage is applied to Ic2d -IN ,some 8.66V DC

If heat sink gets even hotter and amplified thermal voltage surpasses it, Ic2d comparator triggers and turns ON muting FET Q13, muting the amp.
Your customer is overheating the amp.

Does that head include a fan?
Suspect it does not not, add one.

Cut a 3" hole on cabint top or side, and mount a PC fan.
Get a 48V one , such as: https://www.mouser.com/ProductDetail...zpaw7WxQ%3D%3D

80x80mm , 48V DC, youŽll need a resistor to drop extra 7V from main 55V rail , do the Math.
That particular one claims 0.08A consumption so dropping 7V means an 87 ohm resistor, use closest available such as 82 ohm 2W

Repeat Math with the actual fan you get, this is just an example.

YouŽll need to think an air path, so fresh air enters cabinet/chassis and exits around heatsinks.

Show gut pictures for ideas.

[Randall]

Juan, The problem was, as far as I know today, corrected by re-soldering everything in the PS section, most notably the 10 watt 15v rail resistors, which seemed to be especially sensitive to touch after running at close to full power for an hour or so. I couldn't see the bad solder connection, but it stopped cutting out after I re-soldered everything.

At this point, I do not believe it was a heat based design failure. And I do respect your knowledge in this area, but there is no way I would try to re-engineer a sub- par amp such as this by cutting holes and adding fans. It just is what it is.