Roland KC-350 keeps blowing main fuse because of tweeter?

[Kekke466]

Hello everyone

I have yet another Roland KC amp that is blowing the main fuses.
I went through some troubleshooting already and I have determined that the 2SA1962 was shorted, the -VCC measured 3 Ohms, also a very low resistance on the tweeter coil.

I replaced the 2SA1962 today and blew the transistor immediatly. I highly suspect the tweeter to be bad but I would like confirmation.

When I measure the resistance of the tweeter my DMM reads: 1.0 ohm.
The impedance is normally 8 ohms. I know that resistance is not impedance but this seems very low to me and should read more. Could this be the root why 2SA1962 keeps dying and blowing the fuse?

Regards,

Kekke

[x-pro]

Could this be the root why 2SA1962 keeps dying and blowing the fuse?

Regards,

Kekke

It's quite possible.
To test the amplifier without fear of burning it, turn it on with no signal and no load.

[The Dude]

That tweeter should definitely read more than 1 ohm DCR. I suspect it has shorted voice coil turns. That said, the tweeter is capacitor coupled for crossover purposes. I don't think it would cause the amp to blow immediately, unless possibly the cap is also shorted. You may have multiple problems. Also, what do you mean by, "the -VCC measured 3 Ohms". Are you talking about a -VCC to ground resistance measurement? The statement is confusing because a VCC measurement should be voltage- not ohms.

[Kekke466]

That tweeter should definitely read more than 1 ohm DCR. I suspect it has shorted voice coil turns. That said, the tweeter is capacitor coupled for crossover purposes. I don't think it would cause the amp to blow immediately, unless possibly the cap is also shorted. You may have multiple problems. Also, what do you mean by, "the -VCC measured 3 Ohms". Are you talking about a -VCC to ground resistance measurement? The statement is confusing because a VCC measurement should be voltage- not ohms.

Thanks for the help! I highly suspect the tweeter to have shorted windings, it should be close to 8 ohms DCR (measured out of circuit). I meant resistance to ground from the -VCC line, it was the 2SA1962 that was the short. Removing the 2SA1962 from the -VCC line, got rid of the short. I can't find other components that could cause the T2A fuse (Europe here, US is T4A) to blow.

-Desoldered the 2SA1837 driver and that one was still good.
-Desoldered the +53V and -53V lines and tested the PSU pcb without going to the power amp pcb and no shorts occured from the PSU pcb.

Will report back when I get new transistors and test it without load first.

[The Dude]

Also, you may want to upload or link a schematic for your amp for better responses. I found this one (see below), but I don't see a 2SA1962 so it may not be correct.

https://music-electronics-forum.com/...etch?id=868881

Edit: After looking further, I do see the 2SA1962, sorry. It's not uncommon for an emitter resistor to go open when output transistors short, so check R34 and R46. Also check driver transistors.

[Kekke466]

Replaced the 2SA1962, tested without load, tweeter and speaker disconnected and the T2A (230V) blew again. The transistor on the positive rail failed this time, the 2SC5242 giving a short. It's probably best to replace these in pairs, instead of individual.

With the power transistors desoldered, R34 and R46 which are 0.33ohm, both read 0.5 ohm on my DMM. I assume these are not open.

Will desolder driver transistors again and re-check.

[Kekke466]

I took out the 2SA1962 + 2SC5242 and the transistor drivers Q16, Q8 (2SA1837), Q9, Q15 (2SA4793) and now I can power the amp board without blowing fuses. I narrowed it down to these 6 components.

[x-pro]

Replaced the 2SA1962, tested without load, tweeter and speaker disconnected and the T2A (230V) blew again. The transistor on the positive rail failed this time, the 2SC5242 giving a short. It's probably best to replace these in pairs, instead of individual.

With the power transistors desoldered, R34 and R46 which are 0.33ohm, both read 0.5 ohm on my DMM. I assume these are not open.

Will desolder driver transistors again and re-check.

With no load, the output transistors should not burn up if they are in good working order. They may just be out of mode due to defects before them.
R34 and R46 on a multimeter will give you 0.5 ohms. This is normal. Factor in the resistance of the probes with their wire.
For further repair, I advise you to put temporarily instead of 0,33 ohm R34, R46, rated from 10 ohm to 33 ohm.
This way you will be able to properly adjust the output level and check the amplifier for stable modes.
Once all is well, put 0.33 ohms back into the emitter circuits and set the quiescent current (bias). This is done with no load and no signal.
Next, apply a signal to the input, with no load, and see the response at the output of the amplifier.
Then you can connect a resistive equivalent to the output and check for signal again.

[DrGonz78]

Why aren’t we using a light bulb limiter? I always use one in situations where I am replacing shorted outputs. Fuses and transistors are expensive. Early on when I started repairing amps I used the light bulb limiter religiously. There were times where I thought I removed all shorts but the LBL lit up bright, protecting my newly installed parts. Saved me a lot of headaches and that is why I mention it.

edit: I also remember working on a keyboard amplifier with a bad tweeter. I inspected the amp with no load and it was fine. Plugged in the cabinet and smoke! If I had used the bulb limiter it might had protected the amp. It’s just a great tool.