Laney bass amp blows output transistors!

[spy]

Hello,

I'm trying to repair a Laney RBH800 bass amp. I have attached the schematic for anyone who would like to help. The output stage is the second one at the pdf (DRW.NO. 9075-3).

A few weeks ago, the amp fried one pair of the output transistors plus the el.cap C3. I replaced only the fried pair plus C3 and C19 just in case. Also, I rebiased the amp at the correct mV at points A-B. After 2-3 weeks of normal use the amp fried the same pair again!
Is it because I didn't replace all of them? Something like mismatch?
The speakers that are connected to the amplifier are fine.
Is it something else?

Thanks in advance! Any help is appreciated!

[Jazz P Bass]

Which pair did it cook? I will have to assume the PNP side.
Why did you choose to replace C19?
You probably should have replaced the driver of the pair.

[spy]

The weird is that it fried a pair! One PNP and one NPN! Not a side!

I replaced C19 because it's at the other side of C3. Not costly and just in case because C3 was blown.

[Jazz P Bass]

Sounds like you may have a current hog issue.
If it was a Peavey or a Crown or something, the manufacturers typically resell graded transistors.
If you purchase the items from a big house (Mouser, Digikey) chances are you can get "same batch" transistors.
Replace all the outputs & the drivers.
Check, check, double check everything.
The blown zobel cap may indicate the amp was oscillating.
Don't know for sure.

[spy]

C3 is not the Zobel cap! It's at the power supply line. You're looking at the first schematic.

[Jazz P Bass]

Now I'm whicha.
Sorry about that Chief.
Still, I think it is a current hogging issue.
Looking at C3 & C19 I noticed that they are rated at 100 volts.
A higher rating might be a better choice, would'nt you think.
(in case of a rail to rail short, ehh)

[spy]

Which do you believe is the purpose of hogging? Is it normal?

[Jazz P Bass]

Parallel Transistor Current Hogging

Current hogging is bad.
Thermal runaway is the most probable end result.
The amplifier ballast resistors are supposed to help alleviate slight differences.
Here is a quote from Wikipedia:
[If multiple bipolar transistors are connected in parallel (which is typical in high current applications) one device will enter thermal runaway first, taking the current which originally was distributed across all the devices and exacerbating the problem. This effect is called current hogging. Eventually one of two things will happen, either the circuit will stabilize or the transistor in thermal runaway will be destroyed by the heat. Hence current hogging term is related to thermal runaway.]
Link:Thermal runaway - Wikipedia, the free encyclopedia

[spy]

Nice! Thanks for the info!

But what do I have to do to prevent this? I'm a little bit confused now.
It happened twice.

[Enzo]

What you have to do is repair the thing completely. What you have done so far has been enoigh to make it barely function until it croaks again. SO I'd have to suspect a driver or something is damaged. Or the other half of the transistors.

For the price of hte extra $8, I'd replace ALL the output transistors. Then check all of those 0.22 ohm 5 watt resistors, any open?

Your drivers TS5, TS18, at this point I would just replace them. and check each associated resistor, which is to say R22, and all the resistors around TS12.

[J M Fahey]

You *might* have one NPN and one PNP *open* or the corresponding emitter resistors or the corresponding tracks blown, or even cracked solder, anything that opens a current path.
Since the usual test is to check for shorts , an open transistor (or resistor) may go unnoticed.
So you replace the shorted ones, the amp works ... for a while, because they are stressed, only two are carrying the load.
And the ones who blow will be, precisely, the ones you just replaced.
As Enzo suggested, replace all 4 transistors , all 4 emitter (ballast) resistors and drivers too.
Also those Zobel parts, both the resistor and the capacitor.
As a last check, push the amp to medium to high power driving the actual (difficult) speaker load, not only an easy resistive one, and scope it for "fuzzies" or plain old oscillation.
The old "leave no stone unturned" approach.
And lots of good luck !!

[spy]

Thanks for the support, guys!

I'll replace almost everything. 4 output transistors, 2 drivers, emitter resistors, resistor and cap at Zobel circuit, el. caps at the output stage and at the bias point, film caps and D3 & D7 diodes.

Enzo, I checked the resistors around TS12 and all measure at their value. Also, TS12 measure ok. I'll replace R22 as it's a normal resistor and not smd, just in case.

Hope it won't burn the output transistors again, after a month! Wish me good luck!

[Jazz P Bass]

Glad you got it working.
Any chance you could get a good look at the speaker cord?
Just a thought.
Cheers.

[spy]

Jazz P Bass, I haven't got it working, yet! I'm collecting the parts.

I can't find the drivers! One is SM3177A and the other one SM3178A. These are house numbers and I have no idea which are the substitutes for these.
Any idea?

I asked Laney, hope they will answer.

[Jazz P Bass]

I have attached the SM3187/8 datasheet.
That is one strange puppy.
B/C 240V
C/E 160V
E/B 5 V
60W
8A