Laney bass amp blows output transistors!

[Steve Conner]

You can test a power amp using bench power supplies (or laboratory-powerunits as you put it ) I've done it myself several times. Since they have current limiting, they can only cause less damage than the amp's own power supply.

Some transistors in a power amp need a rating equal to the sum of the rails. Others only need a rating equal to one rail. The current limiting transistors (your TS10, TS13, I bet) need even less. I like experimenting with DIY hi-fi amps, and the only TO92 transistors I've ever needed are 2N3904, 2N3906, 2N5401, 2N5551, MPSA42 and MPSA92. The last two have lousy gain, so I use the lower voltage ones wherever possible.

(Except for the input pair, where it can be worthwhile searching for some really low-noise, high-gain types.)

[Stewen]

Laney blows outputtransistors

Thanks for your answers ! Well ,I think there are many transistors that can fit in a special position . I would like to be able to post my version of the Laney bassamp , DB 300 H , but I´m not enough skilled to know how to do that ...... Acouple of weeks ago I tried hours to find it on the web with no luck . Finally I got it from a guy at Laney in
England . I+ll ask a friend to help me with that soon . Regards Stewen

[Stewen]

Hallo again ! Have now connected two labpowerunits to my amp . They have currentlimitation . + pole at one of them is connected to the - pole at the other . So
I can get neg. voltage , zero/ground and pos. voltage . Increased the voltages simultanously to +/-10 VDC as a first step and so the currentlimit . No , well 10 mV at the speaker output . No speaker connected . I get 2,1 VDC over the biastransistor , MPSA 42 , TS 12 in my schematic . Total current through each branch 250mA . Tomorrow I´ll continue to increase the voltages slowly . How and when shall I set the offset adj. and the biasadjust.pots ? There is a testpoint called P1 (Pad), here the base of the second differental-stagetransistor , TS7. What voltage shall I read in that point ? 0VDC ? Thanks for some answers . Stewen

[Stewen]

Hallo ! Have now tested my amp mounted in the cassette . Started with a 25W lamp in series and a speaker connected . Hum , but I could cansel that by turning the biaspot very little clockwise. Then I changed the lamp to 40W/230VAC . Hum again pot got rid of that just by turning the biaspot a little bit clockwise. Then a 75W lamp . Hum again and had it gone by further potturning.... Now also the rely drew and I could connect a guitar and it sounded good !!!!!!! But after turning the amp OFF and later ON again , the amp stopped to work !!!! So far I have checked the new outputtransistors and they seem OK . But something must have happened . The lamp is shining full light so there some current flowing........ Guess I have to take out the board again and search for failed components . It seems to me that this type of amps are very easy to fail due to the directcoupling from input to output . Are there really enough protection - circuits withinthe amp ????
Thanks for some advice and comments . By the way , SPY , how does Your amp work now ?? Regards Stewen

[J M Fahey]

Hum , but I could cansel that by turning the biaspot very little clockwise.

The bias pot is not a "hum cancel" pot.
I think your amp was so much overbiased that it overloaded the PSU causing a lot of ripple which got into the audio signal.
When you lowered bias *only enough* to cancel hum, you were still passing too much current.
You should have measured the voltage across the ballast resistors.

Then I changed the lamp to 40W/230VAC . Hum again pot got rid of that just by turning the biaspot a little bit clockwise. Then a 75W lamp . Hum again and had it gone by further potturning...

.
You just repeated what you did before, still too much current.

Now also the rely drew and I could connect a guitar and it sounded good !!!!!!! But after turning the amp OFF and later ON again , the amp stopped to work !!!!

It started to work, but under stressing conditions.

So far I have checked the new outputtransistors and they seem OK . But something must have happened . The lamp is shining full light so there some current flowing........

Mmmmmmmhhhhhhhh , a full power glowing series lamp usually means shorted power transistors.

It seems to me that this type of amps are very easy to fail due to the directcoupling from input to output .

Yes, that complicates servicing, compared to tubes.
Sorry.

[Stewen]

Laney bassamp blows outputtransistors

Thanks for your answer ! Well , Now I´ve tested all the transistors . As when I got the amp for repair TS 13 is bad again . But now also TS 10 . Those are the protectiontransistors that open if the powertransistors´ 0,22 ohm resistors get to much current . Wonder if I shall change those tiny , surfacemounted to TO92 with same data but 65V instead 0f 45V (CBO) , and 0,5W instead of 0,25W ? BC 546B and its conterpart ? Stwewen

[spy]

The drivers were stressed to say the least when the outputs shorted.
A static check of them is a good way to verify that they are or are not shorted.
A dynamic test will tell if they are functioning as designed. (requires a special test setup)
When I have amps that the outputs failed, I replace the drivers.
Remember that the drivers have to provide current to the base of the ouputs.

Finally I got the drivers and I'm replacing them! Also, I have a new jig in my lab! A series light bulb for my testings! Let's see!

Thank god, MJE15030/1 have the same pinout as the SM3178/7.
MJE15030/31

[spy]

Bad news as the amp doesn't work! Now I have the negative supply at the the output! Output transistors fried again?
The weird is that all measure fine (static measurement) and the bulb in series with the primary doesn't glow at all, like the amp is fine. Also, with the bulb in series, I measure 0.6V B-E both at drivers and output transistors. Any help with this case?

[Jazz P Bass]

O/k, you have 0.6 volt drop B/E.
But what voltage do you have at the bases of the drivers & the output transistors?

[spy]

I have,

drivers
MJE15031
C: 48.8V
E: -45V
B: -44.5V

MJE15030
C: -48.1V
E: -45.1V
B: -45.7V

power
MJ15003
C: 48.9V
E: -45.7V
B: -45V

MJ15004
C: -48.8V
E: -45.1V
B: -45.7V

Seems like, I'm in trouble!

[spy]

Any help, please?

[Steve Conner]

The output and driver transistors are not fried. It looks as if the negative side is turned on hard and the positive side turned off, due to a fault in the NFB loop or small-signal stages. Fix those and get back to us.

[spy]

Ok, I was afraid of that! I'm not familiar with smd transistors. I have a normal iron and I don't know how to take them off. But I'll try and hope everything will go fine.

The good part is that drivers and outputs are not fried!

The amp has two types of smd transistors, S0642 and S0692. I'm not sure if I'll find them locally. Are there any equivalents of these? For example BC-type with the same pinout?

[Steve Conner]

The problem with SMT transistors is that the marking on them is not the same as the part number. As a wild guess, I'd say that they are some cheap second-sourced SMT equivalents of the MPSA42 and MPSA92, which are high-voltage transistors commonly used in audio power amps.

If this is the case, then you may be able to replace them with the MMBTA42/92 (SOT-23 package) or PZTA42/92 (larger package with heatsink tab) which should be easily available. Or use TO-92 devices (MPSA42, MPSA92, 2N5551, 2N5401 etc) and a little creativity.

But you should remove some from the board (carefully heat and pry up one pin at a time) and check the pinout yourself. Checking the hfe wouldn't hurt either: the MPSA42 type transistors tend to have low gain, so that would be another piece of evidence.

Maybe the voltage amp transistor (the BD139 at the bottom of this schematic) has failed short.

[J M Fahey]

So far, so good.
Your drivers and outputs look fine.
The upper pair is turned off and the lower one is fully turned on, that's why you have the static/no signal output voltage slamming the negative rail.
It means the drivers bases are doing that (the outputs just follow them)
*Why?*
Following the schematic on page 5:
The Darlington pair TS15/TS17 is either saturated or shorted, its collector is *very* close to negative rail.
Measure itheir Vce and Vbe relative to the negative rail.
Also TS16 voltages, still referred to negative rail.
The next stage towards the input, is the differential pair, TS3 and TS7.
Measure their emitter and base voltages relative to ground.
Do not unsolder any SMT transistor for checking or replacing ... yet.
It's a mess and the proper service procedure is checking voltages until something abnormal or "impossible" surfaces, and only then start surgery.
Good luck, you are really close to the solution.
I agree those "42" and "92" ended names are SMT versions of MPSA42/92, the replacement parts suggested should work fine, but don't shotgun until your sights are on a valuable target.
You should start investing in SMT resoldering stuff, or some of that magic solder .