Laney bass amp blows output transistors!

[Stewen]

Laney bassamp

OK ! I īm going to order some transistors and other parts tomorrow and replace the damage parts and see what happen . Guess its very clever to use a variac . Think I can borrow one from our radiomuseum .... hope we can be in touch .

[spy]

I can't understand what's going on! Every part measure ok but the problem is still there!

[J M Fahey]

Well, parts measuring is the *long* path ... and it does not cover everything at all.
I would use a lamp limiter, turn the amplifier on, and start measuring voltages.
Only when I find an impossible one I start measuring around to see "why".
I much prefer a lamp limiter because I consider it an "automatic" Variac, which will feed only as much voltage to the amplifier as needed to pass the (limited) current and no more.
Besides letting me free to measure instead of turning that wheel on top
[rant mode OFF]
What you wish to read is
NPN transistors:
Collector more positive than emitter by more than a Volt.
*and*
Base more positive than Emitter by around 0.65V
Just that.
PNP transistors: the opposite.
TS10 and 13 are excluded, because they are protection transistors and they are OFF in a healthy, properly loaded amplifier, specially with signal OFF.
TS16 is somewhat excluded too, because it should only work when clipping.
By the way, its poorly placed there, its collector should go to TS17's base, not to its emitter. Oh well.
Post what rail voltages you get when lamp limited and any abnormal voltages which do not follow the rules above.
Good luck.

[Jazz P Bass]

I would think that you would have changed the drivers, as suggested.

[spy]

No I didn't replace them! They measure ok, check my post #19. But no problem to replace them with MJE15030/31.

I'll do the measurements as you suggested and hope I'll find something.

[Jazz P Bass]

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.

[Stewen]

Laney DB300H bassamp

Very interesting information . TS 13 is damage in my amp . Also TS 17 and one pair of the powertransistors , TS11 and TS 14 and C3 that exploded and finally FS 1 (5A) fuse.
Iīm waiting for the parts .

[olddawg]

Replace the bias pot with the repair. Funky bias pots cause a lot of call backs.

[spy]

I'll start with the drivers and I'll let you know. Hope it's them because the other transistors are smd types! And I don't like the idea to unsolder them with my iron gun.

[Stewen]

Laney DB300H bassamp

OK , Iīll replace that biaspot. I have removed the board from the amp so some cables are disconnected . Do You guys think I can test the repaired board even if these 8-9 cables are off ? I have two separate laboratory-powerunits tha have 0-30 VDC out and possibility to set the current to a special value for inrushcurrent-protection . Donīt you think I can connect these powerunits in series and let the connection be the middle-point (zerovoltage) on the Laneycard ? ( And increase voltage slowly on both units....) Another question : isnīt this amp selfstabilizing . And if so , what transistors are doing that ? I have a little difficulty to understand the signalway from the input to the output . ( Iīm a tubeampguy actually) . What does the diffamp do in the front ? Later it seems like a consertinasplitter , actually three transistors from top to bottom . Please explain a little how the signal passes through the amp.......
Regards Staffan

[J M Fahey]

Just a *very* short simplification, simply to explain SS functionality with Tube terminology:
The actual Phase Inverter is the Differential Pair: "cathodes" together, fed current through a large value resistor; TS3 "grid" receives input signal, TS4' receives feedback.
You get out of phase signals on their "plates" but only TS3's is used, fed into TS11's "grid", the other signal is discarded .
TS11 is a very high gain Class A amplifier, it receives signal on its "grid" , its "cathode" goes to a negative voltage, and its "plate" is loaded with a resistor. In this particular case the resistor is replaced by an "upside down" transistor TS7, but the function of supplying some current to be modulated by TS11 remains.
Since TS11 is incredibly efficient, it not only has a huge voltage gain impossible for tubes, also its "plate" voltage can swing from rail to rail (+50 to -50V), try that with tubes.
Since it has all the signal voltage possible, we need no gain after it, only current amplification , or impedance matching/lowering, so we need only a couple "cathode followers" between this signal and the actual speaker.
Those "cathode followers" are TS8 driving TS 15 and 16, and TS13 driving TS17 and 18.
Hope it helps.

[Stewen]

Laney DB300H bassamp

Thanks very much . Now I understand much better . What do You think about testing the board without a speaker and with several cables not connected . Can I damage something ? I mean , use my two DC powerunits to feed the card . Starting with very low voltage and current limited ..... ? Shall I have zero voltage between the powertransistors whatever + and - voltages I give the amp ? Read somewhere that a low current shall flow through these pairs of powertr. 30-40 mA ???? And do I use that biaspot. to set that current ? Thanks in advance ...

[J M Fahey]

Really you should test the amp as "normal" as possible.
This is not an experiment but bench repair.
You might very well "repair" it and have it explode when fitting those "missing wires" so save yourself time and bitterness and repair it with everything connected, except the speaker, using the actual power supply (current limited with the bulb, of course).
Do not touch the bias yet, you are after larger problems.

[Stewen]

Laney bassamp blows outputtransistors

Hallo ! Have some problem to find this thread and write replies....... are trying again !
Far down on my schematic Laney writes: Cx and Rx are added to reduce shorted current ..... Rx is 330 ohm and Cx 100nF . They are connected in parallell and soldered the collector of TS 15 /emitter of MPSA 42 AND the cathode of diode D9 / base of TS 18 . Iīm going to add those components too. But I have another question : TS 10 on my schematic is a BC 847B and TS 13 a BC 857B . Both rated 45V as max.
Wouldnīt it be wise to use the pair BC846B and BC856B which can stand +65VDC ???
These transistors have other data as the former transistors .....
Thanks for an answer. Stewen

[J M Fahey]

on my schematic

Post it.
I'm late on the payments for my crystal ball and they have remotely disabled it until I cancel my debts.