Proper Replacement Transistors MPSW06, MPSW56

[joltman]

I have got a JBL PB 12 subwoofer that I purchased in 2001. In about 2019, it decided to stop working. Long story for another thread, I have tested the NPN/PNP (Q2 and Q3) transistors in the power supply with an Peak Atlas DCA55 and they both test fine, however, given the scorch marks on the board, I'm hesitant to believe that. I've purchased a Heathkit IT-3121 to try t-shooting the transistors, but I'd like to find suitable replacements for these transistors:

MPSW06 (NPN)
MPSW56 (PNP)

Both of these are showing that they're low noise with about hfe of 100. Here is the schematic for this section of the circuit:




I'm having difficulty finding a suitable replacement for these transistors. Specifically, the Pd of 1W and Ic of 500mA all while trying to stay in the CBE/EBC lead order. I'm looking for advice. Would this transistor be a decent replacement for the NPN?

ZTX455 (NPN)

I'm having very little luck finding any PNP for this circuit in stock anywhere (ZTX555). Maybe the ZTX553??? If there's a better option to replace these, I'm all ears. Unfortunately, I am kind of stuck with the CBE/EBC lead order because the through holes on the PCB are in a straight line, so crossing the leads wouldn't be an easy task. I suppose I could do heat shrink around the legs. I have also seen the onsemi MPSA06 and MPSA56, however they don't dissipate enough wattage in comparison to the current transistors. I also feel like the rail voltage of 15V/-15V is really low for the current transistors, so I might be overthinking this?

I am also going to be replacing the Zener diodes as well, as it's likely they failed as well. The R6 and R9 resistors were badly scorched and they read very bad. Any help would be appreciated. Thanks!

 

[joltman]

Maybe these?

2N5551
2N5400

Or:

MPSA56
MPSA06

I'm worried about the lower continuous current in these. The subwoofer is "supposed" to be 250W. Sure, it'll never reach that. I just don't want the replacements to blow up!. LoL

 

[llwhtt]

MPSW06/56 were replacements for MPSU06/56, maybe you can find those. They're even older than the MPSWxx but you never know what you'll find. Mouser has both the CEN-U07/57 in stock which are the 100V version of the 80V MPSU06/56. The MPSWxx are giant TO-92 whereas the MPSUxx are the larger TO-202.

Craig

 

[joltman]

@llwhtt thanks for responding! In looking at the specs for the CEN-U07/57 transistors, I'm seeing that they have different hfe. I'm new to the replacing transistors game, but I thought that I had to match hfe as best I could? The CEN transistors have an hfe of 20 at 1Vce, 500mA Ic. The MPSW transistors all have a min hfe of 100. Am I wrong to focus on those specs? I'd like to try and learn the best way to effectively replace these transistors. Thanks again!

 

[llwhtt]

According to my handy dandy Motorola data books the hFE spec for the MPSWxx is 60 @ 250ma, the max listed, and the MPUSUxx has a hFE of 50 @ 250ma. I would say that's close enough. I really doubt the current is very much as they are probably only supplying current for some opamps. The MPSUxx transistors will run cooler since they have a heatsink, the collector, and are physically larger. The CEN xsisters have the same spec as the Motorolas.

Craig

 

[joltman]

That is really strange. Fairchild shows the 06 having a min gain of 100, while the Motorola doc shows 60 (just as you said). But both docs show a max Pd of 1W. And the JBL PB 12 does have two IRF640 mosfets. I will try the 4 you recommended and we'll see what happens. Thank you for your help!!

 

[cpt_paranoia]

I have also seen the onsemi MPSA06 and MPSA56, however they don't dissipate enough wattage

That's a very common small signal pair; we had them as lab stock...

And the JBL PB 12 does have two IRF640 mosfets.

Those are likely to be the power stage, running off V+/V-. The circuit we are looking at here is a low current +-18V supply, as mentioned above, likely for running some op amps in the sub driver. Note that R6 and R9 will drop a lot of V+/V-, so the transistors in shunt regulator mode aren't dropping the lot... It may be those resistors that have been getting hot.

 

[llwhtt]

These are the regulator pass transistors for the low current supplies, nothing to do with the +/-70V high current power supplies for the power amplifier. The CEN pair will work fine if there's room for them.

 

[sregor]

You should be able to fit a TO-126 style transistor (may have to put some spaghetti on one of the leads and twist the leads)- BD139 and BD140 are easy to find and match the other parameters. BD139-16 and BD140-16 are hfe >100. but IMHO not a critical factor. (but higher is better)

 

[joltman]

As a followup, I purchased the CEN-U07/57 transistors. I used my Peak Atlas DCA 55 to measure the gain on all 20. I then tried to match the lowest pair. On the DBT, I was measuring 14.1V/14.4V. That is supposed to be 15V on normal line voltage. I checked around the board for my +/- 15V and I was all near 14.1V. I pushed a 150mVrms 50Hz sine through the LINE-IN and used my scope to probe for the signal throughout the circuit. At the speaker terminal + I was seeing it and the scope showed 50Hz. That got me excited and I put it all back together. I forgot to test it off the DBT. After it was together, I plugged it in and the lights on the front turned RED as they're supposed to. I then fed in the same signal, but this time at 75mVrms, and the lights turned GREEN. Next, I turned up the gain on the back plate. I got sound! I messed around with increasing the amplitude on the sig-gen and the freq too. It all worked great!

I've turned off and unplugged the sig-gen from the amp plate. It took about 20 minutes, but the sub finally went into standby! Next step is to put it back into my rotation in the living room. That might be easier said than done. This sub failed back in July 2019. I purchased a couple replacements for it! Maybe the wife will let me bring it upstairs.

Thanks again for all the help! I really appreciate it! Next up, re-capping a Heathkit IT-3121!