Marshall 8100 output transistor

[BStringThumper]

Hi,
I am currently working on a 1994 Marshall 8100. The output transistors are shorted. I have a question regarding the suggested replacements TIP142 & TIP147. The original transistor T65-NPN is rated at 12 amp 120V. The TIP147-NPN is rated at 10 amp 100V. I am concerned about the lower ratings. Has anyone used the TIP transistors? Any suggestions or thoughts would be greatly appreciated. Thanks.
Brian

[Enzo]

I have made this substitution many times. MArshall themselves even has made this substitution in production.

Your transistors will not see 100 volts, they will only see twice whatever the main rail voltage is, so if that is 40v they will see 80v max.

Your 100 watt peaks would be asking maybe 5 amps from the transistors.

[BStringThumper]

Marshall 8100 output transistors

Enzo, thank you very much for the information. I've noticed many posts stating shorted output transistors, was this a defect in this batch of transistors or is the transistor not the best match for the circuit requirements? I will see if I can find the TIP transistors locally. Thanks again.
Brian

[olddawg]

Two things usually cause output transistors to fail in otherwise reliable amps: 1) a low resistance or shorted load or 2) catastrophic component failure. Usually its a bad load condition, bad speaker cable, bad speaker load, shorted speaker, etc.

[Enzo]

You notice many posts about shorted output transistors because no one goes on the internet and finds a forum to ask why his amp is working properly like it was yesterday. This place is like the service counter at Best Buy - only people with problems come here.

Blown output transistors is one of the most common problems in ANY solid state amp.

I have had to write this several times already today, but I'll say again, don't confuse these two things: saying that WHEN an amp has a problem THIS is likely to be the cause, is NOT the same thing as saying THIS happens to these amps a lot.

Let's say you saw 50 posts about bad transistors. What about the tens of thouseands of them that did not blow their transistors?

Why do parts fail? First off, these parts are connected to the outside world. People plug too much load into their amps, people use the wrong kind of cords, people have faulty cords. People sometimes pull cords out while the amp is running, making the amp suffer a momentary short across the output. And cords can collect static charges. On a winter day when you grab the door knob and get a spark? That was a few THOUSAND volts. Get that charge on your cord and plug it in and you just discharged a few thousand volts into your output transistors. They may or may not suffer damage from it.

And a part may suffer some trauma and be weakened, and yet not fail for another month or two. We often never know why an amp picked right now to fail.

It is generally a mistake to start out thinking any problem is systematic. You amp didn;t fail because someone didn't design it very well. Nor was it likely a "bad batch" of transistors. If that were the case, we'd have 8100 threads all in a section of their own because of the hundreds of them we'd see each week. SHit happens, stuff breaks.

[BStringThumper]

Hi Enzo & Olddawg,
Thank you for your reply. I'm learning something new everyday thanks to you guys!
In your experience with these amps, when the outputs transistors fail is there any other components that usually, maybe, sometimes fail with them? I read a post where a guy replaced just the transistors and it worked. That would be great. Unfortunately I found a semi melted bridge rectifier, it still worked, but it's ugly. Upon visual inspection everything else looks good. I've been testing the resistors in that circuit, most are OK, some are low, I assume from being in circuit, I will pull one leg and test. There are 7 other smaller transistors, I will pull and test them. Thank you for the help.

[Enzo]

When outputs blow up, any resistor connected to it is suspect. Any semiconductor connectrd to it is suspect. But other than the outputs, this entire power amp is 7 small transistors, four diodes, and an op amp. I'd just check them all, won;t take long. And any bad semiconductor you find, check any resistor connected to it.

That leaves the caps. darn unlikely a cap is involved.

[BStringThumper]

Enzo & Olddawg, thanks for the help. Here is the update. I replaced the transistors, output transistors, and the op-amp. The amp is working perfectly in clean and OD2 mode, but OD1 has a much lower volume output at same volume setting as the others. The clean channel might be slightly louder. I suspect another op-amp is out. In your experience could you give me your best guess on which one it would be? The reason I ask is I have one TL072 op-amp in my hand. I don't have the M5201 or TL071. I will order them up on my next order. I did try a new pre-amp tube, no difference. Enzo, I did use the TIP 142 & 147, Thanks.
Thanks for any suggestion.

[g1]

There is not much different between the od1 and od2 circuits.
Here is a link to the schem.:
http://www.webphix.com/schematic%20h..._8100_8412.pdf
The od switch is over to the left of the preamp tube and is marked sw3.
OD1 and 2 use all the same IC's so if you are sure od2 is working proper then it could be the switch itself or a bad solder connection.
If Od2 is questionable, then IC3 could be bad.

[BStringThumper]

Hi G-one, thanks for the reply and input. It looks like I'll order up the remaining op-amps. I was thinking IC3, too, it's the first one after the switch, but it seems to be working on OD2. Then again I never heard this amp before I got it. I'm in the process of building a dual op-amp tester. But I feel if you go through the trouble of pulling the component out why not install a new one, hopefully a working new one. If I don't kill it during unsolder I could test it and see if it even worked. Do you know of a way to test this in circuit? Thank you very much.

[BStringThumper]

G-one, thanks for posting the schematic. I haven't been able to do that.

[km6xz]

Did you measure the gain of OD1 and OD2? OD1 is, according to the values in the feedback loop for IC3, supposed to be a lot lower gain. All you know now is that the preamp is doing what the component values would be expected to produce.

Replacing ICs without proving to yourself that it is bad is a risky way of adding additional faults that are harder to find than a correctly wired circuit that just has failed.

[BStringThumper]

Hi km6xz, thank you for your reply. I'm pretty new to this. Is there a way to test a dual op-amp in circuit? I do agree with you, the first IC I replaced I did more damage than repair. I can do them cleanly now. Can you please tell me what test points to check? Thanks for your help.

[J M Fahey]

Check that legs 4 and 8 receive -15 and +15V respectively and that you do not have DC (not more than a few millivolts anyway) on pins 1 and 7.
That's basically it.
Op Amps do not "get weak",they either work or die.
They *may* become noisy (extreme hiss or popcorn noise) but that's something else.

[BStringThumper]

Hi J M Fahey, Thank you for your post. I've learned a lot since this thread, mostly due to the extremely knowlegable and helpful people on this site like yourself.
CONCLUSION: After replacing the output transistors, transistors, and op-amp, the amp worked. It had lower volume on one channel. Well, I had the opportunity to hear another 8100, that's they way these amps are. So.........it's pretty hard to fix something when it isn't broken. Live and learn. Thanks again to everyone for their continued support!