Mr. Fahey, with all due respect, I differ with your assessment of the posts. The only ones that furthered the endeavor were the suggestions for replacements for the original Darlingtons. I took that advice, and the amp works, but draws too much current. So, something needs to be changed. Neither you, nor anyone else, has offered any suggestions as to what might be done to the circuit in order to reign in the current draw of the new output transistors I installed.


Were you in my place, would you not be frustrated?

I did, however, find a source for the same types Marshall used originally, so I may just get them and try them. Since the amp came to me without the originals, it's kinda hard to know what the true original failure mode was. I reckon I'll keep melting lead till something good happens....

I came to this forum sincerely hoping to find the advice needed to fix this amp. I gotta say I'm pretty disappointed, with almost 500 reads and yet no one has taken the time to really help. Oh, well.

Thanks for your reply, J M Fahey.

The TIP102/107 (this IS what I put in there, btw--they are often used in Peavey amps, as I understand it. Got them from CEDist) barely get warm to the touch. I'll take a pic later today when I get home and post it. They each have a heat sink and a small square of aluminum--with grease between each layer. The sink/aluminum are not touching anything but air.

The original parts used TO-3 form, and were mounted to the outside rear chassis wall, so there was nothing compatible with the TO-220 case of the TIPs.

As for overbiasing, I reckon that is really the issue. However, since I'm not a SS guy, I need someone to help me figure out how to correct that. Which components in the support circuit need to change? I posted the schematic with my first post, so if anyone is willing to review it and make suggestions, that'd be great.

Thanks

What I mean is: If I disconnect all six leads to the two Darlingtons that power the speaker (perhaps I should have referred to them as the output transitors), the amp does not blow the fuse.

Since the rest of the amp's circuit, the preamp and driver section, normally draw very little current, I suppose this is a logical result of disconnecting the output transistors. It tells me, though, that the excessive current drain is directly related to the two output Darlington transitors.

Being a tube guy, I just don't know enough about these types of circuits to know if the difference in current draw between the TIP 102/107 pair and the original MJ2501/3001 is normal, or if there is a fault somewhere in the circuit that supports the output transistors. That circuit was made to run the MJ2501/3001 pair, so I'm wondering if I need to change any of the values of the other components in that section.

Here's some more that I've learned working on this amp, in case this helps some of you more knowledgeable chaps formulate a diagnosis for me.

With the power transistors disconnected, the amp runs fine, no blown fuses. The preamp passes signal just fine, as I can take it off of C18 and amplify it otherwise.

I wired in a new rectifier, on the off chance it had a current-related failure mode, but the amp, with power transistors on, still takes out the fuse. So the rectifier is not an issue.

Clearly, then, the current drawn by the TIP pair I installed is greater than the original Darlingtons. This leaves me with two main questions:

1. Can the stock Marshal PT tolerate the higer current? If so, then it's just a matter of finding the right fuse size. If not, then I'm left with no choice but to find the original parts and use them.

2. Do I need to change the values of any of the circuit supporting the Darlintons in order to allow the TIP pair to operate more compatibly with the host circuit? (i.e., resistors, zener, etc.)

Thanks

Actually, I misspoke earlier. I put TIP 102/107 in there. Comparing the data sheets for these to the one for the original transistors Marshall used, the new ones draw significantly more current. I suppose this would explain why the amp will work for a couple of minutes and blow the 500 mA fuse.

Thoughts?

Well, thanks to all your advice, I did get this beast to work! I used the TIP142/TIP147 as was suggested. I mounted them to small heat sinks, the sinks to some small pieces of aluminum (with thermal compound between them), and then to a bit of circuit board material. I mounted that assembly to the chassis back wall with standoffs. So once I got that going, I could inject signal after the blown C18 and get sound. I replaced C18 and the amp worked.

Next I replaced the two 5000 mF power supply caps with a pair of 4700 mF/65V caps, and the amp was much punchier.

The pilot light in the original switch was dead, so I wanted to replace it. However, it uses the older 5-tab Marshall switch, and all I could get is a new 4-tab. I wired it up and got the amp working, but now it blows fuses. (Of course, it only did this AFTER I but it back in the cab!!)

I don't know if my switch wiring is causing the fuses to blow, or if the stronger PS with 21st Century wall voltage means I just need bigger fuses.

I'll post pics later on. I'm too frustrated with it tonight.

These were always mounted on the back of the Marshall chassis. The customer hacked them off, threw them away, and put some random Radios Shack transistors on there that now dangle off the back like plucked out eyeballs.

Thanks for the recommendations.

Hey, y'all, first post here.
I build and repair tube amps, so SS stuff is kind of a mystery to me. In spite of that, I took in a Marshall 2199 Master Lead Combo (30 watt, 2x12) for repair. The customer has hacked off the two output transistors on the back of the chassis. The available schematics online are all rather blurry pdf scans, and I can't for the life of me make out the part numbers for the pnp and npn transistors. I suspect those part numbers would long since be obsolete anyway.

Does anyone know good substitutes for those parts? The operating voltages are +25 vdc and -25 vdc.

I need your collective wisdom, so any ideas would be great.

Thanks

Rich