Need help: Marshall 2199 repair

[BigBoy]

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

[DJPhil]

The big output transistors look like MJ2501 and MJ3001. It fits the profile, as these are TO-3 darlingtons with the appropriate power rating. These are probably not manufactured new anymore, but you might still be able to find them (though they're likely to be overpriced). They were second sourced by a whole bunch of manufacturers, here's the datasheet for the ST Micro version.

The datasheet should give you a great start on finding replacements if you're used to matching spec sheets. If you need more help I can find something, though the pros here could probably tell you off the top of their head what to use. Just let me know if you need more help.

Hope that helps.

[J M Fahey]

Good advice and yes, they are TO3 Darlingtons, MJ2501 and MJ3001; Marshall used them for everything from 12W to 100W.
Today they will be probably overpriced if available.
Since your friend already pulled them from the board and mounted them in the chassis, you might use TIP142/TIP147, of course with the appropriate micas and grease; you'll have to connect them with flying wires anyway.
One caveat: if they are bolted to a *steel* chassis, they will overheat for sure, iron is unsuitable.
If so, you should add an aluminum heatsink.
If the chassis is aluminum (which I doubt), add a piece , say, 4x6 inches or so where they are bolted, to increase thickness and heat dissipation. Put some grease between both sheets.

[BigBoy]

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.

[J M Fahey]

plucked out eyeballs.

PLEASE, please post a picture of that !!!!!!!! 

[jcgss77]

I second that. 


WOW.      :loco

[BigBoy]

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.

[BigBoy]

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?

[BigBoy]

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

[joecool85]

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

When you say that it runs fine "with the power transistors disconnected" do you mean from the heatsink or entirely out of the circuit?  If the latter, how is the amp getting power?

[BigBoy]

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.

[J M Fahey]

To begin with, you must not "upgrade parts to support the higher current" because the real problem is "why is it "eating" more current"?
Two possibilities:
1) The output transistors are overbiased.
2) They have poor dissipation, either because of non-original too small heatsinks, improper mounting, etc.
please post a couple pictures of where the original heatsinks were , what you mounted now.
Those TIP142/147 (*not* 102/107) are probably dead by now, but no big deal, they are cheap enough, order 2 pairs just in ase, as well as extra micas; do you have thermal grease?
To avoid frustrations search for "series bulb lamp limiter" and build one, with a 25W lamp.
Any small mistake will *not* kill the amp ballowing some time to correct it.
Well, that's it !!
You *cam* repair that fine amp and enjoy it for years.
I wonder about the speakers, can you post some picture too?
And whatever's written on them?

[BigBoy]

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

[phatt]

Did you check the pin out of the new output devices? :trouble

Like Valves,, just because it fits in the same holes does not mean they are identical.
Phil.

[Enzo]

All TO3 bipolar transistors have the same pinout.   

All bipolar TO220, TO218, and TO247 transistors have the same pinout - ECB across the front.