Lab Series L3 model 316A

[punkykatt]

Hello all.  By any chance does anyone on the forum have access to a schematic for this amp, or the amp its self to see what the value of R15(color bands burnt off) is on the power transistor PCB. I have searched the net for hours with no luck. Any help much appreciated. Thank you in advance. Punky

[PRR]

If R15 is toast, so is Q8.

And they don't make transistors like that any more.

[shooter]

might be able to cheat with a Darlington pair configuration. 

[PRR]

> might be able to cheat

I mean modern devices, even if TO-3, are not going to live long at that power in stage-amp duty. That was the province of the old hometaxial, "baked potato", transistors. Even so, I replaced a lot of output devices in those days.

Note no protection at all.

LM3886 *might* work, at 8 Ohms, except the LM3886 supply chain is tight this year.

[thetragichero]

here's the service manual: https://www.dropbox.com/s/uibqcescst7jqnh/Lab%20Series%20L3%20Service%20Manual.pdf?dl=0

i might have notes on printed out schematics in the shop that give substitutes, although the l3 i had didn't take much to get it up and running (if i remember correctly it was just a bad output jack.... and the fact that the only thing holding one of the corners together was the tolex). sweet little amp!

[punkykatt]

Thanks Guys for the great info. This amp was (is) my wife's nephew's amp, been sitting in an attic for the past 20+ years. When I fired it up using the light bulb limiter showed no shorts. Firing up on straight wall volts, amp had very low distorted volume. I pulled the chassis and found R15 burnt and R16 swinging by one lead, removed the pcb and R16 fell off the board, the collector on Q8 had a broken solder joint also not making contact with the circuit. I did a diode test on Q8 and did not get the same readings as Q6. All the other components tested ok. According to you Gentlemen it seems to be difficult to find a replacement for Q8. The number on Q8 and Q6 is [RCA 7907]. This amp may be begging to be a tube amp 

[thetragichero]

i would try to repair it before gutting. these lab series amps are some of the best sounding products gibson ever made (helps that moog was also owned by norlin at that time... lots of cool engineering in these and the active rd guitars/basses from that era)

[66Strat]

DigiKey has suitable replacement transistors in stock. TIP29BG ($1.14 each) subs for TIP29B. TIP30G ($4.20 for 5) is the transistor specified in the schematic. MJ15001G ($6.82 each) should be a good replacement for the output transistor.

[punkykatt]

The schematic gives what looks like a part# 992-042759-001(NPN Power T03. On the actual transistor it says RCA 7907  also  759   1A

[thetragichero]

need a hometaxial 3055 and don't have a stock pulled from old solid state organs? there's this https://www.electronicsurplus.com/rca-2n3055hom-transistor-npn-silicon-p-n-2n3055

[shooter]

I pitched about a 100 3055 n the pnp sisters last year
nothing like finding the bad one in a parallel string of 20!! steel fuses makes the job quicker 

[sluckey]

Ed Chambley has a bunch of 3055s. Send him a PM. Tell him I said to give you some.

[punkykatt]

Thank you, thank you, thank, you sooooooooooo  much

[punkykatt]

One more question: With these types of transistors are the mounting dimensions the same on all? In viewing the datasheet for the 2n3055h the base and emitter pins appear to be spaced a bit different  compared to the mounting holes on the one I have? They don`t list any dimensions on the datasheet. Thanks

[DummyLoad]

MJ15003 will also work. You'll want the TO-204 package. No one sells TO-3 any longer (essentially the same thing).

I have a stash of NOS ST brand 2N3055 as well - If Ed doesn't come through, I can send you 8 on your nickle. Doubtful that I'll ever use them, so you're welcome to them. Checking to see if have the TIP30B and TIP29B, you'd be welcome to some of those as well. The TIP29 is a 40V part, the TIP29A is a 60V part, the TIP28B is a 80V part, the TIP29C is 100V part . You can use the TIP29B (called out for) or the TIP29C. Same numbering structure with TIP30 series, so you'd want the TIP30B or TIP30C.

--Pete

[punkykatt]

Thank you Pete, this is going to be a winter project. Right now the wife and I are at out summer cottage in Ct. This week we are packing things up to head to our summer home in Fl. I will be bringing the amp with me. It is good to know I can still get these. It will probably be a month or so before I can dig into this again. I have a lot going on waiting for me down there. Thank you again Punky