Reviving 1978 Lab series L3

[Vinny]

Hi, all,

Good to see some discussion of all types of equipment repair. I have been a fan of simple and loud ss amps since I was left a yamaha 410 as a teenager, some serious Pete Townsend action with that amp.

Trying to revive another which has gone silent, lab series L3, LED comes on but zero sound, speaker is working. so have decided on a near complete rebuild. There are a few puzzles though, cannot identify a replacement for the two TO3 power/output transistors, they are marked as RCA 1A 7836. The real problem is I am not an electronics expert although I have rebuilt a simple tube amp and rebuilt effects pedals, extensive testing is something I try to avoid, I would much rather replace components so that the system is reliable and stable, having got my tweaking fever over with on previous projects.

If anyone has any input( no pun intended) on the power resistors on this amp which are on the preamp, two TIP29 and power amp, TIP29B x 2, 30, 30B plus the two unidentified RCA TO3's please comment.

Thanks and glad to have found the site!

[olddawg]

Hi, all,

Good to see some discussion of all types of equipment repair. I have been a fan of simple and loud ss amps since I was left a yamaha 410 as a teenager, some serious Pete Townsend action with that amp.

Trying to revive another which has gone silent, lab series L3, LED comes on but zero sound, speaker is working. so have decided on a near complete rebuild. There are a few puzzles though, cannot identify a replacement for the two TO3 power/output transistors, they are marked as RCA 1A 7836. The real problem is I am not an electronics expert although I have rebuilt a simple tube amp and rebuilt effects pedals, extensive testing is something I try to avoid, I would much rather replace components so that the system is reliable and stable, having got my tweaking fever over with on previous projects.

If anyone has any input( no pun intended) on the power resistors on this amp which are on the preamp, two TIP29 and power amp, TIP29B x 2, 30, 30B plus the two unidentified RCA TO3's please comment.

Thanks and glad to have found the site!

Why are you assuming that there is a problem with those transistors? If the amp powers up and you aren't blowing fuses and have no DC offset on the speaker, the finals are probably good. Do you have a schematic? Does it have a protection circuit like a relay? I would be checking pwer supply voltages. Does the amp have pre-out, main-in jacks or an FX loop. If so, run the pre into another amp and drive the power amp section with an ipod. Divide and conquer. That will tell you if your problem is in the preamp or output stage. Do you have access to an o'scope? Don't make assumptions. Start with the basics. If you must you can test the junctions of any bipolar transistor with multimeter. Preferably one with a diode test scale. Most do nowadays.

[Vinny]

yeah you're right it could be either board , their are two input jacks on the front panel , a reverb footswitch jack and something labelled ext proc, which I have never really been able to figure out although i tried to see if it was an extention cab jack, it didn't work low distorted signal. so my primitive approach is to look at most likely culprits and replace until it works. I so far have a homebuilt continuity tester and their is a transistor near the input jack PF5102 which seems a bit weak on one side. When you say power the board with an ipod, you mean get an1/4 inch adapter and input?

[Vinny]

Still really want to know the replacements for the power transistors, especially the RCA TO3's or a best guess from anyone, thanks for your advice OldDawg i'll try to advance my testing capabilities

[J M Fahey]

Start by googling as much as possible, trying to find the schematic (I guess Ampix might have something) and post it here or at least link to it, so we all speak about the same.
I think that "ext-proc" jack is some kind of effects loop, but I'll wait for the schematic.
*Do not throw parts at it at random* , worst repair technique and causes more new problems.
To start with something, measure voltage at the hot speaker out, + and - rails, check that you have +/- 15V for the preamp, basic stuff.
Insert a bare stereo plug in that processor jack and touch either of its hot legs, do you get some hum in the speaker?
Good luck.

[teemuk]

Schematic, as a more monitor-and-user-friendly edition of those found from Ampix.

[Jazz P Bass]

Here are the pdf files.
jpg sucks.

[Jazz P Bass]

Still really want to know the replacements for the power transistors, especially the RCA TO3's or a best guess from anyone, thanks for your advice OldDawg i'll try to advance my testing capabilities

A pair of 2n3055's should work just fine.

[52 Bill]

Interesting design. CMOS in the preamp for "tube sound" I suppose. And Jazz P's suggestion of the 2N3055 is correct, but just about any modern NPN power transistor should work.

I agree with all earlier posts, do not just replace stuff until it works. Figure it out and fix the problem. My first test would be J4 which is a single jack FX loop. If the normally closed contact is not making contact, it will kill the signal from the preamp to the power amp.

[jrfrond]

A pair of 2n3055's should work just fine.

I agree. It's a simple quasi-complementary output with a bipolar 36.5V power supply. Not a very demanding application. 2N3055, 2N3773, MJ15003, 15, 22 or 24, MJ21194, 2SD424..... you can use almost anything in there as a sub, as long as you meet or exceed the specs of the original, and all of these I listed definitely do.

The originals must have a number rubbed-off. They were most likely RCA 1A03, which were common back then. The 4-digit number is the production date code (36th week of '78).

Back in the late 70's, we used to experiment with CMOS hex inverters, biased in a linear fashion for that "tube sound", mostly in stomp boxes. Some commercial versions, like the Lab Series and Sunn Beta series actually sounded pretty good. I believe there were some stomp boxes that used them as well.

[olddawg]

yeah you're right it could be either board , their are two input jacks on the front panel , a reverb footswitch jack and something labelled ext proc, which I have never really been able to figure out although i tried to see if it was an extention cab jack, it didn't work low distorted signal. so my primitive approach is to look at most likely culprits and replace until it works. I so far have a homebuilt continuity tester and their is a transistor near the input jack PF5102 which seems a bit weak on one side. When you say power the board with an ipod, you mean get an1/4 inch adapter and input?

"ext proc" would be External Processing" I believe. More than likely it is a stereo 1/4" jack with a send, return line and a common ground. You should be able to test your preamp out signal and inject a signal into the power amp there. I said use an ipod because it is a constant signal source, it is variable, and everyone has one. You can use any signal source you want and interface it however is necessary. I would use a signal generator and a dual trace scope.

[teemuk]

Yes. The "ext proc" is a basic Tip Send - Ring-Return insert.

[Vinny]

Hey Thanks everyone! I did find the ampix schematic and thanks for the pdf version very useful, but really not being an expert in how circuits work, although I know how to avoid being electrocuted and I am persistent and will carefully work my way through your suggestions and let you all know how it works out.

Again thanks a million

Vinny

[Vinny]

Finally had some time to start investigating a bit more, starting to think the transformer is blown. Tried to get a voltage reading off of anything in the power board and nothing, although when checking the power transistors there is scratchy contact noise coming out of the speaker so some voltage must be getting through? I thought maybe my cheap multimeter was not working so i tested on my little fender solid state and the voltage meter works.

[J M Fahey]

starting to think the transformer is blown.

Didn't you say the Led lights?
And that you hear "scratching noises" when your multimeter probes touch he transistor legs?
I'm puzzled.