Gibson G-80

[GibsonPlayer]

I have aquired a Gibson G-80 chassis and a schematic. The amp hums and shuts off and hums and shuts off. I examined the schematic and there was a difference in the Chip in the input channel. the schematic shows a Motorola MC1303L and the actual chassis has a Motorola SC17063L. I am having a hard time finding any parts as this was built in 1971. I would like to replace the IC's and output transistors. does anyone have a good source to obselete parts? some has probably tried to exchange the chip at some point. The PCB is clean and all the parts look good (not burnt) If someone has one would post some layouts or other pics. I would appreciate it! This amp was made in three flavors in the output section mine is the TIP version (TIP 29C,TIP 30C,and TIP 33B. I would like to build this into a Head version it was a 4x10 combo amp. All help is appreciated, thanks !

[J M Fahey]

Hi. Please scan and post the schematic . Don´t change parts at random (specially IC´s) hoping to find the bad one. Very probably the output transistors are shorted (that "hum" is going to kill your speaker). Bye.

[crystaltech56]

Have you Google'd "Gibson G-80 amplifier schematic", (in quotes)? It only comes out to one source, and that seems to cover all factory mods,upgrades and generic substitutions. It's $15.00, plus probably a few for shipping,handling so $20 tops. When you're doing work like this, it seems the wisest choice. Plus, there are many sites that have NOS semiconductors, once you've bought the above manual, just plug them in to google(the p/n) and see what comes up. Even if you are a starving student/artist the $20 would be worth waiting for/spending on what seems to be the factory service manual. CrystalTech56

[GibsonPlayer]

Hi gang here is a copy of the schematic for the version I am working on. It is good advice to replace any parts with the original ones. I think the Motorola MC1303L fix will and replace the transistors will fix it. I just need a source of parts. I replaced a chip in a peavey PA head yesterday and it fixed it. I have a little experience soldering and reading schematics , but I always apreciate the help and experience others have. A source for parts would be a huge help

[J M Fahey]

It´s confusing, but googling MC1303L brings data of a "single channel power amplifier" whichi it very obviously is not. In fact I vaguely remember (from the Stone Ages, at least 25 years ago) something about it, and it as an early op amp type, similar to a dual 709. Perhaps it is so old and so discontinued that they even recycled the name!!. And the SC17063L brings similar confusing resulta. Other possibility is that it was a custom special part number, unrelated to any commercial product. Anyway, first of all, test it; if it works, don´t touch it. The worst problem is that on the power stage. Good amp, classic design, absolutely no short protection and very dead. With the amp off , measure (on the "diode" or "continuity" or "buzzer" resistance scale) the C-E resistance in the Tip33 s and Tip 29, with the red pin on the collector and the black one on the emitter; and on the Tip 30 with the colors reversed. Any short there means a dead transistor, replace them. Be careful with the micas, insulation, etc. If in doubt, pull them carefully from the board and re-measure them: infinite resistance (open) C-E both ways, around 600 mV B-E and B-C one way and infinite resistance the other way. After you´ve replaced them, hook the amp with a 25/40W lamp in series and check voltages  (no speaker connected). You´ll have somewhat less than +-35V on the supplies, and close to 0 Volt on the "hot" speaker out.
Check some pictures that may help:   http://www.geocities.com/nbl968/GibsonG80/


Post your results here. Good luck.