Recapping Gibson G-10 questions

[MickeyB]

This is a 1971 transistor style amp with a cap/can in it that reads 500 mfd 3 times. 4 wires coming out of it. The schematic shows it as 2000 mfd. Is 2000uf correct? For cost purposes, can I run 2 1000uf in paralel? What is an acceptable tolerance? 20%?

[Enzo]

1500, 2000, doesn't much matter. Gibson is notorious for changing amps through the production life. SOme changes are minor, like subbing 1500 and 2000uf caps back and forth. Others are major in that the innards of an amp can become a complete other model.

In the old days, the tolerance on can filter caps was -20/+80% Yes, that is not a typo.

it is just a filter cap, the tolerance is not critical at all.

Can you post your schematic? The schematics I have don't agree with your description - typical for Gibson.

[MickeyB]

Hope you can read it. My scanner is not the greatest and the schem/parts list is faded from being inside the back panel for 3 decades.

[Enzo]

I can read it. Looks like most circuits to me. 2200uf would be more likely to find that 2000 in this day and age of common part values, but it still is not all that critical, and yes you can parallel two caps to make one larger equivalent

[MickeyB]

Treble gets fuzzy distortion above 2

Ok, replaced all the lytic caps. Everything sounds good except when I turn up the treble past 2 it gets a fuzzy distortion and hum. 2 or below its clean and quiet. Any ideas?

[Toddy]

Gibson G-10 conclusions

Mickey,
I am also working on a 1972 Gibson G-10 right now. I also change two electrolytics (C7, C13) because they were shot. I was hoping that when I turned the amp back on it would sound better. Nope! The amp still sounds like someone stuffed a pillow into the speaker. The bass pot is not working either.

I came to the conclusion that these amps just do not, and cannot sound good due to the mismatching of the parts according to the schematic and parts list. None of the transistors even match what the schematic says and some of the resistor and caps values are totally different. Without doing a total rebuild, from scratch to accurately match each individual part...I am just going to work on something else!! Oh and yes, everything in the amp is original except for the two caps that I changed!! And, you can't even give these amps away. Last I saw one sold for less that $50 on that eAuction site.

As they say in Monty Python's Holy Grail..."Run away!!" But if you are going to continue to work on your's let me know how it turns out.
Toddy

[Brazen]

Hey Guys! New to the forum but you all have encouraged me to dig out my ole G10 from high school and try to repair. Works fine but it has a terrible hum when turned on that does not change with volume, so I'm suspecting the caps? After reading a lot here and on other amp forums I'd also like to 3-wire the mains cord. I'm a lil confused if this amp has the "death caps" as it's power goes from the fuse thru the PCB back to the switch... which is a potentiometer, not toggle. Any help would be appreciated.
Cheers, Jim

[MickeyB]

Hey Guys! New to the forum but you all have encouraged me to dig out my ole G10 from high school and try to repair. Works fine but it has a terrible hum when turned on that does not change with volume, so I'm suspecting the caps? After reading a lot here and on other amp forums I'd also like to 3-wire the mains cord. I'm a lil confused if this amp has the "death caps" as it's power goes from the fuse thru the PCB back to the switch... which is a potentiometer, not toggle. Any help would be appreciated.
Cheers, Jim

This is an old thread. You need to start a new one to get replies from the pro's.
I gave up and gutted mine and made a nice tube amp out of it..

Mickey

[52 Bill]

Welcome to the place. Starting a new thread would be a good idea so we don't confuse the two amps.

Works fine but it has a terrible hum when turned on that does not change with volume, so I'm suspecting the caps?

Yes, bad filter caps can cause this, but so can a number of other things.

I'm a lil confused if this amp has the "death caps" as it's power goes from the fuse thru the PCB back to the switch... which is a potentiometer, not toggle.

The schematic shows one and from past experience, unless it has been removed, there is one on this amp.

An important question is what sort of skill set do you have when it comes to electronics? What sort of tools and test equipment is available to you?

[Brazen]

Welcome to the place. Starting a new thread would be a good idea so we don't confuse the two amps.

Yes, bad filter caps can cause this, but so can a number of other things.

The schematic shows one and from past experience, unless it has been removed, there is one on this amp.

An important question is what sort of skill set do you have when it comes to electronics? What sort of tools and test equipment is available to you?

Thanks for the welcome Bill! Ok.... I did replace the main power filter cap and all is well, so I've decided to not replace the rest since it aint broke! Quiet as can be now... no more hum!

I've been a tinkerer since I was a kid.... and I'm a general contractor with pretty good knowledge of electrical work. Good soldering ability and multimeter use as well. excellent shadetree mechanic as well and general do-it-yourselfer.

This amp is confusing, since the one line lead goes thru the fuse to the PCB which I'm sure goes to the combo on/off/volume pot.... then back from the PCB to a soldering lug that ignites the power light then back to the transformer. The other line lead goes direct to the other lead of the transformer.... simple enough.

My concern is, since the one lead doesn't go to a simple SPST switch, but through the PCB, will it matter which transformer lead I choose to make hot or neutral...... AND, if it goes through Caps on the PCB before returning to the transformer. I see the caps to ground in the schematic, so I'm not sure how in hell I can bypass them. Make sense? LOL I'll see if I can post a photo to illustrate the layout.

[Brazen]

And here is the official, legible schematic from Gibson.com.

PDF didn't cooperate here, so here is official the link: http://images.gibson.com/Lifestyle/S...10,G20,G30.pdf

And here is a jpg I created from the official Gibson pdf:

[g1]

Sorry, but that schematic won't work here unless you change the file size or zip it and post it as a zip. There was a problem with the server and some files that were previously posted won't work here anymore.

It doesn't matter which side of the transformer primary goes to hot, it will be fine either way.
The only cap of concern is shown at the very bottom right on the schematic. One side of the cap goes to the switch and fuse, the other side to ground. You want to clip it out or remove it from the circuit.

[52 Bill]

This amp is confusing, since the one line lead goes thru the fuse to the PCB which I'm sure goes to the combo on/off/volume pot.... then back from the PCB to a soldering lug that ignites the power light then back to the transformer. The other line lead goes direct to the other lead of the transformer.... simple enough.

My concern is, since the one lead doesn't go to a simple SPST switch, but through the PCB, will it matter which transformer lead I choose to make hot or neutral...... AND, if it goes through Caps on the PCB before returning to the transformer. I see the caps to ground in the schematic, so I'm not sure how in hell I can bypass them. Make sense? LOL I'll see if I can post a photo to illustrate the layout.

The white neutral line should go directly to the terminal strip by the pilot lamp and connect to one side of the transformer primary. The black hot line should go to the fuse holder and then to the pc board and switch. The switch is a simple SPST, but it is mounted to the pc board. There are no caps that the ac goes through other than the one ground cap that connects the ac line to the chassis.

The only thing that you need to do is to find the one ground cap and remove it from the circuit. I will venture a guess that it is the large brown ceramic cap that is next to the switched control. Check the value of that cap and see if it is connected to the ac line at the switch and to ground.

Connect the green ground wire to the chassis with a nut, bolt and star washer.

[Brazen]

The white neutral line should go directly to the terminal strip by the pilot lamp and connect to one side of the transformer primary.

Check!

The black hot line should go to the fuse holder and then to the pc board and switch. The switch is a simple SPST, but it is mounted to the pc board. There are no caps that the ac goes through other than the one ground cap that connects the ac line to the chassis.

Check!

Connect the green ground wire to the chassis with a nut, bolt and star washer.

Check!

The only thing that you need to do is to find the one ground cap and remove it from the circuit. I will venture a guess that it is the large brown ceramic cap that is next to the switched control. Check the value of that cap and see if it is connected to the ac line at the switch and to ground.

That is the "Cap" in question, although atypical of the era (or Gibson?) the schematic specs C12 as .04 MFD 500V Discap and my amp has a .01 MFD 450V discap. (pic below)

I've already performed the surgery, but I wanted to be sure, and to clarify for others, that I can just snip it loose from the PCB and it's good to go?
Thanks for ALL the help, guys. You ALL have been awesome!
Jimmy

[Brazen]

Complete!

And.... pics of the completed alterations. I have yet to plug it in and power up. Still concerned if I'll need to jumper the leads where the death cap was connected, since it goes through the PCB. If all that did was go from V+ to the chassis (ground) then I think I'm OK.

If it goes through the damn PCB back to the transformer via that brown wire, I think I'll have to run a jumper across those leads for continuity. That would suck! LOL

Thank you all again for the help! Hopefully this ole high school amp will be much safer now!
Jimmy


UPDATE: Ok..... fired it up and no smoke, fire, or shock! lol
New problem/annoyance though! I believe you all call it 60Hz hum/buzz? The amp, after changing out just the large, 2000uF filter cap, was whisper quiet. Now that I've updated it to a 3-prong, grounded line, I have that hum/buzz. It's not "too bad", but a bit annoying. It increases/changes with both volume and treble increases. Any tips?