Just thought I'd share the beginnings of my latest project here. I picked up this old Gibson Skylark Trem off of ebay to restore / fix up. It seems to be the Crestline model, so I guess it was made between 62-64. It's in decent but not fantastic shape, it just needs a little TLC - new tubes, a new speaker, and a new power cord. I don't know why the cable was cut off, but I was going to replace it w/ a 3-prong outlet anyways, so who cares. The seller also said that it "may" need a new output transformer, but I took some measurements on it and it's not blown open or shorted out, so it seems reasonable enough to try it without replacing it first. I plan on using a lightbulb limiter to bring it up slowly, since I don't know how long it's been since those caps have seen any voltage. The caps appear to be original too, and some of them are actually labelled "sprague". 40-year old Spragues? Cool, don't want to blow those things up. Anyways, here's some pics. I'll post soundclips once it's working. Anyone have any suggestions for speakers? I'm thinking about a Weber speaker, either one of their Signature Series speakers, or maybe a Silver Ten if I decide to spring for it.
TZ
Those big brown tubular caps are highly likely to need replacement. They're electrolytic caps, and after 40 years or so are likely to be bad, especially if the amp's been unplayed for a long time. Electrolytics are dependent on applied voltage to maintain the dielectric film of aluminum oxide on the cathode foil. They'll often short out when they've gone a long time unpowered, and if they don't go to a dead short, they'll pop, crackle and sizzle due to arcing through thin spots in the oxide. Sometimes they'll heal themselves after being powered up for a while, but they can blow up violently, so I keep a finger on them when first powering up to make sure they're not heating up. Many folks just replace them straight off before applying power, but I've never done that. If I were fixing other people's amps for hire I would, though.
The Orange Drops will be in fine shape. Old waxed/oiled paper caps tend to go bad.
The resistors look like carbon composition ones, which were standard then. They tend to deteriorate over time, usually going high in resistance, and if they're in a spot where a good bit of current flows (like cathode and plate resistors), they're bad to cause hiss and crackle, too.
It'll be a fun project, and it's a simple device without a lot of parts to check out. Have fun with it! :D
"A cheerful heart is good medicine."
Thanks for the tips, Ric. I'm aware that the big electrolytics are the most likely parts to be problematic, but I'm hoping bringing it up slowly with a limiter will help them be OK. One of them appears to be a double cap (cap can I guess), which might be difficult to find an appropriate replacement for. I wasn't sure what kind of resistors those were, thanks for identifying that they're carbon comps. I do have a schematic, so I'll probably check some of the part values before powering it up for the first time.
TZ
Nowadays electrolytics tend to be a little more compact, so it wouldn't be any problem to replace that double one with a couple of single axial lead electrolytics tied together on the common end.
Multisection can caps are being made again in some common values. JJ is one maker. http://tubesandmore.com/ stocks them.
"A cheerful heart is good medicine."
Absolutely everything that Ric says, except that I would definetely replace the caps, (Just had one blow in my hand powering up an older amp, scared the s*** out of me) they're pretty cheap! Doing the light bulb thing is the same as using a varaic to form the caps to restore the oxides on them and it is kinda of a crapshoot on caps that old.
Hoffman amps and Torres amps should have all that you need in the way of parts. Also, those carbon comp resistors are probably near the end of thier useful life, they are cheap also, look a little cooked to me.
Get a schematic on the amp and see if it gives values for the transformer and power it up with nothing attached to it, and check voltages on the secondaries, it won't tell you how good it is but it will tell you if it is dead.
And remember, HIGH VOLTAGES live here, be careful! (300-500V!) No tongue to check if it's live! :shock:
When I die, I want to go peacefully in my sleep like my grandfather, not screaming......
like the passengers in his car.
If I'd just had a cap blow up in my hand, I'd be paranoid about it, too! Might happen one of these days while I'm feeling for heating. Really, the caps tend to dry up, lose capacity and let too much hum through, as well as being noisy from the leakage and arcing, so it's a false economy to keep the old ones unless you like that sort of noise. Same with the carbon comp resistors. But there are folks who scramble around and buy those things because they want the "mojo" of an amp that sounds like frying bacon.
"A cheerful heart is good medicine."
I wonder if Stiener has one of those old Rickenbacker "Frying Pans" laying around? :lol:
When I die, I want to go peacefully in my sleep like my grandfather, not screaming......
like the passengers in his car.
Thanks for the advice guys. I probably will replace the caps. Along with (at least some of) the resistors. Looking closely at the resistors, some of them are a little ... bubbly. Don't know how else to describe it. Looks like they have pimples kinda, and some of them measure off from what they're supposed to be. I do have a schematic (though I think what I've got here doesn't match it exactly), but I haven't had time to completely go through the amp and compare everything. I haven't done anything yet, other than pick up a new power cord, and order the tubes. After I sit down and go over what's there and what parts look worn out, I'll order those, then really get to work. So what would you guys recommend for replacing the resistors? Should I replace them with newer carbon comps, or will metal film be more reliable?
On the subject of exploding caps.... Last amp I worked on (which was actually the first amp I worked on), I had a cap can blow up in my face on first power on. Scared the crap out of me! I powered it on and was hearing some small popping noises, so I stuck my face down in the amp to look if there was any arcing on the tube sockets or something, when the cap can started spewing a geyser of black electrolyte into the air... Luckily I had my glasses on. Problem was the part was not really spec'ed high enough for where it was in the circuit (was rated for 450V - was seeing a little over 500V). Replaced it w/ one rated to 500V, and switched to the other set of taps off the power transformer (about 10% lower) and haven't had problems with it since.
TZ
Metal film. Definitely. There's no reason for carbon comp to exist anymore.
"A cheerful heart is good medicine."
I've been working on this off-and-on over the past few weeks. I'm not sure it's 100% yet, but it's working anyways. The power supply filters definitely needed to be replaced, prior to doing that, it only made a loud 60Hz hum, and no guitar noises. The cathode bypass caps also needed to be replaced, they were definitely leaky, and that was causing the amp to output low volume. I still think the volume is a little low for 10W, and it doesn't break up as much as I expected it to, but maybe that's just the way this amp is, I dunno. Maybe there's a leaky cap or two left to find. It does sound pretty good clean though, and the trem works well too. Anyways, for grins, I posted a soundclip on my dmusic page. Just me playing short, random, pentatonic licks.
TZ
Cool!
It's great to work on something like that and get it making musical sounds again! :D
"A cheerful heart is good medicine."
