Restoring an old practice amp

[MichaelScott]

A friend of mine was using a beat up guitar combo as his bass practice amp. He is a guitar player and had no real use for the amp so I offered to take it off his hands and fix it up.

I had a feeling it just needed a new speaker but it has turned out to be a little bit more of a project then I had anticipated.

Here is a picture of the front of the amp after I had removed it from the combo.

[Invalid or Expired Link Removed]

The top right corner says CG50B. I am going to take a bet that the 50 = wattages.

[Invalid or Expired Link Removed]

This is a top shot of the amp wires. The flash makes it hard to read the board: "HANA Eletronics Co." CG-50B (Pre) 12.04.98"

[Invalid or Expired Link Removed]

[Invalid or Expired Link Removed]


After I took the amp out I looked at the box and realized that it had a single hole for a 12" woofer and two holes for 3" horns... but the thing had no 3" drivers inside.

[Invalid or Expired Link Removed]

I took the 12" woofer out and it looked really good. I was starting to think that the driver wasn't broken (in which case I was clueless as to what the problem was). I plugged everything in and for some reason it didn't work... Then I noticed that there was a fuse hole on the back of the amp. It looked like it had once had a cover- now the cover had gone missing and the fuse could slide out of it's home with ease.

[Invalid or Expired Link Removed]

I got some cloth gaffers tape and taped up the fuse. Then the amp turned on and I could hear the hum through the speaker.

I cut the high and had my assistant (13 year old brother) wack on my bass while I fiddled withe the volume knobs.

The driver sounded pretty good, and I was starting to think that the distortion I had heard was due to the fuse not being secure, but when I got the volume up to about 3 the speaker started to distort. I stopped the playing and looked at the woofer closer. It was almost impossible to see but one side of the the face was warped a small amount. here is a pic (the woofer looks like it is in good shape but I have circled the warp.)

[Invalid or Expired Link Removed]

So currently I have a few problems with my project:

1) I need a new 12" woofer- a problem because the woofer in there has no markings and I have no clue about what Omh to buy for the replacement.

2) I need to see if I can find a new fuse cover. This isn't that big of a deal because I can just use tape if I can't find one.

3)I need to figure out where the crossover is on the amp board- and I need to replace it with one that will use the 12" woofer down to a lower hertz and then cut to the two 3" I'll install for the high range.

4) I need to buy new 3" horns and re-wire from the new crossovers to the 12" and the 3".

I'll let you guys know how it turns out.

 

[IvanMike]

i'm thinking those are ports, not spaces for 3" drivers

 

[Nightbass]

Hey, welcome to the wonderful world of DIY projects!

(1) Get an 8 ohm woofer. You can't harm anything if you're "wrong", like you would if you got a 4 ohm woofer.

(2) Got any electronics surplus stores in your area? You might be able to find just the fuse cover. If not, get the whole assembly. Rat Shack is another place to look for one.

(3) I don't think there IS any crossover, because...

(4) Those 3" holes are ports. It's a ported enclosure, and the 12" was run full-range.

But since you have hum, I'd work on that before putting any money into the speaker. Get the fuse holder working properly and safely first. The hum is an indication of a bad cap somewhere, and the filter caps may need to be replaced or beefed up, and there may be faulty bypass caps anywhere on the board.

As for the woofer, you could repair it with white glue and tissue paper, if it is split. Once the amp is working properly and you determine it is usable for your purposes, then I'd start looking for a quality but inexpensive stamped-frame woofer. Parts Express has a great selection and when you get to that point, let us know and we'll get down to the particulars. The amp is probably bandwidth limited to begin with, so tuning the cab to 40-50 Hz might not make much difference.

Nightbass

 

[MichaelScott]

Thanks for the replies guys! I didn't even think those were ports.