Amp help plz

[McSpunckle]

So, I picked up the Acoustic 150 on eBay for 100 bucks. Pretty sweet deal, I think. But, naturally, being from the early 70s, it has some things going old, and needs a small bit of care.

First off, the volume controls don't go down all the way. When the knob is physically all the way down, you can still hear the guitar through it. It's usually a pretty distorted signal too.

Secondly, when plugging in a pedal, or unplugging it, even with the volume all the way down, it makes the hum noise. Not how the guitar peaks through-- it's actually got a significant volume to it. What would be happening here is the ground of the pedal would be shorting with the input of the amp... but I can't think of why that would make a hum. Maybe it's related to the volume bleed-through, though.

Thirdly, there's a bit of a hum, even with nothing plugged into it, volume control doesn't change the hum's volume.


The boards are hooked up with little clip things... the edges of the boards are like PC cards, and all the wires are soldered to the connector clip, and the clip slides on over the traces of the board. I know these are getting loose, since I fixed a problem by bending one of the metal contacts in the clip to make a secure connection. I'm thinking it would be best to just remove the clips entirely, and solder the wires directly on the ends of the traces. I'm thinking loose connections could very well be what's causing these problems.

Then, there's the issue of 3-prong conversion. I checked a newer schematic for these amps for when they started using 3-prong wires. It's all the same, just with the ground done straight to the chassis. I assume I can just do that, but hard-wire the ground reverse switch. Or maybe disconnect the neutral from the chassis all together?

Now, transformers. It has a power transformer and an output transformer. If I wanted to replace these (or build a new-production clone of the amp), how could I figure out the ratings of the transformers? I believe they are pretty well un-marked.

[Scruffie]

You're probably gunna need to replace all the electrolytic caps in it... all the problems you've listed seem likely to be caused by old caps such as signal bleed and noise... replace those and your amp will be given a new lease of life (just make sure to discharge them properly and replace them with correct value and rating caps)

[McSpunckle]

Hmm. That does not sound like a big deal at all. Now to do some googling on discharging caps! There's really only one big one, though...

[McSpunckle]

Hey hey... so I took out a preamp board... and there's only two electrolytic caps, but most of the caps are tantalum (according to the service manual). Do these need to be replaced with Tant. caps, or can I use normal electrolytic or even film caps? They're only 1-2.2uF 50V. I'm guessing tant was used for their small size. I took one out, and it tested out close to what it's supposed to be, but pretty sure they can test fine and still be leaky.

I think I'll just replace the filter caps and see what that does first off, though.

[Scruffie]

I've heard conflicting things on tantulums... some say they take longer to dry than electrolytics, others say they're the first thing you should replace, so i'd probably go with replacing them all yes.

I can't see why in the amp they'd have to be tantulum no... just take note of polarity, voltage rating and value and you'll be fine.

[McSpunckle]

Thanks. Next time I order some parts, I'll replace all the caps and see if it helps. Gotta get a cord to do the 3-prong conversion too. I just marked the polarity reverse switch and plug with nail polish.