Troubleshooting/repair an Ampeg portabass 800?

[MechaTomato]

I picked up an ampeg portabass for $10 because the guy didn't know if it worked. I plugged it in, lights turn on, receives input, and outputs via xlr, but does not output through any of the speaker ports. I have tried with working cabs, there is no sound.

I can't find a service manual anywhere (and I wouldn't really know what to look for anyway). Does anyone have any ideas on what I could look for? I currently have the thing opened up but have no idea what I'm looking at

 

[agedhorse]

How's your electronics theory? What scope do you have? Are you familiar with PWM controlled power electronic circuits?

 

[BassmanPaul]

My first question always is does it have an Effects Loop? If yes link Send and Return with a short signal cable to exclude that from the list of possible causes.

 

[MechaTomato]

How's your electronics theory? What scope do you have? Are you familiar with PWM controlled power electronic circuits?

I don't really know anything. I have a voltmeter and can solder well, but that's about it.

My first question always is does it have an Effects Loop? If yes link Send and Return with a short signal cable to exclude that from the list of possible causes.

I don't have a spare cable, but I actually did get it to work somehow, but it's not fully functional

I opened it up and just looked around, unplugged and replugged some things, and then tried it out. And it works. But there are some issues i'm encountering:

1. Pots need to be cleaned, they scratch a tiny bit when moved
2. Not sure if it's related to the pots being dirty, but whenever I touch one of the pots (there are no caps, i'm directly touching the metal), the hums
3. There is a slight hum, even with nothing plugged in. But not really a hum, it sounds more of like a "wind" noise, like a slight breeze.

4. The big issue: after a few minutes of playing, even on low volume, the amp starts to crackle. Loud crackles. Even after I unplug the bass cable from the amp, theres still crackling. I read somewhere that could be an issue with once the internals "warm up", which makes sense. Any idea with this?

 

[agedhorse]

Without tools or electronics education, experience and knowledge, I don't see any safe or effective path to self repair.

 

[mmbongo]

I don't really know anything. I have a voltmeter and can solder well, but that's about it.


I don't have a spare cable, but I actually did get it to work somehow, but it's not fully functional

I opened it up and just looked around, unplugged and replugged some things, and then tried it out. And it works. But there are some issues i'm encountering:

1. Pots need to be cleaned, they scratch a tiny bit when moved
2. Not sure if it's related to the pots being dirty, but whenever I touch one of the pots (there are no caps, i'm directly touching the metal), the hums
3. There is a slight hum, even with nothing plugged in. But not really a hum, it sounds more of like a "wind" noise, like a slight breeze.

4. The big issue: after a few minutes of playing, even on low volume, the amp starts to crackle. Loud crackles. Even after I unplug the bass cable from the amp, theres still crackling. I read somewhere that could be an issue with once the internals "warm up", which makes sense. Any idea with this?

What AgedHorse said is right. However, if you want to give it a shot I'd try replace the output power module first. Then if that's not it, try replacing the preamp. Then maybe the power supply. You can get those boards from Ampeg or Full Compass.

 

[pcake]

first off, with no knowledge you are more likely to damage the head as fix it.

without a spare cable, how do you know the problem isn't the cable?

order a second and maybe third cable. then you can not only see if going through the effects loop helps but you can see if the problem is the cable.

 

[agedhorse]

What AgedHorse said is right. However, if you want to give it a shot I'd try replace the output power module first. Then if that's not it, try replacing the preamp. Then maybe the power supply. You can get those boards from Ampeg or Full Compass.

By that time he could have bought a brand new amp with a warranty!

Reminds me of the story about the guy who was looking for help with DIY brain surgery... how hard could it be?

 

[basscooker]

OP, you're 10 bucks in....

I say shelve it and get it to a tech when you can. Sounds to me like you're poking around a hot circuit.... With bare hands too. STOP. This could get waaaay worse than an inoperable amp!

Close it up, stop powering it on. Even with a bench fee and some proper diagnosis including a repair or three you're still doing okay on cost. Dont make it more expensive, or even dangerous to yourself trying to save a few bucks.

Of course just my opinion.

 

[MechaTomato]

By that time he could have bought a brand new amp with a warranty!

Reminds me of the story about the guy who was looking for help with DIY brain surgery... how hard could it be?

exactly what I did. It was only $10. Ill sell it back off to someone that knows what they're doing. In the meantime, I just ordered a fender rumble 800hd head to compliment the cab/cabs (i bought 2)

 

[agedhorse]

exactly what I did. It was only $10. Ill sell it back off to someone that knows what they're doing. In the meantime, I just ordered a fender rumble 800hd head to compliment the cab/cabs (i bought 2)

I think that was a good call.