Old amp blowing fuses after long period in storage

Hi
Long time since I've posted here.
Got an old transitor bass amp that is blowing fuses after years of inactivity. It used to work fine before being stored away.
What is the best way to soft start it? I suspect the old electrolytics are low.
Thanks for any help
Max

You got your finger on it. Check the electrolytics starting from the psu.

Is there a way of "soft start" an amp like that?

maxkriza said:

Is there a way of "soft start" an amp like that?

Click to expand...

search 'lamp limiter,' there are good instructions on building one safely.  essentially, you're putting an incandescent between the wall and the mains transformer to limit current.

Normally there will not be any need for a soft start for  a device like a guitar amplifier. However as you know the large reservoir capacitors on the psu can draw very large current on start up. On zero time you can consider them effectively as short circuit. So series lamp limiter in the mains is one way. The other one is to use a series resistor immediately after the bridge rectifier and before the reservoir capacitors. It will limit the current. However, this resistor will obviously form a time constant with the capacitors and the caps will take time to charge fully. You can have a timer which its delay time will be (slightly) longer than the time constant of the r/c. It controls a relay which shorts the resistor after turn on, removing it from the circuit. I actually have a small PIC timer for a similar application.

Thanks

I am trying to find a way of getting this amp to work. will current limiting using lightbulb help
reforming the main capacitors?

Is there a procedure you will normaly use with equipment that was not used for many years?

thanks again

Max

The light bulb current limiter and variac to bring up rails slowly are handy for repair/troubleshooting but it may be something more serious than caps needing reforming,.

With a VOM (and power removed) measure ohms across the output power transistors, across reservoir caps, and even PS diode bridge.  A short in any of those places could cause high current draw.

The capacitor reforming issue is associated with sitting but not all that common as a source of failure for a transistor bass amp that by definition can't be very old. 

JR

JohnRoberts said:

The light bulb current limiter and variac to bring up rails slowly are handy for repair/troubleshooting but it may be something more serious than caps needing reforming,.

With a VOM (and power removed) measure ohms across the output power transistors, across reservoir caps, and even PS diode bridge.  A short in any of those places could cause high current draw.

The capacitor reforming issue is associated with sitting but not all that common as a source of failure for a transistor bass amp that by definition can't be very old.

Click to expand...

Oh, I dunno. I started seeing transistor bass amps in the mid-1970s, which would be about 35 years ago. That's 25 years younger than me, so it doesn't seem that old from my point of view, but to an electrolytic cap it's an eon.

The main thrust of your message, though, is right on the money. Something may be shorted. Or both problems may be happening; a badly deformed capacitor may have taken out a diode.

For that matter, check the supply and bypassing caps for shorts. Once in a while I find one that's developed a 100% short, and that would blow a diode, perhaps before it blew the fuse.

Peace,
Paul

Does this happen with any electronic equipment with capacitors? I have some preamps, compressors, eqs, etc. that has not been used for a couple of years. Will they be prone to these sorts of issues?

Thanks for all the replies.

My amp is an old Polytone from mid 70's. It wasn't used for about 10 years.
It is packed with rockwool, so not looking forward to opening it ....
hope it will work again, it was a really nice sounding amp.

cheers
Max