freestompboxes.org

Hi.

I'm working on my friends bass amp, Roland SB-100.
I haven't worked with solid state amps a lot, only with some PA-amps about 10 years ago.
Basically with tube amps since.

Output transistors were blown, so I need to replace them.
I was thinking also recapping the PSU.
It is 30 years old amp so it might be worth doing.

Main filter caps are 3300uf. Being a bass amp that doesn't seem very high to me.
Will it add some extra punch if I increase the value to 4700uf or even 10000uf?
Will that cause any problems? Will it stress th PSU?

As I see it that those reservoir caps will then have some extra current stored and during the lowest and loudest notes PSU will handle that better.

Does this make sense?
Can I increase the value safely, without any future problems?

-T

First Help me understand your Logic?
The power units have blown with less capacity and now you wish to give the PSU more current capacity so that the new power transistors can blow faster than the last time. HUM???

As to the other stuff generally YES bigger filters help with low freq signal but as should be obvious the Amp MUST BE Designed to take it all.

I'd be wanting to understand WHY the output blew Long before I upgraded the PSU. At 10,000uF they start get expensive 4,700uF is fine. If the Amp only has 2 output devices don't waste your money.
Cheers,, Phil.

Thanks or fast reply Phatt.

You cleared me thoughts and sorry for my incorrect "logic".
As I told you it has been a long time when I last time worked with transistor power amps.
Now I got it that those transistors suck all the juice what the PSU can offer and get hotter and hotter and hotter and blam!

My friend told me that he had borrowed it to some punk band and they had played it to external speakers and to incorrect speaker load. Long and loud.

My main guess was that the incorrect load was the main reason to blow those output transistors. It had stressed the amp too much.

Correct me if I'm wrong. That was just my guess that it might have been the cause.
The amp had performed flawlessly before.

Both 5 amp fuses (main supply fuses) were blown.
I tested the old power transistors with dmm and they tested shorted.

I also changed the 5 amp fuses and put that amp on a variac to confirm my measurements and when I just barely turned the variac on to few volts the current meter jumped instantly to full on. I desoldered the power transistor wires and tested it again with variac and it behaved like it should.

I was told by my firend who is an EE, that good subs for the obsolete 2SD555 transistors are
MJ15024.
They are readily available, cheap and will do the job well.
And yes the amp has only 2 output transistors.

Should I just change the lytics to fresh new ones keeping the original values, if there is really no major sonic impacts when increasing the value.
Thank you already for your answers.

-T

Dingleberry wrote:Should I just change the lytics to fresh new ones keeping the original values, if there is really no major sonic impacts when increasing the value.

Look at the end where the rubber bung is. If there is a little raised "pimple" probably with a crack in it then change them, or if anything has leaked out, or if the amp sounds a bit strange when pushed.

Otherwise leave them.

I just repaired an amp from 1956 and checked the electrolytics thinking they probably needed changing, they were still within spec.

You should be safe going up to 4700u, i would also upgrade the power supply diodes in the bridge rectifier while you are at it. If they are axial, use 6A 600v types, or maybe just bung in a 25A bridge & mount it to the sheetmetal somewhere close by.
It sounds like the amp was turned flat out with 4 ohms or less of speakers & run "hard", no wonder stuff died.
i would change the driver transistors as well, as these most likely will have been stressed when the main output transistors blew.
Adjust idle current, soak it for 30 minutes then recheck & adjust idle current again (if there is an idle adjustment).

Yep some good advice here.

*Mozwell* has summed it up well,, these amplifiers get plugged into two speaker boxes but the amp is not rated for it.
If I had a dollar for every blown SS amp story like this one I'd be very rich$

My rule of thumb (which may not always be right)
If the Amp has only 2 power devices then never run 4 ohm loads at high power.
If you see 4 devices on the back heat sink then you have a better chance of driving 4 ohm loads. But always refer to manuals for the load rating.

You can use *poly thermal switches* on SS Amps that you know are going to get flogged. Everything works fine until some drunk idiot plugs in more boxes.
Once the output passes the current threshold the amp stops working until the extra load is removed.
These are totally self resetting little gems that can save a whole sound system.
They where used to protect sensitive drivers but they can also save the Amp.

Cheers, Phil.

You could probably get away with increasing the caps to 4700uF, just make sure the replacement cap's voltage-rating is the same or greater than the originals, if the amp is more than ten years old, I'd replace the caps since they're probably living on borrowed time, because caps from 30 years ago were reckoned to have a useful service life of about ten years before they failed, what happens is that the electrolytic paste in the cap gradually corrodes the aluminium foil metalization over time, which causes a reduction in capacitance, or worse, an internal short-circuit resulting in the cap literally exploding......


As someone else has stated, what I would do first is find out what the root cause of the amp failure was before replacing the caps....