Beta Lead Failure Problem

[theJJ]

Hi All:

I'm a newbie to the site, but a long-time Sunn gear user.  Unfortunately, it has taken a Sunn equipment failure to find you.  I'm wondering if anyone's had a similar experience and if so, what was the identity of the problem and cost of repairs?

I have an enclosed Sunn Beta Lead 4x10, which I've had since 1978; used it many many times for live gigs.  If it is not too expensive, I'd like to repair it rather than purchase a replacement.

I'm not sure if its all related, but here's the story:  Over the past couple months, at random times, the sound got VERY crunchy/distorted, with no sustain.  This would continue for upwards of a minute or two, sometimes with some static or "popping" sounds, then would stop, and everything seemed normal.  This began happening more and more frequently and now there is NO SOUND from the speakers!  Nothing!  Ugh!

Lights for the power switch and amp channels, both on the amp and footswitch, all light as if the amp is working properly, but I'm getting NO sound at all....not even the usual "pop" I would hear in the speakers whenever the amp was turned off.

Anyone have any ideas as to cause, parts needed and cost of repair?  Your advice and experience would be greatly appreciated.  Thank you.

[george]

This may seem odd but if you pound on the top of the amp when it's on - what happens? Bad solder joint if this makes the amp snap, crackle, pop, etc.

Do you get a signal out of the either A/B preamp output jacks on the back panel of the amp? This might help isolate a preamp or power amp issue.

Since the A/B channel lights are on and it doesn't blow fuses - it might be a rather simple fix but you will need some diagnostic tools to fix the amp.

[theJJ]

Thanks George:

It is NOT odd.  In fact, when the crunchiness was happening a "whack" on the top of the amp sometimes helped jolt it back to normal, or it least it seemed to help.  So I hope you are correct and its just a solder issue--although if that is the problem, it apparently held for 30+ years before starting to go bad.

I have not looked at the pre-amp/power amp signals, but I'll check those.

Thanks for your thoughtful response.  I'll try and post an update after I look at those issues more closely.

[george]

Power off the amp and let it sit for 10min - remove the 4 screws from the wooden enclosure and slide the amp out. Remove the many screws that secure the cover to the chassis. You can figure how to remove the 4 screws that hold the power amp board down to the bottom of the chassis. Remove the output board connector. Check out the solder joints on the large power resistors (solder side of PCB) - I bet one or more have failed. If you have a soldering iron and a solder sucker you're home and dry  

[theJJ]

Hi George:

Thanks again for the tips.  I greatly appreciated getting these responses.

Just to follow-up for everyone I tried George's suggestion, but could not get a signal out of the pre-amp output jacks and did not find any bad solder joints.  So I figured it needed more specialized work than I felt qualified to perform.  Ultimately I found what appears to be a good sound electronics shop in the Chicago area, with an owner who is old enough to be familiar with, and enjoys working on vintage gear.  In fact, he told me he used an old Sunn bass head "back in his day."

Anyway, he apparently identified some blown amp circuit problems.  He found the parts and had it fixed in about a week for barely more than $100.  I have it back in my basement studio and its working great!

Thanks again to George, and anyone else who considered my problem.  Keep jammin' on that vintage Sunn gear!

peace,

JJ