Sunn Beta Lead Restoration

[antoniobsr]

Hello and greetings to all. I have general electronics knowledge and just acquired a Sunn Beta Lead SS 4x10. When powered up all it does is hum really loud (60 cycle I believe). Controls have no effect on loudness of hum, unplugging instrument has no effect. I am wondering where to start since this is relatively new to me, and I didn't want to damage the rest of the amp any further. I recently downloaded the manual from musicparts.com which is very detailed, schematic wise and operationally. Upon opening up the chassis there are no burnt smells and no indication of blown/burnt parts on the circuit board. Thanks everyone.

Tony C

[trevorus]

Sounds like a power amp issue. Could be a toasted output transistor.

[Enzo]

REALLY loud? Look at the speaker cones, do they move out - or back - and stay there? If so, the amp has DC on the output. DOn't leave it on long, for the sake of the speakers. You could also disconnect the speakers and just measure for DC voltage with a meter at the output. In that case, what Trev said, likely shorted output transistor and possible some collateral damage. In this case, no guitar signal comes through, just the loud hum.

On the other hand it could be a loss of power supply filtration. In that case no DC on output, just very hummy, but the guitar signal DOES come through. Look then for bad filter caps or broken solder to the existing filter caps.

Of course there are other possibilities like a broken ground connection at the input. In that case the volume control would turn it down.

[wardevans]

outputs

i just restored two of these exact betas with the same exact symptoms on both. bad output transistors. now they both scream like a dream. good luck!

[Don Moose]

The Beta Lead was my favorite SolidState amp.

Do you have a schematic (it's in the service manual)? Drop me a Private Message with your email and I'll send if you need.

Hope this helps!

[antoniobsr]

Beta Lead Restoration

Thanks all,

I do have the Schematics and Service Manual, I will take a look this week and will give further updates. Thanks a lot

Tony C

[LowBassDub]

Would that offer be open to any of us?

I have an old Beta Lead head on the shelf, that is dead and I shelved due to lack of infornation out there. I'd love a peek at the schematics, if it's not too much trouble.

[Don Moose]

You have mail

[LowBassDub]

You have mail

Why thank-you, You have a reply

[Twist]

In addition to the other suggestions, I just had one come into the shop with that symptom. It turned out to be an OpAmp on the pre amp board. It was shorted and loading down the +/- 15 volt supply. The pre amp is definately not service friendly. It's double sided and most of the OpAmps are covered up by the Pots. Check the emitters of Q1 and Q11 on the power amp board for +/- 15 volt.

[augie1]

sunn beta lead combo

mine is great on channel1 but on channel2 it will not reach the full volume as channel1 does. the volume / and master work to get clean or distoted sounds, but the overall volume it about 10% of the total of channel1 almost as if it were simulclassed, and reduced in its output. any help greatly appreciated. contact me please, and i could use the shematics as well, thanks..augie augierokstar@yahoo.com

[jimadcox]

Beta Lead (Combo) Documentation

Thanks all,

I do have the Schematics and Service Manual, I will take a look this week and will give further updates. Thanks a lot

Tony C

Greetings,

I just purchased a Beta Lead Combo and am looking for any documentation, including schematics and owners manual. Would you mind sharing the documentaion mentioned in your post?

Any thoughts on where I can obtain a replacement power switch? Mine is stuck in the "On" position (better than "off" I guess . . .).

Thanks,

Jim A.

[gbono]

Here you go - Index of /site/svcmans/beta

Try cleaning the switch with Deoxit?

[jimadcox]

Here you go - Index of /site/svcmans/beta

Try cleaning the switch with Deoxit?

Hi,

Many thanks for the quick response on access to the Manual!!

Deoxit won't work - I suspect the switch itself needs either a rebuild or replacement. So far, all the local techs I've spoken with indicate a rebuild is to tricky to do and I'd be better of getting a switch replacement, although none of the local techs are familiar with the specific switch required. My preference is to source a new switch and either take it to a tech or have my neighbor/bassist/tech head assist me in swithcing it out.

Thanks for the access to the manual and any guidance you have on sourcing a replacement power switch!

[loudthud]

The correct power switch for a Sunn Beta series amp appears to be a Carling LTA201TRB/125N

The Mouser part number is 691-LTA201TRB/125N and their website says they have several hundred in stock.

Disclaimer: I got this number by measuring the hole in a Beta amp, digging through the Carling Tech website until I found a switch type that required that hole, decoding all the part number options from the data sheet, and finally looking at Digi-key and Mouser. I have not ordered the part to veriify it's the correct one.