Don T......you're needed

[RichBriere]

Morning Don and Sunn-o-philes. This gent needs your help in fixing an amp. The may already be on here and I expect that he'll be in today. He'll be wearing jeans, a denim jacket with a hole in one sleeve and carrying a dozen red roses.

OK Kevin Smith....ksmith1@telus.net...say hello to everyone.  

RB

[Don T.]

Good morning Rich & Kevin:
 So, What's the haps today?

[XoFFoX]

Hello gentlemen,
THis is Kevin.  I sure hope you can help Don T. I have an old Sunn Beta Bass amp that is in need of some repair. I've been working on it for some time now and I'm at my wits end. THis thing has the strangest power supply setup I've ever seen and believe me I've seen lots. I used to be a troubleshooter for an Electronics mfgr, actually the same one that builds the Otari RADAR which is now IZ technologies RADAR. I've been laid off for 2 years now and I'm trying to eek out a few measly bucks repairing my friends music gear. If you have schematics for this unit it would help a lot. I am used to working blind (w/o schematic ) but this one has too wierd a pwrsply. It seems to have some CMOS IC's that so far I can tell are part of some switching circuit for the inputs. When the instrument is plugged in a LED is supposed to come on for that channel but that's no longer working and I also had to replace a power transistor due to my own carelessness.
Can you help

[XoFFoX]

Hey man . . . I need my wake up coffee. Steppin out for a few minutes. I'll be back in a while to check in with ya.
11:11
[/code]

[Don T.]

Hello:
 The schematics and component locator guide is part of the service manual that can be downloaded from this site. You will find that the cmos ic's are hard to find. The led's on the front panel are controlled by the foot switch. If no footswitch is plugged in then both led's should be on. They are hooked to the powersupply + - 15v so if one fails to lite then you may have a power supply problem on the powerboard.
Questions:
1. Does the poweramp section work - signal to "poweramp in" on the back panel?
2. Do both red and green led's lite up without a foot switch plugged in?
3. Does the second half of the preamp section work - signal to "accessory return" produces sound through the master level control (amp section)?
4. Do either of the preamp channels work?

Can you give me more details about the symptoms?

[Don T.]

Hello Kevin:
 The power supply is beyond simple. The +- 15V is regulated on the power amp board and sent to the preamp. With the powerboard towards you (heatsink away) and components up, the TO 220 regulator ICs are on the right. Main Vcc +- 45V supply is fed to the regulators through the brick resistors. The voltage is determined by the Zener diodes. It is a simple uncompensated voltage regulator that will sag when the current goes up ( due to base/emitter current). If the power amp works and the led's on the preamp are off then either there is a short on the pre or the interconnect cable is not sending power to the pre.
Well I guess I'll wait for more info so I don't prattle on.

[RichBriere]

Scotty, I don't care about the Zenor Diodes! Can you get the Dilitium Crystals up and running or can't you!  The Romulans are climbing up our butts and I need to get to Warp factor 5 power in 7 seconds!!  

[XoFFoX]

Hi again,
Well . . . where do I start OK ...
When I first saw the thing it wasn't working then the second time I saw it . . . it was dead. However I got the impression that the LEDs only come on if you plug into a particular channel.
The LEDs were on when it was working then after it quit the LEDs stopped as well.
I believe the amp section was intact but there was a problem while I was troubleshooting and one of the 2N3055's went poof. After the Xsistor died the thing would draw high current and start to burn certain resistors. I have not revisited the amp section since as even after I replaced the burnt transistor it still draws high current and heats up resistors or blows fuses.
We don't have a footswitch for it and no the LEDs do not light even with Bass plugged in.
I think it's a power supply problem because of this.
The Vcc power for one of the CD4069's is connected to chassis ground directly through circuit trace. This is immensly puzzling to me.
There also seems to be two separate grounds in the circuit. There's chassis ground and then there is a ground for the preamp brd. Or it may be a negative power source I'm still not sure.
I don'y know if preamp channels work. I figured if Leds don't light that nothing is working on preamp section either.
Where do I find Beta Bass schematic
I have been to site before and I dont see it listed here unless it is the same as another model

[Don T.]

TRY AUXILLIARY POWER - TAP THE WARP DRIVE!

[Don T.]

Kevin;
 Try this        http://www.sunnamplifiers.com/svcmans/beta/

[XoFFoX]

ummm
There is power to the pre brd for the op amps ( +- 13v or so)
The power feed from the TIP30 looks funny.
Base voltage -14.5
Collector voltage -42 or so
Emitter volts at -0.7

[XoFFoX]

If U dont hear from me for a bit its cuz I'm DLing the schemes for the unit.
THanks man this will really help.

[Don T.]

Kevin:
 You will have to solve the power amp problem first. Remove the fuses from the powerboard and see if it still draws current. If so, rectifier or filter cap may be bad, if not, then pull the power transistors. If it still draws current then the protection diodes may be shorted. The predrives emmiters should be at 0V or there abouts. Make sure you have current available for both +- 15V supplys.
Fortunately all the parts used are readily available (no proprietary numbers) except for the cmos distortion IC's.

[XoFFoX]

Gottcha chief . . . I'm onnit !!!
Current is well behaved when power transistors are removed.