Sunn Beta Lead

[andyjazz]

I'm working on this Sunn Beta Lead. The amp was making the loud humming noise, not affected by volume knobs. I tested the power transistors and three out of six were bad. Also went through and just did a diode check on every diode/IC I could find on the power amp board. Everything else tested OK. Got new power trans's turned the amp on and a resistor fried. I'm having a real hard time seeing on the schematic what value of resistor it was. Also it looks like it took out the diode (Q16) next to it. Also I can't read on the schematic what diode Q16 is. I can barely read anything on this faded out thing! Any other places to get a schematic that a guy can read?

[Jazz P Bass]

Did you try to contact Fender .
I know they have the schematic.
Q16 is a PNP transistor.
I cannot read the number. 2n4 something.
It looks like a high voltage TO-3 can type.
2n5416 is the compliment to Q7 (2N3440)

[Ronsonic]

Undocumented feature, that probably has nothing to do with your blown output section, but can be a headache - The reverb drive circuit is not entirely stable without a load and will oscillate without the reverb pan connected. This will upset the power supply to the preamp section and introduce hash. You can tack a resistor like 2Kish across for testing without having reverb hooked up.

[andyjazz]

thanks for the help guys. Q16 is either 2N4882, 2N4888, or 2N4889. This transistor is definitely bad. It tests an "open" across two of the legs. As far as I can tell the resistor that fried is tied to this thing. the resistor spot is marked R17. I can't tell from the shemo what it's value should be and I'm guessing this is fairly important seeing as how it's Vdrop is feeding Q16. I'll check with Fender and see what I can find out about a better schematic. I guess these amps are kinda "something." Worth fixing anyway. Also thanks for the reverb tank issue. I have one that I'm going to install on this thing once (if) I get it running.

[Jazz P Bass]

R17 & R47 are 15 ohms.

[Enzo]

Transistor Q16 is a 2N4888/2N5401 according to the schematic, but I'd believe a 2N5416 more readily. 2N5401 are common, I stock them. SO if you have no 2N4888 use those instead if replacing a 2N4888. I'd believe it at Q5 or Q12.

R17 is 15 ohms, but it is associated with Q8, not Q16.

I am not sure how you are testing the transistors. Bipolar transistors check like diodes from base to collector and from base to emitter. It is normal for it to read "open" from collector to emitter.

This schematic is a sin. I scanned mine, I think it is a little better. Here:

[andyjazz]

thanks alot for your help Enzo. Yeah i explained this wrong, I was testing so many IC's. I guess some have different Vdrops than others as part of there design? This particular IC tested a short. It also had a bunch of brown ooze leaking out of it (It's an old-time "can" type). What I can expect to see is continuity and/or a Vdrop? Since there was no Vdrop I was suspect. I've read a few things about testing these devices - guess I'll go dig some more. The "good" power transistors that tested had a Vdrop and the bad ones were either a short or open-I just replaced all 6.

Now that the amp has new power T's I am not at all confident that replacing these few burnt components will get the amp working. There is always the question of what caused these things to burn in the first place. I've been down this road before - I think I find the problem and it just leads to more problems. I'll keep you posted.

[Jazz P Bass]

To increase your confidence in getting this amp up & running, may I suggest again that you build yourself a "lamp limiter".
It is a simple circuit where one of the main leads goes through the lamp.
Solid State amplifiers are very unforgiving if you missed a shorted transistor or open resistor/diode.
The LL will "limit" the current that the amp draws from the mains.
This will enable you to make some test voltage measurements which may help define what (if anything) is still wrong.

[gbono]

Now that the amp has new power T's I am not at all confident that replacing these few burnt components will get the amp working. There is always the question of what caused these things to burn in the first place. I've been down this road before - I think I find the problem and it just leads to more problems. I'll keep you posted.

Does the preamp section work okay? You can easily test this independent of the power amp.

These amps fail for a number of reasons: one very common problem is people try to drive a 2 ohm load with this amp

[Enzo]

What are you calling "IC"? There is only one IC on this power amp, that 4558 there. And that is a plastic DIP as far as I know. Three-legged cans are transistors. And I don't think I have ever seen anything leaking out of one of those metal transistors. I would more likely believe it was some glue used to keep the part steady, or possibly some contaminant. But anything is possible.

No transistor should measure shorted, especially out of circuit.

All it takes is one component to fail, and other parts can burn up. I fix burnt up amps all the time.

[andyjazz]

An IC can be a diode array, a transistor array, a zener/transistor array, a bunch of different things. If it is a diode array it could be tested as a diode array....or am i wrong? But what does it matter? half the time I'm looking at some fucking part that looks like one thing but it's not. Unless you have the experience to know what to most of us is a bunch of abstract numbers you don't know. So you get some faded out piece of shit schematic that you still can't read........... I know enough about electronics to make me dangerous I guess.

I started doing this about a year ago out of desperation (I got laid off almost 3 years ago, no full time work, I have 5 part-time jobs including the I-can-fix-your-amp-in-my-basement business) I'm sinking further and further into debt-I can't find a real job and I'm starting to realize I'm just not very good at this stuff. I'm a musician who knows something about electronics not an electronic technician who can play music - which is probably my problem. Although I have some electronics background and I've had a few successes most of the time it seems I'm just floundering around in a sea of electrical terminology of which I understand about 30% and I'm constantly refering to old manuals because I just don't remember everything about what a fucking CMOS or a MOSFET is all about.

I'll see what I can find out about the pre-amp section. I ordered some 2N5401's yesterday and I'll be shocked if I put in the replacement parts and the amp works. I'll probably just be out another bunch of $ -I already have like $50 in parts into this thing- and then the guy will be like "Why doesn't my amp work?" and I'll say "Because it's broke" and he'll say "Well you can't charge me anything if the amp don't work." And then I'll see if I can get some $ to recoup some of my losses and tell him to go find a real repair shop where he'll pay 3X as much to get his lame-assed Sunn amp working so he can re-live his fucking teen-age KISS guitar hero wet dream...............Oh I'm not bitter at all

[nashvillebill]

I can tell you are definately frustrated, I've been there chasing problems myself. And stories like yours--spending many hours of work and lots of money in parts, unsuccessfully--make me glad I'm not in the amp repair business. I'm just a hack when it comes to fixing amps.


By conventional nomenclature, IC's are integrated circuits such as 8 pin or 14 pin opamps, 555 timers, and the like, where many components have been miniaturized and placed within a single monolithic chip. Single discreet components like the common 3-legged transistor are not considered IC's. I don't think rectifier bridges are commonly considered IC's, though maybe in some catalogs they might be filed there.

Enzo is extremely knowledgeable and very helpful, this site has a few other folks that are also very very good at helping guys walk through their repairs. I've noticed Enzo will bend over backwards to help folks as long as they are willing (to learn and follow directions), respectful, and patient.

[gbono]

I suggest this book: Electronic Troubleshooting by Jerome Oleksy (get the second edition). You might want to look into basic electronic classes at a local community college.

BTW I have a very good friend with an MSEE from the University of Edinburgh - he is pumping gas for a living - this country is in a depression and you're not alone. Hang in there.

[andyjazz]

I'm going to send this thing on down the road I can't do anything more with it. It's driving me crazy.

When I said I have some electronics background I wasn't kidding. I have a VoTec electronics degree. Basic electronics? Give me a break! All that did was make me good at math! Didn't do jack for troubleshooting amps. We never even had a schematic reading class! Education is a fucking joke! 10K for a VoTec degree and all I can find is a part-time job washing dishes.

I can't read the schematics to see what diodes I'm supposed to be replacing after I find they're bad-if even thats the only problem-which I'm sure it ain't. I can't get good readings from the transformer to see if the V coming to the board is right. I dunno maybe I got some bad connections but I can't see on the schematic what the V should be anyway. I've spent 3 hours now trying to see what number Q? and Q? is now that I know that they're bad but I can't afford to keep ordering shit from Digikey only to have nothing work anyway.

I know you people mean well.....I have been able to gleen some useful information from this forum-much more useful than some fuckwad VoTech teacher.......seriously, thanks for your time, but I've had it. I'm done.

[Jazz P Bass]

I feel your pain!
My first repair was a Luxman.
It fought me to no end.
I did everything right by doing it all wrong.
NTE parts, bogus, counterfeit Toshiba output transistors.
On top of that I did not know "how" a power amp output section worked.
(I still don't)
Bottom line.
I learned a lot of stuff.
The amp worked.
I did not give up.
(4 months, off & on to repair it.)