Sunn Beta Lead Guitar Amp Repair

[Bud]

Hi All,

I've been repairing my Sunn Beta Lead guitar amp that fried and took out a whole bunch of components on both the preamp and power amp boards. I have the power amp board working to where I can play through it for as long as I want with no problems.

When I connect the preamp board (some chips replaced now), it will pass a signal in both channels, but will make a loud pop in the speaker on power-down.

I've been replacing the caps on it, but so far I still get the pop.

One thing I've noticed is my supply voltages to the op-amps should be +15 and -15, but I instead have +15 and -13.1. I'm not sure how op-amps work as my electronics expertise is roughly at "hobby" level. But would that 2 volt differences in supply to the op-amps result in a 2-volt DC at the output causing the popping sound? I don't have a working o-scope, so I can't see if there's DC at the output, but it certainly *sounds* like there is, at least transiently at power-down.

Anyone ever work on this amp before? Or is anyone good with op-amps that can answer the voltage difference question?

I've uploaded JPGs of the pre-amp schematic. Best quality I could find.

Here is a link to the PA schematic, where the +/-15vdc is generated:
http://music-electronics-forum.com/attachments/40035d1469741079-sunn-alpha-slave.gif
(alpha slave model, but the PA is the same on both amps)

Thanks to all,
Bud

 

[73's de Edd]

Sir Bud . . . . .

This is your initial evaluative homework assignment . . .should you choose to accept . . . .a la Mission Impossible.

The power supply for the pre amp is being derived from the snippets that I have extracted from the power amp schematic.

Note that there is a disparity in the assigned value of + and – polarity outputs from the main power supply at bottom left corner.
See if 40 or 45 is correct and how well balanced they are being .

If we find them balanced then you look at the manner of derivation of the + and- supplies from its markup schema.
It is critical for the GOOD health of the YELLOW arrow marked filters.

In the end, if you still find the negative supply being down as it is now, it can be micro adjusted upwardly in ~ 0.7 volt increments. by the addition of each of the 1N4001-2-3-4-5-6-7 type of diodes that I have shown being in series in the GREEN dot outsetted schema.


Go ye forth and see . . .Wha a a a a s s s s s up ? ?
( But . . . . . I really think that your problem is going to be impulse related. )

TECHNO REFERENCING . . . .




73’s de Edd

 

[Bud]

Sir Commander,

Thank you for considering me for this mission. I have accepted and am deployed, armed with a parts bin full of 1N400X diodes.

During my initial recon per the mission plan, I encountered an unforeseen contingency, perhaps as a result of some bad initial intel. Thus I am camped outside the border of target's territory awaiting a go/no-go directive on the diode infiltration.

Here is the new intel:

At PS output we get a nicely balanced +/- 44.6 vdc *UNDER LOAD*

However, if I measure with no load per the schema instruction "DC VOLTAGES TAKEN WITHOUT LOAD AND WITHOUT INPUT" then I get no meaningful reading -- meter just kinda floats between 1 and 5 volts. If I move the negative lead of my meter from the chassis to the transformer center-tap, then I get expected readings. So the front-end of PS continues to work when not under load, but no +/- 45 nor +/- 15 vdc are present at the test points.

I've checked the instruction "DC VOLTAGES TAKEN WITHOUT LOAD AND WITHOUT INPUT" against another copy of the schematic (a bad JPG I have labeled "BETA SERIES") and the directive is the same.

Are there any operatives we can sneak into the target's camp to discern why the voltages disappear when measured per the schema instruction? We could call in Charlie's Angels, but I fear that might distract the troops from their ultimate objective ;-)

Thoughts?
Seriously, thank you so much for your help and also for the fun way in which you presented it.

Bud

 

[Bud]

Well, I worked some more on the preamp board today. I replaced all the 2.2uF NP and 10uF caps along the -15vdc line and re-tested. Preamp still wonky.

The foot-switch logic appears to work, as I can switch between Channel A, Channel B, and "BOTH" and all the correct diodes light up at the right times.

However the Drive and Level pots for Channel B, when turned up, pass the signal for Channel A. Channel A pots have no effect in this case. If I put the Drive pot all the way up going for an "overdrive" tone, I get it, but it's LOUD LOUD LOUD, even with the Level and Master each at about 2. I don't remember it being that loud. If I turn the amp off while still on the Channel A--the green LED--I get a fairly good POP from the speaker. But If I hit the foot-switch first and change over to Channel B--red LED--it powers down nice and quiet.

The schematic says to take all voltages "without load." I'm taking this to mean with no speaker (or resistor) connected to the speaker jack. However, if I do this, all dc voltages drop to almost zero. This seems to go against what the schematic is saying.

The "ground" of the preamp board seems to float from the chassis ground, but looking at the preamp schematic, I guess this is correct?

If I remove the 14506 and select Channel B manually with a resistor. I can get some signal to pass through it using the correct potentiometers, but the signal is very small.

Using a resistor to select Channel A with the 14506 removed, Channel A is very hissy, and controlled with the correct pots, but the signal passing through is not anywhere near as loud as with the 14506 in place switching things.

I'm not sure what all this means--esp. the speaker having to be connected in order to take measurements when the schematic says otherwise.

Thoughts anyone?
Bud

 

[73's de Edd]

Sir Bud . . . . .

Looking at the Power amp schematic and the separate Pre amp circuitry, it appears to me that all grounds are chassis related.

You can even see all connector plug grounds, going to chassis ground.

The ONE exception is the shielded cable run where the low level audio input to the preamp is shown as a floating ground . . .with its symbol . . . at the plug in point .

They want to avoid any potential floating ground hum inducing loops, so the actual ground is made at the end of that shielded line.

Nowhere, on these two schematics, do they show the routing of + and – 43VDC from the power supply over to the preamp. How is this done ?

With this being a DIRECT DC coupled to the speaker design of amp I see the need of the speaker being connected in. BUT with no need of audio input to the input jack of the preamp, or into the power amp.
And you do have a grounding between the amp and the preamp. .

Then, on my two snippets submitted the raw +43 should be on the Q1 collector and a +15 outputted on its emitter.

Likewise down on the Q11 collector there should be a raw -43 and a -15 output on its emitter.

Is that being a fact now ?


73’s de Edd

 

[Bud]

Hi Edd,

Thanks for answering.

>>Nowhere, on these two schematics, do they show the routing of + and – 43VDC
>>from the power supply over to the preamp. How is this done ?

There's no +/- 43VDC on the preamp board. It's used for the finals only (and as a source for the +/- 15VDC sent to the preamp).

>>With this being a DIRECT DC coupled to the speaker design of amp
>>I see the need of the speaker being connected in.

Thank you! I thought I was going crazy. This is how I see it, too, and is indeed how it works. I don't know why SUNN has that instruction on their schematics. Typo? That's why I checked a 2nd schematic, but the same instruction was there, too. Weird. I feel better about ignoring it now.

>>BUT with no need of audio input to the input jack of the preamp,

Yeah, maybe "without load" means to refrain from hammering an E7#9 chord into the inputs ;-)


>> And you do have a grounding between the amp and the preamp. .

I don't quite get where that's supposed to happen. I figured the pots mounted to the case would take care of that. So, with the board un-mounted, I thought to run an alligator clip from the chassis to the pot case. However, the pot case itself does not electrically connect to anything else on the preamp board. So I'm stymied as to how to ground the two boards together or even if I'm supposed to. If I measure between the ground pin on any IC on the preamp to the chassis, there is 1.6 ohms. If I pull the speaker cable wire it goes to infinity. I *guess* the 1.6 ohms is the speaker cable. This is why I thought the preamp was floating with respect to the power amp. I have to say . . . I still don't quite get it. You see a good spot on the preamp to connect to ground? I'd trust your suggestion here.


>>Then, on my two snippets submitted the raw +43 should be on the Q1
>>collector and a +15 outputted on its emitter.
>> Likewise down on the Q11 collector there should be a raw -43 and a -15 output on its emitter.
>>Is that being a fact now ?

Yes, other than the fact that Q11 is still at 13.1VDC because I haven't yet installed the diodes. Now that you agree about the speaker needing to be connected despite what the Sunn schematic says, I'll install the diodes. That will make me feel better about the voltages, but like you said in your 1st email, I agree that it probably won't solve the preamp weirdness that going on. I should have time to pop the diodes in later today.

It's getting there. A little at a time. I have more caps on their way to me, too. I might as well change all the elecrtolytics while I'm at it. Only a few left . . .

Thanks for all your help. And for the moral support on reading the Sunn schematic,
Bud

 

[73's de Edd]

Sir Bud . . . . .

In reference to that schematic . . . . I initially converted from a two piece to a Unitized copy . . . .with a vest . . .no less !

Then a bit of “ NASSER “ digital area pixel dithering to minimize prior J-peg crusting, then throw in some mathematically averaged digital prestidigitation to then impinge upon having it being almost totally eliminated.

Then do a black and white balancing, with some required intermediate contrast restoration.

A final . . . Sala-gadoola . . . . " Bippity Boppity Boo " . . . . . . and we then have . . . . . .

Le Schematique . . . .

https://s11.postimg.org/r7pxnjfnl/BETA_BASS_PRE_AMP_FNL.png

In looking at the now, much mo’ bettah, one piece schematic, an initial cursory design analysis reveals 3 analog sections at top, three across the bottom and a digital interfacing to them within the center area.


Thasssssit . . .for now . . . .


73’s de Edd

 

[Bud]

. . . and we then have . . . . . .
Le Schematique . . . .

Whoa Edd! That's some serious Bippity Boppity Boo and Prestidigitation on the preamp schematic. Soooooooo much easier to read. Excellent job! I'll have to see how it differs from the Beta Lead schematic, but in going thru it just now I don't see a ton of differences. I'm certainly able to work off of it. I'm sure others finding this thread will appreciate your work as well. Good scans of these schematics are hard to come by.

If things still don't work after I get the supply up to -15VDC with your diode trick, I'm not sure what's next for me. Signal tracing, I guess. Hard to do since my 'scope is also out of commission. Old Tektronix. I had it going last year for about a day after changing out a cap. Whole thing needs to be recapped and it might be better for me if I pick up a digital 'scope off Amazon. I'm open to hobbyist-level recommendations here. Something < $300 preferably.

I'm also going to look more closely at your awesome PNG of the preamp schematic to see if there's a way I can bypass the channel switching circuitry. I still suspect something's bad in there, esp. the way the A Channel pot is controlling the B Channel drive levels.

I'll post my findings!

Thanks for all the help and moral support (and prestidigitation).

Peace,
Bud

 

[Bud]

Likewise down on the Q11 collector there should be a raw -43 and a -15 output on its emitter.
Is that being a fact now ?
73’s de Edd

Just to clarify my previous answer, the collector voltage is ~ -35vdc, though the raw is present on the other side of said resistor. We'll see how it reads after I add the diodes, which I'm getting to soon . . .

 

[Alec_t]

I haven't analysed the circuit in detail, but I'm not surprised that you're getting cross-coupling between channels A and B. Resistors R104,5,7,8 provide a direct connection between the channel inputs. (Why??).

 

[Bud]

I haven't analysed the circuit in detail, but I'm not surprised that you're getting cross-coupling between channels A and B. Resistors R104,5,7,8 provide a direct connection between the channel inputs. (Why??).

Hi Alec,

Thanks for your interest and input in the thread.

I think those resistors are there so you can plug into the the "BOTH" input and then use the foot-switch to direct the signal to Channel A, Channel B, or both.

But you make me want to check caps c112 and c113 to make sure one isn't shorted.

Thanks for jogging my ol' brain.

Peace,
Bud

 

[Bud]

UPDATES:

1.)
Well, thanks to Edd's suggestion to put 1N400x diodes in series with the CR5 Zener, the negative supply voltage to the preamp board is now at -14.5vdc (up from -13.1vdc). I could add another diode to get it to -15.2, but I'm thinking this is close enough.

2.)
I may have been wrong above about the channels being crossed. I hadn't played thru the amp in awhile (er, cuz it's been broke!) so I was remembering the LED colors backwards, i think. Indeed I noted them backwards above. So, testing more carefully this afternoon, after balancing the supply voltage per #1 above, this is what I've found:

a.) Plugging directly into the Channel A front-end input OR into the "BOTH" input, I can get no signal to pass through the preamp.

b.) Plugging directly into Channel B front-end input, I can get signal to pass, but on power-down I get an awful pop at the speaker. Not a nightmarish one, but it's not right.

c.) Plugging into the "BOTH" input (front of amp) using the same settings as in (b) above, no signal passes thru the preamp.

I'm thinking something on the A side of the preamp is grounding the input signal, and also causing dc to leak to the output on power down. I'm checking over the schematic now to form some hypotheses about what component that could be. Open to your thoughts as well. I wanna find the little bugger and sit it under a hot lamp and grill with endless questions about why it has wrought such havoc in my life.

 

[73's de Edd]

Sir Bud . . . . .

Parte Deux . . . . .


You should be able to test your logic functions just my plugging a phone plug in and out of the three input jacks and seeing if the respective LED’s respond.
Then plug in the foot switch and check it out to see if it canges in the same manner..

When that is completed, come back and I will fill you in on the audio flow.

The redrawn input jacks are now reflecting the spaced apart stereo contacts, with the tip being the audio input.
Then upon plug insertion, the rear contact is getting shorted out to ground to create a logic low for the respective GREEN or RED SQUARE logic lines, routing over to the FUCSIA logic control chip.
The same is being true of the combined Channels A and B jack.


Mark Up Schema . . . . .
Errata . . .pin 9 IC 103 Logic square shud be GREEN vice RED.

https://s14.postimg.org/hxmkg2rvz/BETA_BASS_PREAMP_MARKUP.png



73’s de Edd

 

[Alec_t]

I'm thinking something on the A side of the preamp is grounding the input signal, and also causing dc to leak to the output on power down.

Have you checked for DC on any of the outputs?

 

[Bud]

Have you checked for DC on any of the outputs?

Hi Alec,

Thanks for replying. My o-scope is out of commission, but I checked for DC on the output with my multi-meter, which showed okay, but it will not reveal transient DC. Right now all I have is my ears for the transients (and a very forgiving speaker) and I can hear the pop on Channel B when I power down. So I'm guessing DC is hitting the outputs in that scenario, at least. How much is hard to say. It's a loud pop with fast attack, so definitely not right. But it's not nightmarishly loud.

 

[Bud]

Mark Up Schema . . . . .

Edd, there are no words for this! I opened your file and stared at it for a half hour with my jaw on the floor. This is great!

I had thought last night that I'd build an audio signal tracer today (since my o-scope is down) and I was already wondering how I'd accurately trace the signal, given my limited abilities. I know *generally* where to look, but I was afraid that would not be good enough. This will be a great guide when the time comes. Thank you, Edd, I really appreciate this.

I ran the tests you suggested and an interesting finding occurred. That being, with the foot-switch removed, everything works as before (all LEDs light at the proper times), EXCEPT there is no POP in Channel B on power down. Certainly a clue!

Preamp passes Channel B, seemingly normally (mind you, I'm not cranking anything yet till I get the matched transistors in the output), but Channel A still won't pass signal, no matter which input I use. The channel will hiss louder as I turn up the volume, but no signal.

Another interesting finding is that Channel B will pass signal even when my guitar is plugged into the "BOTH" input, which is not the case with the foot-switch connected. Gotta be another clue.

So, in summary, whether the foot-switch is connected or not, and no matter which input the guitar is plugged into the LEDs do everything they're supposed to do, both on the amp display and on the foot-switch display.

Thanks for all your help, Edd!

Peace,
Bud

 

[Bud]

I've had the opportunity to work some more on my amp. (Edd & Alec, please scroll up as I'm triple-dipping here and I have messages to each of you above).

Using Edd's awesome signal path diagram, I first encountered signal loss at R113 which, after testing, I indeed found to be open in one direction. Realizing the melt down was violent enough to blow out the wire inside the pot, I feared for what lay ahead. Difficult to find pot -- 1M dual split shaft linear taper with the two pots offset 180 degrees for PCB mounting. Oy. I'm gonna have fun hunting that down.

So, I bypassed that with a "setting" of roughly 50% and put my little homemade audio signal tracer back to work. Didn't get very far down the path of Channel A -- nothing at the output of IC101, pin 1. I could hear a little signal with the power off, but as soon as I powered on, the signal was gone, so I figured the op-amp was sending it to ground.

Pin 6 of IC 101 read 13.2VDC which wasn't supposed to be there. Without the right tools, it took me 75 minutes (I kid you not) to get the IC and all the solder out of the 8 holes, clean things, up, and install an 8-pin socket. I saw a guy on YouTube with a de-soldering gun do the same job in 5 minutes. Might have to get me one of those puppies.

After verifying the traces and connections (and no shorts) to the IC socket, I popped in an LM1458 I was lucky enough to have on-hand. Back to my trusty audio probe -- voila! -- signal at pin 1 of IC 101.

Played my guitar thru the amp for awhile, manually switching channels by plugging into A, B & Both frond-end inputs, everything acting as expected LED-wise and sound-wise. No popping in the speaker at power down in any scenario. Had the feeling this would not be the case with the foot-switch connected, however. So, testing that, everything works the same, except I do still indeed get the speaker pop on both channels on power-down.

So my next step is to figure out what's causing the speaker pop at power-down when the foot-switch is connected.

Side note: R25 -- 5W power resistor on PA board runs MUCH MUCH cooler now with Edd's diode idea in place. I may add one more diode to bring the -14.5 up to -15.2, since the opposite regulator is generating just about +15.2. That'll balance it out even more. (Thanks for the tip, Edd!).

Getting there, though! A little at a time