Sunn(0))) Amp Beta Bass Repair

[wirelessliquid]

When testing the preamp circuit I'll use the power supply with a +15V 90mA and -15V 90mA, I need to to give the premap circuit a better to test to see if anything is running hot on it, I didn't notise anything before though.

On the amplifier circuit when testing it I'm testing it at +30V 40mA and -30V 40mA, in the bottom of the circuit diagram it says it should draw 90mA. Does this mean +/- 30V 90mA or +/- 30V 45mA? I notised when I try to bias it as the voltage goes down the current shoots up but I don't want to put to much current in and break my circuit so I havn't tested it at +/- 30V 90mA. Could I be testing at too low a current the bias wont let me get the +/- 1.8V?

I'm waiting for a transistor tester to arrive in the post that can test up to working voltages to see if theres leakage, it should also tell me the gain of the transistors so I will be able to check if they are good. I've checked almost every resistor on the board and defiantly all of the emitter resistors I can't find any bad ones. I've also been checking for any caps that have shorted out and still not seeing any.

[wirelessliquid]

Beta series preamps draw more current than you would expect on the -15V to run the footswitch and the LEDs. A couple of the opamps act as power supply regulators for the CMOS analog circuits so their outputs are around +/- 4V. The CMOS channel switching logic runs from -15V to ground. Note the resistors in the collectors of the +/- 15V regulators. The Voltages across those resistors tells you what the current draw should be (when the preamp is connected).

If the channel switching IC is bad, it will get hot. Before you replace it, check that the channel LEDs follow what jack a cord is plugged into. There are CMOS inverters that actually detect when a cord is inserted by looking for a ground on the Ring contact of the stereo input jack. These can go bad and not give the MC14508 the right inputs. If the chip is bad and you can't find a replacement, look here: https://forum.sunnstillshines.online...29374#msg29374

Cheers I'll keep it in mind is the channel switching IC on the amplifier board?

[nevetslab]

It would be +/- 30V @ 90mA (same current flowing thru both supplies....actually flowing out the -30V port and into the +30V port, as current flows from low to high potentials. It could be that you need to increase the current range on your supply to get the +/- 1.8V at the base of the pre-drivers. I would monitor the voltage across the emitter resistors of the power xstrs Q9/Q10/Q18/Q19, as I wouldn't expect the power amp circuit to be pulling 90mA....unless you're also powering the preamp from the amps' +/- 15V outputs.

The Driver stage Q8/Q17 can pull around 40mA. All the preceding stages are low current. Seeing the supply voltages go down as the current goes up, you've hit the current limit of the supplies, and it folds back, lowering the output voltages. Increasing the current limit to 60-70mA, does it still fold back to get your bias voltage at the Q7/Q16 bases? Your clues are looking across the emitter resistors of Q7/Q16, Q8/Q17 & Q9-Q10/Q18-Q19. Sunn set the emitter resistor values so it would work, and their nominal bias at the bases of Q7/Q16 show +/- 1.7V. It might be better a bit lower. The higher the base voltages become, the more you are turning on the output stage, and it will pull more current. Normally, just a tiny bit of current will flow in the output stages Q9-Q10/Q18-Q19.

[wirelessliquid]

I have a feeling not increasing the current enough may have been my problem then as I had them at 30-40mA either side of the power supply rather than 90mA, hopefully this will allow me to get +/- 1.8V.

In the drawing it says to test the circuit without it being connected to the pre amp board with the 90mA draw, it doesn't look like it would be expected but this could be the case? Maybe IC1 causes most of this current draw? as there is a large difference between having it in the circuit and having it out the circuit

My transistor tester arrived and it showed the HFe of the transistors in the output state to be correct against the datasheet so I have a feeling it must be a low current, I'm going to turn it up and hopefully not destroy my circuit once again

[nevetslab]

Have you already replaced IC1? I do recall one of the diodes on the clipping indicator ckt IC1A...CR6, I think it was had failed. IC1B provides a slow DC integrator to continuously drive the amp towards 0VDC Offset...at least that's it's intended purpose. With it removed, the DC offset will land where it does, usually less than +/-100mV, and more likely less than +/- 25mV on the output of the amp.

If +/-1.8V on Q7/Q16 bases don't cause too much sudden increase in supply current, then good. If it does, the Sunn spec indicates +/- 1.7V, and maybe a bit less if you're still seeing too much current increase. Bias nominal would be where you just start seeing output current to flow, as seen measuring across the output stage emitter resistors. (you can measure from emitter to emitter across the NPN/PNP pairs).

[wirelessliquid]

I've been testing without it the past couple times but I have like 20 of them spare, was cheaper to get them in bulk, I'll make sure to pop it back in.

Would you measure the voltage between the last emitter of the NPN and PNP? What should I expect to see nothing?

[nevetslab]

I'd shoot for about 10mV between the NPN emitter and the PNP emitter. That would result in 15mA per output pair, or 30mA thru the output stage. You'd have about that flowing in the driver stage Q8/Q17. Some companies set the bias with a distortion analyzer, as the bias effects the crossover distortion. I had to stop to look at the photo and layout to see what the driver package was. TO-3's. Yeah, you'll be fine a bit of driver current. They have to source the base current to the outputs anyway, so they can handle the current on signals when the outputs aren't turned on, driving speakers.

[wirelessliquid]

Okay so I tested it only to find IC1 is getting really hot and Q16 is getting hot but just seems to be running warm as it hasn't broken, turning the bias pot doesn't seem to change the +/-18V at all. The bias pot will let me almost turn off the current draw on the negative supply but turning it the other way just makes the positive supply current pull way high.

[wirelessliquid]

Emitter to Emitter
Q7 - Q16 14.15V
Q8 - Q17 1.1V
Q9 - Q18 0V
Q10 - Q19 0V

I've got time to get back in the swing of fixing this now I'm back from holiday

[nevetslab]

Emitter to Emitter
Q7 - Q16 14.15V
Q8 - Q17 1.1V
Q9 - Q18 0V
Q10 - Q19 0V

I've got time to get back in the swing of fixing this now I'm back from holiday

With Q8-Q17 emitter-emitter measuring 1.1V, that seems ok, but with Q7-Q16 emitter-emitter measuring 14.15V, it doesn't sound possible, since that's Vbe of WAY more than 0.7V for either Q8 and Q17. I'd expect Q7-Q16 E-E being about 2.3V. Something is still wrong here. At first, I suspected the bias pot is bad, but since turning it in one direction causes the positive supply current to go way high, there's still one or more bad semiconductors in the output stage, or now there's an open circuit or more from the multiple removal/re-installing of parts. Sorry this is being such a PITA.

And, with IC1 running really hot.....removing it for now might be the thing to do. Q2 will still get bias via R22 and R39 (10k + 27 ohms to gnd). Was the output buss way far away from 0V? That would drive IC1B to producing a correcting voltage, with a 27 ohm load, so that could be the reason for that.

What happens if you jumper Q5-Q14 collectors together? I'd now expect no voltage between Q8-Q17 emitters. And, no voltage between Q8-Q17 bases.

[wirelessliquid]

I had a play last night, seeing as the output stage looked shot and took all of the TO3 transistors out. All of the MJ2955 had gone leaky and one of the 2n3055 had gone leaky. When testing the new mj2955 with my transistor tester it shows the whole bag of them I have to be leaky, all with HFe of around 200 whereas the datasheet shows them to be HFe 20 - 70.

I've ordered some new ones from someone else guess its what I get for using parts from ebay¬.¬ I think this is what has been causing the +/-18V and I think I destroyed the two 2n3055 by turning the bias pot with the leaky opposite side. I'm hoping none of the tracks have gone open circuit at the moment I'm getting better and quicker at soldering this think and not destroying the pads now.

I've removed IC1 for the moment and currently testing the other transistors against there datasheets to make sure they are all functioning correctly.

[J M Fahey]

I LOVE 2N3055, used them from 1969 to about 2004/2006 so some 35/37 years to build some 8000 amplifiers ... and yet I had to give up on them.
From then on, ALL FAKES, and even worse, the few real ones which are bought straight from Importer/Distributor are *down*graded from original specs.

Old ones from 60´s to 80´s were Hometaxial, which are "slow" (ft 800kHz) and ROBUST, SOA (safe area operation) is large.
Then process was changed to way faster and more economic Epitaxial process which makes "fast" transistors (>3 MHz) but WIMPY.

Real snowflakes

The point is even if you buy 2N3055 from Mouser (I used to buy straight from Arrow or Avnet, both huge worldwide suppliers) so you get real ones ... they are NOT the same as what you find in old Peavey, Sunn, Acoustic, etc. by any means.

And to boot, getting them from EBay means you have a Death Wish

I suggest you use at least MJ15015, MJ15003 or MJ1502x (what Enzo stocks as "universal" replacement) and bought straight from Mouser or Digikey.

You are NOT saving a cent buying elsewhere.

[nevetslab]

I LOVE 2N3055, used them from 1969 to about 2004/2006 so some 35/37 years to build some 8000 amplifiers ... and yet I had to give up on them.
From then on, ALL FAKES, and even worse, the few real ones which are bought straight from Importer/Distributor are *down*graded from original specs.

Old ones from 60´s to 80´s were Hometaxial, which are "slow" (ft 800kHz) and ROBUST, SOA (safe area operation) is large.
Then process was changed to way faster and more economic Epitaxial process which makes "fast" transistors (>3 MHz) but WIMPY.

Real snowflakes

The point is even if you buy 2N3055 from Mouser (I used to buy straight from Arrow or Avnet, both huge worldwide suppliers) so you get real ones ... they are NOT the same as what you find in old Peavey, Sunn, Acoustic, etc. by any means.

And to boot, getting them from EBay means you have a Death Wish

I suggest you use at least MJ15015, MJ15003 or MJ1502x (what Enzo stocks as "universal" replacement) and bought straight from Mouser or Digikey.

You are NOT saving a cent buying elsewhere.

Thanks, Juan! No wonder his readings continued to be squirrely. Now let's see if fresh solid output xstrs do for this. MJ15022G's & MJ15023G's will work just fine here, including the drivers Q8/17.

[J M Fahey]

Yes, those are GOOD.

Cost is not only transistors themselves but possible putting 40/45V DC on speakers, burning voice coils and requiring full reconing or straight replacement.
We are talking at least $50/70 per speaker.

Way more than a couple extra bucks spent on good transistors.

[nevetslab]

Yes, those are GOOD.

Cost is not only transistors themselves but possible putting 40/45V DC on speakers, burning voice coils and requiring full reconing or straight replacement.
We are talking at least $50/70 per speaker.

Way more than a couple extra bucks spent on good transistors.

Early on in this thread, wirelessliquid indicated he's only tested the amp without a load, with continual failure, so I/We trust speakers won't get attached until this amp is finally an amplifier again. For sure don't need burned-out voice coils! I've got him running it on a dual tracking 0-30V lab supply so it folds back when current cranks up.