Sunn(0))) Amp Beta Bass Repair

[g1]

I took reading of all the test points on the circuit and what they should be on the schematic if this help at all.
Actual Voltage
tp 1 = 0.8VDC
tp 2 = 6.4VDC
tp 3 = 10.07VDC
tp 4 = -15.98VDC
tp 5 = -18.54VDC
tp6 = -17.21V

Schematic Voltage
tp1 = 2.2V p-p
tp2 = 0.7V p-p
tp3 = 0.9V p-p
tp4 = 60v p-p
tp5 = 60v p-p
tp6 = 60v p-p

these are both vastly different which is confucing me as all semi conductors are working and I can't find any shorts

Please do not confuse AC and DC voltages listed on the schematic. If it says DC on the schematic, then use your DC meter range. If it says p-p on the schematic, then those are AC voltages with specified AC signal at input and controls adjusted as specified. To (roughly) convert p-p numbers to what your meter will say on AC range, divide the p-p number by 2.8
Also, I'm not sure if you are running straight off the wall voltage now, but if not, you need to mention it when posting any voltages.

From what I can tell, the voltages on the schematic in the rectangular boxes are p-p AC signal voltages. There are DC voltages listed on the schematic, but I don't think they are in boxes, so you have to look for them.

[wirelessliquid]

Please do not confuse AC and DC voltages listed on the schematic. If it says DC on the schematic, then use your DC meter range. If it says p-p on the schematic, then those are AC voltages with specified AC signal at input and controls adjusted as specified. To (roughly) convert p-p numbers to what your meter will say on AC range, divide the p-p number by 2.8
Also, I'm not sure if you are running straight off the wall voltage now, but if not, you need to mention it when posting any voltages.

From what I can tell, the voltages on the schematic in the rectangular boxes are p-p AC signal voltages. There are DC voltages listed on the schematic, but I don't think they are in boxes, so you have to look for them.

Yeah that has confused me a little, I read it on the instruction on the schematic as all voltages as DC, I tried reading with AC to compare but I was getting some weird stuff out I'll have to take these voltages again in that case

At the moment the voltages taken were from using a power supply, I'll mention if I'm taking them off the wall but I'll be using my light bulb current limiter to try to stop any short circuits

[g1]

That is ok but you need to specify when you are not using wall voltage.
Comparisons to the schematic are not valid unless wall voltages are used. You can look at relative differences I suppose, but you should include your voltage source (bulb limiter etc.) when posting voltages if not connected directly to wall AC.

[nevetslab]

Calculated voltages/currents thru the power amp circuit

Yeah that has confused me a little, I read it on the instruction on the schematic as all voltages as DC, I tried reading with AC to compare but I was getting some weird stuff out I'll have to take these voltages again in that case

At the moment the voltages taken were from using a power supply, I'll mention if I'm taking them off the wall but I'll be using my light bulb current limiter to try to stop any short circuits

Working from the published DC voltages on the schematic, I’ve prepared a list showing approximate voltages to expect at each of the stages in this Sunn Power Amp circuit, based on using the +/- 30VDC lab supply powering the amp, and no load, and all of the parts functional.

Q2 base = 0VDC
Q2 emitter = -0.6VDC
Q2 collector= 6.7VDC
Q2 current = 3.06mA

Q3 base = 0VDC
Q3 emitter = -0.6VDC
Q3 collector=6.7VDC
Q3 current = 3.06mA

Q4 base = 10.0VDC
Q4 emitter = 9.4VDC
Q4 collector= 28.8VDC

Q12 base = 5.1VDC
Q12 emitter = 5.7VDC
Q12 collector = -28.8VDC
Q4/Q12 current string = 685uA

Q5 base = 28.8VDC
Q5 emitter = 29.4VDC
Q5 collector=1.7VDC

Q14 base = -28.8VDC
Q14 emitter = -29.4VDC
Q14 collector= -1.7VDC
Q5/Q14 current string =4mA

Q7 base = 1.7VDC
Q7 emitter = 1.2VDC
Q7 collector= 30VDC

Q16 base = -1.7VDC
Q7 emitter = -1.2VDC
Q7 collector= -30VDC
Q7/Q16 current string = 2.14mA

Q8 base = 1.2VDC
Q8 emitter = 0.6VDC
Q8 collector= 30VDC

Q17 base = -1.2VDC
Q17 emitter = -0.6VDC
Q17 collector= -30VDC
Q8/Q17 current string = 40mA

Q9 base = 0.6VDC
Q9 emitter = 10mVDC?
Q9 collector= 30VDC

Q18 base = -0.6VDC
Q18 emitter = -10mVDC?
Q18 collector= -30VDC
Q9/Q18 current string = 30.3mA?

The output stage voltages called out on the schematic lead to some inaccuracies, and are subject to where the bias pot R42 is set. If R42 is adjusted for 375mV-500mVDC across the emitter resistors for Q8 / Q17, which also is the base voltage on the output xstrs, the voltage across the emitter resistors for Q9/Q18 & Q10/Q19 will vary, but typically around 10mVDC or less.

[wirelessliquid]

I'm surprised with +/- 18V at the bases of Q7 & Q16 you didn't see high current being drawn. Only way I could think that would happen would be if the emitter resistors opened on the driver and outputs.

Does your schematic show DC voltages on components thruout the schematic? On mine, for instance, it shows -1.7VDC on the collector of Q14, which is the base of Q16. It also shows 1.2VDC on base of Q8/Emitter of Q7, and -1.2VDC on base of Q17/emitter of Q16. Those are your target voltages, which in adjusting the bias pot R42 should achieve. With regards to the 1mV across R18, that sounds typical. I've always been adjusting by watching the voltage of the emitter on the previous stage.......that across R17 & R47, where a typical bias voltage would be between 375mV and 450mV. Another means of adjusting bias is to watch the mains current of the amp being adjusted, and set it to where the mains current just begins to increase. You could do similar, watching the current readings on your lab supply as you adjust R42. I'd monitor both the voltage on either/both R17 & R47 while watching for current increase. This adjustment controls the crossover distortion, setting the output stage bases close to but not at the point of where they turn on and begin conducting current.

100W @ 4 ohms is 20V RMS, same as 50W @ 8 ohms.

Sorry about the solder pad lifting issues. All of us go thru that horror, and seek ways to combat it during service. That aspect is a real PITA, worse on some gear than others due to the quality of the PCB. I've often had to resort to using short EZ-Hook cables to clip parts in and out to avoid the constant heating of the solder pads until the solution is at hand.

Yeah I've found these voltages on my schematic but there aren't many, just on a few transistors. The weird thing is when adjusting on this pot the current will draw very heavy on the positive voltage until it trips on the power supply, if you give the positive the extra current flow then the positive side steadies out and the negative side shoots up massively and vice versa. I'll have to check for an open resistor on the drivers but nothing obvious on the board, nothing popped and no bad smells.

Okay yeah that make sense

Yeah i guess the 30 year old PCB doesn't hold up to the heat to well today, So far I've managed to keep them on the PCB for now if they pop off completely I'll have some trouble. Any of them I'm unsure about I make sure to check there no resistance over the track to that component in case the track breaks

[wirelessliquid]

With TP4 being -16V, I suspect R13 is open (emitter resistor for Q5), or Q5 has failed. That would be consistent with the output sitting at -17.2V. As you previously had +18V and -18V at TP4 & TP5, you may have open emitter resistors now in the output stages. Verify R16/R46, R17/R47, R18/R48, R21/R50.

With TP 1 (base of input xstr Q1) at 0.8V, I'm wondering if that's a result of IC1B trying to compensate the negative output voltage at TP6. I'd lift R22 out of circuit, or, if IC1 is on a socket, remove it.

With the output TP6 at -17.2VDC, seeing TP5 at -18.54V is good, as that tells you Q17 & Q16 are ok.

Looking at the voltages stated on the schematic, it shows 500mV across the emitter resistor R38 of Q14, which is 3.3mA. You should have that same voltage across Q5's emitter resistor R13. All the essential DC voltages are shown on the schematic to guide you, though I certainly understand the frustration level when so many aren't in agreement. That's part of the learning curve in understanding how these things work. With feedback in the system where a circuit isn't working, I've had to open the loop and make connections to force DC stability to find the parts that aren't working, even when my DMM is telling me parts suspect show good semiconductor junctions. We haven't gone there yet, but you're getting close. Remember, we're still trying to recover from having the supply voltages reversed. You're making progress.

I think R13 will be the likely suspect but I'll have a look tonight as it may be Q5 also, I'll make sure to double check these resistors aswell.

Yeah I added a socket for IC1 as I didn't want to solder one in and damage it, make life far easier , Theres some sockets on the board for some of the transistors I may buy some of these and pop them in, the transistors that regularly blow already have these types of socket. I'll pop IC1 and see if it makes a difference

Yeah I checked the output stage none of them have blown up and there no being turned on hard which is a good sign I guess

It's fustrating but It's fun to work out how this thing function and to fault find on it. I need a bit of help understanding the full circuit I can see a few little circuits in the schematic itself but there's a few little bits I'm unsure what they do. I've ordered a transistro and diode tester that should show the amplification of different transistors and there characteristics which can test my transistors at high voltages than my fluke so I should be able to see if any of them have gone leaky

[wirelessliquid]

Working from the published DC voltages on the schematic, I’ve prepared a list showing approximate voltages to expect at each of the stages in this Sunn Power Amp circuit, based on using the +/- 30VDC lab supply powering the amp, and no load, and all of the parts functional.

Q2 base = 0VDC
Q2 emitter = -0.6VDC
Q2 collector= 6.7VDC
Q2 current = 3.06mA

Q3 base = 0VDC
Q3 emitter = -0.6VDC
Q3 collector=6.7VDC
Q3 current = 3.06mA

Q4 base = 10.0VDC
Q4 emitter = 9.4VDC
Q4 collector= 28.8VDC

Q12 base = 5.1VDC
Q12 emitter = 5.7VDC
Q12 collector = -28.8VDC
Q4/Q12 current string = 685uA

Q5 base = 28.8VDC
Q5 emitter = 29.4VDC
Q5 collector=1.7VDC

Q14 base = -28.8VDC
Q14 emitter = -29.4VDC
Q14 collector= -1.7VDC
Q5/Q14 current string =4mA

Q7 base = 1.7VDC
Q7 emitter = 1.2VDC
Q7 collector= 30VDC

Q16 base = -1.7VDC
Q7 emitter = -1.2VDC
Q7 collector= -30VDC
Q7/Q16 current string = 2.14mA

Q8 base = 1.2VDC
Q8 emitter = 0.6VDC
Q8 collector= 30VDC

Q17 base = -1.2VDC
Q17 emitter = -0.6VDC
Q17 collector= -30VDC
Q8/Q17 current string = 40mA

Q9 base = 0.6VDC
Q9 emitter = 10mVDC?
Q9 collector= 30VDC

Q18 base = -0.6VDC
Q18 emitter = -10mVDC?
Q18 collector= -30VDC
Q9/Q18 current string = 30.3mA?

The output stage voltages called out on the schematic lead to some inaccuracies, and are subject to where the bias pot R42 is set. If R42 is adjusted for 375mV-500mVDC across the emitter resistors for Q8 / Q17, which also is the base voltage on the output xstrs, the voltage across the emitter resistors for Q9/Q18 7 Q10/Q19 will vary, but typically around 10mVDC or less.

Thanks alot buddy I'll see if I can get my circuit to match these voltages I'm not sure I'f ill be able to read the current draw on these

[nevetslab]

Yeah I've found these voltages on my schematic but there aren't many, just on a few transistors. The weird thing is when adjusting on this pot the current will draw very heavy on the positive voltage until it trips on the power supply, if you give the positive the extra current flow then the positive side steadies out and the negative side shoots up massively and vice versa. I'll have to check for an open resistor on the drivers but nothing obvious on the board, nothing popped and no bad smells.

Okay yeah that make sense

Yeah i guess the 30 year old PCB doesn't hold up to the heat to well today, So far I've managed to keep them on the PCB for now if they pop off completely I'll have some trouble. Any of them I'm unsure about I make sure to check there no resistance over the track to that component in case the track breaks

Yet, with just those few voltages shown, you can derive the rest. And, a couple they gave tell you someone didn't record them accurately, or did so at different times. For example, the two voltages stated on Q14: -44.5VDC on the emitter, and -43.3V on the base. That suggests 0.8V Vbe on an MPS A43. Not likely. With a DMM, which normally sources 1mA constant current with 2 to 3V potential in it's Diode Test Mode (or 2k Resistance mode), we'll see 0.665 typically, which is the Vbe of that junction. Even so, making minor corrections on the list, you'll get the ballpark figures. To arrive at some voltage values, you're calculating IR values for a voltage, and subtracting that from another known voltage, such as finding the collector voltage of Q2 & Q3. You can also see with power transistors, the Vbe is lower @ 1mA current than that of a signal transistor. Such as an MJ15022 NPN Pwr xstr....Vbe @ 1mA = 0.518V.

With regards to your finding current on one side or the other jumping high while adjusting the bias pot, that's NOT normal, unless your current settings on the supply is low. Normally, both sides will increase in the same degree, unless you have a bad xstr or more still in the circuit, or an open resistor. If this was happening while you were showing +/- 18V across the bias circuit/collectors of Q5/Q14, then yes....abnormal condition exists, and find out where!

[wirelessliquid]

Okay so I did a test,
IC1 removed
Checked:
R13 Good
Q5 Good
R16/R46, Good
R17/R47, Good
R18/R48, Good

I tried to get the voltage on the collector of R14 to -1.7V but it was stuck at -18ishV only going up and down by about 50mV before current tripping, although with IC1 removed it didnt seem to trip as much, I left R42 in the center position to get the voltages form the board, I wasn't sure how to check the currents so I left this out and Q7 as the legs were to short and flipping the board was pretty hard to get voltages, I've put ish on some voltages becasue they were all pretty much the same and it was hard to get the reading with the board flipped in one hand.

Your prediction followed by my reading:
Q2 base = 0VDC -0.31VDC
Q2 emitter = -0.6VDC -1.13VDC
Q2 collector= 6.7VDC 6.89VDC

Q3 base = 0VDC -0.48VDC
Q3 emitter = -0.6VDC -1.16VDC
Q3 collector=6.7VDC 7.36VDC

Q4 base = 10.0VDC 9.51VDC
Q4 emitter = 9.4VDC 8.87VDC
Q4 collector= 28.8VDC 28.7VDC

Q12 base = 5.1VDC 5.00VDC
Q12 emitter = 5.7VDC 5.63VDC
Q12 collector = -28.8VDC -28.94VDC

Q5 base = 28.8VDC 28.71VDC
Q5 emitter = 29.4VDC 29.31VDC
Q5 collector=1.7VDC -15.62VDC

Q14 base = -28.8VDC -29.00VDC
Q14 emitter = -29.4VDC -29.62VDC
Q14 collector= -1.7VDC -18.73VDC

Q7 base = 1.7VDC ?
Q7 emitter = 1.2VDC ?
Q7 collector= 30VDC ?

Q16 base = -1.7VDC -18.67VDC
Q16 emitter = -1.2VDC -29.82VDC
Q16 collector= -30VDC -18.14VDC

Q8 base = 1.2VDC 18.6ishVDC
Q8 emitter = 0.6VDC 18ishVDC
Q8 collector= 30VDC 29.8ishVDC

Q17 base = -1.2VDC -17.6ishVDC
Q17 emitter = -0.6VDC -17ishVDC
Q17 collector= -30VDC -28.9VDC


Q9 base = 0.6VDC 18.6ishVDC
Q9 emitter = 10mVDC 18ishVDC
Q9 collector= 30VDC 29.8ishVDC

Q18 base = -0.6VDC -17.6ishVDC
Q18 emitter = -10mVDC? -17ishVDC
Q18 collector= -30VDC -28.9ishVDC

Hope this helps

[nevetslab]

Are Q16 readings instead: base = -18.57VDC, emitter = -18.14VDC, collector = -29.82VDC? Something is definitely pulling the upper half of the voltage gain stage Q5 collector 'south'. At first, I thought Q16 was bad, but think you've mixed up the readings.

The currents are all calculated from the published voltages and derived voltages based on what 'normal' semiconductor junctions would yield.

Ah, I see a mistake I made, in suggesting you lift up R22. If you did, that's why Q2 isn't sitting at 0VDC. With IC1 removed, which you said is socketed, R22 & R39 provides the bias & ground reference to set the base at 0VDC, unless there's a problem with Q2 & Q3. Or, all that is due to the output being pulled 'south'. The collectors of Q2 and Q3 are joined by R26, so Q3's base, being pulled low, pulls it's emitter low, and Q2's emitter follows, pulling Q2's base low. All controlled by the output bus via feedback network R7 & R31.

I'm looking at Q8 & Q9. Both of those parts bases and emitters are high. With the question marks ??? on Q7, and their emitters being 18V above ground, you must have open emitter resistors in the output stage. That would account for those being pulled high, and the rest of what controls the output stage being pulled below ground. Is Q7 inaccessible, or perhaps it has failed, allowing Q8, Q9 & Q10 to be pulled up. Q10 & Q19 would read the same as Q9 & Q18 on a good day.

[nevetslab]

Just checking to see where you're at with Q7, Q8 & Q9, as that's where there is a problem, based on the readings. No problems taking a pause on the project.

Steven

[wirelessliquid]

I've been checking resistors and struggling to find the problem with any of them, I'll check Q7 - Q9 when I have a chance. Been really busy with work and college had to take a little break as to get ready for going on holiday next week, should hopefully have a chance tonight or tomorrow to have another go

Tommy

[wirelessliquid]

I've been looking at the circuit today and every component after TP3 is good

I was wondering about the 90mA draw when testing the circuit idle, is that +15V 90mA and -15V 90mA or 90mA total = +15V 45mA -15V 45mA, I've been testing 90mA total, I havn't gone above this as I dont want to break my cirucit
Should I test 90mA per channel?

[nevetslab]

If you're testing the preamp circuit using the lab supply to source the +/- 15V, 90mA seemed like that was in the ballpark for the current flowing thru the two pass xstrs Q1 & Q11, when the power amp is sourcing the preamp. It would usually be equal current, as the path is from -15V to +15V. If there is a difference, it's probably from utility circuits, where either side is taking that current to ground. Did you find any of the audio IC's running hot, or having DC offset? Normal status is near 0VDC on the output of the Audio IC's.

If all the parts beyond TP3 are measuring good, but, if you are still getting voltages like you stated on Q8, Q9 and Q10, then it could be a part(s) that is leaking/not working under circuit potentials, and simple DMM diode junction tests aren't indicating anything is wrong. Those conditions you stated are incorrect, which led me to wonder about the emitter resistors being open on those xstrs.

[loudthud]

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