Sunn(0))) Amp Beta Bass Repair

[nevetslab]

I don't think it would be CR2 that's the culprit but thereno harm in checking. The emitter on Q6 is attached the the output buss @ 0V which is a direct connection to the emitter on Q15 which is at -1.4V. the collector on Q15 is 0V which is opposite to the collector of Q6 @ 1.4V. I think Q15 also has its CE reversed. the collector of Q15 should be 0.4V to 0.7V lower than the base of Q16.

Time to put the Ossiloscope into use to get the perfect bias, I do need to clean the output on my func gen. Little adjustment and I should get the 1mV over 0.33Ohm resistor.

I'll be checking the pre amp board properly next to make sure nothings getting hot and see if I can get some sound out of my amp

Ah, you're right...I missed that! I was also puzzling over the base voltages of the two limiter xstrs, then saw the pre-bias string on each, having a 39k resistor to the appropriate supply rail, and the steering diode to Gnd, to account for the near 0.2V/-0.2V potentials at the base. At any rate, neither limiter xstr is conducting. I'll have to ponder the collector voltage on both parts some more.

Cool........you're almost done!

[wirelessliquid]

Well with Q15 reverse I think thats the amplifier board all good, I biased it using my meter but I couldn't get the scope working with it as when its attached to the preamp board there's an oscillation on the input signal so somthings not right. guessing its the preamp from when I reversed the power supply polarity by accident.

If anyone has a decent preamp schematic or board layout it would be very useful.

[wirelessliquid]

Well with Q15 reverse I think thats the amplifier board all good, I biased it using my meter but I couldn't get the scope working with it as when its attached to the preamp board there's an oscillation on the input signal so somthings not right. guessing its the preamp from when I reversed the power supply polarity by accident.

If anyone has a decent preamp schematic or board layout it would be very useful.

[nevetslab]

Well with Q15 reverse I think thats the amplifier board all good, I biased it using my meter but I couldn't get the scope working with it as when its attached to the preamp board there's an oscillation on the input signal so somthings not right. guessing its the preamp from when I reversed the power supply polarity by accident.

If anyone has a decent preamp schematic or board layout it would be very useful.

I just downloaded the Service Manual you had attached on your original post, and while the Bass Preamp Schematic is in two pages, here, I would suggest printing out the pages and then carefully slice off the overhang on one of the two drawings, as there is overlap at the middle of the schematic, and paste the two together into an 11 x 17 drawing (that at one time was an 11 x 17 fold-out page). Same with the layout drawings. There's a good circuit theory of operation presented at the beginning of the manual, and while it's written with respect to the Lead Preamp, which has the reverb circuitry, a quick look at the circuits, they look identical in topology, and may only differ in some component values in the EQ section, and of course with the addition of the Reverb circuit for the Lead version. I haven't stared at it long enough to know. But, they do also give you layout for both preamp versions. Probably as good as you'll find at this point in time.

I wasn't aware of the preamp using CMOS Hex Inverters following the input stage of each channel. No idea how those fared with the supply reversal. I'd imagine you can still find 74C04 CMOS Hex Inverters and the MC14506 BCP Dual Expandable And/Or Inverter gate...though that one might take longer to find. Assuming these aren't current production IC's, it leaves you with searching thru alternate sources, including ebay if the parts aren't functioning.

As you have oscillation occurring, have you tried plugging into the Power Amp jack itself, which interrupts the Preamp from feeding it. Does the oscillation go away on the output of the power amp? If so, then, you can work on the preamp circuit with a phone plug wired to short the power amp input (Shorting Plug), so it dead-patches the power amp. Then, you can go thru from input thru output of each channel and see where the oscillation is occurring.

Do you have a X10 scope probe? Sometimes a X1 probe will cause high impedance circuits to misbehave when loaded by them.

If you had to replace all of the IC's in the preamp, I hope you took the opportunity to install IC sockets, assuming there weren't any installed originally. That will make life easier when trying to find where the oscillation is occurring, by being able to lift out IC's that are suspect.

[wirelessliquid]

I've attached the schematic and the PCB layout, knocked them together in paint and they print on A3 pretty good, not perfect but better than half a page

Would you recomend testing each chip or powering the board up and seeing what gets hot, I dont think they would have coped too well. I'm not sure where the electrolytic are in the circuit but I guess its good to check every polarised and active component incase it had a full supply reversal.

If anyone knows a good place to get replacement components that would be very useful

I'm pretty sure I've got a X10 lead at home, should I use it on the input or output? I've only got the 1

Yeah that was my plan, I did this with the IC in the last circuit

[nevetslab]

[ATTACH=CONFIG]51080[/ATTACH][ATTACH=CONFIG]51081[/ATTACH]

I've attached the schematic and the PCB layout, knocked them together in paint and they print on A3 pretty good, not perfect but better than half a page

Would you recomend testing each chip or powering the board up and seeing what gets hot, I dont think they would have coped too well. I'm not sure where the electrolytic are in the circuit but I guess its good to check every polarised and active component incase it had a full supply reversal.

If anyone knows a good place to get replacement components that would be very useful

I'm pretty sure I've got a X10 lead at home, should I use it on the input or output? I've only got the 1

Yeah that was my plan, I did this with the IC in the last circuit

Those two drawings came out great! I copied them to my Sunn folder.

As to IC's getting hot, that's also a good indicator that the part is shorted. You'd also find the low voltage bipolar supplies being dragged down as a result. If you have +/- 15V on the preamp, then you probably don't have any IC's hard shorted. I'd first check with the DMM in DCV 20V range, to see if the outputs from the 4558 Dual Op Amps (pins 1, 7) are all at / near 0VDC. Typically +/- 10mV or less. The 74C04 Hex inverters are different....they are single-supply devices, and are powered at 8VDC, so the outputs will be half-way or there-abouts, so you'll see around 4VDC on their outputs. Each set of 3 circuits are AC coupled between the driving op amp and the receiving op amp with capacitors, while DC-coupled between them.

You only need to be looking at the output of the IC's. I'd start from the preamp output(s). Insert a shorting plug into the 'From Accessory' jack (which is the input to your power amp). The two preamp channels are summing together just ahead of IC112A, which is where the Master Level is located (100k Lin pot in the feedback loop). Turn the Ch A and Ch B Level pots up fully CW. With your scope probe, see if there's oscillation present at junction of R203 (150 ohms) and the Master Level pot R204 / 'To Accessory Jack'.

If you find oscillation there, then with additional phone plugs (open, closed, don't matter), insert one into the Channel A 'From Accessory' jack, which will interrupt the Ch A connection to the Master level summing amp IC112A. If oscillation still there, then do the same with Channel B 'From Accessory' jack, and remove the plug from the Ch A jack, which re-connects Ch A preamp and interrupts Ch B preamp. If oscillation still present, then, verify it goes away with each channel when you turn the Level controls down CCW. Those level controls don't turn the output down to zero like a volume control does, they just reduce the gain to nothing, but the op amps are still able to pass signal if they're oscillating.

It may be just one (or more) IC's that are oscillating, and the common coupling path is the power supplies. I don't see the presence of local power supply bypass caps at each Op Amp or near by. But, they're just RC 4558 Dual Op Amps, which are not real fast, and usually not prone to oscillation. I'm not sure about the 74C04 Hex Inverter IC's, though. I've never seen them used as a linear signal amplifier stage as Sunn has employed them.

The FET switches Q103, Q104 and Q105, Q106 are the channel switches controlled by IC106, and apparently controlled by the foot switch. I don't know if there's a local switch on the front panel to do that. It might be if oscillation is further up the system ahead of the tone circuits, that it's in the Drive Circuits which have the 74C04's in them. Anyway, this will get you started. Good hunting.

[wirelessliquid]

Been a little while I got really busy ended up fixing a digidesign rack 003+ for a mate, never done SMD stuff before but got a win

So I tested the preamp board on the bench using the dual supply and was getting this reading from it,+/-15V @ 0.06A. So no shorts which is a win but with that current draw I dont think any of these IC's have turned on.


A quick test on the power for each IC shows this;
(Note all had around -3V to -4V on the op amp inputs and the correct +/-15V)
IC112 - PIN1 -4.25V, PIN7 -3.74V
IC110 - PIN1 -3.70V, PIN7 14.37V
IC111 PIN1 -3.77V, PIN7 -3.74V
IC108 PIN1 -3.76V, PIN7 -3.75V
IC109 PIN1 -3.75V, PIN7 -3.54V
IC107 PIN1 -3.75V, PIN7 -3.77V
(these two op amps had almost nothing on there inputs)
IC101 PIN1 0.45V, PIN7 0.428V
IC102 PIN1 -0.06V PIN7 -0.04V

IC105 PIN14 7.22V PIN7 -3.7V
IC104 PIN14 6.75V PIN7 -3.7V
IC103 PIN14 -3.78V PIN7 -14.89V

IC106 PIN13 -3.7V PIN5 -3.78V

Looking at all of the IC's they all have this -3.7V on the ground pin. I dont really know where to starting looking for this as all of the ground is conencted together.

EDIT: I'm pretty sure the grounding issue is what has caused those ossiclations in the amp stage

[nevetslab]

Been a little while I got really busy ended up fixing a digidesign rack 003+ for a mate, never done SMD stuff before but got a win

So I tested the preamp board on the bench using the dual supply and was getting this reading from it,+/-15V @ 0.06A. So no shorts which is a win but with that current draw I dont think any of these IC's have turned on.
[ATTACH=CONFIG]51162[/ATTACH]

A quick test on the power for each IC shows this;
(Note all had around -3V to -4V on the op amp inputs and the correct +/-15V)
IC112 - PIN1 -4.25V, PIN7 -3.74V
IC110 - PIN1 -3.70V, PIN7 14.37V
IC111 PIN1 -3.77V, PIN7 -3.74V
IC108 PIN1 -3.76V, PIN7 -3.75V
IC109 PIN1 -3.75V, PIN7 -3.54V
IC107 PIN1 -3.75V, PIN7 -3.77V
(these two op amps had almost nothing on there inputs)
IC101 PIN1 0.45V, PIN7 0.428V
IC102 PIN1 -0.06V PIN7 -0.04V

IC105 PIN14 7.22V PIN7 -3.7V
IC104 PIN14 6.75V PIN7 -3.7V
IC103 PIN14 -3.78V PIN7 -14.89V

IC106 PIN13 -3.7V PIN5 -3.78V

Looking at all of the IC's they all have this -3.7V on the ground pin. I dont really know where to starting looking for this as all of the ground is conencted together.

I agree with your thoughts on the grounding. There's apparently no ground from the power supply to the preamp board ground. I see three ground connections on the right-hand side of the layout drawings, so you'll have to check to see where all of the circuit components that go to the ground plane are connecting to, and get them connected up and returned to the power supply ground. I wouldn't have expected this, but, it may just be a broken wire or broken trace. I've been bitten by those issues enough times. Glad to hear no bad IC's, and that all are suffering from this common connection issue. Good look in the hunt.

[wirelessliquid]

Yeah, I'll try a different ground spot on the board but I had the ground on the power supply conencted to the gound on the channel B input

[Reader]

Yeah, I'll try a different ground spot on the board but I had the ground on the power supply conencted to the gound on the channel B input

Since the preamp ICs seem to have a strange grouding issue, how about an investigation like this. With all power off connect your continuity meter to the IC ground pin and then probe around with the other test lead to trace where the ground actualy goes (or does not go)?

[nevetslab]

Since the preamp ICs seem to have a strange grouding issue, how about an investigation like this. With all power off connect your continuity meter to the IC ground pin and then probe around with the other test lead to trace where the ground actualy goes (or does not go)?

A variation of this method......since the IC's do NOT have ground terminals.....select the ground points of the different preamp stages' components that go to ground, and do the same.

[wirelessliquid]

Its been a while since I've had a chance to post, but looking at the baord the ground to ground checks out on the meter with all grounds being the same. I've got most of the IC's off the board not and most of the op amps are blown. I'm not sure if the 3.7V was coming from one of the OP amps as my mate said hes seen this happen a few times with bad chips making a bad ground.

I've had to get teh 14 Pin chips from ebay MC14506 & 74C04-4069, to make sure there good before placing them back in, how should I test these? like is there a circuit I could build with some LED's to test they would or can you do a test with a meter?

[nevetslab]

Its been a while since I've had a chance to post, but looking at the baord the ground to ground checks out on the meter with all grounds being the same. I've got most of the IC's off the board not and most of the op amps are blown. I'm not sure if the 3.7V was coming from one of the OP amps as my mate said hes seen this happen a few times with bad chips making a bad ground.

I've had to get teh 14 Pin chips from ebay MC14506 & 74C04-4069, to make sure there good before placing them back in, how should I test these? like is there a circuit I could build with some LED's to test they would or can you do a test with a meter?

Sorry to hear the outcome on the power supply reversal did that much damage on the preamp that was attached. I can't recall if the two pass xstrs Q1 & Q11 survived....I had thought if they did, the +/- outputs wouldn't have come up and the preamp components remain ok. So much for hoping.

The 74C04 Hex Inverters are easy to test. Any chance you have a Proto-board....one of those white plastic things loaded with rows and rows of terminal sockets for plugging in IC's, components to build test circuits, where you have power and ground rails to attach parts to and such? That's the easiest way. Wiring up a 14 pin IC socket on some perf board is a bit more tedious, but will do also for this part. Though, if you have unpopulated the preamp PCB of the op amps, using that board, disconnected from the power amp would make a good test bed. Though in this case, Sunn set the 74C04 Hex Inverters as linear devices and not logic devices. No matter. With one plugged in, starting with IC104 1/6, connect signal to the at R118/C106. The output at pin 2 will be phase-inverted, then at pin 4, it will invert again, and at pin 6, it inverts that again. That same input signal also gets to the hex inverter at pin 13, and at pin 12, it will be inverted. The last two inverter stages of IC104 aren't used, and their inputs grounded. You'd do the same on IC102...same drill.

Now, also using your PCB as the test bed, you can test the MC14506. Ah, there's IC103, working together with IC106, being that MC14506. Logic circuits aren't one of my solid foundations, for some reason, so I have to move slow with the basics so I know what to expect. I do own a circuit tester, but it's at another shop setup, that's been inactive for several years, so I don't have it. With one of those, you plug your unknown device into it, hit the go button, and it looks up the part in it's memory, tests the part and tells you good or bad. Here, we have to learn what it does, I'm afraid. But, I think Sunn provided the Theory of Operation on this part of the circuits....checking the manual. Yes....looking at the manual, starting with Page 5...it shows the block diagram of the preamp, then discusses it on 6 & 7. On page 7, they explain how the MC14506 works in their switching circuit. I'd follow that.

When you replace the op amps, use IC sockets, in case Sunn didn't already do that. Makes swapping out parts so much easier. With fresh op amps installed, that 3.7V level may now be gone, so with fresh logic IC's as well, I'd think re-installing the fresh parts will have the preamp back up and running without having to learn all this digital logic.

[wirelessliquid]

Yeah I think 1 or tow of the op amps survived but I don't expect them to be great after all the others died so I think its best to replace them, I've got loads spare. It was the positive voltage that leaked through to 45V and all of the 14 pin chips are positive supply so i expect them to have taken a beating aswell. A propper test should show how they are.

Yeah I've got proto-board (bread board) so it looks like any input signal you place onto it should be 180 degree phase reversed? that should be simple to test

I've done some work on logic circuits before, easiest way may be to place it on a bread board with its powered supply and using jumpers to 3.5V - 5V to test all of the logic gates using a logic probe? although I'll have to get my hands on a logic probe.

I've brought risers for all of the IC's to try and make this job alot easier. Will the risers cause any grounding issues or noise as its not a strait contact with solder, this could make the amp sound a little worse right? I'll leave them in for now to get it running properly first

[nevetslab]

Yeah I think 1 or tow of the op amps survived but I don't expect them to be great after all the others died so I think its best to replace them, I've got loads spare. It was the positive voltage that leaked through to 45V and all of the 14 pin chips are positive supply so i expect them to have taken a beating aswell. A propper test should show how they are.

Yeah I've got proto-board (bread board) so it looks like any input signal you place onto it should be 180 degree phase reversed? that should be simple to test

I've done some work on logic circuits before, easiest way may be to place it on a bread board with its powered supply and using jumpers to 3.5V - 5V to test all of the logic gates using a logic probe? although I'll have to get my hands on a logic probe.

I've brought risers for all of the IC's to try and make this job alot easier. Will the risers cause any grounding issues or noise as its not a strait contact with solder, this could make the amp sound a little worse right? I'll leave them in for now to get it running properly first

Yes....from the mass failure, I'd pull the remainders out and start fresh, as you surmised.

Risers? IC sockets I presume. Hadn't heard that term before. No problems in using them, unless they're old and corroded. If they're decent quality, hold the IC's in firmly, you're good to go. Good soldering on all them, and you're as good as Sunn had designed it.

I'd be tempted to install the risers, install fresh IC's thruout and give it a go, dispensing with the pre-test. Years ago, I bought a nice HP Logic Probe, Logic Pulser and Logic clip in its' case.....I can't recall when I've actually used it. I've used a DMM DC Volts mode instead, just because that was the most used instrument on the bench. I've gotten more use out of the HP Logic Lab unit, which has a number of the proto-boards, and has 5V logic supply, several logic sources that can be turned on and off, patched where needed.