Sunn(0))) Amp Beta Bass Repair

[wirelessliquid]

Ignor my last post I figurered that out haha, test leads the wrong way round

Tested the circuit and I'm getting the correct + and _ 15V out on the rights pins,
Tests points 1, 2, 3 look good but TP4 is 18V rather than 60V is this normal.

[g1]

That's 60V peak to peak signal voltage. If you are set up however they instruct for the AC signal test, and your TP1,2,&3 are correct (AC p-p), then no, 18V p-p at TP4 is not normal.

[nevetslab]

Ignor my last post I figurered that out haha, test leads the wrong way round

Tested the circuit and I'm getting the correct + and _ 15V out on the rights pins,
Tests points 1, 2, 3 look good but TP4 is 18V rather than 60V is this normal.

[ATTACH=CONFIG]50588[/ATTACH]

Looking at the schematic again, is your DCV at base of Q7 about 1.8VDC & base of Q16 about -1.8V? That's about what you should have. As I recall, we were still setting up the amp to run off of your lab supply, which can provide about +/- 30VDC, with current limiting, so you can prevent killing the output stage. We had Q8-Q10 & Q17-Q19 removed, so your output of this Voltage Amp (not power amp at this point) is from Q7 & Q16. You should still be able to get full voltage swing, up to the limits of your lab supply.

You would NOT be able to get the 60V P-P at this point, since we're NOT yet connected to the brute-force power supply of the product. Remember?

The output at the zobel network L1 & R52 might show oscillation, since we're not pulling base current from Q7/Q16 that drive the output stage. But, you should be able to get full voltage swing. With +/- 30VDC supplies, I'd guess you might swing 40-45V p-p at TP4.

[wirelessliquid]

I'll check that tonight

I thought this would be the case

What do you mean by a voltage swing?

[nevetslab]

I'll check that tonight

I thought this would be the case

What do you mean by a voltage swing?

Q7 & Q16 have virtually the same P-P output voltage at clip as you would with the output stage installed, ignoring the drivers and outputs are 1 & 2 Vbe junctions down from Q7 & Q16.

[wirelessliquid]

On the base of Q7 & Q16 I'm seeing 18VDC & -18VDC

Vbe looks to be about 0.5V

I have installed the output stage at this point and it looks okay, I'm still seeing the 18Vp-p rather than 60Vp-p

[wirelessliquid]

The pre amp is pulling around 4mA on -15V and about 50mA on +15V

[wirelessliquid]

I should note the schematic is from a Alpha series amplifier where this is a Beta amplifier, the power board is almost the same though, F2 & F3 are 6A on my amplifier where as they are 10A on the schematic, for 100W output is around 18V * 6A could this be that 18V?

[nevetslab]

On the base of Q7 & Q16 I'm seeing 18VDC & -18VDC

Vbe looks to be about 0.5V

I have installed the output stage at this point and it looks okay, I'm still seeing the 18Vp-p rather than 60Vp-p

With seeing Q7 and Q16 so far north and south, either bias xstr Q13 hasn't been trimmed to pull the bases down to around +/- 1.8V, or Q13 is open. That's why your voltage swing is so low. Also, with that much voltage at the bases, you have the output stage turned on hard, and your lab power supply would be showing you're pulling a lot of current. Too much current, and you run the risk of killing the output stage again. I suspect smoke has already appeared.

I would disconnect the output stage until you have the bias circuit working. With the trim pot R42, you should be able to pull the bases of Q7/Q16 down to maybe +/- 1V or less, and on the high side, run them up to maybe +/- 2.5V.

[nevetslab]

The pre amp is pulling around 4mA on -15V and about 50mA on +15V

I don't have the schematic of the preamp thus far, but I'd be using your lab supply to source the +/- 15V to run it, rather than from the power amp's +/-15 V supplies at this point. With the current imbalance, I'd say you've suffered some failures in the preamp. The quickest way is to find the IC's who's outputs are NOT sitting at or very near 0VDC (+/- 25mV or less, typically), if they are in the signal path. There may be some used for driving LED's or other utility functions, and their output voltage would be other than 0VDC. I'd need to see the schematic. If there are any dip tantalum caps on it used for power supply bypass, they like to fail short, but it doesn't sound like that's the case. Nominal current flow on all signal Op Amps is the current will be nearly identical.

[nevetslab]

I should note the schematic is from a Alpha series amplifier where this is a Beta amplifier, the power board is almost the same though, F2 & F3 are 6A on my amplifier where as they are 10A on the schematic, for 100W output is around 18V * 6A could this be that 18V?

For 100W output @ 8 ohms, you need 28.3V RMS, which is 40V pk, or 80V pk-pk 60V pk-pk is 30V pk, or 21.21V RMS, which into 8 ohms is 56.3W.

Typically for a 100W amp, you'd need about +/- 50VDC. Under full power, the supply sags, dropping down to maybe +/- 45VDC or less. So, your amp is maybe rated 50-60W output.

[wirelessliquid]

Should I just turn R42 until this trims Q13 properly then? the bias in the service manual states that I should see 1MV over R18 after 10 seconds of power. I'm also having trouble with some of the pads lifting up now as I've soldered components in and out so many times. The output stage was never turned on hard, the output transistors didnt' heat up at all, no smoke either

With the output stage disconnected how can I test on the test points as the reading will not be the same?

that's a typo there 40mV and 50mV sorry, on the power board when I add the Op amp IC it increases the current draw on either side by +20mV and +40mV,this should also have an equal increase in current draw

The output for this amplifier is to a 4Ohm load, says 4ohm 100W output on it

[wirelessliquid]

I removed the output stage to check it still worked and checked the semi conductors and all appears to be okay so I put the output stage back in to do a test to see if it was the Q13 trim on R42. trying to get the base of Q16 down to -1.8V creates massive current draw which trips the current protection on my power supply, the amp will only sit stable on the power supply with the basse of Q16 at -18V.

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

[nevetslab]

Should I just turn R42 until this trims Q13 properly then? the bias in the service manual states that I should see 1MV over R18 after 10 seconds of power. I'm also having trouble with some of the pads lifting up now as I've soldered components in and out so many times. The output stage was never turned on hard, the output transistors didnt' heat up at all, no smoke either

With the output stage disconnected how can I test on the test points as the reading will not be the same?

that's a typo there 40mV and 50mV sorry, on the power board when I add the Op amp IC it increases the current draw on either side by +20mV and +40mV,this should also have an equal increase in current draw

The output for this amplifier is to a 4Ohm load, says 4ohm 100W output on it

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.

[nevetslab]

I removed the output stage to check it still worked and checked the semi conductors and all appears to be okay so I put the output stage back in to do a test to see if it was the Q13 trim on R42. trying to get the base of Q16 down to -1.8V creates massive current draw which trips the current protection on my power supply, the amp will only sit stable on the power supply with the basse of Q16 at -18V.

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

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.