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SX-626 problem, at half volume DBT kicks in

Where I am going with troubleshooting is to get the center voltage and bias set correctly, so the amp does not clip at high signal levels. When the driver and output transistors on this power amp architecture are replaced this has to be done from scratch. In order to get this done properly the power supplies have to be right (within 10% anyway). It is probably not getting a dead short, but with the center voltage and bias not set correctly the amp will clip asymmetrically way before the output reaches its rated output power level. This can cause excess current through the output transistors, and blow the fuse. If the amp is being over-driven by a larger than expected input like a smartphone or CD player, or even turned up too much with a normal signal it will blow the fuse. This is not a failure, it is normal protection activity to keep the amp from going up in smoke. The only way to really know if the amp is performing up to its rated output is to get the power supplies right, set the bias and center voltage close to norms, then put dummy loads and an oscilloscope on it, drive it with a sine wave and measure it, readjust, remeasure, etc.
It can be adjusted 'close enough for government work' with a DMM, but to get the most out of it is beyond kitchen table hobbyists. I am sure that is why the procedure is not in the normal Service Manual. This is not meant to be disparaging, just stating facts, forgive me if it seems too blunt.

FWIW, the owner may just need a more powerful amp for the levels he wants to listen to with his speakers. :dunno: Not much you can do about that :rolleyes:.
 
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This amp is rated 27 WPC at 8Ω. That will get pretty darn loud (as you said) with efficient speakers. If the owner is seeing 110 W Music Power in the SM thinking it will compete with a 'real' 110W amp he will be disappointed.
 
Thanks for your help, I really appreciate it!

I’m dedicated to getting the power supply board working properly. I hope I have the correct Zener diode in stock, if not, I’ll order it which will push this into next week. I’ll let you know.

I was wondering why they didn’t have the amp adjustments in the service manual.
 
bradfro said:
Thanks for your help, I really appreciate it!

I’m dedicated to getting the power supply board working properly. I hope I have the correct Zener diode in stock, if not, I’ll order it which will push this into next week. I’ll let you know.

I was wondering why they didn’t have the amp adjustments in the service manual.
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You are most welcome! I am glad you did not take what I said the wrong way. If you do have the proper equipment I am willing to walk you through it, it is not that difficult, just finicky. We can get it close enough with a DMM, so no one can't tell its not perfect without a scope. I actually have only seen a couple of people on AK interested enough to go Whole Hog with it.
 
i find 2 meters makes the job much easier as you tend to trade the voltages against each other . no idea if all cap coupled amps behave the same or not .
 
I put in the 14v Zener diode, pin 10 is still stuck at 12 volts. There are 2 wires on pin 10. I removed one wire and now the voltage is at 24v on pin 10. The wire I removed goes to the meter drive assembly which is correct in the schematic. The other wire goes to the tuning dial lights board. This seems wrong. The schematic only shows one wire.
 
bradfro said:
I put in the 14v Zener diode, pin 10 is still stuck at 12 volts. There are 2 wires on pin 10. I removed one wire and now the voltage is at 24v on pin 10. The wire I removed goes to the meter drive assembly which is correct in the schematic. The other wire goes to the tuning dial lights board. This seems wrong. The schematic only shows one wire.
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Pin 10 is supposed to be at +12vdc. It is un-regulated, it just has a resistor (R8) to drop the voltage because it has a constant load. When you remove part of the load the voltage goes up.
 
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Schematics are not the same as a wiring list, they show electrical connections only, not necessarily wires. A wiring list will have every wire in it, where both ends physically connect, and if necessary, how it is routed through the chassis.
 
Is pin 7 near +13vdc?
Is pin 5 close to +30vdc?
Those are the regulated voltages the higher zener should have improved.
 
Crap, I thought I found a miswire . The wires on the a power supply board have been messed with by a previous tech

ok, pin 10 11.9v
Pin 7 12.9V
Pin5 28.3 v
Pin3 57.2v


I do have a oscilloscope
 
bradfro said:
Crap, I thought I found a miswire . The wires on the a power supply board have been messed with by a previous tech

ok, pin 10 11.9v
Pin 7 12.9V
Pin5 28.3 v
Pin3 57.2v


I do have a oscilloscope
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Those look like good numbers. You are not on dbt, correct?
 
Lets get the center voltage close to correct then. Black lead to chassis, measure with red on dcv.
Volume minimum, input to Aux, no input signal.
Measure dcv on the collector of Q9 on the heatsink, write it down and divide by 2, write that down.
Move the meter lead to the + of C13, the output capacitor, adjust VR1 for that voltage to match the second number (you calculated and) wrote down.

Repeat process with Q10 collector, + of C14, and VR2.

Then we will do bias.
 
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Bias is next, same input and volume settings, black on ground, measure vdc, millivolts:

Emitter of Q11, carefully adjust VR3 for 16mVdc, let it settle and readjust. This sets the bias current to 32mA.
Recheck center voltage on C13 +, as these will interact. Go back and forth a couple times to get them to minimize interaction.

Emitter of Q12, carefully adjust VR4 for 16mVdc, recheck center voltage on C14+, go back and forth a couple times to get them to minimize interaction.
 
Next step, play music at moderate volume for 20 minutes or so until the heatsink is warm (not hot) and recheck center voltage and bias on both channels. If it sounds good and is stable give it a workout. You have done a major rebuild, replacing most of the 'likely to be tired' components. If he continues to blow fuses it is not likely to be the amps fault. If you have 2 50W or so 8Ω dummy loads and a sine wave source you can do the final adjustments and output power verifications. Have some spare parts readily available (output transistors, driver transistors, emitter resistors, and fuses) just in case. That process is like firing a proof load in a gun. If it is in good shape it can handle it, if there is a weakness it will come apart.
 
Thanks, I got one channel adjusted properly last night and then Perry Mason was on tv (my wife and I are Perry Mason addicts).

I'll finish up the other channel tonight. I do have dummy loads that I've been using. I will let you know how it goes.
 
Bias adjusted for both channels at 16mv and center voltages are right on. With the scope I get 10 volts peak to peak which I think is 12 watts per Channel . Any higher volume and the dim bulb starts to glow and all the voltages start dropping Voltage on the output collector at this point is 44volts and half voltage is 23.4. Bias is 77mv. Heatsink is warm. When I move to mains, the volts go up to 54.8 and 27.5
 
I got up the courage to crank the volume to 30 volts peak to peak without any noticeable clipping. The volume was all the way, it wouldn’t go any higher. This is a 1khz signal using the dummy load. I’m using my phone as a signal generator. I cranked the signal generator to full output and I got 40 volts peak to peak before the wave started to clip. No fuse blown. How about that?
 
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