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Class D Switching Power Amplifiers and Power D/A conversion

Sony XM-1600GSD repair
Sony XM-1600GSD repair
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Old 4th November 2017, 07:52 AM   #1
lazzer408 is offline lazzer408  United States
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Default Sony XM-1600GSD repair

This was originally my amp. Sold to a friend. It failed. I got it back and found the legs to one of the rectifiers had broken from mechanical stress. I replaced the rectifier.

The amplifier will turn on and play audio for about 10 seconds then begin playing intermittently on and off every second or so. Pretty random.

When the amp cuts out, IC5 will heat up quickly. When the amp starts playing again, the temperature of IC5 will drop quickly.

I've replaced IC5 as well as a few other transistors that I hade on hand. In the front end of the amplifier stage, after IC5, I've replaced the 5401s and the two C3200 (subbed in 5551 with BC legs swapped to comply with the 3200 pinout).

+/- rails to IC5 are stable.

Amp still has the same symptoms.
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Old 4th November 2017, 09:49 AM   #2
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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This is not a class D amplifier, it is a simple class B split voltage amplifier.

IC5 is a line amp feeding the power stage. What voltages have you got on pins 1-3 and 5-7 when in fault mode? Should be zero volts at all times plus any signal.
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Old 4th November 2017, 09:55 AM   #3
lazzer408 is offline lazzer408  United States
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I will try and measure but it cuts in and out so quickly that I might not be able to get an accurate reading. It is not lighting any of the protection LEDs.

It may be easier for me to check for DC (or even audio) at the base of Q18. I have a feeling something in the input stage of the power amp is leaking current back to IC5. Would that help?

If it's not class D why is there a low-pass filter after the output stage?

Last edited by lazzer408; 4th November 2017 at 10:15 AM.
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Old 4th November 2017, 10:03 AM   #4
lazzer408 is offline lazzer408  United States
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There's things in this schematic that are wrong. One example is Q19 emitter labeled 0v but it's tied to Q33 collector labeled -57.2v. Or Q2 collector showing -17.6v but it's tied to Q4 collector showing -18v. These errors are all over.

Q21 labeled 2n5401 in schematic but KTA1275 was used on the PCB.
Q33 labeled MPSA06 in schematic but KTC3228 was used on the PCB.

Last edited by lazzer408; 4th November 2017 at 10:10 AM.
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Old 4th November 2017, 10:33 AM   #5
JonSnell Electronic is offline JonSnell Electronic  United Kingdom
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Low pass filter?
230uH is to keep the amplifier stable at higher frequencies and to reduce HF oscillations when unstable as in all other well designed Class B solid state amplifier output stages. It has a 3db point into a low impedance load far higher than the audio spectrum.

Class D is a switching output stage. We convert linear audio into FM chopped mark - space square waves using an integrator, sending "digital" signal to the output stage that switches the power stages on or off, so no power is dissipated in the output transistors, ideally!

Anomalies in the schematic is usual. When on the Sony Service Training Course, these are explained and updated drawings are supplied. Keeps the non Service Agents, on their toes! I have mislaid my notes from the course, sorry. Well it was 15 years ago and most of these have since been destroyed with faulty speaker wiring.

Q21 2N and Q33 MPS are North American manufacture that have similar characteristics and probably Sony fell out with Ti. Who knows, politics maybe or just production costing.
Hope this helps and keep methodical with your fault finding. The Zero voltages should be Zero at all times.
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Old 4th November 2017, 06:39 PM   #6
lazzer408 is offline lazzer408  United States
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Quote:
Originally Posted by JonSnell Electronic View Post
Low pass filter?
230uH is to keep the amplifier stable at higher frequencies and to reduce HF oscillations when unstable as in all other well designed Class B solid state amplifier output stages. It has a 3db point into a low impedance load far higher than the audio spectrum.

Class D is a switching output stage. We convert linear audio into FM chopped mark - space square waves using an integrator, sending "digital" signal to the output stage that switches the power stages on or off, so no power is dissipated in the output transistors, ideally!
It's a subwoofer amp that doesn't have to play above 200hz. This amp was advertised as class D. I'll probe the output gates as see if there's a PWM signal present.

On the 3001 and 6001 versions of this amp, I see the updated components were labeled correctly in the schematic. Early production revision is my guess.

Last edited by lazzer408; 4th November 2017 at 07:00 PM.
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Old 5th November 2017, 12:31 AM   #7
lazzer408 is offline lazzer408  United States
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60hz sine fed to amplifiet input. Top trace is amplifier output. Bottom trace is Q18 base. The signal at Q18 base is not very high.

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Old 5th November 2017, 12:50 AM   #8
lazzer408 is offline lazzer408  United States
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Something else I noticed. FET 10, 11, 12 have strong gate drive. FET 7, 8, 9 have weak gate drive. Gate drive is lost with the amp cuts out. Something else I'll have to look in to.

The amp is most definitely class D.
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Old 5th November 2017, 05:10 AM   #9
lazzer408 is offline lazzer408  United States
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Q21 overheating seems to be the cause. If I place the cool tip of a screwdriver against the transistor, the amp begins playing again. Q21 and Q33 both run very hot. Larger devices were used on the PCB then what's indicated in the schematic.

Back to this...

Q21 labeled 2n5401 in schematic but KTA1275 was used on the PCB.
Q33 labeled MPSA06 in schematic but KTC3228 was used on the PCB.

I may have to look for a better substitute for the A1275 and C3228.
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Old 7th November 2017, 04:27 AM   #10
lazzer408 is offline lazzer408  United States
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Before I replace every transistor in the front end of this amp, any ideas for the cause of Q21 and Q33 excessive heating?

I've replaced Q18, Q19, Q22, Q31, EC57, EC61, and IC5

Here's the schematic. Pg 14

sony_xm-3001sxd_6001gxd_sm.pdf - Google Drive
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