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Synq 3K6 fed with 20hz DC squarewave: said poff!
Synq 3K6 fed with 20hz DC squarewave: said poff!
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Old 7th November 2020, 10:05 AM   #1
Imbecillen is offline Imbecillen
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Default Synq 3K6 fed with 20hz DC squarewave: said poff!

Well, I bought broken Synq Digit 3K6. The previous owner used it for electronic-research in some way. Someone accidentally fed it with 20hz squarewave when it was connected to a load as following: 10ohm @ 20Khz and 50Mohm @ DC. It said "poff!"...

Now, I like to fix things. So I've come to the conclusions:
1. PSU are shorted somewhere, I'll try to measure the semiconductors today.The fuse in the house trips when trying to start the amp.
2. Right sided amp module have shorted semiconductors for sure, measured dead. Both Mosfets and rectifier diodes.
3. PWM modules from amp PCB's are ok! Not sure about the PWM module on the PSU.

I have in fact two of these amps, one is working. So I can try some components by changing them in between.

My questions:
What are the most likely broken components I should look for? If the semiconductors are shorted, should I look for something else in particular that might as well have been "poffed"?

Service manual and schematics:
Synq 3K6 service manual - Google Drive
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Old 7th November 2020, 05:38 PM   #2
RBMK1500 is offline RBMK1500  Germany
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I own 3 3K6. Buy a cheap variac (adjustable Transformer) and Infrared Thermometer or camera. Fade in the short circuit slowly by increase voltage.



Used them one time on a cheap 4stroke power alternator.
This cause defective mosfet in PFC Part of two of 3 amps.
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Old 9th November 2020, 12:05 PM   #3
Imbecillen is offline Imbecillen
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Oh, this seem to be a very common way to kill these amps! The power really needs to be stable for them to work correctly. Have you noticed any changes that can be made for them to be more durable? I've read that one can change Q1 and Q2 transistor to make the PSU a bit more stable...

If I buy an adjustable transformer and use it, can I even give it power at all before the fuse in the house trips? I would understand this approach if the PSU where functional, but now I can't even start the amp.

I have found out that the first two big capacitors are shorted on the PSU though, would it be an idea to just change these and try again, or are there likely more components that need to be changed?

Thanks for your answer!
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Old 9th November 2020, 04:13 PM   #4
RBMK1500 is offline RBMK1500  Germany
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Shortet capacitors ? Did they blow up or crack Up ? If not, solder them out and check them single again. If you try to find out the problem by using a variac and you say output amp mosfets are already broken. I would disconnect the amplifier modules from capacitor bank. Replace alle broken fuses. Connect the amp to the variac while measure the current. I have a cheap chinese 500W Variac. In Case of the short circuit it soon will start to buzz even at Low voltages. You now can Look for warm or hot parts. Best is a ir camera, because some parts are Not easy to Touch for temperature check. Keep in mind that the variac has no disconnection to main power like a Transformer with primary and secondary coil windings.
If all defective parts in psu are located and replaced. The psu pwm circuit start to work from around 70-80V ac
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Old 9th November 2020, 04:20 PM   #5
RBMK1500 is offline RBMK1500  Germany
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I made some YouTube clips how i test the PSU Board after changing the defective PFC mosfet.


SYNQ 3K6 load test - YouTube Load Test



SYNQ DIGIT Class D Amplifier SMPS power supply troubleshoot Fehlersuche - YouTube PFC Mosfet Signal
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Old 9th November 2020, 09:24 PM   #6
Imbecillen is offline Imbecillen
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Previous owner told me it said "Poff!". But everything looks good at least, no blown/swollen caps, bad smell or burned components.

I'll resolder and remeasure the components I've measured to be "shorted". I know some components get weird measures while placed on the PCB, thanks for the tip though!

Great information about your work. I've actually seen your videos before and ordered the updated Q1 and Q2 mosfets to change them on my other fully functional amp before it breaks down as well. I've understood it as these parts are the main issue when giving these amps unstable power, if so it's an easy fix for them to become more stable?

The Variac seem to be a great tool, but are there any alternatives? I don't know how much I'll use it if I buy one. It's a bit of a one trick pony right? Meanwhile, here's a list of my measured (diode mode) components still on PCB, if it's saying something:

PSU:
D1 measure 512, 528
D3 measure 0.0
D31-34 measure 377
Q1-Q6 measure 0.0
E3 measure 0.0
E5-E6 measure 0.0
E1-E2 measure 1.0
E19-23 measure 1.0
E24-E28 measure 1.0
E8, E9, E29 measure 1.0

Amp-modules:
D7, D8 measures 0.0 on right module, 407 on left module
Q21, Q24 measures 0.0 on right module, 407 on left module

Last edited by Imbecillen; 9th November 2020 at 09:38 PM.
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Old 10th November 2020, 01:19 PM   #7
Eva is offline Eva  Spain
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This amplifier, I assume powersoft clone, uses a trick for MOSFET commutation, for low loss in old MOSFET, that only works for audio-like signals and loads. The circuit will suffer a lot when the load demands high current at low voltage.

Incidentally, the trick in essence looks like attaching the 2nd PFC MOSFET with 10cm wires to the PCB, electrical resistance is not substantially different, but the rate at which the current flow builds up is dramatically different.
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Old 10th November 2020, 01:48 PM   #8
Imbecillen is offline Imbecillen
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Quote:
Originally Posted by Eva View Post
This amplifier, I assume powersoft clone, uses a trick for MOSFET commutation, for low loss in old MOSFET, that only works for audio-like signals and loads. The circuit will suffer a lot when the load demands high current at low voltage.

Incidentally, the trick in essence looks like attaching the 2nd PFC MOSFET with 10cm wires to the PCB, electrical resistance is not substantially different, but the rate at which the current flow builds up is dramatically different.
Very interesting, this would explain a lot why these amps failure at low voltages when driven hard. So your last line would be a solution to this problem if I understand you correct? A VERY simple solution in that case! 10cm wires to G, D and S?

Or are you meaning that the trick are used already by design?
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Old 10th November 2020, 08:48 PM   #9
Imbecillen is offline Imbecillen
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Update!

I've been desoldering some components now and the electrolytes seem to be ok all of them! The blue disc caps though, four of four desoldered are dead. Interesting! I'll continue desolder disk caps and measure them. They are after all cheap to change so that's on the good side.

Mosfets and diodes will also be desoldered of course..
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Old 17th November 2020, 12:11 PM   #10
Imbecillen is offline Imbecillen
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Second update:

PSU:
Blue disc caps are probably ok, just hard to measure with my instruments.

D3 are shorted, NFE48G U1560 KA
Q3 and Q5 are shorted, IXFH44N50P TS1817 S23984
The rest seem okay!

Since I've desoldered all the mosfets and the diode, does anyone have an idea about what's good to replace them with? I've read about these mosfets regarding this specific amplifier:
IRFP460PBF and IXTQ36N50P

Any input?

Amp-module:
D7 and D8 are shorted, BYC8-600 PJA1827 D5 4163
Q21 and Q24 are shorted, K60N30 CE8P1Y

Any inputs on replacement components for these?
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