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Old 14th Nov 2023, 10:10 pm   #1
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Default Help Understanding Betamax PSU Schematic

Hello,

I know this is a simple one but this isn't really my area.

As I understand the schematic below, this is a SMPS and the IC is driving the primary winding of the Drive Transformer, which then drives the main transformer at the top of the schematic section. That bit I'm fine with.

What I'm struggling with is how the BJTs work (especially the top one) as by my basic understanding I have it in my head that the top BJT conducts but I can't grasp the purpose. The lower one (Q102) switches the converter transformer to ground when the primary of the drive transformer is energised, but I can't work out the function of Q101, unless the coils are wound in opposition but it's not indicated on the diagram.

I know this is something very simple, and it looks a bit like an amplifier circuit but I've not played with anything like this for years and having a brain fart!

I get the rest of the diagram!

Cheers,
Ben
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Old 14th Nov 2023, 11:07 pm   #2
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Default Re: Help Understanding Betamax PSU Schematic

If I'm not mistaken it's called a half bridge. The voltage on the primary of the transformer T102 alternates polarity as Q101/102 switch, driven by the control IC, with the junction of the two capacitors at half the supply voltage. It seems to be a common configuration in switch mode supplies.
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Old 15th Nov 2023, 12:01 pm   #3
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Default Re: Help Understanding Betamax PSU Schematic

Thanks so much @Keith956, that makes sense.

I guess this arrangement was used rather than something as simple as a FET drive was because the tech’ when this was designed wasn’t up to speed yet? Seems a bit perverse to add another transformer on to the BOM when later on this could be simply driven direct?

Thanks
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Old 15th Nov 2023, 1:01 pm   #4
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Default Re: Help Understanding Betamax PSU Schematic

Yes I guess in the days of Betamax, power NMOS fets were expensive compared to bipolars.

Would be interesting to know what IC1 is. It looks like a MB3759 which I came across in a HP switch mode supply recently.
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Old 15th Nov 2023, 2:05 pm   #5
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Default Re: Help Understanding Betamax PSU Schematic

That architecture was still used even when power FETs became affordable. The capacitors on the left act like a reservoir capacitor and a DC block capacitor. One transistor switches the far end of the transformer primary up to the high voltage positive and then lets go. After a delay, the other transistor switches that same far end of the transformer down to the high voltage negative, and lets go. After a delay the cycle starts again.

The SMPS transformer sees no net DC in the primary which is good for it. It's a forward converter on both pulses, into full wave rectifiers. The full wave rectifiers act as snubbers converting stray switching transients into useful output power. Snubbers in the primary area only have to sissipate fast transients allowed by the leakage inductance of the transformer.

Whatever sort of transistors you use, this is an excellent architecture. Because the two power transistors act as a totem-pole, transformer isolation of their base drive is needed, hence the driver transformer.

Pulse width control of both half cycles is controlled to regulate the output.

David
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Old 15th Nov 2023, 4:36 pm   #6
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Default Re: Help Understanding Betamax PSU Schematic

Quote:
Originally Posted by Keith956 View Post
Would be interesting to know what IC1 is.
Sony often used TL494 in their earlier SMPS circuits. Certainly appears to have similar pin functions as shown and at one time most semiconductor manufacturers offered a direct replacement.
Rich.
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Old 15th Nov 2023, 7:44 pm   #7
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Default Re: Help Understanding Betamax PSU Schematic

Right again, it’s TL494CN for the aficionados.

Thanks,
Ben
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Old 15th Nov 2023, 8:15 pm   #8
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Default Re: Help Understanding Betamax PSU Schematic

Sorry should have said thanks to David, that clears up matters and I understand the circuit now.

Well those two caps out (4.7uF) , no leaks one seems to have gained a extra uF or two but has an ESR of 45Ohms, the other tests fine but I’ll replace them both anyway.

There’s a lot of scorching around the top of the drive transformer so I’m going to check those rectifying/snubber diodes now.

Thanks,
Ben
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Old 15th Nov 2023, 9:00 pm   #9
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Default Re: Help Understanding Betamax PSU Schematic

That looks like the Sony SLC7 psu to me, those two capacitors were part of the psu mod/repair kit which was available back then
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Old 15th Nov 2023, 9:53 pm   #10
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Default Re: Help Understanding Betamax PSU Schematic

Good shout! Yes it is, this one is totally dead and was found with a blown main fuse so I’m fairly certain the PSU did something spectacular.

Also a rotten backup battery and board but I think that can be cleaned and repaired.
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Old 15th Nov 2023, 10:05 pm   #11
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Default Re: Help Understanding Betamax PSU Schematic

See this link https://www.palsite.com/slc7tech.html
I am fairly sure Colin McCormick of this forum (I think) would have more information for you.

I repaired quite a few of these machines back in the day, but then I had access to Sony spares and mod kits available via SES as we were not Sony official dealers, SES were pretty good for Sony technical information so long as we were buying spares from them
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Last edited by Red to black; 15th Nov 2023 at 10:18 pm. Reason: not sure if Colin Mc is a member here or not?
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Old 15th Nov 2023, 11:22 pm   #12
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Default Re: Help Understanding Betamax PSU Schematic

Quote:
Originally Posted by RobustReviews View Post
Good shout! Yes it is, this one is totally dead and was found with a blown main fuse so I’m fairly certain the PSU did something spectacular..
If it's the SMPS that's caused the main fuse to blow then the bridge rectifier, Q101/2 and C109/110/111 are worth checking for s/c.
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Old 16th Nov 2023, 5:27 pm   #13
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Default Re: Help Understanding Betamax PSU Schematic

Quote:
Originally Posted by Keith956 View Post
Quote:
Originally Posted by RobustReviews View Post
Good shout! Yes it is, this one is totally dead and was found with a blown main fuse so I’m fairly certain the PSU did something spectacular..
If it's the SMPS that's caused the main fuse to blow then the bridge rectifier, Q101/2 and C109/110/111 are worth checking for s/c.
Thanks Keith, finally got the bits desoldered and tested, Q101/102 are both short C-E so I think that completely proves them well and truly "shot through".

I'm replacing the 4.7uF's as a matter of course anyway, I have plenty of caps lying around so bar the expensive 100uF/400V one (tested perfectly on my tester) I might just review all the bits on the boards. This is going to be a museum piece and possibly and exhibit one day so worth getting it all in tip-top condition for another number of years!

Thanks,
Ben
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Old 16th Nov 2023, 10:01 pm   #14
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Default Re: Help Understanding Betamax PSU Schematic

I seem to remember a higher capacity is allowed in this circuit, but most important is that the capacitors should be able to deal with the ripple current. I think Panasonic ED series will be allright. Physical size is also a rough indicator of the amount of ripple current a capacitor can withstand.

The transistors are also a bit critical. I'm not sure whether Sony used selected types, but make sure to not use ISC or other Chinese rebranded rubbish as they will blow up again. Isn't the orginal 2SC2335? I think they're NEC orginally, with the most common second source Samsung / SEC which are fine. A European substitute would be BUT56A which should be fine as long as not some rebranded rubbish.

If you don't want to rely on old stock transistors, maybe some modern CFL-inverter types (MJ series, BUL series) might be suitable substitutes.
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