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Old 6th Apr 2019, 6:59 pm   #1
samjmann
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Default Technics SU-V60. Voltage regulator

Hi all,

I've been looking a slightly knocked about example of this amp. To protect the now hard-to-replace SVI4004 current amplifier, I've removed it from the amp whilst testing. On first examination pins 1&2 of the SVI4004 had been shorted together on a previous repair. One transistor in the voltage amp is short circuit (Q417). What is really puzzling me is the operation of the LT regulator, I've attached a snippet of the circuit below. The gate/source of each FET are shorted. What's the function of these FET's. Is it some type of electronic smoothing to cancel line ripple?

I've got 0v on the emitter of each regulator with a 48v+/- input. All the transistors check out OK. No heavy loads on either 18v +/- rails.

I've read on some of the audio forums that this amp is of the 'current-dumping' type, of Quad inspiration no doubt. It's supposed to be a pretty good amp, the figures look quite good for it.

The full circuit is on Hi-Fi engine, the snippet is just the regulator

Thanks for any advice on this.

SJM.
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File Type: pdf New TextMaker Document.pdf (233.1 KB, 44 views)
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Old 6th Apr 2019, 7:47 pm   #2
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Default Re: Technics SU-V60. Voltage regulator

The FETs with the gate-source connected are constant current sources. They sometimes get drawn as current source symbols or as diodes. They used to be made in a range of current values.

Theoretically you could do this with ordinary FETs but the variation in Idss from part to part is a major problem. These parts are from a selection process.

David
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Old 6th Apr 2019, 8:40 pm   #3
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Default Re: Technics SU-V60. Voltage regulator

Thanks RW for the reply. As I have no base voltage on either Q701/2, I suppose that the FET's must O/C. The FET'S are both 2SK246 not expensive! I'll replace both the 33uf's as well. Both Q701/2 test perfect out of circuit.

Cheers, SJM.
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Old 7th Apr 2019, 12:07 pm   #4
orbanp1
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Default Re: Technics SU-V60. Voltage regulator

Hi SJM,

As David suggested the FETs act as current sources and they have quite a variation in their Idss parameters (It is the Idss value of the FET that gives the current of the current source).
The Zeners are 18V 370mW devices, so you would want not more than 5mA to 8mA across them. The base current through Q702 is negligible.
You should measure the Idss current of the replacement FETs before you install them. Just connect the gate to the source, feed the drain source from a bench supply with something like 5V - 10V, and measure the drain current.

Regards, Peter

Last edited by orbanp1; 7th Apr 2019 at 12:24 pm.
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Old 7th Apr 2019, 4:07 pm   #5
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Default Re: Technics SU-V60. Voltage regulator

Thanks Peter for the help. The 2sk246 has a rated Idss of 1.2mA, this is what Cricklewood state on their site. On the circuit the original 2SK301 has a Idss of 20mA, again figures from Cricklwood. If this was fitted then it would put more than the 5-8mA advised into the 18v Zener. Perhaps Technics revised what they were fitting during production.

When testing should I fit a 470ohm in series with the bench supply when testing to give a little protection to the FET?

Thanks again for the advice.

SJM.

Last edited by samjmann; 7th Apr 2019 at 4:08 pm. Reason: Typo error.
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Old 7th Apr 2019, 10:21 pm   #6
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Default Re: Technics SU-V60. Voltage regulator

Hi SJM,

As mentioned, the FETs have quite a variation in their parameters.
Generally FETs of the same model number are ranked according to their Idss values. Do check the datasheet.
For the 2SK301 the Idss groups are:
P: 1ma - 3mA, Q: 2mA - 6mA; R: 5mA - 12mA; S: 10mA - 20mA.
For the 2SK246:
Y: 1.2mA - 3mA; GR: 2.6mA - 6.5mA; BL: 6mA - 14mA.
So pick the FET accordingly.

The 470 Ohm resistor would just drop some voltage, the FET should not be damaged without it when you measure the Idss.

Regards, Peter
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Old 8th Apr 2019, 10:10 pm   #7
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Default Re: Technics SU-V60. Voltage regulator

Thank you Peter for the information. I can see now that there is quite a wide variation in spec for the same transistor, so careful choice is needed!

I'll post again when I fit the replacements. These are 1.2mA-3mA Y class 2SK246's.

Regards, SJM.
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Old 14th Apr 2019, 6:54 pm   #8
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Default Re: Technics SU-V60. Voltage regulator

I've fitted the two FET's and both the supplies have come to normal levels. In the Voltage Control amp, Q417 was s/c collector-emitter, it's 2.2ohm emitter resistor had risen to 6ohms. These have also been replaced. At the mid point of each Voltage control amp I have now 6.2V. This should be 0v, but as both channels exhibit the same characteristic with the power amp Ic removed, I can only assume that this is correct.

Next step, to refit the power amp IC, then gently power on with a lamp limiter and variac...

SJM.
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File Type: pdf Voltage amp..pdf (364.9 KB, 10 views)

Last edited by samjmann; 14th Apr 2019 at 7:10 pm.
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Old 15th Apr 2019, 12:18 pm   #9
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Default Re: Technics SU-V60. Voltage regulator

Hi SJM,

I could not find a datasheet for the SVI4004, but here is a reverse engineered circuit of the chip:
https://elektrotanya.com/svi4004_sch.pdf/download.html

The circuit (together with its driver in the SU-V60) looks funny at the first glance, but I have not really spent any time on it to understand its working and to see if those 6.2V voltages in your circuit are correct or not.
Best course of action is to plug it into a simulator and see how does it work. (I might just do that.)
No chance to burn down anything in there!

Regards, Peter

Last edited by orbanp1; 15th Apr 2019 at 12:46 pm.
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