Technics SU-V4X strange problem – help needed!

[Ivan Holmes]

Hi all,

A mate of mine recently bought a Technics SU-V4X on eBay, advertised as fully working. It was definitely not fully working – it just went into the safety routine.The seller was being very difficult so rather than getting a return my mate settled for a partial refund.

I was left with the task of fixing the amp. I checked Q501-504 and they were all fine. I then removed the chip and opened it to find that it had exploded inside. I bought an SVI2004 replacement (which was unsurprisingly an SVI2004C) from AliExpress (only place I could find one) and fitted that in.

The safety light still comes on. After having a look at it on the meter, I found that pins 1 and 5 (left channel) were at about 32 volts each and pin 3 was at 15v some times but 0 at others. On the right channel, the corresponding pins were at the correct voltages (1.8v, -1.8v and 0v)

I've swapped Q501-504 around and tested them all again and nothing has changed. I've also replaced pretty much every capacitor in the left channel, metered all the resistors and removed and tested D501. I just can't seem to find anything wrong!

Does anyone have any ideas as to what it could be? The chip perhaps?

I've managed to blow the fuse so I can't work on it for the next couple of days while I wait for a new one to arrive.

Thanks for any help anyone can give and let me know if anything needs clarifying (or you want any more measurements).

Cheers,
Ivan

 

[JonSnell Electronic]

The first thing I would do, before fitting the replacement chip, is to check all of the safety resistors and replace with the same type if faulty. Then fit a new chip, I presume you checked the voltages of IC501.

 

[SVI2004A]

32V at pins 1 and 5 and 15 out at 3... something’s not right in that ic

 

[rsavas]

Hi Ivan,

Check out this thread where a member used my STK-0050 module as a replacement.
STK-0050 replacement for SX-780 and others | Page 21 | Audiokarma Home Audio Stereo Discussion Forums

Cheers
Rick

 

[Ivan Holmes]

Hi everyone,

Thanks for replying to my message and offering suggestions.

Jon: the voltages on IC501 all seemed to be about right, the problem was on the other side of the diode (D501) if that makes sense.

Baily (SVI2004A): I quite agree... do you think I could flip it round and see if the problem moves on to the other (right) channel?

Rick: Nice job on that replacement module, it looks great! Unfortunately it is compatible with the Technics SU-V4 rather than the SU-V4X, which is quite different, it seems. I haven't been able to find any modern reproduction of the SVI2004A.

Cheers
Ivan

 

[Ivan Holmes]

Sorry, left out a bit of info in the last post.

Jon: I've already fitted the replacement chip, and I've tested any resistor I can see and they all look fine, which does point to the chip. I think I'll get the supplier to send me another one as this one is pretty clearly broken.

Cheers
Ivan

 

[SVI2004A]

Yes, install in reverse see if the issue follows as I’m sure it will

Shouldn’t be well over 2V difference from inputs to output.

I’m kinda sitting ok for unobtanium - been collecting over the years

 

[Ivan Holmes]

!

Wow, look at that! I guess with so much Technics stuff you kind of need spares to match.

Will install in reverse when my new fuses arrive.

Have you ever considered retrofitting the syncro bias circuit into an SVI2004C? I assume there's some reason it can't be done...

 

[SVI2004A]

Should look into it.... iirc they did something like that.. SVI1001A (SA-007) essentially is a lower powered version of SVI2004C... which they had externally set up the Synchro bias...
MA27W-A diodes were used....

I will have to find the schematic... I put in SVI2004A in my 007 amp and de commissioned the external Synchro bias since it now was internal.

 

[Ivan Holmes]

Thanks for this great info. I had a quick look for the SA-007 service manual but turned up nothing, I would be very interested to have a look!
I have to say your knowledge on these amps and chips is really impressive.

I suppose it's all a moot point until I've got it working, but it certainly is very interesting indeed.

 

[Ivan Holmes]

I thought I would do an autopsy on that chip to see if I could figure out what was wrong with it.
Well, it seems pretty clear to me that the left channel output and input transistors are blown along with the 0.3 ohm 'resistor' to the speaker output... a certain Aliexpress seller will be giving me a full refund I think.

An externally hosted image should be here but it was not working when we last tested it.

I've ordered another, from a different seller. Hopefully I have more luck this time. I'll be opening the new one before I fit it to make sure it's working alright.

Cheers,
Ivan

 

[SVI2004A]

Yeah C version there... opened Emitter resistor and bbqed npn output and driver and I’m curious of -V supply continuity to Collector - pin 4

What was the original ic like inside?

 

[Ivan Holmes]

The -ve supply was not continuous to pin 4, the meter said there was a 0.8v drop between them as opposed to something like 0.02v on the right channel.

The original IC had a dead short on the transistor on the bottom right – not sure where that is in the schematic off the top of my head. It caused the right channel NPN driver to heat up so much that it melted the solder holding it down and slid over the synchro bias circuit removing it from the board.

I'll send a picture of the remains when I get home but I've messed around with it a bit trying to find what was wrong. I think the original destruction is pretty clear though...

 

[SVI2004A]

Blockslide

Sounds like that amp was driven hard for some time with no ventilation (in a cabinet) or the xfmr side was higher then Volume side (causing fluid to not be behind power ic)

They’ll keep sounding mint but run so hot, causing the top block to unsolder and slide into the Synchro bias circuit... this effectively results in a short parallel with Collector-Emitter of npn (top) transistor, and is bad news for the pnp taking the hit of the short... often brutal results...

If your mate is saying he has never heard sound from that amp... you can bet your balls the seller did it... and you’d have to know that sound suddenly stopped...

 

[Ivan Holmes]

I unboxed it with my mate – we never heard it. Seller is not a decent guy at all – he claimed that he'd had a tech look at it.... not likely!

Amazing how they can reach something like 300ºC and still work. It's a real testament to the quality of Technics stuff of that era.

I just hope I can get it fixed to be honest. I'm looking forward to hearing it after having read so much praise of it online.

 

[SVI2004A]

Especially if you get an A version or a very rare SVI2004B which still was Synchro Bias

 

[Ivan Holmes]

Yes, that would definitely be ideal. I've ordered one purporting to be the A version but I am quite sure it's not. The availability of these parts is very scarce indeed these days (as you will be aware).

I will probably just have to look into recreating the synchro bias circuit.

What's the difference between the SVI2004A and SVI2004B?

 

[SVI2004A]

The B run was a 2nd run on SVI2004A - there were no circuit change unlike the C type.

SVI2004B usually seen in SU-Z600 and SU-Z610 ... I have a few of these... I did open one years ago to see if it had Synchro Bias which they did..

Which makes me scratch my head... the SVI2004A from their spare parts run was a C version worth an A labeling on a 1986/7 run (note divots on cover inside of screw points) after there was SVI2004B...

SVI2004B seen here in a Z600

8**

The c run in the A cover... this was in a V4X that had gone through the same thing... interestingly it never shorted output devices but did knock off the Emitter resistor... there were no Collector Emitter shorts.

 

[Ivan Holmes]

Thanks so much for all this info. It really just isn't available anywhere else at all.
It's very odd that Technics would do this, with all the different versions of the chips. Plus, the synchro bias circuit doesn't look to be at all expensive to include... I guess we will never know why these decisions were taken.

Thanks also for the picture of the blown SVI2004C. All of this is bringing me closer to understanding the way that these chips work (and break).

 

[SVI2004A]

All good...

Tbh I think the manufacturing plant had changed...
Substrates from A & B were different from C substrate... not only the loss of the Synchro bias - the tracks themselves... in the A and B version they are copper all the way... in the C run they’re alloy - with thin laid copper Emitter resistors.

SVI3200 series were same in manufacture as the C type