Technics SU-VX800 amplifier

Thread Starter

Marius83

Joined Dec 28, 2017
380
Hi.

I got this Technics SU-VX800 from a friend, he was going to throw it.
At first the power button was not working, i thought hey, easy fix.
Replaced the power button, it "turns on" but the "current drive" lamp is not on, so the amp does not work.

I took a closer look, found the four power transistors on one of the channels had a short circuit.
Replaced these. Still no difference.

Took an even closer look, found four burned resistors and a little capacitor, i also noticed the soldering around one of the four
small transistors who usually gets hot, were loose..
I then replaced all these components with new ones, four resistors, a little capacitor and also the four "hot" transistors just because i could see they had been so hot.

Got it back together, turned it on and hoped for the best.
The current drive lamp did not light up, and then my new resistor at R414 started to smoke, followed by the new capacitor at C405 blowed up.
So i turned it off, hehe.

Is there a Technics specialist here.. who could give me a tip about the next thing to check.. or so...? :)
Would be very appreciated!
Would have been fun to get this thing up running without leaving it to a service workshop.
 
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MrChips

Joined Oct 2, 2009
21,890
What? No takers?

Ok, clear out the workbench. Get out your DMM and warm up the oscilloscope.

First step - Do the visuals. Look, smell, touch. (Oh. You've done that already).
Next - Get the Service Manual.
Make a list of all the components you have found suspicious and you replaced.
R414
C405
What else?
Locate and mark these on the circuit schematic.
 

Thread Starter

Marius83

Joined Dec 28, 2017
380
Thanks

The service manual is online, i have now printed it :)
http://www.maxdat.eu/_data/13=Kapcsolasirajzok/Technics SU-VX800.pdf

Here is what i have replaced:

Power transistors:
Q510
Q508
Q518
Q516

Resistors:
R510
R514
R448
R414

Capacitor:
C425 (!!) not 405, didn't see cause of the capacitor waste

Small transistors which had been hot:
Q651
Q653
Q652
Q654

After turning it on, R414 was smoking and C425 blew up.


As i was looking down in it today, i also found another faulty component wich had not caught my eye.. :oops:
Q420E is cracked on the top, see picture.
This is a little transistor right.. but could this cause the C425 to blow up?



C425 :p
 

MrChips

Joined Oct 2, 2009
21,890
Nice photos.
Yes, I knew there was no C405.
Are you certain that you installed C425 with the correct polarity? That looks like what happens to a capacitor when the polarity is reversed.
 

MrChips

Joined Oct 2, 2009
21,890
Let's see. Where are we?

Disconnect any inputs and speakers to the amp.
Turn on the power and measure the following four voltages with respect to GND.

- on one leg of R505 (should be about +60V)
- on the other leg of R505
- on one leg of R506 (should be about -60V)
- on the other leg of R506
 

Thread Starter

Marius83

Joined Dec 28, 2017
380
Yes I am certain.. as long as the markings on the new one was correct..

The one I replaced had not exploded, but it was half way split like it had been hot.
 

Thread Starter

Marius83

Joined Dec 28, 2017
380
I forgot to ask but since you mentioned + and - i did put my DDM to Vdc...

Turned it on, put the black one to GND.. I'm on thin ice here, but i thought you meant like the chassis of the ampflifier, ground, right?

Well.. when i touched one leg of R505... at once R513 smoked, and right after that R509 was glowing red... so i turned it off..
Turned it on again, R509 glowed for a couple of second, then went dark.

Did I do something wrong? :(
 

Thread Starter

Marius83

Joined Dec 28, 2017
380
Oh my god I feel like an idiot now, I actually put the tester on one leg of C505.. is it even possible.:mad::mad:

Well I can still measure R505 then.. but tell me, am I doing it the right way as I described over?
 

MrChips

Joined Oct 2, 2009
21,890
Sounds like you are destroying components one after another.
What is the history of the unit?
Was it always used on 230VAC?
Are you certain that the unit is set for 220-240VAC mains?
 

MrChips

Joined Oct 2, 2009
21,890
If your DMM is set to VDC there is little chance of causing damage unless you accidentally short your test probes to some other junction.

Chassis is usually GND. If it isn't there is still no harm done in connecting your -ve lead to chassis.
 

Thread Starter

Marius83

Joined Dec 28, 2017
380
Sorry, my leads in my DMM was set in the holes to measure Ampere.
I guess that caused the problem, making connection between GND and C505 .....:oops: :oops: I'm getting so angry at myself.
I have now replaced R509 and R513.

The history of the unit from the owner is that the power button was not working, so they used to put a little credit card piece between the button and the front, then it came on.
The last time it was used was at a party, i guess then the damage happened.
But the guy who had it couldn't even remember it stopped playing..

At the back it's two choices for choosing voltage 230 or 240 Volt.
I has always stood on 230, i measured my wall socket now, it's showing 236 Volts
 

Thread Starter

Marius83

Joined Dec 28, 2017
380
So, after my little idiot incident... replaced R509 and R513, I turned it back on.

And I have caused a new problem... one step forward and two back..
Power transistor at Q507 shooting flames...





Soo... could i have shortened the transistor with established connection between C505 and GND ?
 

MrChips

Joined Oct 2, 2009
21,890
Yikes! Things are going from bad to worse.:eek:

Remove Q507. Don't replace it just yet.
If any of the power transistors go up in smoke, don't replace them for the moment.

We still have some power supply voltage checks to be completed before we start replacing components.

It is not my style to replace any components that just look bad. Usually components go bad for a reason. We have to pinpoint the reason for a component to go bad.
 

MrChips

Joined Oct 2, 2009
21,890
Remove both Q507 and Q509.

After you remove Q507, measure the voltages at what would have been Q507 at pins 1, 2, and 3.

Do the same at the pins for Q509.
 

Thread Starter

Marius83

Joined Dec 28, 2017
380
Going worse when i'm doing things wrong..:oops:

I understand what you are saying.
I will remove Q507 and Q509 and then measure the voltage on the pins.. DC, between one and one pin to GND ?
 

MrChips

Joined Oct 2, 2009
21,890
Correct.

One of the second things we do with technical repairs is to check all power supply voltages.
We need to check +60V, -60V, +18V, -18V, +5V with respect to GND.

Collector (pin-2) on Q507 should be +60V.
Collector (pin-2) on Q509 should be -60V.

We will come back later and check for Q507/Q509 bias and DC balance.

Voltage at R651/C651 should be +17.8V.
Voltage at R652/C652 should be -17.8V.

Voltage at C706 should be + 5V.
 

Thread Starter

Marius83

Joined Dec 28, 2017
380
Ok.

Removed Q507 and Q509

Turned it on. R513 and R509 instantly went up in smoke.

Measurements:

Q507 collector (Pin2) +63 Volt
Q509 collector (Pin2) -63,3 Volt

C651/R651: +17,33 Volt
C652/R652: -17,33 Volt


Having a hard time finding C706, i've looked on the schematics, is it on the "front" circuit board?
When i was standing looking for C706 on the main board, the front panel started smoking, right behind the "Voltage control" and "Direct current" lights :eek:...so i disconnected the flat cables, as i was still looking for C706 on the main board.

If I have got it right, the C706 is on the front circuit board, and since it's smoking...do you have another place I can measure the +5 Volt ? :rolleyes:
 
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MrChips

Joined Oct 2, 2009
21,890
We need to stop the smoking.
Replace R513.
Do not replace R509.
Can you remove Q501?

Does the "VOLTAGE CONTROL" light turn on?

Can you take some photos of the main PCB?
Take the photos in overlapping sections so that we can see the location of components.
 

Thread Starter

Marius83

Joined Dec 28, 2017
380
Yes, the "Voltage Control" light has turned on every time i turn the amp on.

I have now replaced R513, and leaved the R509 as it is.
I have also removed Q501

I have not turned it on yet after this, should i try? :)
Should I also put in the flat cables for the front PCB before I turn it on ?


Here is two pictures of main PCB, you can see they are overlapped in the bottom.
If they're not good enough let me know i will take some more.

EDIT: I see the pictures didn't appear the way i was thinking, but the upper picture contiues out the rear side and starts on the left on the other picture.


 

Dodgydave

Joined Jun 22, 2012
9,463
C425 is blown, it's a smoothing capacitor for IC703 5V regulator to feed the front pcb relay/leds chip.

Q501 sets the bias for the Left channel.

From reading your posts, i think you had your meter on amps, and therefore blew up the good channel, ideally check the DC supplies, 60V+/-, 5V, and as long as the DC bias is set as per the manual, the voltage at the speaker outputs( L501,502) should be Zero, if not then the speaker protection chip IC501 will put the relays off.
 
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