Technics SU-CH7 amp smoking resistor r573

Thread Starter

cwouter

Joined Nov 23, 2016
24
Hello,

i'm the proud and first owner of the Technics set since 1992 and it has not been used since 5 to 7 years. I wanted to pass it on to my daughter and installed everything. At power on i noticed smoking coming from the amp and after 5 seconds i turned it off again. I opened and cleaned everything. Then i did another power on and i noticed smoke coming from the resistor marked with R573. It's located close to the fan, see pictures. I further cleaned that specific spot and switched in on again, and off when i noticed the resistor itself is burning and becoming black. In total i had 3 times power on, two times just for a second or so. As far as i noticed this is the only malfunction and the display and response was correct. I didn't play any music but i am confident it will.

I guess i am looking for the confirmation i can do without this resistor, than i would need to know what it is for. If it's critical for the planned usage i need to find a way to replace it. Though i'm a amateur (no engineer) i think i should be able to do the job myself.
The set will be played for playing streaming (spotify) from the an aux or dat input. No cassete, no cd, maybe radio. So i will only be using the amp and radio components. If it's only to operate the fan, than i could think of another way of cooling it or instruct my daughter not to use it at high volume... hmm this sounds very risky ;-)

Thank you very much for your support!!

cheers WouterIMG_20161123_085338.jpg IMG_20161123_085455.jpg IMG_20161122_144735.jpg
 

wayneh

Joined Sep 9, 2010
16,102
If you can search and find the service manual, it might have a schematic. That would help immensely in figuring out what's going on. I would assume the part is essential. But, it's possible it supports a function you don't care about.

It's unlikely that the problem is isolated to the resistor itself. You could just replace it and hope for the best, but I think the odds are against you.
 

Thread Starter

cwouter

Joined Nov 23, 2016
24
read the other treads on this amp and was hoping
R!f@@
would get involved. From the service manual i did find the following information "r573 ERDS2TJ330 1/4W 33" I think the 33 refers to OHM. it's part of the main (B) printed circuit board. I tried to find a source where i could order the ERDS2TJ330 but no one had stock or search results. I assume as the r573 is dying i could just put the set on and start using it while assessing what the actual malfunction is right?
 

AlbertHall

Joined Jun 4, 2014
8,186
It is indeed supplying power to the fan. It is (or was) 33 0.25W and any resistor matching that spec will do as a replacement, but the resistor is a symptom not the cause of the fault. Replacing it without finding the cause will result in the new resistor burning too.
Check C574, C575, D576 and the fan for a short circuit. These components are connected in parallel so if one of them is short they will all read short in circuit. Check any one (whichever you find is easy to get to) of them in circuit and if it reads short you will need to disconnect/unsolder each in turn until you find the short. The fan, I think, is connected by a plug/socket so that should be easy to disconnect.

The schematic is available free here:
http://elektrotanya.com/technics_su-ch7_sm.pdf/download.html
 

Thread Starter

cwouter

Joined Nov 23, 2016
24
Thank you, yes i checked the manual from below link and to wayneh point it's listed on page 15 as part of the main board.

The cleaning might have actually caused the defect: I did disconnect the fan as part of the cleaning process as well i put a vacuum cleaner on it, which made the fan spinning at high speed while being connected but with power off.

So then AlbertHall if i understand you well i need to use a multimeter and make sure the resistors still have resistance. If not i need to unsolder them untill the short circuit is resolved, then replace the unsoldered ones. Are the C574, C575. d576 all equal spec?
Why would i disconnect the fan (again)?

http://download-service-manuals.com/en/manual.php?file=Technics-2118.p
 

AlbertHall

Joined Jun 4, 2014
8,186
Not quite, no. Find one of C574, C575. D576 which you can connect the multimeter across on the continuity buzzer setting if possible, otherwise the lowest ohms range will do (all this with the main power disconnected). If you get a buzz or ohms reading less than 1Ω then one of the components is short and that is the cause of the problem.

One of the things which may be causing the short is the fan so unplug that and see whether the short you measured has gone. If it has then the fan is faulty.
If the fan is OK, to discover which remaining component is short you disconnect at least one lead of one of them and check whether you get the short reading across the disconnected component. If not proceed to the next component.

Eventually, you have found at least one shorted component, and the short has gone from the board.

Then you need to replace R573 and whatever shorted components you found. I would also recommend replacing Q574 as it may have been damaged. However, it is obsolete so it may be difficult to find and I can't decide from the datasheet what the connections are so I haven't found a substitute yet.
 

wayneh

Joined Sep 9, 2010
16,102
The cleaning might have actually caused the defect: I did disconnect the fan as part of the cleaning process as well i put a vacuum cleaner on it, which made the fan spinning at high speed while being connected but with power off.
Oooh, I've heard that can be very bad for certain types of fans. If you can post the model number, maybe someone here can tell us if this was bad thing.
 

Thread Starter

cwouter

Joined Nov 23, 2016
24
hmm, will start assessing further tonight. The fan motor is the MDN-4RB4MRC. it's out of stock, even substitutes are as far as i am aware of.
Brand Name Matsushita
Part Number MDN-4RB4MRC
UNSPSC Code 26100000
the
Part Number MDN-4RB4MXA is likely the same product.
 

AlbertHall

Joined Jun 4, 2014
8,186
There is probably a good substitute but don't worry about it until you have discovered which part(s) is/are faulty.
 

wayneh

Joined Sep 9, 2010
16,102
hmm, will start assessing further tonight. The fan motor is the MDN-4RB4MRC. it's out of stock, even substitutes are as far as i am aware of.
Brand Name Matsushita
Part Number MDN-4RB4MRC
UNSPSC Code 26100000
the
Part Number MDN-4RB4MXA is likely the same product.
From what little can be found online, that looks like a normal DC motor. It would generate power when spun by a vacuum cleaner. I have no idea what the consequences of that might be.

But if the fan spins freely, I wouldn't worry about it too much. Your burnt resistor is more likely from a failed electrolytic capacitor.
 

Thread Starter

cwouter

Joined Nov 23, 2016
24
Hello,

I managed to locate all 3 of them on the board. I can't reach C574. The C575 and D576 gave values of 23,4 (on scale 200 ohm). I then disconnected the fan and no values. I then opened the fan, cleaned it and isolated the motor, then connected the socket again. Then measured again and values of 17,8 showed.

The faulty and burned R573 shows a consistent 33.0

Not sure what to conclude from this but I believe it is a good sign. Was thinking of unplugging the fan and power up for few seconds to see if smoke would come from the R573.

Any suggestions or further information on how to proceed? Thanks!
 

AlbertHall

Joined Jun 4, 2014
8,186
So no short circuit there, we haven't yet found the fault.
Next check Q574 for shorts - check between the three pins (3 measurements).
 

Thread Starter

cwouter

Joined Nov 23, 2016
24
Thank you guys for thinking with me... I really appreciate!

I measure with the DMM on the 200 ohm setting but both of them give no change, so 1. For the Q574 I can't reach the middle leg so I can do only one measurement. For the "d373" I assumed it's the d573 as it's neighbouring the smoked r573. It also didn't gave a value when connected to the DMM. The fan is connected.
 

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Thread Starter

cwouter

Joined Nov 23, 2016
24
Also check D373 on the diode position. It should read about 0.6 one way and open (0L) the other way.
I checked the manual and indeed there is no d373 so I'm sure it should be the d573. Than I don't understand how it'll read a different value 'the other way'. Also what does the "L" refer to in the (0L)?
 

AlbertHall

Joined Jun 4, 2014
8,186
You are right it is D573. I knew what I meant!
Does your meter have a diode test position?
You need that to test the diode. The normal resistance range won't produce enough voltage to turn the diode on and we need to know that it does conduct.
 

Thread Starter

cwouter

Joined Nov 23, 2016
24
Did some reading on the DMM. Using the symbol for diode it reads 1080 for the d573 and 1020 for the Q574. Both read 1 the other way.
 
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