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Technics SU-300MC Head Amp crackling issue

T

Timloud

New Member
Hi folks

I'm getting frustrated with a Technics SU-300MC PrePreAmp which has persistent crackle and a big transient when the needle hits the record on the right-hand channel. Left channel is fine. It's crackling when in the bypass mode (ie not using MC), so not sure what the problem is, bar a transistor.

I've swapped out all the caps except C701 and C702 as I was missing those, and swapped out transistor TR401 on the relay as the 2AS720 is in the list, and swapped for a KSA1013. I think my soldering was OK and continuity seems fine

Tim
 

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Timloud said:
Hi folks

I'm getting frustrated with a Technics SU-300MC PrePreAmp which has persistent crackle and a big transient when the needle hits the record on the right-hand channel. Left channel is fine. It's crackling when in the bypass mode (ie not using MC), so not sure what the problem is, bar a transistor.

I've swapped out all the caps except C701 and C702 as I was missing those, and swapped out transistor TR401 on the relay as the 2AS720 is in the list, and swapped for a KSA1013. I think my soldering was OK and continuity seems fine

Tim
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If it’s crackling in bypass mode, the issue isn’t the unit, it’s your turntable, cabling, or stereo.

Replacing components that are being bypassed and aren’t on won’t help.
 
Right, thank you Ivandazende

So after some more testing, the unit still seems to be the fault.

Switched into bypass mode (but is that still using components? My reading of the schematic may be wrong), it makes noise regardless of which amp it's plugged in to and the cabling, again only on the right channel. I've sprayed the right RCA jack with contact cleaner, but I'm a bit stumped
 
Relay is a good idea. Try bypassing it with some wire and see if your issue goes away. Looks like you've have to get creative if you want to replace it.
 
Thanks both,

I've cleaned out the relay with wet and dry and contact cleaner and it hasn't made any difference - also tapping doesn't seem to make much difference.

I also noticed that if I take about half the batteries, it stops. It uses 6x D cells and I swapped these out for another brand, but that didn't work either. Could that point to a capacitor issue still?
 
Hmm. I looked at the schematic and it’s a true bypass. Doesn’t go through a signal component. Other than the switch itself. Check all the wiring.
 
Thanks ivandezande.

Still stumped by this. Wiring is OK and continuity fine although the casing has rust so there's been some moisture around - not sure how much that's affected things.

With the batteries all in, switched to bypass, on my DMM I get a reading ~4V in the right hand channel consistently, left hand is 0 (and silent). Also, and I may be reading this wrong on the schematic, but at the checking point where the battery meets the first two capacitors C701, C702 there are some voltages of +/-4.5v - I get +4 and -5. When I switch it on, these are 9v in both - something wrong somewhere?
 
Are replacement transistors easily available?

If so, I advocate for simply replacing all of them. I often saw fairly old units back from the field in my old job and on something small like this, I would just replace what I could.

Also check the zener diode D701. See what value it is on the BOM and verify it is holding at that voltage.
 
RVT1K said:
Are replacement transistors easily available?

If so, I advocate for simply replacing all of them. I often saw fairly old units back from the field in my old job and on something small like this, I would just replace what I could.

Also check the zener diode D701. See what value it is on the BOM and verify it is holding at that voltage.
Click to expand...
Did you ever figure it out, I have same issue, right channel. I replaced all the caps and resistors with no luck. I did take switch apart and got finally got the thing to work but still have the crackle. All my voltages on the transistors are good, and I swapped the channels with no luck, swapped the diodes as well. Trying to find those transiters or equivalent is pulling my hair out. I hope you stumbled on the issue. My bypass is fine, taking the switch apart helped a lot, i wonder if more work on the switch needs to be done.
 
@basko93 Mine seemed like it was centered on the switch. On the pins for the right channel (middle set I think?) there was the -4V DC I talked about above. I think the copper contacts in the switch were skewed somehow and bridging something on one side and when I opened the switch up, I struggled to get those contacts back into place and to work normally. Eventually I found a guy on the 'bay with a very similar Alps switch (though he was selling them as hitachi) and I used the contacts in that to replace them, and it is currently working fine - but if there's nothing plugged into it and its just outputting, it still has loads of clunking, maybe it was always like that and needs the ground from the TT

My box was rusty as hell though, and I had tracks lifting and all sorts of fun and games before I was happy with it. Good luck! Sounds way better than the stock MC preamp on the SL-10 once it's working OK
 
Timloud said:
@basko93 Mine seemed like it was centered on the switch. On the pins for the right channel (middle set I think?) there was the -4V DC I talked about above. I think the copper contacts in the switch were skewed somehow and bridging something on one side and when I opened the switch up, I struggled to get those contacts back into place and to work normally. Eventually I found a guy on the 'bay with a very similar Alps switch (though he was selling them as hitachi) and I used the contacts in that to replace them, and it is currently working fine - but if there's nothing plugged into it and its just outputting, it still has loads of clunking, maybe it was always like that and needs the ground from the TT

My box was rusty as hell though, and I had tracks lifting and all sorts of fun and games before I was happy with it. Good luck! Sounds way better than the stock MC preamp on the SL-10 once it's working OK
Click to expand...
So, it is the switch, i will have to look into it again. This makes sense cause i have literally gone and checked everything else. I will pull mine again and see if i can get it resolved. Thank you.
 
Timloud said:
@basko93 Mine seemed like it was centered on the switch. On the pins for the right channel (middle set I think?) there was the -4V DC I talked about above. I think the copper contacts in the switch were skewed somehow and bridging something on one side and when I opened the switch up, I struggled to get those contacts back into place and to work normally. Eventually I found a guy on the 'bay with a very similar Alps switch (though he was selling them as hitachi) and I used the contacts in that to replace them, and it is currently working fine - but if there's nothing plugged into it and its just outputting, it still has loads of clunking, maybe it was always like that and needs the ground from the TT

My box was rusty as hell though, and I had tracks lifting and all sorts of fun and games before I was happy with it. Good luck! Sounds way better than the stock MC preamp on the SL-10 once it's working OK
Click to expand...
Well, i got a switch and it took care of the right channel output. I still have that clunking and a small amount of noise in the right channel, also if i turn the selector switch it makes a thump. I remember exchanging the transistors and diodes from one channel to the other but i still think it's the transistors, I just cannot find substitutes. Shame because my left channel is perfect. For such a small unit it is driving me nuts.
 
It still sounds a lot like the problem I had.

If you're in MM/bypass mode, with one DMM probe attached to ground, are you still getting +/- DC anywhere - particularly in the switch pins on S1-2, ie the set of 10 in the middle of the board? Also, are you getting DC in the right channel output plug, sleeve etc? If so, I think that means something is being bridged still within the switch.
 
Timloud said:
It still sounds a lot like the problem I had.

If you're in MM/bypass mode, with one DMM probe attached to ground, are you still getting +/- DC anywhere - particularly in the switch pins on S1-2, ie the set of 10 in the middle of the board? Also, are you getting DC in the right channel output plug, sleeve etc? If so, I think that means something is being bridged still within the switch.
Click to expand...
Yea, I am. Right channel, like -1volt at rca terminal and about 1 volt at s1-s2. I put new sliders in that switch, but the body is the original part, the new one i bought was a 5 position. Not sure i can find exact switch and my traces are not doing well at this point. Did you ever find a correct new switch?
 
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No, similarly I bought a NOS Alps switch that was pretty close, took the copper contacts out, cleaned the body of my original (maybe lubed?) and put it back together. I didn't crimp back the pins on the top of the switch housing until it was working properly. Traces on mine are terrible also, had to do some creative bridging :rflmao:
 
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