• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

Technics SP 15 runs too fast (Not Run-away Speed)

M

Mr_Technics

New Member
My Technics SP 15 runs a little too fast a start up when I first turn it on. I have tried re-capping, looking for bad solder joints, breaks in the traces, made all voltage and tracking adjustments but nothing fixes the problem. I don't think the problem is in the MN6042 chip because once the speed settles down it is pitch perfect until I turn the table off and back on.

So what happens is...... when I first turn the table on 33 runs at about 47rpm / 48rpm. As soon as I hit any pitch adjust buttons the table immediately settles down to 33 1/3 rpm and stays there until I turn the table off. I hope this is a known issue and I can get some advice. I actually have 2 tables that do the exact same thing and would be fairly excited to fix them.

Again, this is not the runaway speed issue. It is something a little different.

Thanks
 
Register to hide this ad
Strange problem. I have two SP-15s with defective 78rpm speed.

Have you tried hitting those speed switches & circuit board pots with deoxit? I remember seeing plenty of those pots when I opened one of mine up. I'm assuming you've already checked all of the circuit board connectors.
 
The Technics switches use silver plated contacts inside the switches on many of their turntables. Internally, they go 'black' with oxidation and can cause all sorts of issues sending partial signal level states to downstream logic.

I would be first eliminating them by removing all 7 of them, carefully dismantling them and cleaning the internal contact plate and tactile contact dome. I use Silvo metal polish on a cotton Qtip, then I clean off all the residue in an ultrasonic cleaner, lubricate with a tiny amount of contact cleaner, reassemble each switch and re-install on the PCB. Also clean the pitch lock switch.

I have some pictures someplace of this process when I last did it.
 
Last edited:
Sounds like a bad MN6042. FI and FO should be 262.082kHz 262.080kHz at 0.0% pitch.
 
Last edited:
Hello there,
Remember that the service manual has an error, the voltage test point should be TP22, not TP18.

regards

Angus
 
JP said:
Sounds like a bad MN6042. FI and FO should be 262.082kHz at 0.0% pitch.
Click to expand...
I do have a friend with an oscilloscope. Is that what is needed to test the chip? I was hoping the chip was not the problem and I have been telling myself it is ok because once the table has settled down to 33 1/3 it stay there until it is turned off. The pitch never fluctuates.

I will try to test the chip in the next few weeks and let you know.
 
restorer-john said:
The Technics switches use silver plated contacts inside the switches on many of their turntables. Internally, they go 'black' with oxidation and can cause all sorts of issues sending partial signal level states to downstream logic.

I would be first eliminating them by removing all 7 of them, carefully dismantling them and cleaning the internal contact plate and tactile contact dome. I use Silvo metal polish on a cotton Qtip, then I clean off all the residue in an ultrasonic cleaner, lubricate with a tiny amount of contact cleaner, reassemble each switch and re-install on the PCB. Also clean the pitch lock switch.

I have some pictures someplace of this process when I last did it.
Click to expand...

Looks like I have some cleaning to do. I will try cleaning the pots as Northwinds and illini suggested as well as take the switches apart and clean those. Thank you all for the speedy reply and I'll let you know how it goes.
 
Redikilowatt said:
I remembered a thread initiated by @bangsezmax about this problem that could help.

Recapped my Technics SP-15 and it's still messed up

Roger
Click to expand...
Thanks for the link. There is a post in there about someones table running at 48rpm like mine but the fix involves bypassing the pitch control. Right now my table fixes itself if you touch any pitch button and remains fixed until you turn it off. It even remains fixed until you turn it off for a long time. If you turn it off and right back on it still runs at 33rpm. Fingers crossed the fault is somewhere else on the table and I won't have to bypass the MN6042 chip and I will still be able to adjust the pitch.
 
The pitch buttons connect directly to the MN6042.

FO will equal FI at 0.0% pitch, which should be 262.080kHz (corrected above as well). You can see this with a scope. I can all but guarantee you it's not holding speed as well as you think - flutter is likely quite high.
 
restorer-john said:
Here you go:

View attachment 979143

View attachment 979144

View attachment 979145

View attachment 979146

View attachment 979147

View attachment 979148
Click to expand...

restorer-john said:
The Technics switches use silver plated contacts inside the switches on many of their turntables. Internally, they go 'black' with oxidation and can cause all sorts of issues sending partial signal level states to downstream logic.

I would be first eliminating them by removing all 7 of them, carefully dismantling them and cleaning the internal contact plate and tactile contact dome. I use Silvo metal polish on a cotton Qtip, then I clean off all the residue in an ultrasonic cleaner, lubricate with a tiny amount of contact cleaner, reassemble each switch and re-install on the PCB. Also clean the pitch lock switch.

I have some pictures someplace of this process when I last did it.
Click to expand...


Okay. So I did clean out the buttons but it did not fix the problem. However, man was it dirty in there! I recommend everyone do this to their table. I was able to sneak the plastic cover off carefully without removing it from the board. With a tiny flat head screwdriver you can unclip one side and lift the cover up and off. The only button I could not remove was the 45rpm selector.

Thank you for the advice. It didn't fix the problem but well worth the effort.
 
One other symptom I forgot to mention is that when you hit pitch - the display readout alternates between +0.1 and 0.0. The other pitch buttons display correctly. Pressing + displays +0.1, +0.2, +0.3, etc. and the return button returns to 0.0
 
Hi there. You say you tried recapping ? What did you recap - everything ? I had a Technics SL-QL1 with a similar issue and I tried several fixes including the replacement of the drive control IC but nothing fixed it.

What did fix it, however, was replacing the capacitors in the frequency generator ( FG) circuit. I can see from the schematic there are several around the drive control IC101 - have you changed those out ? If so, have you definitely got the right values - I there's at least a couple of polar caps in there.

Good luck.
 
Mr_Technics said:
One other symptom I forgot to mention is that when you hit pitch - the display readout alternates between +0.1 and 0.0. The other pitch buttons display correctly. Pressing + displays +0.1, +0.2, +0.3, etc. and the return button returns to 0.0
Click to expand...

Bad MN6042. It couldn't be clearer.
 
JP said:
Bad MN6042. It couldn't be clearer.
Click to expand...
Okay... so I bypassed the MN6042 today and I get some weird results. My table does run at the desired speeds but the pitch display acts a bit odd. When I first turn the table on, a few digits are illuminated more brightly than they should be. The table does run at all the desired speeds but when I press the pitch adjust buttons the display will react occasionally illuminating one or two segments but will sometimes turn off completely with the exception of the decimal point. Once the digits have disappeared no amount of button mashing will get them back on again. However, when I turn the table off and back on again, they are back.

I feel like when the MN6042 is bypassed the display should be blank with the exception of the decimal point and the speed readout (33,45,78). My table displays bright garbage until you hit the pitch adjust buttons and then goes blank. This all seems to correlate to my initial problem where the table ran fast when I first turned it on but settled down once I pressed a few pitch adjust buttons.

Am I wrong? What should the digital display look like once the MN6042 has been bypassed? Might another problem on the board be causing the digits to illuminate?

Thanks for all your input.
 
You must log in or register to reply here.
Back
Top Bottom