• 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

SL-1200 MK1 Woes (Negative Voltage at TR-14 collector)

zombiemodern

zombiemodern

New Member
Hi AK,

Was wondering if anyone here could help with an SL-1200mk1 that's seeing speed irregularity, particularly on power cycles. This is now a DIY project — a third turntable I don't really need but due to the rarity I'd love for it to operate properly. I've taken this to a local tech figuring it was beyond me, but he only swapped the zener diode and charged me $200 which to me was a total ripoff so we parted ways. According to the service manual, the issue is likely that I'm not seeing 20v at the collector of TR-14. Currently I'm seeing a range of negative voltage from -17 – -24v. Unit still measured the same as the collector after trip to the tech.

So far I've:
-Cleaned all pots and switches, oiled the bearing
-Checked the solder connections at the speed switches, refreshed some
-Checked power supply with DMM
-Replaced all electrolytic caps slowly and carefully with new caps from Newark (Nichon, Panny, etc, decent stuff). Axial looked swollen, none were replaced by tech.
-Swapped in a third zener diode from Newark

All this, and no change in the reading I'm getting at TR-14. It looks like speed is slightly more stable but that could be confirmation bias at this point. Does anyone know what this issue could be??

Couple of thoughts:

- I'm measuring wrong or something is wrong with my cheap DMM. Currently I'm using the DC Volt setting w/ sensitivity of 200v, red lead on the collector of TR-14, black lead on Pin 7 for ground. DC batteries test fine.

-Something is wrong with a transistor in the voltage regulation circuit.

Honestly at this point I'm at a total loss of what could be wrong. I'd hate to have to get a new board as I've already sunk too much into this with the useless trip to the tech. I'm just a hobbyist with cheap gear so any and all help from more experienced members is appreciated. Recapped board shot attached.

ul9PBK5.jpeg
 
Register to hide this ad
Start by checking to see if touching (not turning) VR1 changes the voltage. I'd think the wiper could be making bad contact. If not this, move to checking transistors in the VR circuit. Also verify the value of R29, it looks like it got hot.
 
maxhifi said:
Start by checking to see if touching (not turning) VR1 changes the voltage. I'd think the wiper could be making bad contact. If not this, move to checking transistors in the VR circuit. Also verify the value of R29, it looks like it got hot.
Click to expand...

Thanks for the suggestions, will start by checking contact of VR-1. Assuming for the transistors in the VR circuit I should pull from the board for a more accurate test?
 
zombiemodern said:
Thanks for the suggestions, will start by checking contact of VR-1. Assuming for the transistors in the VR circuit I should pull from the board for a more accurate test?
Click to expand...

Start by checking in circuit using the diode test function of your multimeter. You can look up online how to do this. If anything is looking odd you can pull that one and check it more thoroughly.
 
maxhifi said:
Start by checking in circuit using the diode test function of your multimeter. You can look up online how to do this. If anything is looking odd you can pull that one and check it more thoroughly.
Click to expand...

Hey again,

Checked touching VR-1 with varying degrees of force, no change. BTW, the voltage is consistent at TR-14, and responds to turning as one would expect, just well below the target 20V per SM. Visually it seems to be geometrically intact and similar to VR-2 and VR-3.

Checked R29 with the Ohm setting, one leg pulled out, got a measure of 325ohms, SM specs this part as 330ohm, assuming this is in spec?

Checked all transistors in circuit. Pulled 7 and 9 to get accurate reads (they’re attached to resistors) got good read outs. Also pulled TR-14 since it had a poor soldering job, fixed the joints, measured fine.

At this point I’m really stumped! I’m also confused as to the proper voltage test point. I’ve been testing collector of TR-14, black DMM lead attached to pin 7, but the SM seems to indicate that the proper test point is TR-13’s collector. With this, I get a reading of 0v from the DMM, transistor tested fine w/ diode test.

Not sure if I should shotgun replace some more parts or just consider it lucky it still spins at this point. Thanks again for any help!!
 
How are you measuring voltage?

This regulator is in the negative rail of the power supply. You need to measure between the positive terminal of the filter capacitor, and the collector of Tr14 to get the regulated voltage.

The negative terminal of the main filter capacitor is not circuit ground, and will vary according to the load on the supply.

Edit:

Never mind I see you're doing it right.

Try measuring voltage at the base of TR12 and see if it is stable, then TR13, then TR14. Maybe this will help identify where things go wrong.

The zener stack really should be stable.
 
Last edited:
maxhifi said:
How are you measuring voltage?

This regulator is in the negative rail of the power supply. You need to measure between the positive terminal of the filter capacitor, and the collector of Tr14 to get the regulated voltage.

The negative terminal of the main filter capacitor is not circuit ground, and will vary according to the load on the supply.
Click to expand...

That could be my issue, I’ve been using the red lead on TR-14’s collector pin, black lead to pin 7 which I assumed to be ground based on my reading of SM (like I said I’m stumbling in the dark lol). Power is plugged into board, board is unplugged from motor. Which pin should the black lead be set to? Positive lead of axial C16?
 
zombiemodern said:
That could be my issue, I’ve been using the red lead on TR-14’s collector pin, black lead to pin 7 which I assumed to be ground based on my reading of SM (like I said I’m stumbling in the dark lol). Power is plugged into board, board is unplugged from motor. Which pin should the black lead be set to? Positive lead of axial C16?
Click to expand...

I edited my post above.. pin 7 IS the positive terminal of C16, so unless you're using the wrong pin, you're doing it right. I just added a few more ideas to my previous post.
 
maxhifi said:
Try measuring voltage at the base of TR12 and see if it is stable, then TR13, then TR14. Maybe this will help identify where things go wrong.

The zener stack really should be stable.
Click to expand...

OK so I tried that, same process, common black lead on pin 7, power on. Left VR-1 alone for these tests, assumed I’d get some variation on TR-14 with fiddling.

TR-12 -25.4V
TR-13 -0V (whatever that means)
TR-14 -21.8V

Bizarre, as TR-13 seems to pass the diode test. one note is that I have had issues getting readings with this DMM, might be me, might be the cheap leads, or just a crap meter.


Upon inspection it looks like the local tech pulled TR-12, not sure if that’s helpful. Markings on the newly installed transistor are A1015 GR331
 
What do the collector and emitter of Tr13 read

Also, look up the pinout for 2SA1015 and make sure it is installed properly. Some older Japanese semis have a couple leads reversed versus new ones, and maybe he got it wrong.
 
maxhifi said:
What do the collector and emitter of Tr13 read
Click to expand...
On diode test? Reading in circuit, let me know if I should pull.

TR-13
C>E wild swings from 1600mv down to 600mv. Held probes in place, voltage continued to rise from 600 to 1300mv till I stopped test.
E>C OL

other test points:
B>E 729mv
E>B OL
C>B OL
B>C 718mV

does this indicate a failed TR-13?
 
zombiemodern said:
On diode test? Reading in circuit, let me know if I should pull.

TR-13
C>E wild swings from 1600mv down to 600mv. Held probes in place, voltage continued to rise from 600 to 1300mv till I stopped test.
E>C OL

other test points:
B>E 729mv
E>B OL
C>B OL
B>C 718mV

does this indicate a failed TR-13?
Click to expand...

I mean the voltage on the three terminals of TR13, with the board powered up, the black lead of your meter to pin 7 of the connector, and the red lead to EBC of Tr13 respectively.
 
maxhifi said:
I mean the voltage on the three terminals of TR13, with the board powered up, the black lead of your meter to pin 7 of the connector, and the red lead to EBC of Tr13 respectively.
Click to expand...

Oops
C: -0v
E: -25.8v
B: -25.3v

edit:
Read collector on a lower threshold setting (20v max), got -0.14v
 
I think the best thing to do here is just order a new Tr12, Tr13, Tr14, and all the diodes, and just replace them until the voltage is stable. All your capacitors are the right polarity, I can't really help with more remote troubleshooting now that we see it isn't something obvious.
 
maxhifi said:
I think the best thing to do here is just order a new Tr12, Tr13, Tr14, and all the diodes, and just replace them until the voltage is stable. All your capacitors are the right polarity, I can't really help with more remote troubleshooting now that we see it isn't something obvious.
Click to expand...

No worries thanks for your help so far!! I'll order those up with suggested replacements in other threads. Do you think it's worth replacing VR-1 based on your hypothesis about wiper? I see SM seems to default to replace and adjust vs adjusting that part. If so do you have any tips on how to source that open air pot (preferably on Newark)?
 
zombiemodern said:
No worries thanks for your help so far!! I'll order those up with suggested replacements in other threads. Do you think it's worth replacing VR-1 based on your hypothesis about wiper? I see SM seems to default to replace and adjust vs adjusting that part. If so do you have any tips on how to source that open air pot (preferably on Newark)?
Click to expand...

Pick a one turn sealed pot to replace the old one - those old open air ones are inferior anyway. Same resistance value, just look for one with a similar footprint.. You can kludge it a little, it just has to fit.
 
You must log in or register to reply here.
Back
Top Bottom