Nak ST-7 circuits or repair info please

MadMutt · 1st September 2007, 11:45 PM

Hiya,

I have a Nak ST-7 here that has no display at all.
All else works fine.
Was hopeing someone here either has a schematic or can point me to the fault.

Thanks....

anatech · 2nd September 2007, 01:17 AM

Hi MadMutt,
Sorry, that's one I don't have yet.

The fault should be easy to troubleshoot.

Firstly, does it work otherwise? If so ....

Look for bad solder connections.

-Chris

MadMutt · 2nd September 2007, 04:14 AM

Yup, otherwise it works 100%

Ta, shall pull the main board (oh what fun) and start a joint hunt.

Sounds like you think is a common problem ?
Are there others I should be tending to while I'm at it ?

anatech · 2nd September 2007, 04:25 AM

Hi MadMutt,
Change the memory battery (or capacitor). I can't remember which it is. There was a service note to change the diode to the memory battery (or capacitor) to a schottky type to prevent memory loss at start up. The details escape me, but it was a "do to all" type campaign to get all of them no matter why they came in for service. Your's is probably fine.

I can not recall wether this was an LED type, or an FL display. If you see a glass tube, it's an FL display. Often with FL types, either the heater connections become intermittent or open (craked connections), or the (approximately -30 VDC) supply fails. It is regulated and can be anywhere from 20 to 35 VDC. Normally negative unless they switch the positive when driving the digits.

I doubt it's an LED display because you are more apt to lose segments rather than the entire display.

Another thought is this unit may have an AC fail circuit that mutes the display. Normally a small glass diode charging a 1 uF capacitor in the power supply. If I had the diagram I could tell you.

-Chris

MadMutt · 2nd September 2007, 04:33 AM

Excellent, ta for the pointers.

Memory is a cap, display is Fl.

Transformer has 3 tappings - 30vac - 15vac - 3.3vac this one has a gnd (?) with it

anatech · 2nd September 2007, 04:59 AM

Hi MadMutt,

Quote:

Memory is a cap, display is Fl.

Great! Replace that cap. 5.5 VDC unit, right?

Quote:

Transformer has 3 tappings - 30vac - 15vac - 3.3vac this one has a gnd (?) with it

Wish I had the manual.
Normally the 3.3 VAC tap is biased to - 20 ~ 30 VDC -ish. The regulators often go open. Either the digit pins will have a + supply or the heaters will be tied to a - supply with the segments going to + 5 VDC.

Measure around the power supply with your meter. Can you please post a photo of the power supply area?

-Chris

MadMutt · 2nd September 2007, 05:54 AM

Will start with overall pic.
More to follow....

MadMutt · 2nd September 2007, 05:58 AM

Power supply

MadMutt · 2nd September 2007, 06:02 AM

Display board.
Excuse the ticks, it's so I can keep track of what I've checked...

MadMutt · 2nd September 2007, 06:06 AM

Ohh, should also add, I have a 'spare but not' st7 here for comparison purposes.

After a bit of playing around (not easy to pull apart in some areas), I think I've narrowed down the fault to the actual display board.

When I use the display board from the 'not spare' unit, it works fine.
Guess that means the vfl is shot.
Don't spose there's any hope of replacements out there ?

1st September 2007, 11:45 PM	#1
MadMutt diyAudio Member Join Date: Nov 2006	Nak ST-7 circuits or repair info please Hiya, I have a Nak ST-7 here that has no display at all. All else works fine. Was hopeing someone here either has a schematic or can point me to the fault. Thanks....

2nd September 2007, 01:17 AM	#2
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Nice tuner Hi MadMutt, Sorry, that's one I don't have yet. The fault should be easy to troubleshoot. Firstly, does it work otherwise? If so .... Look for bad solder connections. -Chris __________________ "Just because you can, doesn't mean you should" © my Wife

2nd September 2007, 04:25 AM	#4
anatech diyAudio Moderator Join Date: Jun 2004 Location: Georgetown, On	Hi MadMutt, Change the memory battery (or capacitor). I can't remember which it is. There was a service note to change the diode to the memory battery (or capacitor) to a schottky type to prevent memory loss at start up. The details escape me, but it was a "do to all" type campaign to get all of them no matter why they came in for service. Your's is probably fine. I can not recall wether this was an LED type, or an FL display. If you see a glass tube, it's an FL display. Often with FL types, either the heater connections become intermittent or open (craked connections), or the (approximately -30 VDC) supply fails. It is regulated and can be anywhere from 20 to 35 VDC. Normally negative unless they switch the positive when driving the digits. I doubt it's an LED display because you are more apt to lose segments rather than the entire display. Another thought is this unit may have an AC fail circuit that mutes the display. Normally a small glass diode charging a 1 uF capacitor in the power supply. If I had the diagram I could tell you. -Chris __________________ "Just because you can, doesn't mean you should" © my Wife

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2nd September 2007, 04:14 AM	#3
MadMutt diyAudio Member Join Date: Nov 2006	Yup, otherwise it works 100% Ta, shall pull the main board (oh what fun) and start a joint hunt. Sounds like you think is a common problem ? Are there others I should be tending to while I'm at it ?

2nd September 2007, 04:33 AM	#5
MadMutt diyAudio Member Join Date: Nov 2006	Excellent, ta for the pointers. Memory is a cap, display is Fl. Transformer has 3 tappings - 30vac - 15vac - 3.3vac this one has a gnd (?) with it

2nd September 2007, 06:06 AM	#10
MadMutt diyAudio Member Join Date: Nov 2006	Ohh, should also add, I have a 'spare but not' st7 here for comparison purposes. After a bit of playing around (not easy to pull apart in some areas), I think I've narrowed down the fault to the actual display board. When I use the display board from the 'not spare' unit, it works fine. Guess that means the vfl is shot. Don't spose there's any hope of replacements out there ?

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