drive gone crazy

[hiwiho]

Its me again - my nakamichi drives me crazy! I simply won't get it running when its cold. It plays about a minute, then spins the disc up and down, partially reversing and the feed motor might even drive the pickup to the outer end without stopping. If I don't switch it of, it would destroy the drive (or one of the gearwheels - as happened before). As i only had distorted nf on the rca's , I soldered out the mute transistors - now, when the drive's gone crazy, it crackles and hisses.

I resoldered the whole digital section - no effect
I changed the ribbon from pickup to pcb - no effect

One thing is strange. When I touch the ribbon cn-102 (link to schematics below - it connects the detector diodes with cxa1081) with my fingers, me being not earthed, the player starts going crazy. He doesn't do it when I use some isolating material to touch it or when I touch the case with my other hand. This very much seems like a grounding prob of the pickup, but can that be??? Is the detector circuit always that sensitive? Any capacitors fault?

Once the player got crazy, i have to switch it off for hours or to unplug the mains for some seconds. Even if i take out the disc, the player buzzes, crackles and hisses...

Any help??? I start running out of ideas! Unfortunately i've got no oscilloscope...

schematics: http://tinyurl.com/4rlht

matthias...berlin...germany

[PMik]

A wild guess

Check the state of C202 (1000µF/35V-)

if wornout it could drag the -5V and -9V supply down

[hiwiho]

Thanks!

Hmm, C202 only gets about handwarm, just as C204 and the big psu caps 3300/25. Its glued to the pcb, so I cannot see whether its a ran-out cap.

I checked out the -5/-9Volts on some points and have them in a variation of .1/.2volts, but the point given at D120 (-32V dc) has only -28.5 to -29volts and around the zener zd102 i have -21 instead of -22.5 and -27 instead of -28.8.

Might that be a sign c202 and /or c204 are weak? They're not costy, so I could change them anyway.

matthias

[PMik]

If any doubt of prober function, change them...
That since they are in-expencive to buy

[hiwiho]

Well, i'll do that tonight. Maybe i'll be back tomorrow or on sunday

[hiwiho]

short update:

changed both c202 and c204 for new ones rated 1000/35/105°c. Works better, but still not stable in cold condition (my room 17°c), so next week i'll change all the psu electrolytes assuming they (all: taicon 85°c) are past their best days.

The voltage drop to values given in the manual was obviously caused by measuring in operation. They're still about the same now.

[Extreme_Boky]

Hi Hiwiho,

Unfortunately your CD player has a provision for a full tracking and servo adjustments. You'll need an oscilloscope!

Start at pin 2 of U101 and check the waveform, it should be 1.5V p-p while CD is playing (any music CD will do fine). If this checks okay, proceed with focus and tracking adjustments. you'll need a test CD (Technics for example). Instructions usually come with the test CD.

[hiwiho]

update2:
Today was the first time in the last 3 days it went crazy again. Being switched off since yesterday, the mains plug remained plugged, and perhaps 40seconds after start the disc remained rotating, but the signal got lost completly...the player sounding like searching a station on an analogue am tuner: grizzling, sweeping.
But unlike before, i could stop both noise and disc with the stop-button. A reload of the disc meant no spinup. Unplugged the mains for some seconds and now it's playing erversince.

@ extreme_boky: Yes, the player offers full adjustment resistors, but the manual says I need a special switchbox (schematic included in service manual) thats plugged to a multipin-serviceconnector in combination with frequency counter, 2channel-osc and signal generator. I lack all of these could just get my hands on a 1channel-version...
And I don't really think its the pickup. When it runs, I've got no clicks, no skipping and fast tracking even on poor cdr.

I ordered replacement electrolytes for all taicons in the ps and hope it's done when finally changed. At least Pmiks guess meant some 80% improvement on the naka. Thanks for that. I knew danes never lie

matthias

[PMik]

np!

European (skilled) engineers havnt used this cheap trick in four decades and why not?
A leason was learned and wrote down in the black book of what not to do......it simply fails to fast
Its just the fact that C202 is under great stress, when used in this fashion to gain double voltage

[kevinkr]

I am not a big fan of voltage doubler topologies either half wave or fullwave, but just about every computer on the planet that operates on 100V - 120V with a switching supply uses one. (fullwave) Like anything else it is a matter of picking the appropriate component for the job - I have a now retired pc that ran continuosly for about 8 years and that switcher still works fine...

Without knowing the ac line voltage the voltages measured on C202/C204 are not conclusive. Generally there is also a tolerance on the values given by the manufacturer, and distortion of the ac mains waveform may also slightly affect the value.