REALISTIC STA-78 No Stereo Light

[JB8]

I get the same readings when in AUX function - all four positions in fact. I checked the polarity for the LED and it was correct as well.

 

[petehall347]

did you check D702 cathode voltage yet ?

 

[JB8]

Sorry, Pete, I got a forward bias reading of .56 and reverse bias reading of OL. I'd never checked a diode so I went with the resistor idea of pulling one lead from the board then testing with the other leg still attached. Let me know if this isn't the correct method for testing the voltage.

 

[petehall347]

meter on dc volts . black lead to chassis red lead to D702 cathode which is the end with a band .

 

[JB8]

Got it. Should I reinstall the end I pulled, and then I assume I'm testing for DC volts with power on, correct?

 

[petehall347]

yes solder it back .power on for testing .

 

[JB8]

Thank you, I reinstalled the diode, cathode measured .05 v with power on (black on chassis ground, red on end of diode with band)

 

[petehall347]

now set on AC volts .black to chassis red to fuse holder F701.try both ends of fuse holder ,

 

[JB8]

Both ends read 12.12 v AC with power on, black lead on chassis, red on either end of F701.

 

[petehall347]

now measure AC volts again at D702 Anode

 

[JB8]

12.20 v AC at D702 Anode (feels like we're doing a remote appendectomy, I like it!)

 

[petehall347]

now to be sure check D702 cathode dc volts again .

 

[JB8]

.04 v DC, with power on, measuring D702 cathode.

 

[petehall347]

c708 has gone short .or D702 has forgotten how to be diode .
power off then measure ohms across c708 . leave in the board for this . if its not shorted replace D702 .

 

[JB8]

C708 checked out fine, checked it while installed and it read 481 ohms so I'm assuming no short?

Any good leads on a replacement of D702? First time sourcing/replacing a diode. Is this still the preferred option? (pretty sure this was a suggestion by our friend @avionic from another AK thread)

https://www.mouser.com/ProductDetai...=sGAEpiMZZMtvcUztdGSumJS725njxsbLoeZvehNmL9U=

 

[petehall347]

1n4007 should do it .

 

[avionic]

1N4004 would suffice. 1N4007 is overkill but will get the job done.

 

[petehall347]

was going to suggest that first but changed my mind .. also was going to say borrow D702 to see if anything is is bad but might end in trouble .. best just get a bag of diodes . they are cheap enough .

 

[JB8]

Thank you both, your assistance and patience have been exceptional.

I'll get an order placed this evening and post an update once the repair has been completed. Onward!

 

[JB8]

Two quick updates/questions:

I had another STA-78 arrive this week and it's incredibly clean, didn't even have to take it outside to blow the dust out once I took it out of it's case off, which is rare! At any rate, just for fun,I checked the cathode voltage for D702 and it was ~14 v DC, nice to know what to look for once my diode order arrives and we get the new part installed.

Lastly, this new unit is functioning exceptionally well but, in my opinion, doesn't look very exciting with it's original incandescents. On the other unit, the main focus of this thread, I've got the LED's installed, but also recently added the 47 ohm 2 W resistor to cover the voltage drop from 12V to 8V - is this necessary? I trust the crew here and saw this technique in BassBlasters rebuild thread, so I'm assuming it's the way to go, but would love to know what the risk is, if any, if I simply swap in the 8V LED's for the 4 lamps in the array plus the one in the meter.