Strange SX-1280 Short

[ivandezande]

Standard restore, it was working decently before. I'm using a 70W halogen bulb in my DBT. When turned on it's bright as day, and I've checked the entire path and I cannot understand how there would be a short before the soft-start circuit where the relay is for sure not engaging....
I took out the fuses to the different power supply sections and I felt like maybe it was dimming, but I wasn't trying to leave it on any long to find out.

 

[Watthour]

How long was the unit left on? If only for a few seconds, the caps may not have charged fully.

Are there any audible relay clicks while the unit is powered? That's usually 7-10 seconds minimum.

Also, 70W might just be too small for a 1280. Try bumping to a 100W lamp for a brief period, and possibly 150W.

 

[The Fuxtor]

Halogen bulbs are ok to use in a dbt??
Looking great otherwise!

 

[Watthour]

Halogen incandescents still use a tungsten filament.

 

[zebulon1]

Disconnect the big caps and try it.

 

[ivandezande]

Thank you are for the replies, I think I'm just not giving it enough time as Watthour said as I connected a working 1250 and it took a century to dim even a little.

 

[kevzep]

Thank you are for the replies, I think I'm just not giving it enough time as Watthour said as I connected a working 1250 and it took a century to dim even a little.

Go higher wattage with the bulb, 100watt maybe even 150....

 

[rcs16]

I monitor the soft start relay coil voltage, at where the the V drop R connects to the coil, to see what's going on. The longer the S/S relay takes to energize, the longer you have current running thru the big power R and the thermal cutout. I'd say 10s max, it is usually <1s
The relay coil ckt has a power reduction ecap, that has to charge up as well, thus slows down the v rise rate.

 

[avionic]

working 1250 and it took a century to dim even a little.

The longer the S/S relay takes to energize, the longer you have current running thru the big power R and the thermal cutout.

Is the thermal fuse and ballast resistor still seviceable.

 

[rcs16]

Asking me? if so I say yes

 

[avionic]

Asking me? if so I say yes

Sorry. No, I wasn't asking. Just using your statement as a reference. If he was waiting a long time for the bulb to dim. And the SS relay hadn't by-passed the ballast resistor. The ballast resistor probably got pretty darn hot.

Perhaps the dim bulb prevented it from getting hot enough to open the thermal fuse.

 

[avionic]

A shorted output would cause the SS relay not to " set "..

 

[markthefixer]

That much of a current draw means:

1. possible polarity goof on main caps, check with fresh eyes (pictures looked ok, but my factory reference pix didn't show the colors of the wires well enough)
2. shorted outputs
3. outputs turned full on HARD: open circuit thermal diode, reversed thermal diode, idle current pot set to maximum resistance.


The RELAY for soft start runs off of the FU3, FU4 (green brown ct green)
regulated DC (+/- 68v DC reg) source's power: C5, C6 +/- 88v

And don't be afraid of the bulb glowing brightly, you can leave it on for say 10 seconds or so without incurring significant damage - the bulb is absorbing the power, so get busy with DMM.

Oh, and with this big of a unit, use 150 watts of DBT (2 x 75 in parallel).

 

[ivandezande]

She powers up ok and the soft start relay energizes, now the speaker relay isn't clicking even with offset at zero...go figure. But that's another problem.
Thanks everyone!

 

[zebulon1]

So? What did you find with the issue in post #1??

 

[ivandezande]

So? What did you find with the issue in post #1??

Nothing, I just didn't give it enough time/not a big enough bulb.

Once off the DBT the protection relay clicked in. Now I just have to tackle some hum issues.