Delco 981968

[Cruiseomatic]

Fuse wouldn't work since the jumper wire caused the choke to smoke so a resistor is best. Or I would think? Could the transistor becoming weak which is causing low volume? If I put a fuse inline with the resistor, What value fuse would I use? It would almost have to be a small current one. I am getting around 12.5 VDC on both sides of the burnt resistor. As for the speaker, I was using a factory speaker from my '89 F150. Shouldn't of, But I assumed it was 8-ohm.

On the test, I must have read the directions wrong. I thouht the "broken" radio was the one that was to be affected. Either way, She got aligned (mostly) and she WAS working, Now just get the resistor replaced and she should be good. Although, When I was checking the resistors, All the caps under the board looked new. Only ones that were looking bad were ones on the volume and one in the back. Covered in wax, casing cracked but still worked. I did replace one cap, It is 1000 mfd. 35V and the solder I used has a ton of flux in it. Is it true I can use mineral spirits (paint thiner) to clean the flux up? Thanks for the help guys.

[bandersen]

I was just thinking that since 0.47 ohms is such a low value it's almost the same as a short. Perhaps you could measure the current draw and use a fuse slightly higher ?

I use lacquer thinner to clean flux. It's pretty potent stuff so don't get it on plastic parts and such, but it gets the job done fast. Alcohol works too, but not as fast.

[Einar72]

Another factor you might check is the 600-ohm bias pot setting. The factory manual states "with 14 volts applied and a 10-ohm speaker connected, adjust for 1.5 volts between the collector (DS-501 outer case) and chassis ground". This pot is often corroded, and I've found no continuity between the two major metal parts of this pot, causing a mis-biased condition and lotsa heat.

http://en.wikipedia.org/wiki/Load_line_(electronics)

The pot sets where the transistor operates along the load line. I know this is confusing, but try this analogy:

Think of how you would center a steering shaft in a car so you have the same amount of rack travel when you turn in either direcftion - mis-adjusted, the shaft will top or bottom out after an unequal amout of turning by the operator. The amp is the same way - you want the signal amplification to be centered along this useful area of the transistor - and the bias pot sets the circuit to allow this.

Lotsa luck...

[old_coot88]

Originally Posted by bandersen

I was just thinking that since 0.47 ohms is such a low value it's almost the same as a short. Perhaps you could measure the current draw and use a fuse slightly higher ?

If memory serves, that resistor is in the emitter leg of the transistor, analogous to a tube's cathode resistor, is it not? As such, its ohmic value would be critical (in conjunction with the bias pot) in setting the transistor's operating point. And it served a dual purpose as a fuse. We used to stock them in .33, .47, and .56 ohm to cover the various iterations of those hybrid radios, 'cause the resistor has to be replaced with the correct value.

[mstaton]

Gm cars from the 80's and 90's still use 10 ohm speakers. I have a 90's Pontiac speaker 4x6 that is 10 ohm

[bandersen]

Originally Posted by old_coot88

If memory serves, that resistor is in the emitter leg of the transistor, analogous to a tube's cathode resistor, is it not? As such, its ohmic value would be critical (in conjunction with the bias pot) in setting the transistor's operating point. And it served a dual purpose as a fuse. We used to stock them in .33, .47, and .56 ohm to cover the various iterations of those hybrid radios, 'cause the resistor has to be replaced with the correct value.

Oops, I didn't read the thread carefully and assumed it was a tube radio