Wacky 5150 schematic

[Enzo]

CR21,22 are themselves clamps around a signal line called "clamp." The clamp is the momentary mute activated by channel switching. it is connected right to the preamp output at the post control selector relay K2B.

The signal there can be pretty hot, so those two diodes are in reverse. They prevent the signal at that point from exceeding 24v either polarity. The only reason for that as far as I know is to protect the JFET from over voltage.

By the way, I would change the other one too. I don't care if it tests good.

OK, here is the failure mode as I see it. That diode shorts. Now +24VDC is on that section of signal path where clamp goes. Turning the volume controls down grounds that point, so the 24v rail is grounded through the pot. The return current path is through the pot grounds - the circled ground - through R77 back to the 24v power supply ground - the circled rake. 24v across R77 is about 12 watts.

I bet if we had turned both post controls all the way up, or even half way, that R77 would not have burned. Things wouldn't have been right, but no burning. The post controls prolly would have been scratchy.

[Valvehead]

interesting--as now i get a lot of noise and a delay when i switch channels......and very low volume still.


i will change the other diode now.



which JFET are you talking about Enzo- could that cause the "low volume" ? so if the signal exceded 24v---what would take the hit?

[Enzo]

Look at the schematic. The JFET is right next to the diodes on the drawing - Q7. I was thinking more about the burning resistor and not your low volume. I should have mentioned it. COnsidering that one of those diodes was shorted, 24v was on the drain of that JFET, so every time it turned on, it would up trying to short 24v rail to ground. I don't doubt that JFET is stressed or bad. Just remove it and see if the amp wakes up.

Every time you switch channels, that JFET turns on for a brief moment, grounding the output of the preamps. This prevents loud noises when you switch. If it fails and sticks on, then you will lose most of your signal. The amp will work without it, but you run the risk of the loud noises. But to see if it is causing the trouble, remove it. It is a J174.

[Valvehead]

once again-you were correct Enzo.

I took the jfet out and had full volume.

The local vendor of course doesnt have that one,.....on the way from Mouser.





enzo = genius