Help Please

[Steve_in_NV]

I have a problem that just started and is intermittent (the best kind).  On the C4S board D1&D2 an the "A" side go out D1 on the "B"side goes out, with a thump and no audio.  I resoldered some connections and gave it the twice over, any suggestions? Thanks.

[Doc B.]

Hi Steve,

Start with giving us some voltage measurements.

[Steve_in_NV]

While it was in the failed mode just as I was hooking up the meter to terminal 18 (gnd).  The HLPM led's came on, so I retouched that solder connection and will watch.  When everything is on the voltages all appear to be in spec.  I will try again when it's failed.

[Prairie Dog]

Hi Steve,

I had the exact same symptoms in my right channel roughly a month ago: intermittent loss of audio and only D2, B side LED lit.  I'll spare you the whole boring troubleshooting story and get straight to the fault I found: Q2 on the B side (MJE350) went flaky.  If I touched the +ve probe of my DVM to the metal case of Q1, the amp would come back to life for a day or 2, then the fault would return.  I put in a new Q2 and haven't had any issues since.  Funny thing, the same fault appeared in the left channel about a week later.  I went straight to the MJE350 Q2 and it's been good ever since.  Hope this helps...

[Steve_in_NV]

Yes, considerably, thanks! Both my channels are failing also but I figured once one was fixed the other would be no problem, Thanks again.

[Paul Joppa]

Check your powerline voltage, and check for reasonable ventilation of the amps. Overheating can cause the MJE to fail. (That's why the new board under development has good-size heat sinks!)

[dmannnnn]

I had the same problem with my 300b paramounts.  I think the cause may have been higher than normal voltage in my house lines.  As soon as I touched the DVM probe to R1, the amp would come back to life.  I replaced the transistor with a beefier part, heatsinked it and haven't had any problems since.

See this post from the old archive for the part no.

http://db.audioasylum.com/cgi/m.mpl?forum=bottlehead&n=138024&highlight=dmannnnn

[Steve_in_NV]

Check your powerline voltage, and check for reasonable ventilation of the amps. Overheating can cause the MJE to fail. (That's why the new board under development has good-size heat sinks!)

Line voltage 119.7  The feet I have are appx. 7/8 high (not my feet) the one's on the base.  I think I have enough ventilation.

[Paul Joppa]

Line voltage 119.7  The feet I have are appx. 7/8 high (not my feet) the one's on the base.  I think I have enough ventilation.

OK, I have to agree, 119.7 is fairly close to the design target of 120v! Guess that's not the problem. It was worth a shot though. IIRC, new Paramounts have been going out with the beefier MJE5731A (note that it mounts backwards from the MJE350!). Check the Zener string for correct voltage at turnon, before the shunt reg kicks in. There have been a couple incidents where a Zener shorted and put extra stress on the other components.

As dmannnn noted, if the '350 has overheated it will likely have a discolored appearance.

[Steve_in_NV]

Well I changed the transistors I upgraded to MJE 5731A series hope this solves my problem.  Thanks to everyone who posted, thanks to Paul for the heads up that it mounts backwards I downloaded the datasheet not that I didn't trust him, just wanted the info.  Horn Shopee speakers should be here in a couple of weeks, burning in a new set of JJ-2A3-40 tubes.  Want everything just right for horns. Again, thanks.