Right side not working on Extended Foreplay 3

[J Leng]

My Extended Foreplay 3 has just stopped working on the right channel although it's been working fine for 6 months.  The channel gave a few small pops with the music playing, now it only pops. 
All the LED on the right channel board are out and one of the A LED on the middle board is out, whilst the other is glowing much brighter.  I've swopped the left and right tubes round, but this did nothing, can you help please.

Thanks

Johnathan

[Grainger49]

Johnathan, the first thing to do is resistance checks.  If the resistances check are correct, within +/-10%, then turn it on and do the voltage checks.

Get the intended readings for both from the "Extended" instructions.

Then post the results that are not what was expected.

[J Leng]

Thanks, as soon as I get time I'll do the checks.

J

[Doc B.]

Sounds like it could be a loose connection. First check your cabling. If that is all OK check all the solder joints on the bad channel side with a magnifier. If they all look OK, try reheating them anyway, to reflow the solder.

[J Leng]

Just found the problem one of the wires had broken, soldered it back on and all is fine.  Thanks guys!

J

[J Leng]

The fault has been happening on and off for the past 3 weeks now.  I've traced a Voltage fault (0v)to pin 12, then Idid a resistance check and this was fine right back to R5, now I'm not sure what to do?

J

[Grainger49]

The fault has been happening on and off for the past 3 weeks now.  I've traced a Voltage fault (0v) to pin 12, then I did a resistance check and this was fine right back to R5, now I'm not sure what to do?

J

To be certain, you soldered the broken wire back on December 13^th and have had an intermittant problem afterward?  

Now Terminal 12 which is supposed to be at 150V DC, reads 0V DC when there is a problem?

If so, that is the plate voltage coming into the cathode follower stage for the right channel.  There is a jumper from Terminal 21 bringing the voltage over.  It then jumps to tube pin 6 of the right channel tube.  That would shut off the right channel if there was no voltage coming in.

Check the jumper from T21 to T12.  Rewet both ends.  Then measure resistance back to T21 from tube pin 6.  This method keeps you from applying any force to the suspect terminal while making the measurement.

[J Leng]

I've resoldered the joints and the resisitance is measures 0. I did carry out this procedure before I posted and the channel was still not working. 

[Grainger49]

If your problem is as described and reiterated by me in my post, reply #6 above, the problem is with the high voltage supply coming from terminal 21.  The source of this voltage is the center, octal, tube pin 5.  This pin feeds both tubes.  Since you do not have a problem with the tube attached to terminal 32 the lack of continuity must be between the octal socket pin 5 and terminal 12.

You did not answer either of the questions I asked so I'm not sure I'm barking up the right tree yet.

[J Leng]

Sorry for not being clear, yes I fixed the wire and had the intermittant problem afterwards and yes terminal 12 is reading 0v when there is a problem. 

[Grainger49]

Then my "Reply #8" suggestion seems to be a good starting point.  If Terminal 21 has 0V when you have the problem it can only come from the terminals I listed.  That assumes the Extended doesn't change the measurements at those points.  You will have to look back to where the shunt regulator feeds the right channel in the Extended instructions.  That is a likely source for your problems.

[J Leng]

The problem is still present, I have resoldered all the joints on the shunt board and the problem is still there.  I thought it could be from the right power supply so I swooped the right and left supplys to the shunt board, but the noise was still on the right channel.  I have moved the tubes around this hasn't worked, can anyone help?

[Caucasian Blackplate]

Please measure the DC voltage at the following pads on the center PC board on the side with the fault:

"I"
"O"
"k"

Since you have 0v at the output, there is very likely an issue with the C4S portion of that feeds the offending terminal.  Since you have swapped B+ feeds from side to side (a very astute action I might add), you have eliminated the power supply as a cause. 

I would also set your meter to measure resistance and then check to see that none of the three legs of the MJE350 show a short (do this with the preamp off).  It is also possible that the jumper that goes from the "O" pad to the shunt regulator and out to the circuit has somehow broken.  It would be most likely that the wire from "O" to the offending channel is the issue, as a bad connection from "O" to the shunt regulator would have you seeing higher voltage than 150. 

I hope this helps.
-Paul

[Grainger49]

Just a note:  Measure resistance after it is unplugged, powered down and wait a minute for the voltage to discharge from the power supply capacitors.

It is a safe habit to get into measuring the power supply voltage before getting into an amplifier.  Just get to the biggest caps in it and carefully see if the voltage on the capacitor's terminals is under 30V.

[J Leng]

The Voltage checks are as follows:
I = 228v  (when I touched the pin the led that was out lit, as did all the led's on the right board, this state lasted for 5-10 secs)
O = 0v when led's out and 148v when lit
K = 3v led's lit and not lit

I think I was understating the severity of the pops and crackles, they are speaker destroying.