Troubleshooting FP2

[TadS]

Hi All--

I am having some problems with my Foreplay 2 w/C4S and looking for some group wisdom.  I bought the kit 13 (I think) years ago, and it has been trouble-free since I first built it up.  I have typically gotten a little static on power-up if I turn on the amp too soon, but now the right channel is constant in popping and static.  I can't even turn it on with out blowing the rail fuses in my power amp.  I have tried crossing RCA-cables between my different components to identify the FP as the problem.

Since I can't use the amp/speakers as a measure of the noise, I disconnected the amp from the system and measured the VAC at the output RCAs.  Is there a better way to measure noise at the output besides measuring the VAC?  I am getting 30-200mA (w/ the attenuators turned all the way down), where I get 0-2mA in the left channel. 

I have switched out tubes and re-flowed all the joints around the right tube socket and ground busses, but I am still getting voltage in on the R output RCA.  I have been afraid to plug it back into the amp so I am just going by the VAC at the output.  I have not worried too much about joints that are common between the channels since the L side is fine.  Also, I have not tried swapping the CCCS boards from channel to channel, but was hoping I might get an idea from the forum on what I could try next.  With Thanksgiving holiday, my honey-do list has been keeping me from focusing too much time on the FP, but I suppose testing the C4S boards may be next.  However, I have low confidence that the boards are the problem.  Thanks, for you insight!

--TadS

[Paul Birkeland]

Hello Tads,

Could you check the voltages on each channel? 

Specifically, you can measure the voltages on the metal tabs of the MJE350's and MJE340's as a sufficient starting point.

Lots of popping would generally be a bad solder joint, failing socket, or iffy tube, but the voltages can help narrow it down.

-PB

[TadS]

Wow PB--

That was a quick response.  I am with you and still suspect a loose connection somewhere, but I have reflowed most of the joints (all on the right channel side).  As I mentioned, the left side has no crackle and static.  The tube socket is a suspect that I had not thought of.

Concerning your recommendation, to measure the voltage on the "tab" do you mean the VDC across the body of the IC (I think that is the collector [pin 2]) to ground?  Thanks, for your input.

--TadS

[Paul Birkeland]

Hello again TAD,

Yes, the collector voltages are what is most informative.  The metal pad on the transistor is tied to this internally, or you can measure the solder track/pad coming from the center leg of each of the MJE-350's and MJE-350's.

[TadS]

Yahoo!  Thanks, PB, for making me take a closer look at the C4S boards.  The LEDs were intermittent on the left VA board.  That is where the loose joint was.  I reflowed all the joints on the boards and now back to the silent background.

--Tad