Foreplay 3 Hum

[John EH]

I have two Extended Foreplay 3's.  After getting a new DAC and doing some really critical listening I noticed the right side from my Dynaco ST70 outputs had a tiny, tiny crackle in it.   Then after a minute or two I get what I think is a hum (could be buzz) for about 5 to 10 seconds then it stops.

I did everything.  Swapped every tube in preamp and amp, changed cables, etc.  No joy.  Then I swapped Foreplay's.  The swapped foreplay is dead quiet.  No crackle no hum.  So it seems the problem lies in the first Foreplay

Because the hum seems to happen at regular intervals and seems to last for a similar amount of time each occurrence I'm thinking capacitor.  Like a discharge.

I did recently change the pots in the Foreplay.  I wonder if a crappy pot or solder joint could do this?  Pots are new sealed Japanese 50K's so I doubt that.

[Paul Birkeland]

Try swapping tubes between Foreplays first. 

Crackle, hum, silence sounds like something with a thermal cycle.  It could just be a loose connection on that side.

-PB

[John EH]

Try swapping tubes between Foreplays first. 

Crackle, hum, silence sounds like something with a thermal cycle.  It could just be a loose connection on that side.

-PB

Not the tubes for sure.  I have some eBay 23 stepped attenuators that I just figured out rin rg and gnd on.  No directions.  I'm going to go upside down later to install them and I'll poke around and see what's what.

[John EH]

I changed the pot on that side and that isn't it although it is somewhat less pronounced.

[Paul Joppa]

Is it old? Is the power line voltage above 120v? Does it still have MJE350s without heat sinks on the regulator board?

[John EH]

Ok I think I got it.  Not sure which of these things did it but something did it.

- Changed the pots to Chinese eBay stepped attenuators.  I know that wasn't it because it did it after this.

- Coupling cap on that side (Auricap) pulled right out when I gave it a tug on one side.

- Power supply cap dislodged on one side (probably not it and probably snapped loose when I gave it a slight twist

- The ground solder joint on the terminal board running away from the pots looked colder than my ex wife.  Got some desolder braid and cleaned it up and "tried again".

- My RCA connectors on my cable were woefully loose.  Tightened them up a tad with a squeeze from some needle nose. Now they fit snugly.

- Interconnects to the amp seemed to minimize it when you give it a twist.  Tightened them and cleaned them with alcohol and an eraser.  Really looked  crappy.

And now it's good.  If I press my ear up to the speaker I can still hear the faintest of goofiness but I have a 1 foot rule. If I can't hear it more than a foot away it doesn't matter.

So I think I can call this a success.

[John EH]

Is it old? Is the power line voltage above 120v? Does it still have MJE350s without heat sinks on the regulator board?

Old.  No heat sinks.

[Paul Joppa]

We have had some incidents with those transistors when the power line voltage is very high for a long time (years), especially if the preamp is on for extended periods every day - they dissipate a lot more heat when that happens. Look for brown heat damage to the PC board.

[John EH]

We have had some incidents with those transistors when the power line voltage is very high for a long time (years), especially if the preamp is on for extended periods every day - they dissipate a lot more heat when that happens. Look for brown heat damage to the PC board.

No damage.  My home is 100v (Japan) but I use a hefty step up.  Line v is about 116v.  This Foreplay has seen very little use over the years and never been left on for extended periods.  I think it was one of the things in my post however again I can still here more crap in one side than the other but not audible over a foot away. 

Should I change the 350 or replace it with something else?

Were the issues similar to what I described?

Thanks,

John

[Paul Joppa]

The issues were (IIRC) erratic failures, not all the same. At less than 125 volts you are probably good for another couple decades - no sweat. I was just making sure - this was a rare but really hard to diagnose issue.