My 700C did a very similar thing. At some point after EchoWars initially fixed my non functional 700C it would start the crackle after being switched on for anywhere from a minute to as much as 10 minutes. Then it would carry on for a while, maybe 15 minutes. (I took extensive notes while Glenn and I were trying to track it down). I sent several boards out to him which he "shotgunned" but the problem remained. I don't know what it was because he did extensive work to it and I was happy and since I am not an electronics person it didn't matter to me., but it is now dead quiet always.
Here are the notes I took of my 700C's issue: I don't know if this will help you any, but EchoWars spent considerable time solving this so it, if it is the same issue as mine it may not be easy to track down. As a note, I do not want this to paint EC's work with anything negative at all. He is highly competent and fully stood by his work and the additional work needed to solve the static was all done for free. I consider myself extremely lucky to have found him when I did and get my amp/preamp back to as good or better than it sounded in 1975 when I purchased it.
Hi Glenn, here are a few observations of the static issue.
There isn’t really any obvious pattern to this issue except for the following but even that is variable in intensity and frequency of occurrence. On power up the static usually occurs very intermittently from immediately to up to 5 minutes. It usually comes in short bursts lasting 3 to 10 seconds with from maybe 30 seconds up to a few minutes of silence between, but occasionally the static last much longer. Sometimes it is just a few pops here and there. The volume level of the static changes as well and more rarely there is a slightly different sounding static that seems to come on the left channel only and can occur by itself or accompanied by the much more common static coming through both channels. Usually the initial “episode” of static is finished in about 10 minutes and it seems to settle down, perhaps for hours, I don’t know. I have noticed it several hours later if the music stopped, but it isn’t usually noticeable with music playing, however it sometimes comes through loud enough to be heard over music being played at relatively low volume.
The static occurs with the headphones or through the output to the 700M and while the 700C volume level doesn’t matter, the 700M’s level will change the volume of course. Because of this I set the M’s levels low, (around 2) and turned the C’s volume up to around -14db. This served to minimize the static to a less noticeable level. Note, this issue has absolutely nothing to do with the M. The static is not there playing the M without the C.
Various controls that affect the static:
Balance
Attenuator
Mode: Left and right
Controls that do not affect static:
Volume
Inputs,
Tape monitor
Other controls are unknown if they are affected or not.
This is about all I can tell you. It is easier to hear this issue with headphones turned up but plugging into any amp and speakers should suffice too.