Kenwood 700c crackle and pop

bhames

New Member
I followed a few great threads and restored a beat up 700c. It fires up and sounds really good but more often than not on startup there will be some background crackling sounds coming through both channels. All the switches have been thoroughly cleaned and adjusting them while the crackling is occurring doesn't change the crackling sound. Eventually, there will be a mild pop (or two or three) and the crackling goes away. If I leave the preamp on (which I have done for days to avoid this situation), the problem does not seem to resurface until I power off and on the unit.

I pretty much followed EW's thread (http://www.audiokarma.org/forums/showthread.php?t=92741) and (http://www.audiokarma.org/forums/index.php?threads/kenwood-700c-restoration.547198/) for replacement of parts.

Getting ready to pull back the covers again but looking for some advice as to what to troubleshoot based on the symptoms.

Much appreciated.
 
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 EW'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.
 
Last edited:
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.

Thanks for the info. Maybe EW will jump in as to what he fixed on your 700c.
 
suggest listen carefully if the crackling is about the same and at same moment for both channels. If so, there must be something common to both channels (grounding, or the power supply)
 
I would suggest taking notes on exactly which things effect the static and which does not. The fact that my volume knob had no effect, even on zero (-infinity) the static was always there but that the Attenuator would drop the volume of the static by -15db and -30db seemed to indicate somewhat where the issue was originating from in the circuit. Check my notes and see if yours does the same as mine, and if so that would seem to be a strong indicator it is a similar component.
suggest listen carefully if the crackling is about the same and at same moment for both channels. If so, there must be something common to both channels (grounding, or the power supply)
On mine, most of the time (over 90%) the crackling and popping was on both channels simultaneously, but not always. I had remembered hearing it from only left and only right occasionally. It was a real head scratcher but not unsolvable to EW. Mine has not made any sound like this since I got it back.. Knocking on wood now!
 
crackling is a poor connection .it could be anywhere .even inside a transistor . narrowing it down is best move first as stated above .
 
the problem does not seem to resurface until I power off and on the unit.
i missed that bit earlier .. best to start looking at power switch but only if fault in both channels .
 
If the unit is left on for awhile (warmed up) and then you power it down, then back on, does it crackle? ...or only when it's cold?
I will test again but if memory serves me correct, if I turn it off and back on (when warm), I am likely going to get the crackling.
 
If the unit is left on for awhile (warmed up) and then you power it down, then back on, does it crackle? ...or only when it's cold?
After having the unit on all night, I powered it down, waited 15 seconds and powered it back on. I didn't hear/experience the crackling. I repeated the procedure and received the same results. No crackling.
 
Unless you notice otherwise, the two knowns are that the issue is common to both channels and is temperature related. What petehall (post 6) and gslikker (post 4) mentioned are where I'd start troubleshooting as well. Have any freeze spray? Also, which components did you -- and which did you not -- replace during the restoration?

All capacitors per the threads I originally mentioned along with a few transistors per EW suggestions. This problem existed before the work I performed so I believe my work didn't cause (nor solve) the issue. Yep, I have a can of freeze spray at the ready. Given that the preamp seems happy when warm, I am hoping freeze spray will bring to light the bad part.
 
Okay, you've narrowed it down further. What's common to both channels that you didn't replace and is affected by temp? Start there. And just for sh*ts and giggles, has this unit -- before and after you restored it -- ALWAYS been plugged into the same wall outlet?

Different wall outlets before and after.
 
Cool. One less variable. First check the point to point wiring for loose connections then with freeze spray and magnifying glass at the ready, I'd start with the PS board. If the unit is cold, you can also "tap" the PS components with a wooden or plastic screwdriver handle to check the integrity of solder joints.
Thanks for the advice. I was thinking of going for the Power Supply board first as well. I will report back once I have some info...
 
ok, so I opened up the 700c and went toward the power supply. Old age is setting in and accordingly I forgot that I replaced darn near every transistor and all zener diodes on the board per another AK thread talking about hissing with a 700c. So, I gave a shot of cold spray on the the few remaining transistors on the board. The crackle didn't change. I did a bit more simple diagnostics and started moving the mode knob to see if I could isolate the crackle to a channel. While doing so, I got a few minor 'pops' when going from 'R' to 'L' and back. Interesting. So I started tracing wires back from the switch and when I lightly touched them, I heard additional crackle. Hmmm. Loose connection? I kept lightly pushing wires moving away from the switch and landed on three connections on the power supply board that sit parallel to one another. I gently moved the connectors back and forth and the crackle intensified. I unplugged the unit and pulled the connectors. A couple of pins were somewhat tarnished. So, I took a few minutes with some metal polish and q-tips and brightened them up nicely. Plugged them back in and fired it up. No crackle even when I pushed on wires and gently moved the connectors. This might be the culprit. So even though I did 'clean' the pins when I went from board to board replacing capacitors, I didn't do a good enough job. So, I am going to go back through and clean all of the pin connectors again (properly this time). And then kick myself hard for not paying enough attention to EW's repeated comments across multiple threads about getting the pins very clean.....

Once I get all the pins cleaned, I will update again if this truly was the issue.
 
Fwiw...when I've had a situation where I needed to unwind the wire from a post -- and I've had those -- when I reconnect it, I form the end of the wire in a loop, slide it over the post and position it right over the pcb solder pad for that connection. Get a small blob of solder on the iron and where the post meets the board, heat it up and then with my free hand, use a pair of needle nose pliers, or a small flat head screwdriver, and push the wire as far into the melted solder as possible so it's mating with the board solder pad. On a couple occasions I've desoldered and removed the post, then ran the wire through the top side of the board and soldered the wire's end directly to the board solder pad. I actually considered removing all posts and soldering wire directly to the board when I redid my 9100, but I'd already spent more time on the project than I'd planned, so I nixed the idea. Sounds like you nailed it. Fingers crossed.
I have done a similar practice for 'pin and wire wrap' connections. With the 700c the plastic connectors make working with the boards a snap (pull the connector, unscrew the board) but obviously the pins are more susceptible to corrosion/bad connections.
 
Problem solved. I went through and cleaned every molex pin in the preamp with metal cleaner (and a lot of q-tips). Quiet as a church mouse.
 
Actually.....the problem wasn't completely solved by cleaning pins. The 700c worked wonderfully for a couple of days and then the crackle and pop returned. Frustrating. So, I bit the bullet and brought it in to a local tech that I have used in the past not wanting to debug this thing for days. He called about 2 hours later and said it was fixed. He found a bad solder joint on one of my new power supply board transistors. I was a bit embarrassed/mad that I caused the issue but glad the problem was solved. 1 day later the problem resurfaced. I thought about it for a while and even if I created a bad solder joint, the problem was there before I did any work on the unit (i.e. something that I didn't touch). From my previous troubleshooting I found that if I touched a grouping of connectors on the PS board the problem was amplified (which prompted me to clean all the pins in the first place). I opened 'er up again and went right for the PS board. Sure enough, almost all of the pin solder joints of the problematic connectors were bad (cracked, etc.). I had to use a magnifying glass to see the problems. Naked eye didn't cut it. I went through the solder joints of all the boards on the bottom of the 700c and fixed a few more suspect joints. Put it back in service and we are on day 3 without any issues. I am crossing my fingers this was the problem.

So although I am upset with the tech for missing this issue after being scolded about a bad solder joint I created on the same board, I am thankful that he at least led me in the direction of looking at solder joints. If I use him again, I will casually bring my findings to his attention (not rubbing his nose it in) but to let him know we are not all perfect and miss things every now and again.
 
Back
Top Bottom