Help Saving 400-CX SA-100 and 100-R Tuner

Thanks for the info on the paint. I would point out however that even with both Aux 1 level controls up full, the sound should be balanced. It may be that if when the level controls are both equally backed off from full up the sound balances out, it may be because you are then operating the main volume control at a higher position where it tracks more evenly between the channels.

Dave
 
I know there were different versions of the 400-CX. Is there any way to tell about when this was manufactured? Mainly because everyone that sees or hears it asks. I tell them early 60s most likely but don't know that to be accurate.
Also, I will work on the tuner at some point. I have other irons in the fire at the moment.

Bill
 
Power Transformer has an EIA code on it below the Part #. Should be 6 or 7 numerals. Post the # and the manufacturer and approx week of the year it was made can be determined. Then add 2-3 months from that week in the logistics chain to the Store. If the 1st 3 numerals are 926 it was made by TODD Electric in NYC. Logistics train is shorter as FISHER was on Long Island . In any event get the number posted and a rough estimate can be made.
 
The original power transformer was missing on his unit, Larry. The original can caps may be gone now too, but don't know that for sure. Controls might be the next best place to check for production dates.

Dave
 
I have been absent for awhile. I think I have a slight issue with the SA-100. I haven’t done much to investigate as I wanted to see what would make most sense as far as order of troubleshooting for isolation.
I have a slight hum in the amp. I have swapped inputs from preamp and it remains in same channel(A).
There is a dead spot in that Level pot. I could replace it but the hum seems to be there and vary with the level of of output. So, I don’t think it’s the pot itself.
I was thinking about swapping tubes from one channel to another, one at a time to see if I could get hum to move but wanted to see if something made more sense to try first?
I appreciate any input. Thank you.

Bill
 
Dave, it is only channel A. No it is not. It gets louder as I turn the level up. Clockwise. I hope that is what you meant.

Bill
 
OK, if the hum is gone when the SA-100's level controls are turned full down, then the hum is likely not originating in the SA-100. With the SA-100's bottom plate attached, and no interconnects plugged into the SA-100, turn it's level controls up full. Is there any sign of the hum then? If not, then the hum is definitely not originating in the SA-100.

Dave
 
Dave, I did just that. Disconnected and no sign of hum. Amp is good. That’s great!

Bad news is it must be in the preamp. The 400-CX.
So, I have tried different sources and inputs. It is same. I swap A/B channels on one side and the hum does move. I was mistaken when I said it remained in channel A.

Bill
 
So it's only in one channel? If so, which one? And, does the hum level vary with volume even when a high level source is selected that has no input connected? Does the hum go away when the preamp volume control is turned full down? Is the hum present the instant the unit is turned on? Or does it only come in after the tubes have warmed up? Knowing details like this are vital to help determine where a problem might exist in a preamp circuit. Finally, is this a new noise? Or was it noticed right after you installed the new power transformer?

Let us know!

Dave
 
Dave, let me try to answer each of your questions.
Yes it is only one channel. Channel A(Left)
The hum does not vary with volume, with or without a source. Actually, hum seems to be a constant level. If you have music playing, you can over come the hum if you have the level on amp turned down low but I know it is still there.
Hum is not present until tubes warm up.

Let me know If I can provide more info

Bill
 
OK. Is it 60 Hz or 120 Hz in nature? 120 Hz is one octave higher than 60 Hz hum, with the latter being the type you hear from an ungrounded input.

Also, with the preamp running and the hum present, turn the unit off by unplugging it (not with its AC switch). Notice the sound the instant the unit is unplugged. Does the hum go away when unplugged? Or is it still present?

Dave
 
Dave, I believe it to be 60 Hz hum. I also unplugged just the preamp as you directed above and hum immediately stopped.
 
First check then is to see if any of your transformer wires are running close to any Left Channel audio circuits, and also make sure that the relevant transformers wires are twisted as well.

Dave
 
Dave, I am attaching a couple of pics. Let me know if you see anything off...I don’t know if any left channel wiring near transformer.
Transformer wires were loosely twisted?
Both channels run near that transformer but not sure if that is an issue?

Bill
 

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Bill. Twist up all the pairs out of the transformer so there is no slack (3-4 turns per inch if you have enough length). Make the runs as straight as possible from any other wiring. This cuts down on the 60Hz interference from the AC lines out of the transformer when they get near audio circuits. The Red with the Yellow stripe is the center tap for one of the sections. Bring that out separately from the other two reds going to the rectifier. Those two red get twisted tightly while having enough length to get soldered to the tube socket. You may have to extend the leads to get the needed length.

The Black and Brown leads coming up from the audio output section and passing between the transformer and the Rectifier(marked with Arrows). If it were mine, I would move them over away from the transformer to the main bundle and back to the audio section (RED LINES, I'm not looking at a schematic right now). Placement of those (even shielded) next to transformer is not best practice. (Probably original placement but still). That and with the Transformer leads not twisted enough can overload those lines with EMF causing a hum. The black and brown leads are from V4 & V9 for Center Ch. (V4) and L&R (V9). I could very well be a combination of both problems. Twist up the pairs 1st, then test. If still there, try moving the output leads to the main bundle and getting them away from btwn the transformer and rectifier.

Clipboard01.jpg

Larry
 
In addition to Larry's great comments, also look a the white dropping resistor you are using drop the voltage for the rectifier heater and indicator lamps. That resistor is a transmitter antenna for 60 Hz energy, so moving it as far away as possible from the audio circuits will help as well.

Dave
 
Thank you both for taking the time to look at this and advise me.
I will remove the two reds and twist them up as best as I can. I’m hoping I don’t have to add extensions but can if necessary.

I will also twist the yellow and green??

Then try it.

Dave, you mentioned the white resistor. The bright new white one? If I were to move it, any suggestions?
I probably didn’t notice the hum initially, due to using smaller little speakers.

I’ll post up progress. Thank you.

Bill
 
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I've been following with interest....
With regard to the white resistor behaving like a broadcast antenna...I see a wirewound white resistor.
Would a non-inductive wirewound resistor or a carbon resistor emit less RFI?
 
Heyraz, I’m not sure? Let me know if I need to change anything out?

I twisted the transformer wires and tried it again. Still humming. It’s still about the same, not horribly loud but definitely noticeable.

I can try to reroute the cables as suggested by Larry.

Bill
 

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