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yet another hum elimination question

L

lootheer

New Member
I'm trying to eliminate a 60hz hum in a 1960 Motorola console stereo. I've replaced the power caps, the rectifier, the 12AX7s, and put in a grounded plug. I will be replacing the power tubes with a matched set of EL84s in the coming months (likely the JJ EL844s), but somehow I don't expect this to fix the hum issue either.

The hum:
  • is heard immediately upon turning on the unit, with cold tubes
  • is relatively quiet, but audible throughout the room
  • is heard with the volume turned all the way down; it only amplifies at very loud levels
  • is still heard when the input from the tuner/phono is disconnected
Could this be a tube heater circuit issue? A bad power transformer? Dirty power? Mismatched power tubes?
Or is this level of hum considered normal for a system this old?

Thanks for any info or other hints!
 
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Sounds like your transformer may be vibrating. Check that all the bolts that go through the transformer are tight. Make sure that if this is an upright transformer loosen the screws that are holding the end bells to the chassis. This is vibration and not due to the circuit since the hum starts right after you turn the amp on and the tubes have not yet warmed up. You should replace all the caps on this unit except for ceramic caps. A bad coupling cap could cause hum, and cause other damage.
 
A hum can be caused by many things so you need to rule out the most obvious things which were posted previously. If those solutions dont cure it try pulling the driver tube/tubes and see if the hum goes away. If there then the hum is probably in the power tube or power supply circuit. If hum goes away then check the drivers for problems like cold solder joint, intermittent short, poor contact with tube pins, etc. good luck
 
Just a few more bits of info:

  • The hum is still present when I remove the rectifier
  • hum is also present when I remove other tubes, notably the power tubes, one at a time, and all out at once
I can feel the power transformer jolt when the power is first turned on, but I can find no mechanical vibration after that, either by touch or by listening in with a mechanic's stethoscope. The old rectifier did have an audible rattle; perhaps there is magnetic induction from the transformer?

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IMG_0665

  • Media owner lootheer
  • Date added
Motorola SK32W amplifier under chassis. New power caps. For orientation, the power transformer...
IMG_0660

IMG_0660

  • Media owner lootheer
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Motorola SK32W layout/tube diagram
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Fr_2_size400

  • Media owner lootheer
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Motorola SK32W amplifier
 
I would suggest a thermistor at the junction of the power cord and the switch. This will smooth out the "jolt when turning on the unit. Also will drop the Wall voltage a couple volts. A cl80 will probably do nicely.

Do you have schematic you can post???

+1.....Dump the grounded plug. They weren't designed to be grounded, and a grounded cord can/will induce a ground loop / him of a 60Hz variety. Use a polarized plug with the wide blade on the neutral side.

Larry
 
knockbill said:
I agree with Mark,, disconnect the ground wire from the chassis, and see how it is...
Click to expand...
I'll have to go get a ground lifter plug to try this again, but the hum was there before I put in the three-prong. I'm already considering replacing that hardware store 3-prong appliance cord with an emi filtering power socket, regardless. I do have the unit currently plugged into a home theater surge protector strip.
 
Schematics and diagram below.. thanks everyone.

I replaced all the old electrolytic caps, including the coupling caps. I left the old power can in, though it is not connected anywhere other than to the chassis ground. There are still a few electrolytics to be replaced in the tuner/preamp, as well as the crossovers in the speakers, but I don't think these would be the source of my hum.
 

Attachments

I know this sounds like an obvious question, but is the hum coming from the speakers? Does it still hum if there is a resistor connected as the speaker load?
 
kward said:
I know this sounds like an obvious question, but is the hum coming from the speakers? Does it still hum if there is a resistor connected as the speaker load?
Click to expand...
I'm not sure I understand; are you asking if the hum can be heard with the speakers totally disconnected (a mechanical hum) or if it goes away if I replace one speaker at a time with a dummy load?

With no audio input (switched to the aux input with nothing connected) and the volume maxed out, the hum does respond as any other audio input, i.e. it gets louder or softer depending on the bass control, left/right panning, etc. I'm not sure how useful that is for diagnosis, since the hum at full volume could be part of the noise floor, rather than the problem hum.
 
So it is through the spkrs and not a mechanical hum through the chassis from the PS Trans.

I just thought a bad new cap could cause a load on the PS trans and heat would tell you what's wrong.

Caps in tone controls area may cause a hum also.
 
If the 5U4 is pulled and there is still a mechanical buzz it has to be the transformer . Loosen and remove the screws holding the transfomer to the chassis and slide a piece of foam between the transfomer and the chassis and if it still buzzes tighten the four bolts holding the transfomer together . Then if it still buzzes buzzes your out of luck .
 
Your Main filter cap replacements look ok for Sections C15D, C15C, C15A. However C15B looks like you just connected the new cap to the old cap's POS terminal. In which case if that section in the can was bad and causing the hum, it would still hum if bypassed with another cap. ISOLATE the POSITIVE Terminal of the new cap with all the resistors, caps, and wire leads FROM that CAN CAP section.

motorola-Clipboard01.jpg
 
The hum was still present in the speakers after removing all of the tubes. More significantly, after closer investigation, I found the hum most prominently in the right channel. Disconnecting that speaker minimized the hum to almost noise-floor levels without the tubes; it didn't seem to do as much with the tubes. Bad output transformer?

The right channel speakers are located right in front of the tuner/phono amp in the console, which has at least one large electrolytic cap awaiting replacement, if that has any significance.

edit: I switched the speakers at the terminal and the hum moved accordingly; not sure the tuner/phono preamp is involved.
 
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larryderouin said:
Your Main filter cap replacements look ok for Sections C15D, C15C, C15A. However C15B looks like you just connected the new cap to the old cap's POS terminal. In which case if that section in the can was bad and causing the hum, it would still hum if bypassed with another cap. ISOLATE the POSITIVE Terminal of the new cap with all the resistors, caps, and wire leads FROM that CAN CAP section.

View attachment 739141[/QUOTE

Thanks for such a close look! It isn't very clear in the photo, but that C15B is actually connected to a terminal strip mounted on one of the can tabs; the old can is not connected in any way other than the tabs to the chassis. Even so, I'm tempted to take the old can out entirely just to be sure.
Click to expand...
 
I haven;t looked up the schematic for this set, but recall a similar problem on my Dad's Motorola mono hi-fi with pp 6V6 output section. The hum drove me crazy, and I did almost all of the things suggested in this thread. I finally solved the problem when I checked the resistors in the phase inverter circuit - a couple had drofter way off nominal value. Hum disappeared after I replaced the off-value resistors.

Jim
 
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