Airline GAA-2940 - Power Supply 60 Hz Hum

[cecilk]

I'm repairing an Airline (Montgomery Wards) GAA-2940 console and after noticing the sound quality was a little off tone-wise and distorted, I have tested and replaced all electrolytic capacitors in the circuit (many were way off tolerance and quite leaky).

The console now works fine - AM/FM both play and have decent reception. Tape inputs work fine. I'm waiting on a stylus so I can't test the turntable yet.

The problem? There is a noticeable 60 Hz hum when volume is at 0 on all channels (tape, phono, AM, FM). I've plugged the unit in to a few different outlets and turned off different lights and other components that I have in the room just in case there was any issue with interference.

From what I understand, a 60 cycle hum that is present on all channels usually indicates an issue with the power supply.

• C18, C17A, C17B, and C17C are all new Nichicon caps. I over-spec'd them by accident so they are rated for 250v (I'm simultaneously working on a bunch of tube AC/DC radios and wasn't thinking).
  
• I've replaced R96, R97, and R98 with 1% metal film resistors.
  
• Power supply voltages are all pretty good - Not "spot on" but within range (-17v reads as -20.8v, +17 reads as +15v, +13v reads as +12v, +9.1v reads as +8.8v).
  
• I've tried varying the value of R96 to "balance" the positive and negative voltages (+17v and -17v) and got them spot on but this had no effect on the hum.

My next venture is into the Amplifier circuit and tone stack - unless anyone can see something I'm missing in the power supply?

 

[toxcrusadr]

There is a lot more hash in the power lines and in the air than there used to be. Not only fluorescent lights. I have trouble with FM noise killing weak stations just because I have the TV or cable box powered up next to the (vintage) receiver. So make sure you test with all of that stuff off.

Also you could try a ferrite bead on the power cord to kill stuff coming in that way. I know it sounds like a 60 Hz hum so this may not work. Are you sure it's 60 Hz though? I get a high pitched buzz out of my shop system when all the fluorescents are on - definitely not 60 Hz it's a continuous buzz so it acts in all ways like what you're describing.

 

[cecilk]

Are you sure it's 60 Hz though?

Yes - I am sure. I used a frequency counter app on my phone to test the sound coming out of the speaker. It measured ~60 Hz.

 

[cecilk]

Would adding a three prong power cable help?

 

[toxcrusadr]

Is the power cord polarized? If not did you try reversing it?

I don't know if the 3 prong would work since we don't really know where the hum is coming from. You could try just clipping a wire from the chassis to the center screw on the outlet (that should be ground in a 3 wire outlet) and see if it helps. If you have a meter you might check for any voltage difference between the two first, just to be on the safe side.

 

[cecilk]

Tried reversing non-polarized power cord. No dice.

I'll try the 3-prong test and see if it helps.

Otherwise, I'm also noticing that a lot of the internal wiring is just all open and non-shielded. For example, the output transistor pins are attached to 3-4" long wires that stretch across the inside of the chassis to the amplifier board - where they loosely cross over some of the +17v and -17v wires powering the board. Wondering if maybe the wiring just needs to be cleaned up.

 

[mbz]

Often, power supply hum is caused by a voltage regulation transistor leaking, causing increased ripple, ie, twice mains, 120Hz.
60Hz hum is more environmental and possibly the rectifier, you could test and/or replace SR1 and SR2. Certainly move unit to
another room and try different power outlets.

 

[cecilk]

Often, power supply hum is caused by a voltage regulation transistor leaking, causing increased ripple, ie, twice mains, 120Hz. 60Hz hum is more environmental and possibly the rectifier, you could test and/or replace SR1 and SR2.

Thanks - I'll take a look at SR1 and SR2. I traced with Oscilloscope and there is definitely visible ripple at that point of the PS.

Certainly move unit to another room and try different power outlets.

I will once I get it put back together - it's a bit heavy to move! ^_^

 

[mbz]

I traced with Oscilloscope and there is definitely visible ripple at that point of the PS.

measure with the cro the peak-peak voltage. 100mV or so is ok. More than 1Volt would be a concern

 

[Tom B]

Missing grounds, reversed filter caps can also cause hums. Make sure all board grounds are secure. I would think ground issue on the PS, or tone section as the most likely. Confirm the ground points on the boards and use a jumper wire to place another ground, in case one came loose or there is a crack in a circuit trace

 

[cecilk]

Update: I checked all board grounds. Checked the tone board very carefully. Replaced the SR1 and SR2 rectifier diodes with 1N4004 diodes. Tried grounding chassis to third prong of outlet. Tried a couple of different outlets both in the room and via extension cords, in other rooms. The ripple of the PS seems within reason.

The hum persists.

Attached is a visual taken with Oscilloscope from speaker outputs. It seems to be a combination of 60 and 120 Hz hum.

I have traced the circuit and it does appear that the source is in the power amp. Everything right up until the output transformers looks fairly clean. Also, the hum appears in the negative feedback loop going from the output transistors back to the primary of the OPT.

I'm thinking this could be coming from the DTG110 germanium output transistors. I have heard these can become noisy due to overheating. After reviewing numerous substitution options - I'm thinking of trying the silicon based MJ15016. With a bias adjustment, I have read this should be a good substitute.

Any thoughts on this diagnosis or this transistor substitution?

 

[cecilk]

Oh - here is schematic of amp section for reference.

 

[mbz]

The 60Hz is of concern, suggests the grounding on the AC side may be suspect.
You could try converting the grounding to a star system, I'm assuming it's a chain with multiple ground points

 

[cecilk]

The 60Hz is of concern, suggests the grounding on the AC side may be suspect.

The AC side does connect with the turntable motor - perhaps I could try disconnecting the turntable power to see what effect that has on the hum.

 

[larryderouin]

250V caps on a 20V system is a lot of overkill. 50V caps are plenty enough voltage overhead on them. 35V caps even better. Give it a little voltage headroom to compensate for the increase in line voltage, but not "the sky's the limit". They could be a cause, but unlikely.

 

[cecilk]

250V caps on a 20V system is a lot of overkill. 50V caps are plenty enough voltage overhead on them. 35V caps even better. Give it a little voltage headroom to compensate for the increase in line voltage, but not "the sky's the limit". They could be a cause, but unlikely.

I tried subbing in two 35v caps - no effect on hum.

 

[cecilk]

Disconnecting the turntable from the circuit did not affect the hum.

I ordered a few MJ15016 Silicon PNP power transistors and will try subbing them in for the DTG110's with a little bit of circuit mod to ensure they turn on and do not overheat or go into thermal runaway.

Really hoping those old germanium transistors are to blame. I'll report back.

 

[mbz]

There should be "no" 60Hz hum if the rectifiers are doing there job.
These have been replaced, so known good.

Earlier you mentioned transformer wires passing close to Vdc wires, hope
you sorted this out as best you can. Check/measure R96 470ohm/1W, in
circuit should be ok.

Try and trace the GND wires(?) on the AC side ie, Changer GND and lamps and also
the psu GND at C17A. Ideally there should be separate wires from each of these
"star points" to a chassis tab (star centre). However what you probably have is
a direct chassis connection at each location.

Hums can be caused by two different voltages in the GND circuit, for example
one near zero voltage at the change switch and a different near zero at C17A.

 

[cecilk]

Earlier you mentioned transformer wires passing close to Vdc wires, hope you sorted this out as best you can.

It's a bit of a mess - 120Vac line voltage wires running right next to Vdc wires running close to 6Vac wires - all on two sets of criss-crossing seven lug terminal strips. When I get the new output transistors I'll see if I can try to clean that area up and keep the AC wires as far away from DC wires as I am able.

Check/measure R96 470ohm/1W, in circuit should be ok.

Yes - R96 is a new metal oxide resistor that tested spot on before it went in.

Try and trace the GND wires(?) on the AC side ie, Changer GND and lamps and also
the psu GND at C17A. Ideally there should be separate wires from each of these "star points" to a chassis tab (star centre). However what you probably have is a direct chassis connection at each location.

What I did find is that each of the two lamps only has one wire running from it to the 6v power source. They both are grounded to the chassis directly - there is no return wire that goes back to the "star point" ground. However, disconnecting the lamps and their housing clip from the chassis (and thus breaking the circuit) did not affect the hum.

Hums can be caused by two different voltages in the GND circuit, for example one near zero voltage at the change switch and a different near zero at C17A.

I did disconnect the turntable altogether with no affect on hum.

 

[cecilk]

Thank you, by the way, mbz, for tracking with me this far. I appreciate all the help and advice from you and others!