Marantz 4140 hum - AC Rectifier?

[sdrobbins]

Good evening! I am just finishing a restoration of my Marantz 4140 and I am bothered by a hum (60hz I believe) that is present at zero volume and throughout (both speakers). The hum will increase considerably with nothing connected to the selected input, but with shorting plugs or a device connected, it there at a constant low level. It is not really noticeable at normal volume listening through speakers, but it bothers me to the point where I feel it should be corrected as low volume headphone listening may be hampered. The amp also seems to be susceptible to RF interference from household lights and appliances, so I am thinking it is a power supply filtering issue? Unfortunately, I cannot say for 100% if the hum was there before my work, as the amp initially had several issues.


Before I get asked, yes I have checked and rechecked every ground connection, tried with bottom plate on and off, and switched the plug. I have carefully refurbished the power supply board with all new electrolytic caps, diodes, and transistors (voltage adjusts fine to 35vdc), and thoroughly checked solder joints and connections. No luck so far;(


Really, after many hours of cross-checking, the only thing I can think to try is to replace diodes H851-H854 (U11C/U12C) on the AC Rectifier board as it is my understanding that they can cause hum issues. I have pulled them and they tested fine, but I have no replacements on hand (only 6amp/200v?) and trying to decide if they are worth ordering. Also, I have not been able to determine the specs for U11C/U12C for replacements, which is another problem.


Would anyone happen to know a good/safe replacement for these diodes? Or if you happen to have any other ideas in regards to the hum I would be very very grateful.

 

[Steven Tate]

Did you replace the filter caps?

 

[sdrobbins]

Did you replace the filter caps?

Yes, the filter caps were replaced and bypassed. I am fairly certain I bumped the capacitance up to 22,000uf.

 

[Steven Tate]

Was the hum there before the rebuild?

Edit- just saw that you couldn't check before. I'm not going to be much help. My diagnostic skills are shallow. The symptoms sure sound like a grounding issue, though.

 

[sdrobbins]

I'm not going to be much help.

No worries, I appreciate any thoughts. I am still trying to find some reference to a replacement for the U11C and U12C diodes, but I really cannot find any information to help in a proper substitute.

During my work, I did need to replace the volume pot with one from a 4270. I initially thought the problem was due to that work, but I have been over it so many times, and I used a 0.1uf cap to ground the pot as per the original design.

I will keep looking, but I am definitely running out of ideas of other causes. Unless the 4140 is just be extra susceptible to RF interference due to so many inputs, but I somewhat doubt it...

 

[KutzlerTrans]

Are you near Neon lights?? Mine hums when the neon lights are on. I even heard it through my speakers without the stereo on. The induction was that strong. I rerouted my speaker wire and it went away, And no i wasn't drinking that day...

I have heard JStang say to flip your Mains and brakers once in a while. I

also, did you put extra large input film capacitors anywhere near AC? try holding a shield between stuff and see if it gets better.

Missing ground on shielded wires? Stupid stuff like that. Does it do it with headphones?? Speakers off)

 

[sdrobbins]

It does hum with headphones and speakers. I have also used the amp at two different houses and the result was the same. Not unlike several vintage amps, it pops with my bench fan and crackles with my work light, so it is possible that I am just picking up interference in the line, which the 4140 is not the best at filtering out. I have a line conditioner around somewhere, so that might help too.

I will be back it later today and will look into shielding and a missed ground. Hopefully something will jump out at me before I go completely off my nut

 

[bryans12v]

Was the 1000uf axial replaced?

 

[KutzlerTrans]

Did you try bypassing your power switch?

The only other thing i can think of is finding out if it does it on the pre amp OR the main amp, being they are two different power supplies.
Do you have a pre amp you can use to bypass yours?

Did you re-flow the Power supply solder joints or just check them?
I would think you would see AC coming from the rail supply or the 35V? pre amp supply voltage.

You may have mentioned this but, did you check for any zenor diodes, and if you replaced them, did you check the voltage drop or line voltage to see if they were correct?
Ask me how i know this?? I missed a few on one of my first rebuild and it hummed its own tune.

Btw, The chassis isn't grounded to the wall socket, is it?

 

[sdrobbins]

Was the 1000uf axial replaced?

All electrolytic capacitors were replaced, including the 1000uf axial.

Did you try bypassing your power switch?

The only other thing i can think of is finding out if it does it on the pre amp OR the main amp, being they are two different power supplies.
Do you have a pre amp you can use to bypass yours?

Thanks for all of that, it does give me some things to think about. I did install a new power switch and safety cap, but that was after I noticed the hum. No the chassis is not grounded to the wall socket.

On the power supply board, I did replace all power supply diodes (besides those U12C/U11C on AC ripple board) and resoldered the board, carefully checking the traces. Can't imagine I missed anything there, but I am starting to second guess myself.

I did try separating the main amp/pre amp, and I believe I noticed the hum in each set-up. But that was a while ago. I think I will check again... I might be getting tunnel vision on the power supply board, perhaps I am missing something obvious on the main amp boards, or preamp. Fun on the bun!

 

[petehall347]

just for fun try flipping the power plug through 180 degrees

 

[bryans12v]

If you have another unit try running the pre-outs to another amp and/or from another pre-out I to the main ins. This may help narrow down where it is coming from. If it hums both ways I'd focus on the power supply. If you have a scope check the dc supply.

There's always a possibility that a trace had lifted or cracked. I take my time and even I had to repair a trace or two.

If you handwrote diagrams or took pictures before hand, double check all cap polarity. If you need any images, I can take whatever you need.

Like kutzlertrans has mentioned, try some insulating material near any A.C. lines, especially primary side of the transformer.

Does the hum get louder when phono is selected?

 

[KutzlerTrans]

If you have a diode bridge, Try jumping all four diodes with a .01 and .1 film. There is a resistor value you could use , but there is a formula for it and you would need to know secondary impedance of your transformer.

Run some .01 films off of the transformer secondary to ground. it may have some ceramic caps installed already. They might help remove rf noise from the line.

If you have some bigger diodes, You can try jumping every diode with some 1n4004 diodes. It may have an inefficient noisy diode that cuts off too soon. I never did it but i cant think of why you couldn't do it. Just unplug your amps before testing it.

I would think elevated AC would be seen.

 

[sdrobbins]

Thanks so much for the comments thus far. I imagine a scope would be helpful here but I have been lacking all these years – damn economy…

I had a chance to split the preamp and main, and found the hum to be present on both setups. Not surprisingly, the additional hum at higher volumes (when an unused input is selected) was not found when using the 4140 as amp only.

So back at the power supply I went, but still with nothing jumping out at me. I checked for polarity of the new caps throughout (aside from the hidden tone amplifier board), and rechecked the p800 traces/solder joints and found nothing out of place. So all together the PS board has all new caps, transistors, diodes and has been completely resoldered.

I also pulled and rechecked bridge diodes H851-H854 and noted a voltage drop of 0.45-0.5 V each, which does not raise any huge red flags to me. Again I am not sure of what would be a safe replacement for these.

So turning to leaky AC voltage, I must admit that I am in some uncharted waters here. I did take some measurements, but I am unsure on what to expect here as normal. I found 0.023v AC on the new main filter caps, which feed into the two power amps. DC was a little high on the collector of H801 (49.72v DC), but it also had 0.05v AC. Similarly, there was a small amount on J802 of 2.2mv AC and around 10mv AC on the negative terminal of the 1000uf axial.

So all in all, I am still scratching my head. Not being an expert, I want to suspect that the AC rectifier diodes are getting tired and creating a buzz, one constant and another that is further being picked up in the preamp and input wires? (I noted how noisy these got if they went anywhere near the main filter capacitors on the underside).

 

[KutzlerTrans]

Did you put led bulbs in it?

 

[sdrobbins]

Did you put led bulbs in it?

I did actually, for both meters and indicators. I had this thought too, but I could not think of an incidence where this caused a hum...but maybe? I did try disconnecting the lamp power coming in from the transformer, but the hum was still there regardless.

 

[KutzlerTrans]

One thing at a time.
Try jumping your old filters over the top of your new ones, but.... i am wondering about this...

the filter caps were replaced and bypassed. I am fairly certain I bumped the capacitance up to 22,000uf.

What value were the stock ones?

The question of the day...
Was this problem here before you worked on it?

Is it the stock transformer..or is it a 120/240 unit??

Do you have a min/max feature on your dvm?

Did you try pulling off the p800 power supply to other boards before running just the main amp.. my thinking is that if you have a phono amp board or something putting out noise, your main amp could be picking it up.

 

[sdrobbins]

What value were the stock ones?

The originals filters were 20,000uf/50v and the best available replacement I could find at the time was 22,000uf/50v. I tried with bypass caps and without (no difference in hum). I am not sure if there would be an easy way jumper in the old filters? Unsoldering the new caps would be a PITA...but I think the new replacements do need to be ruled out as a potential cause.

Was this problem here before you worked on it?

I really cannot say for sure, when I got the unit the previous owner did a bit of a hack job trying to put 2 volume pots in to replace the original quad pot, and it had a bad tantalum on the tone board. Since the hum is fairly subtle without a critical ear or headphones, I cannot say for sure when it come up or if it was always there. After the new volume pot? Or somewhere along the restoration?

Is it the stock transformer..or is it a 120/240 unit??

Do you have a min/max feature on your dvm?

As far as I know it is the stock transformer. Apparently, I do have a min/max feature on my dvm.

Did you try pulling off the p800 power supply to other boards before running just the main amp.. my thinking is that if you have a phono amp board or something putting out noise, your main amp could be picking it up.

I like this idea, I should have thought of it as well I will look into this tomorrow...I am also starting to wonder the possibility of getting a new capacitor that is bad? But chances are it is something obvious that has been staring me in the face all this time...

 

[KutzlerTrans]

Or the worse scenario, two things are causing it and your temporarily only eliminating one at a time.

 

[sdrobbins]

Tried a few more lines of investigation and this 4140 is still putting up a solid fight. Most people would probably just live with the small hum, but I appear to have a mental defect

I have ruled out the new main filters as a cause; the makeshift reinstallation of the originals made no improvement.

I also tried disconnecting power to the preamp board, phono, and vari-matrix boards with no luck. Also, I looked yet again at the power supply board and tested the new caps there with an ESR meter.

So??? More still to come it seems. Since the hum is fairly equal on both/all channels, I still want to say power supply, or some problem wire or ground that I just cannot find. The new/used volume pot? Or perhaps input selector (I do notice slight cross-talk between inputs, which is not unusual for these older units, but I also notice a trace amount of volume with the knob all the way down).