Low 100hz Hum After Recap Philicorda Organ

[anotherjim]

Ah yes, x2 680uF should be enough for that amp.

I obviously don't know for sure, but...

...there are x2 68uF on the input side of the amp in the bias network, C601/603. They would let in hum if they decided now was a good time to fail.

There are a couple of socket mounted switches on the way to the internal speakers. If high resistance they may have made the speakers quieter but handling/vibration has now improved them.

You may have inadvertently "improved" the chassis ground connections and get a ground loop hum that was hidden by a higher resistance connection. I've actually managed to do that one by fixing a buzz when touched problem. Cleaned up the bolted chassis ground paths, fixed the buzz but then got more hum!

If there is no hum in the audio when you tap off from the amplifier output with the internal speakers disconnected, then I can only imagine that there is an induction pickup loop formed between the speaker ground and the wires in between to the other ground points. Those old speakers are quite sensitive, so it wouldn't take much to be audible.

[PRR]

Turn the amplifier plug around in the wall outlet.

(If you can't do that there, then never mind.)

[sarakisof]

...there are x2 68uF on the input side of the amp in the bias network, C601/603. They would let in hum if they decided now was a good time to fail.

i did a full recap of power supply,pre,amp section so i had replaced them too.

There are a couple of socket mounted switches on the way to the internal speakers. If high resistance they may have made the speakers quieter but handling/vibration has now improved them.
You may have inadvertently "improved" the chassis ground connections and get a ground loop hum that was hidden by a higher resistance connection. I've actually managed to do that one by fixing a buzz when touched problem. Cleaned up the bolted chassis ground paths, fixed the buzz but then got more hum!
If there is no hum in the audio when you tap off from the amplifier output with the internal speakers disconnected, then I can only imagine that there is an induction pickup loop formed between the speaker ground and the wires in between to the other ground points. Those old speakers are quite sensitive, so it wouldn't take much to be audible.

maybe, who knows...

Turn the amplifier plug around in the wall outlet.

had done this too, no change...

I also tried different speakers, had a pair of 4ohms laying around and wired them in series for 8ohms, same little hum there too...

Guys i have tried almost everything, i have to think and remember what i did steb by step the second day when i changed the last big caps at the lower secondaries rails section.
Do you think i should resolder all caps one by one like it was before? Pain in the ass and i think hum will be still there after this. 
I will replace C706, C707, C708 (those were the caps i replaced the last day) to see if there are changes, maybe i have a bad new cap who knows. I will also replace R707 and R708 those two 5.5W resistors that getting extremelly hot and have burnt a bit the PCB under them. The fact that hum appears at zero volume and does not get affected by volume knob what tells me according to schem?

[sarakisof]

Something else, as i dont want to open new thread, i found one of those yellow axial caps tottally cracked, thats why all B notes failed even transistors replaced..
Can you tell guys, those yellow (mullard?) caps are polypropylene or polystyrene?

[willienillie]

Almost certainly not polystyrene.

[anotherjim]

+1 ^
Any of the poly's except polystyrene.

[sarakisof]

Thats what i expected, just asked cause found in a german/french forum some giys saying they are styrene lol and i was confused for a while.
Furthemore, is it so critical to use 5.5W (come like that from factory) and bigger wattage for R707 and R708? I used classic ceramic boxes, 5W, not enough space for 10W ... Those are the two that are getting too hot ...

Page 12 : http://www.peel.dk/Philips/Philicorda_GM752_service_manual.pdf

[tubegeek]

is it so critical to use 5.5W (come like that from factory) and bigger wattage for R707 and R708?

8 - 4 = 4 V drop across 47 ohms. 47 / 4 = 85mA.

47 x .085 x .085 = .34, or approx ⅓ watt. (We do need to allow for current surges when filling that 100uF cap at turn-on though. But that's quite the under-rating @5W.)

8 V drop across 100 ohms = 80 mA (which means that the audio circuit is only pulling 85 - 80 = 5 mA off the 8V supply) .08 x .08 x 100 = .64 W  or approx ⅔ Watt.

I'd say you are well within limits using 5W resistors here. They really SHOULDN'T be getting super hot either, right? A third of a watt just isn't much.

[sarakisof]

. They really SHOULDN'T be getting super hot either, right?

Well, they are getting hot too, like the older ones, but i guess i am ok with them being 5W instead of 5.5W.

[amptramp]

I had a similar problem with a Univox U65G amp - I replaced the main filter capacitor and the hum got worse.  I haven't bothered to change it back yet because I never use it but capacitance and voltage rating are not the only things to consider, the ESR (equivalent series resistance) is a major factor.

[sarakisof]

Well i am back again, wondering about this thing:

is it so critical to use 5.5W (come like that from factory) and bigger wattage for R707 and R708?

8 - 4 = 4 V drop across 47 ohms. 47 / 4 = 85mA.

47 x .085 x .085 = .34, or approx ⅓ watt. (We do need to allow for current surges when filling that 100uF cap at turn-on though. But that's quite the under-rating @5W.)

8 V drop across 100 ohms = 80 mA (which means that the audio circuit is only pulling 85 - 80 = 5 mA off the 8V supply) .08 x .08 x 100 = .64 W  or approx ⅔ Watt.

I'd say you are well within limits using 5W resistors here. They really SHOULDN'T be getting super hot either, right? A third of a watt just isn't much.

The 5W ceramics that i used as replacements seem to get too hot again, like the original in the circuit (and maybe more). The original used were 5.5W wirewound i think.

They are R707, R708. I


Page 12 for full:
http://www.peel.dk/Philips/Philicorda_GM752_service_manual.pdf

.They really SHOULDN'T be getting super hot either, right? 

https://youtu.be/c6SCjvcLCtI  -------> 6.04

Are you sure i am ok within limits using 5W ceramic there?
What does this "8-4=4V" you wrote stands for? +8, +7,+4 in the schem do not stand for Volts...

[sarakisof]

Help me identify this i got lost. I get those measurements:

+7 point: 33V.
+8 point: 23V.

So   for R707: 33-23=10V., so 10/47=0.21A. so P=2W.
and for R708: 23-0=23V., so 23/100=O.23A. so P=5.3W. !!

So obviously i am not well with 5W or i do smthn wrong?

[tubegeek]

I assumed that when there was a node on a power supply marked "+8" it meant 8V above ground. If not, then I dunno WHAT you're supposed to get at that point, but use whatever you actually measure as the basis for your wattage calculations. It's just Ohm's Law and the formula for Power.

Volts = Current * Resistance

Power = Current^2 * Resistance

Where I put 8-4 I was calculating what I thought the voltage drop across the resistor was supposed to be, but I guess I misinterpreted the schematic markings. Either that, or your voltages are way too high.

[sarakisof]

If you look carefully above +2 node it says 9.1V so +"X" cannot be volts. All my voltage readings are fine the organ works nice, my only concern comes about those 2 resistors overheating.
If that helps i get 16 VAC at top secondaries and 24 at bottom ones. So i guess it makes sense that i get 23VDC atfter top diodes (+8 node) and 33VDC at bottom diodes (+7 node).
So are my maths correct?

R707: 33V-23V=10V so I=10/47=0.21A. so P=0.21x0.21x47=2W

R708: 23V-0V=23V. so I=23/100=O.23A. so P=0.23x0.23x100=5.3W

Could that explain that factory 5.5W woundwire resistors read 200 C degrees overheating in the youtube video above?

Those resistors are responsible for spring reverb driving ...

[tubegeek]

Math is correct.

I'd be very comfortable running a 5W resistor at 2W indefinitely. 5.5 W resistor at 5.3 W, not so much.

But I'd be shocked if that had anything to do with the him.

[sarakisof]

No nope for the hum, this is very low audible only when stuck your ear on speakers and may be caused from bad new caps.. Resistors discussion is made only for better and healthy general functionality.
So i guess i have to go with a 10W one, have no enough space there though, i see...

[PRR]

> I assumed that when there was a node on a power supply marked "+8" it meant 8V above ground.

Looks like instead of noting the nodes as A B C etc they numbered them 1..8. Yes, confusing.

2.2W in a 5W part is not a problem. If the space is too tight for 10W parts then it is just too tight for good airflow; but it wasn't made to live forever and these are not expensive repairs even if you must do them every decade.

[sarakisof]

True. Also read a similar thread on diyaudio in which you PRR discussed about better airflow or even place them on chassis and use wires to the solder pads, or using cooling paste etc.

.but it wasn't made to live forever and these are not expensive repairs even if you must do them every decade. 

It is not damaged/burnt or not working resistors, they were measuring perfect (-being woundwire-actually more accurate than the new ceramics replacements), mostly just because i saw signs of a little black/heat pcb underneath and that they were a bit darker than the others. Also, because right next to them it is placed the small transformer for spring reverb and from one side it gets too hot due to those resistors. I just want to make things better there as much i can. What could you suggest?

[anotherjim]

There are some ways to reduce the harm from hot power resistors.

Standoff - raise the body of the resistor away from the pcb. Ceramic beads are available that stack and fit over the resistor leads to protect them. This reduces the heat damage to the pcb.

Heatsink - similar to any other heatsink but clamping over the resistor body - it carries heat up and away.

Heatshield - a wall in between the source and the sensitive components. Often made of aluminum, but copper has been used as it can be soldered to the ground plane and then is also a reliable electrical shield.

Deflector Shield - an active device that repels harmful radiation by duplicating its frequency and presenting an opposed polarity. It usually requires a functioning warp-core to power it.

It's especially important to keep electrolytic caps safe from hot neighbors - there is nothing like heat for reducing the lifespan of an electrolytic.