Low 100hz Hum After Recap Philicorda Organ

[sarakisof]

Hello everyone, i found at flea market a Philicorda GM751 organ transistor version, got home, powerd on, it was partially working, i have to fix reverb, replace some transistors some transistors and clean some contacts. But before those, i decided to replace first all electro caps of power supply with new ones. And here comes the really weird issue. After full recap, i get a very low (i can hear it only if i place my ear on the speakers) 100/120hz hum now! It is really low and stable at zero volume and does not get affected by volube knob. The strange thing here is that before recap, there was no hum at all    The old 60s caps were extremelly leaky, got fluids all over the place, but no 100hz hum at all!

Here is the schem: http://www.peel.dk/Philips/Philicorda_GM752_service_manual.pdf

(i read that it is the same with 752 as my 751 is the transistorized).

Have measured all voltages and they are all fine, well except for one point - see pic below instead of 6V i get 33V. Also measured all resistors of power supply they are all fine, but R703, R707, R708 5.5W get extremelly hot - don't know if that has smtng to do and if schem is correct there (??) though.

Any thoughts?

[GibsonGM]

Hi,

You may want to go back over all the solder joints, connections, and component values on that lower transformer winding.   If you should get 6V, but read 33V at that point, I suspect something is shorted.   That would also explain the hum.    Just a guess, hope that helps!

[sarakisof]

Thanks GibsonGM for reply, but as i went back to check i realised smthng weird, look at the two pics i attached above: According to schem GR703 and GR704 diodes are not supposed to connect each other, but if you look at the second pic, in the pcb layout they get connected! Am i missing smthng here?

[sarakisof]

Actually if you compare those two, smthng does not make sense at lower winding of transformer +4,+5,+6,+7,+8. I am confused now   

[GibsonGM]

Hmm, you sure that schematic is for this particular model of organ??  R708 is connected to GR703 + GR704, but should have R707 in series first, only be connected to GR704, and so on.  There is very screwy stuff there for sure.

I think this might go to another model or something.    If all you have is hum and it works, it's probably traceable back to the power supply, and the fact you just did the caps.   Bad new cap, bad solder connection, missing ground, bad diode...bad something.     But based on that PCB, this wouldn't even work! LOL

[sarakisof]

.
I think this might go to another model or something   

Those two pics are from the same service manual i attached link in first post.  My model is the 751 and my pcb is same with the pic's pcb. So maybe the schem is wrong and i am correct by getting 33V there..
Pffff think that before recap it was competely silent, no hum at all!

[anotherjim]

Not sure if the diodes being joined really makes any more difference to it except reduce ripple out of the RC dropper/filter. Expect engineering changes, even if its the correct scheme.

The pcb layout doesn't reference the transformer secondaries at all, unless the wiring looked hacked by an earlier repair, assume it's as factory.
Are you certain the AC voltage selector is correct for you? An overdriven transformer will hum. It might also just be working harder with better capacitors, and new ones are not always perfectly formed until some hours after first powering up.

[willienillie]

Any wires get moved while you were working on it?

[sarakisof]

The pcb layout doesn't reference the transformer secondaries at all, unless the wiring looked hacked by an earlier repair, assume it's as factory. 

i know but i repeat this pcb layout is inside factory service manual i attached in 1st post.
http://www.peel.dk/Philips/Philicorda_GM752_service_manual.pdf

.Are you certain the AC voltage selector is correct for you?   

amongst others it has 245V and 220V, i live in EU 230V so i left selector as it was, at 245V.

. It might also just be working harder with better capacitors, and new ones are not always perfectly formed until some hours after first powering up. 

i was thinking of that but see smthng i didnt mentioned above.
As i was tired the first day i got it in my hands i had time only to replace the first three big filter caps(3x1000uf) and some other smaller ones, all of them in the "first power stage" - if you look at pcb pic there are all in the left side of the wall ("wall"=2xAC128s block). By my last power on before go to bed, there was no hum at all, only normal low "wind" hiss when volume at max.
So organ worked like charm like that for 2-3 days.
Yesterday i found time to replace the other last caps in the right of the wall (3x1000uf and 3x680uf) and after that i got hum!!!
So problem appeared after last caps (and if i remember well it must be after the 3 last 680uf's) - i soldered back the 3 old 680ufs but no diference, still hum. So the only thing i havent tried is to solder back the other 3 1000ufs. But i think 100/120hz hum has to do usually with filter caps, BUT in the first day that i changed the filter caps i didnt got hum so in this point i start to loose my mind!!
And smthng last i just tested: speakers switch to off, pulled some alligators from organ output (phili uses those Dutch 5pin) and guess what: no 100hz hum at all. So maybe smthng at output amp stage?

PS. 1.I left yellow liquids and leakage from old caps there on pcb cause i couldnt find an effective solution to remove them totally. Tried with razor knife but it was really hard and didnt make hufe difference after all. It is a mix of sticky adhesive mixed with cap fluid dont know if this has snthng to do.
2. The hum is really low you must stick your ear tobthe speaker to hear it, but it is there and it was not before.

[sarakisof]

Here the repairs i ve done so far

[tubegeek]

It looks like some of the original caps were insulated from the chassis (those yellow plastic rings.) Are any of your replacement caps touching the chassis? I'd expect something much more spectacular (blown caps, blown diodes) than a tiny bit of hum, but it's a place to look.

Tube amps sometimes have a "hum balance" adjustment pot in the heater circuit, it's possible that your new caps are different enough from the originals that they require a little tweak of that pot to null out the hum more effectively than it is now. Look for a screwdriver-adjustable pot behind a hole on the back or top side of the chassis.

[PRR]

> Tube amps sometimes have a "hum balance"

The top voltage seems to be 23V. I don't have to open the whole plan to guess this is not a tube amp.

[sarakisof]

Tubegeek it is a solid one as PRR mentions  furthemore those yellow plastic rings are common on old organ combos Ducatti or Falcon caps used them for leakage purposes..

I concentrated on output amp  C607, R619, R620, R621 they go to speakers switch - speakers but all components are fine.
Talking about pots actually i remeasured my pot for voltage calibration on AC178 base. Supposed to be 18.6V and i get 18.7V i mean i have tested every possible thing and all are perfect. 3 days ago it was dead hum silent, now why? Could it be that i used the PT as a seat to hold my mobile phone so to have stronger light from my mobile's flash light? I had some calls while it was sitting on the trafo, could this affect smthng electromagnetically magic voudou ?? dont know i think i will live with that..
Dont know if you saw this -mentioned above- but R703, R707, R708 5.5W get extremelly hot - don't know if that has smtng to
do, i know this is normal for this organ's supply check here at 4.20 and 6.10
https://youtu.be/c6SCjvcLCtI
i have the exact same version.
What else should i check, diodes? But volts are fine ughhhh..
Could i test with different speakers? I have 2 same 4ohms i could wire them in series to get 8ohm as it is now but i m too lazy now,could the speakers produce a low 100hz hum?

[sarakisof]

Ok verified, tested also with headphones dedicated out and no hum again, so both line and head outs no hum. When i switch to speakers mode i get that low hum. Could amplifier section cause 100hz hum?

[tubegeek]

> Tube amps sometimes have a "hum balance"

The top voltage seems to be 23V. I don't have to open the whole plan to guess this is not a tube amp.

Sorry! I saw "old organ" and I got ahead of myself.

if the parts list specs them as 5 Watt resistors, they're expected to get pretty hot, that would probably be normal. But if there's a short pulling excess current through them they'd be EXTRA hot! The outer case of the new caps certainly is a possibility - having a short that doesn't show up on the schematic - if the old caps were insulated from the circuit board and the new ones aren't.

I am not convinced that what you found stuck to the chassis is electrolyte leakage out of the old caps: I think it's glue to hold the caps in place, that's used quite often.

Look at the caps with the yellow sleeves: their (+) terminal inside the sleeve is meant to connect to a pad on the board, and the body of the cap (which is usually connected internally to the (-) terminal) is kept up off of the circuit board so it doesn't short to the PCB traces. If there is a break in the insulation (the plastic skin around the cap) you could have the (-) terminal making contact someplace with a (+) voltage on one of the board traces.

If no load is connected at the +6 point, then the voltage would drift high - it's the voltage drop across R701 that takes it down from a higher voltage.

Re: electromagnetic cell phone voodoo: naaaah.

[willienillie]

if the parts list specs them as 5 Watt resistors, they're expected to get pretty hot, that would probably be normal. But if there's a short pulling excess current through them they'd be EXTRA hot!

There are scorch marks under a couple power resistors.  Were those replaced?  Burnt-black PCB material contains carbon, and can be conductive.

[duck_arse]

I'm confused. GR701 on your circuit is shown as connected anode to trafo, but there is no nothing connecting there on your baord layout. it also appears you have a wire wired there, and another [grey] wire below that 150R resistor, where the layout dia is blank. how many wires?

[sarakisof]

The outer case of the new caps certainly is a possibility - having a short that doesn't show up on the schematic - if the old caps were insulated from the circuit board and the new ones aren't. Look at the caps with the yellow sleeves: their (+) terminal inside the sleeve is meant to connect to a pad on the board, and the body of the cap (which is usually connected internally to the (-) terminal) is kept up off of the circuit board so it doesn't short to the PCB traces. If there is a break in the insulation (the plastic skin around the cap) you could have the (-) terminal making contact someplace with a (+) voltage on one of the board traces.

OK you are stuck there there is no possibility for that, look here http://www.peel.dk/Philips/Philicorda_GM-751.html
there are 4 caps with those plastics, i had three. I have seen in other Philis there were 2 or whatever, just whatever stock caps the factory had. I have seen a Phili used only those blue caps, no plastic at all. Problem is not there, what board traces, there are no traces on board, only bottom.. Apart from that my caps have enough space, they dont touch each other.

I think it's glue to hold the caps in place, that's used quite often.

yeap i said it is a mix of glue with those "ill leakage spots old cap puke".

If no load is connected at the +6 point, then the voltage would drift high - it's the voltage drop across R701 that takes it down from a higher voltage.
I'm confused. GR701 on your circuit is shown as connected anode to trafo, but there is no nothing connecting there on your baord layout. it also appears you have a wire wired there, and another grey wire below that 150R resistor, where the layout dia is blank. how many wires?

You mean in my picture of power supply or pcb layout from pdf i have attached above? If you the black n white pdf pcb layout it is from service manual, they have left all wire pads unconnected, it is just for components on board ... Wiring is OK, it is as it should from factory. I can post pics from the guts later. Anodes of diodes go to trafo, guys i get correct voltages everywhere and the organ works like a charm, only prob is a little hum i get on speakers.

There are scorch marks under a couple power resistors.  Were those replaced?  Burnt-black PCB material contains carbon, and can be conductive.

Yeap thats true, is the only thing i didnt do, i have not replaced them, this will be my next try.

Did you guys read what i wrote above? From the line and headphone outputs the signal is ultra clean, no hum at all. I get that low slight 100hz hum only with speakers, so prob is in amplification section? . The hum is there at zero volume and doesnt get affected by volume knob.
Look at page 12 of service manual: http://www.peel.dk/Philips/Philicorda_GM752_service_manual.pdf

[anotherjim]

I think that amplifier is always going to hum. What surprises me is the lack of filter capacitance on the "+7" supply. I'd have thought there would be at least 470uF cap on that & I'd use 1000uF, but maybe the diodes won't handle the switch-on surge. If they blow, it won't harm to replace with something stronger like 1N5401.

They joined those diode cathodes together in a cheap attempt to reduce the ripple.
Before they joined them, the ripple would have been 50Hz.

Do not expect anything Philips made to be either technically perfect or comply with the printed service manual. Updates were not issued by reprinting the manual but by issuing an addendum sheet which only got sent to authorised service centres. These either got ignored or lost, just like the errata sheets they used to put in books.

[sarakisof]

Thank you anotherjim for the "psychological support"    But the weirdest thing here is that i did not got hum before, even after having replaced filter caps with new ones. So,

.I think that amplifier is always going to hum.

why it did not hum before? I remember clearly it had only a slight normal hiss when volume at max. I remember well cause i told myself "wow now thats a unit from 60s that didnt hums neither 50 or 100hz,amazing!" and went to bed first night with that soft hiss in my ears. I was happy.  Hum appeared 2 days after when i replaced the last 3x1000uf's and 3x680uf's in the end side of pcb. 

EDIT: anotherjim look at the boacknwhite pcb layout above. There, and in my pcb too of course, GR703 and and GR704's cathodes get connected together so, "+7" is connected with the two 680uf's. The schem has errors  But i tell you, the unit was dead silence, clear no hum at all, smthng changed after having replace those last caps!