[Jamie]

This is the link to the schematic for a Philco 620.
http://www.nostalgiaair.org/PagesByModel...013804.pdf

My question is about part no 52 - a 16MFD E-cap.

Does the negative end of the e-cap go to chassis ground, or is this one of those models where it is reversed? If it is positive to chassis ground, how would I know by looking at this schematic?

[morzh]

No, why would it be reversed? As usual, minus to Chasis, plus to the resistive divider.

[Jamie]

Ok thanks. I don't know why it would be reversed. I was reading some old threads about a different model and the Cap was reversed on that one. I'm experiencing some hum with this model 620 and I'm trying to track it down. I was just double checking my installation of the caps. Thanks.

[gregb]

Hi PuhPow:
Yup, like he said. Did you rebuild all of the bakelite blocks and replace all of the caps? Is the hum all the time or is it tuned? Could be a tube, maybe.

Gregb

[Jamie]

Tubes check good, and I swapped in a totally different set too...just to be sure. I rebuilt all of the bakelites, replaced all the other caps and I think I checked all the resistors. I've gone through all the wiring, everything looks kosher. The hum is not tunable. It is constant. There is one large resistor that's drifted pretty high, but I don't have a replacement that's high enough wattage ( I think it was no 47, but I can't recall for sure and I'm not at my bench now). Voltage on all the tubes seems to be acceptable levels.. maybe a little high, but nothing way out of whack.

[PAradiogeek]

Puhpow,

It looks to me like the main filter caps are the two 8 uF caps in the dual-cap item #67. Have those have been replaced? You say the hum is not tunable, and is constant. Does the hum change when changing the volume? If so, the problem is before the volume control. If not, it is after.

Good luck.
Jon

[morzh]

Also,

In case the hum does not depend on the volume level, does the speaker have that anti-hum wire?

[Raleigh]

I'm not positive, but I think the schematic shows a hum bucker in the voice coil. I've heard that'll increase hum if it's wired backwards and reduce it if it's wired the way it's supposed to. Reversing the speaker leads could reduce or increase the hum, depending if it's right in the first place.

[Raleigh]

To your original question, how could you tell which end of the e-cap part 52 is connected to ground, check where the other end is connected, which is B+, dropped a little by a 15K ohm resistor. Since B+ is the highest voltage in the radio, ground is lower voltage than B+, even with the voltage drop across resistor 48. You would connect the - end of the e-cap to the lower voltage, which is ground.

Note that in this radio, ground is not the lowest voltage, which is always the center tap of the power transformer. That tap is connected to ground through what looks like 257 ohms of resistance, so ground is at a higher voltage. If, say, cap 68 connected from ground to the center tap had been an e-cap, the + lead would go to ground because ground is higher voltage than the center tap.

[Jamie]

Thanks for all of the responses from everyone. I'll try to answer every question..

Jon, all of the caps have been replaced, including the filter caps. The hum is constant, regardless of volume.
Morzh, the speaker has three wires. I do not know which one is the anti-hum wire. I've not heard of that before.
Raleigh, I can try swapping the speaker wires. The wires had to be replaced when I restored the radio. I recall being very careful and marking each wire, but I suppose I could've made an error.

The resistor that's out of whack is #48. It's a 15k resistor that's drifted up to about 35K. I believe it's a 2 watt resistor and I don't have one in stock. If I bridge the 8mfd e-cap #67 with a 20mfd cap, the hum disappears. Could this be linked to the bad resistor?

[morzh]

It is hum-bucking coil,

http://www.radiomuseum.org/forum/backgro...akers.html
(search for "Hum compensation methods").

it is serialized with voice coil and picks up the field from the field coil.
As the field coil sometimes double up as a filter it will have some AC ripple current going through it and it will cause some humming in the speaker. By connecting the hum-bucking coil in series you will compensate this hum. It has to be right polarity, as the wrong one will exacerbate the hum.
It is clearly seen in a speaker if it is present, but is only used in those radios where the field coil is used for filtering. In radios such as 20 or 90 where there is a separate choke coil for filtering and the field coil does not experience any much ripple, the hum-bucking is not used.

In your case the speaker could benefit from it as the field coil is the filtering coil at the same time.

[Jamie]

Morzh,
Thank you so much for that information. That link you provided is like the answer from above! ... I can quote two sentences that mirror my experience perfectly -

"A comparable effect could be obtained, in a less elegant way and by introducing an additional component, by increasing the value of C1 from 4uF to 33uF in order to substantially reduce the ripple current across the field coil."

..and, this is exactly what happened to me,.,...

"...but no hum had been observed during the restore procedure, it appeared only after inserting the chassis in its cabinet."


Thanks!!!

[morzh]

Well.....this might work but I have to warn you (and I think this was raised in that thread at the link too) - you have to be careful about increasing the first cap value. What happens is it reduces the ripple and the rectified voltage goes up, and might go up quite a bit so now you have to check your B+ and other power voltages and see if they ventured too far from the specs.
In any case higher voltage will make your tubes run hotter.

[morzh]

In fact I wonder how hard it is to make a simple shading ring which would be a piece of thick copper wire arranged into a shorted single turn around the central pole of the Field Coil. It would as much as almost eliminate the hum magnetic field produced by the field coil.

[Mondial]

Jamie, did you check or replace resistor 65 and cap 66? Thes parts in combination with the field coil comprise a resonant trap which is tuned to the 120 Hz ripple frequency. If the resistor is open or high, or cap is off tolerance, the circuit will not be tuned to the ripple freq and you will experience increased hum.

Placing the speaker in the cabinet results in increased hum because of the acoustic baffle effect. The low freq response of the speaker is increased and therefore the hum becomes more apparent and noticeable.