[morzh]

Now, after being intimate with the condenser bank for 3 hrs, after I turned on my Philco 20, I think (no I cannot vouch for it, but I am pretty sure of it) the AC buzz is more pronounced.
I actually did not hear much of it with the volume turned all the way down before the change.
Now I hear it, and it is quite noticeable.
Interesting that when swapping the two output 71A tubes places, it gets louder in one position, and quieter in another.

Now, I am 100% sure I did everything right, and the values are correct (two caps of 1uF/400V (pos. 1-2 and 1-4), one cap of 1.5uF/250V (pos. 1-3) and 0.15uF/50V (not 0.13uF, but it is today's value) in pos. 2-6).

I am not sure as to the reason. It is possible that due to large size of the old caps and their intimate (I seem to like that word, don't I) coupling with the Chassis through the can walls, there was some effect.......don't know.

I am thinking of trying larger values in filtering positions first.

Has anyone experience similar effects when you changed your banks?
I mean, all is new now - all caps and resistors, and all seemed to work better before the bank got updated.

[TA Forbes]

Are you positive that all your filter caps are sufficiently grounded?

[morzh]

What do you mean by "Grounded"?

None of the caps is actually connected to GND (Chassis). They are connected across the full-wave rectifier output (those 1uF ones) and across the resistive divider, but the rect. output connects to GND only through 187 Ohm resistor, not directly.

Or do you mean what I said before, that they were capacitive-coupled to GND via surface area between the caps (due to them being large) and the metal can?

Otherwise, if you know a trick how to "GND" them - tell me. But according to the wiring and the sch. - they were not connected directly to the Chassis in any way.

[Radioroslyn]

If you want to try a bigger cap don't use it in the first position, the input cap as it will raise the dc output of the power supply and cause the BC resistor to run very hot. It's ok on the 2nd or 3rd caps.
Terry

[morzh]

If you want to try a bigger cap don't use it in the first position, the input cap as it will raise the dc output of the power supply and cause the BC resistor to run very hot. It's ok on the 2nd or 3rd caps.
Terry

Yes Terry, they actually rely on higher ripple and then dropout on the choke to keep the voltage about 260V at the choke output.

I actually tried (I have connected in series 4 electrolytic caps, 10uF/100V - this is what I have to experiment with) to connect it to the position right after the 80 diode tube, and the voltage jumped about 30V (320V i/o 290V).

Connecting the caps across the position right after choke (2nd cap, pins 1-4) does not change the voltage, but makes the buzzing noise less pronounced.

-----------------------

I actually found the reason for extra buzz - the short piece of wire shorting pins 6 and 4 broke, but in such a way so I did not see it. So the output of the choke was simply not filtered. I found that by checking voltages on the caps and seeing the voltage on the pin 4 being about 2 or 3 Volts. After I re-connected it with a big solder blob, the buzz subsided and the quality of the music improved.

I still have some buzz, but it is about the same it was before, and I think I will beef up that 1uF cap after the choke, maybe to 5 to 10uF, and see what happens (once I get me some good caps - ordering them from Mouser does not make sense unless order is large - they rip you off on shipping).

[gvel]

My TRF buzzed horribly, until I added line filter caps. Here's a good read:

<!-- m --><a class="postlink" href="http://www.justradios.com/safetytips.html">http://www.justradios.com/safetytips.html</a><!-- m -->

If you don't have a "safety" cap handy, use whatever you have of correct value and at least rated a 630V. If this solves your problem, order some safety caps and change out the temps.

[morzh]

Gvel

Thanks.

This radio does not use line filtering, and, to be honest, I am not sure why it would need it.
I can try it, just to see if it does anything, but line filtering does not remove AC buzz.
(As I just wrote the buzz subsided after I found a disconnect).
It will filter out very high frequency noise, which you won't even hear.

What you refer to, I mean the article, is "Line filtering" caps, or (in case of switcher-supply) Y-cap. In the Switching Supplies (I designed few for very bad line conditions, like in India, where 270VAC is not uncommon where it should be 220VAC) they are used, and for a good reason.
Y caps are used to bridge the supplies from the "Hot" side to "Cold", to short-circuit common mode hi-freq. currents (going through chopping pulse XFMR), to keep the supply from radiating EMI (the one detected by an Antenna and a spectrum analyzer), and those are Y-rated, as failure will connect a customer to the AC line directly.
The line caps are used, together with a Common-mode chokes, also in the Switchers, to get rid of the conducted emissions, (detected by using LISNs and the spectrum analyzer).
That is all this is usually to make a device pass FCC/ETSI/CISPR22 and such.

[gvel]

I still have some buzz, but it is about the same it was before

Sorry...I've never heard AC hum referred to as a buzz.

Line filters were around long before switching power supplies. Further, a lot of radios used line filters, to remove "local interference" from finding it's way into the radio's front end, via the line cord. Back then - "safety caps" were not available - so wax/paper caps were used. Today, when these are replaced, most use X or Y rated capacitors. Others don't bother and substitute with a poly cap.

My TRF, an Amrad Model 70, originally did not have a line filter either. Like I mentioned, it had a horrible "buzz". After I was enlightened, I added a line filter, and my problem was solved. It's so quiet now, I hear very little in between stations.

[morzh]

Gvel

Thanks, I will try that.
I have a couple of Y-rated 2.2nF caps and an X-cap of 0.47uF from one of my projects.

Actually, I could also try a CM choke (to get rid of common mode RF signal on the AC line), and use two X-caps, one before and one after the choke.

A GND wire is usually helpful when using Y-caps, but that was not provided - these radios used 2-prong plug with no 3rd GND wire.

What I also think is good - I have some NTC thermistors I used to protect my supplies from the inrush current upon plug-in, and although Power XFMR itself is a protection, and the tube diode provides for no in-rush for filtering caps, once you introduce an input cap of 0.25-1.0uF, you will see a spark on plug-in, so an NTC thermistor might do some good here.

PS. What the authors forgot to mention that in the very first picture (with 2 Y-caps to GND), where the chassis is not Grounded (and it is usually not), the chassis is actually under floating potential of about 60V (the caps form an AC divider).

Of course, the current is very weak, and not likely to shock you, but it will give you an unpleasant "pinch" if touched.

This "weak" current is actually enough to kill some devices connected to each other. For example these 2 Y-caps without the protective GND were (and still are, in the same India) responsible for RS232 interfaces burning in then desktop computers (and now laptop computers) in Eastern Europe, when connected to local properly grounded devices.