[R-390] R390-A Power On B+ Voltage and Choke Filter

[Larry H]

Charles Steinmetz wrote the following on the R390 List on Tue Sep 16 08:54:50 EDT 2014:

> If you switch the radio on from cold straight to Slow AGC, depending on how fast the power supply comes up 
> and how long it takes V506A to warm up, C551 could see the full power supply voltage across it for a short
>  time.  Because the radio uses a choke-input power supply (search the archives for "choke input" for 
> discussions of the implications of this), it is possible for the B+ to be >400v for a short while at startup.  This 
> is worse in radios in which the tube rectifier has been replaced with solid state diodes.......

I certainly agree that the initial voltage will be higher with SS recs, however, the purpose of the 'swing choke' input power supply filter in the R390-A is to eliminate a high voltage surge at 'power on'.  I have done indepth testing on my 2 R390-A's with 26Z5's in this area and have found that the B+ at C606A (1st filter cap) rises quickly to a maximum of 255V in Standby and slowly goes down to 242V, where it holds.  The attached picture of my scope trace of this shows that is the case (the A trace is the C606A point and the B trace is the F102 point and trigger scope point).  Turning a R390-A on by switching immediately to AGC yields a little lower initial maximum voltage (250V), as you would expect and is an identical scope trace.  

Link to scope trace picture:  http://s29.postimg.org/a2yqc2tyv/IMG_6239s.jpg

Here's why:

The R390-A uses a 'swing choke' input power supply filter, the immediate load on the supply in standby is 6.7 ma and in AGC is 16.9 ma.  This load is from: Standby: 4 ma - 150V reg, 2.7 ma - audio resistors to gnd; AGC: standby current + resistors to gnd: 2.2 ma - RFamp, 8 ma - IF amps 1, 2, and 3.  I have verified that my 2 rx's do not have any undo load (ie: leaky caps, gassy tubes, or additional resistors from B+ to gnd or anything else).  The AC input is 117 VAC at my home.  The reason that the additional 10.2 ma load in AGC only reduces the initial maximum voltage by 5 volts is the way a 'swing choke' input power supply filter works.

The following is an excerpt from Norman H. Crowhurst's book, 'Basic Audio' from 1959.  I like this explanation of a swing choke as it is short and clear:

> Another kind of filter circuit employs the so-called "swinging" choke. All smoothing chokes employ iron cores 
> with air gaps that prevent saturation. By properly choosing the size of the air gap, a special action is 
> produced. At low load currents, the core is not saturated, but for higher current it progressively approaches
> saturation, which makes the circuit act as a capacitor-input filter. Capacitor-input filters produce  higher 
> output voltages; hence, the output at the filter can be made to rise with increased load current.

> At small load currents, the inductance of the choke is sufficient to make the filter behave as a choke-input
> arrangement, and the output voltage is not more than 0.637 of the alternating peak voltage. As the current
> drain increases, the choke begins to saturate, and the rectifier starts pulse-feeding the capacitor at the 
> output end of the choke. The circuit then begins to act as a capacitor-input filter and the output voltage rises.

> Because the current is increasing at the same time, the output cannot possibly reach the peak value of the 
> applied a-c because the drain effect will cause dips between the peaks, but the average voltage can rise with 
> a carefully designed filter of this kind. This is useful because it will serve to offset the voltage drop in the 
> supply circuit that always tends to reduce the output voltage with increased load current. If the rise produced 
> by the swinging choke just offsets the losses produced by increased current through the rectifier, the power 
> transformer, and possibly a further smoothing choke, the output voltage of this kind of filter will be almost 
> perfectly constant as the load current is changed.

Link to the picture of the associated graph from Norman's book: http://s9.postimg.org/usu46ncgv/IMG_6241s.jpg

As you can see from the voltage versus current graph, it takes very little current to hold the output voltage down.  That's the whole purpose of the large 12H swinging choke as input.  As the current load increases, the inductance reduces to a low 2H.  The way I verified I have no leaky caps is I removed all the tubes except the recs, plugged it into my variac, while measuring the voltage and current at F102, brought it up to 255V, in AGC I read 12.9 ma, as it should be (16.9 ma - 4 ma for the regulator).

I know that 1 book may not convince you that this is true, so you can google 'choke input filter' and find many articles written about them.  I found the following:

1. Aiken Amplification - Randall Aiken, 1999
2. Wireless World - Cathode Ray, 1957
3. Power supply design - Henry A. Pasternack, 1995
4. Basic Electronics - Albert Malvino, David Bates, 2008
5. Analog Electronics - Ian Hickman, 1990, 1999

So, if your R390-A's are working correctly and using 26Z5 recs, it will never see more than 255V DC on any capacitor (even C551 because there is no agc voltage at initial power on).  If you measure the voltage at F103, it will reflect the voltage at C606A -2 volts at initial power on if L601 is working correctly (it's probably ok if it measures 125 ohms (you could easily measure L601 and L602 in series for about 245 ohms)).  

The way I scoped C606A was to unplug it, wrap a wire (bare on both ends) around pin 3 and hang it outside the rx after plugging it back in.  Be careful not to short anything out with this risky connection.

This is why I believe the designers rated the 5 electrolytic filter caps C603 and C606 in the power supply mounted in the 2 cans on the audio deck at 300V and not higher.  They knew what they were doing.

Now if your AC input is higher than 117VAC, it would adjust upwards by the percentage.  125 is about 9% higher, so your C606A would be about 9% higher, or 277V.

If you are using SS recs, your initial C606A voltage I would estimate could be up to 40V higher, but I don't have any way to test this.  This will certainly add additional stress to your R390-A, but I don't know how much.  I do know that you will need to watch C606 A & B and C603 A & B for excessive leakage if they are still 300V caps.

Regards,
Larry J. Haney