[Hammarlund] HX-500 trials

Tue Apr 26 23:12:30 EDT 2016

     The rectifiers are vacuum tube diodes with an indirectly heated 
cathode.  The voltage on the plates according to the voltage chart in 
the handbook is 845V, voltage on the cathodes, and input to the filter 
is 790V, voltage to the final plates (filter output) is 780V but that is 
a voltage marked on the schematic.  Normally, the output voltage a 
rectifier with a choke input filter is lower than the voltage on the 
plates but is better regulated with load. If the filter is a capacitor 
input filter the voltage is higher with no or low load but goes down 
faster with load.  The handbook specifies a 20K ohm/V meter for the 
voltage charts but in the alignment procedure specifies a 
Hewlett-Packard 410-B. While the 410-B has much higher input resistance 
(122 megohm) it should not make much difference for the DC voltages 
except possibly at the grids of low level tubes. I think it is specified 
mostly for RF measurements.
    The voltages are so much higher than specified and are so 
consistently high that it makes me suspicious of the meter. The meter 
may read high on all ranges but differently on some.  If it is a VOM at 
least an approximate test is to measure a couple of fresh batteries. 
Although alkaline batteries will read higher than their specified 
voltage it won't be that much higher and zinc-air cells of the sort sold 
for hearing aids will be very close to their rated voltage and are 
steady.  While these are not precision standards by any means they will 
show up a global error in the meter. The voltages given by the OP are on 
the order of 7% high, more in some cases.  I would certainly check the 
meter as well as repeating the measurements with another meter, even a 
cheap one from Harbor Freight will do. If they do not read sensibly the 
same I would not trust the meter.
     I have not gone over the adjustment and calibration procedure but 
it seems complex and I would undertake it without trustworthy tools.
     FWIW, the mention of solid state rectifiers is a good one to follow 
up. Tubes have a definite voltage drop, it is not linear for vacuum 
tubes.  Solid state rectifiers have a constant drop per diode, maybe 1.5 
volts where the tube may have a drop of 20 to 100 volts. Mercury arc and 
gas type rectifiers also have a constant drop, about 15V in the case of 
mercury tubes, so they can be replaced by a string of diodes where 
vacuum tubes really can't be unless the load (and therefore the voltage 
drop) is pretty constant. However, the substitution of solid state 
diodes would not explain the high filament voltage. Unless something 
extraordinary has happened to the power transformer (and I can't imagine 
what it could be) the high AC voltages suggest the measuring instrument 
is out of whack. Certainly something to eliminate before doing a lot of 
tail chasing.

On 4/26/2016 7:17 PM, Kenneth G. Gordon wrote:
> On 26 Apr 2016 at 18:38, Richard Knoppow wrote:
>
>>       There is something strange about this.
> Boy! I certainly agree with you, Richard! :-(
>
>> The ratios of correct to
>> measured voltages vary.
> Yes. Odd.
>
>> Of course, if the voltages are wrong the
>> currents will be wrong too which may account for it.  I found a handbook
>> and schematic at Rigpix. The instruction book states the voltages are to
>> be measured with a 20K ohm/V meter so I don't think using the wrong sort
>> of meter will account for it.
> No.
>
>> However, if you do have another meter or
>> can borrow one it would be worth checking just to make sure the meter
>> itself is reading correctly.
> Agreed.
>
>>      It would be interesting to know what your line voltage is. If you
>> said I missed it.
> So did I. What is it?
>
>>      The difference in DC resistance of the chokes is probably normal
>> tolerance.
> Yes, it is.
>
>> Also note the HV choke is a swinging one intended to provide
>> some regulation to the supply.
> Yes, again.
>
> Now, there are several things about the stuff below that makes me wonder:
>
>>> AC voltages out of the transformer are 900 VAC, 350 vac, 6.8 vac.
> OK. Here is one: the AC voltage is 900 VAC, yet the DC voltage is also 900 VDC? Unless
> things are exactly "correct", this cannot be so. There MUST be some differences between
> these two voltages.
>
>>> Filter chokes test OK -- although I don't have my LCR bridge.
> Don't need one: an ohm-meter will do for this purpose.
>
>>> According to the schematics, the HV choke should have a resistance of
>>> 160 ohms, mine is 172.  LV choke should be 250 ohms, mine is 271.
> Those are within tolerances. However, here is a problem: Assuming that 900VAC output is
> connected to a full-wave rectifier, the DC output voltage at the rectifier, before it ever
> reaches a filter will be 1.414 X 900 or 1272.6 VDC. 230 mA drawn through your choke of
> 170 oms would mean a drop of only 29.2 VDC. THis is still over 1200 VDC.
>
> Are the rectifiers solid-state diodes? If so there is very little voltage drop with those. If it is a
> tube or tubes, then there is significant voltage drop across those, unless they are mercury
> vapor rectifiers or argon types (3B28).
>
>>> LV circuit is a CLC filter.  At the output of the LV rectifier,
>>> voltage is 380 vdc, should be 270.
> 350 X 1.414 = 494.9 VDC, 320 X 1.414 = 450 VDC plus
>
> The whole thing is weird...
>
> Ken
>

-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL