Been working on this old radio for almost 20 years off and on notwithstanding heart attacks, cancer, and various other maladies. Finally got the radio completely back together again. Ran voltage checks. Need some feedback to make sure I am on the right track. The RF 6K7 plate is 213 vdc, sg is 114 vdc. The 6A8 plate is 213 vdc, sg is 114, osc plate is 237 vdc. IF 6K7 plate is 230 vdc, sg is 114 vdc. The 6Q7 plate is 192 vdc. The 6F6 plate is 225 vdc, sg is 230 vdc. I am thinking about changing resistor 29 (70k) to higher value to drop some of the higher voltages. Also, might change resistor 17 (20k) to 24K. Will this help restore voltages to correct values? Are the present voltages too far out of bounds to functionally operate?
The voltage on the 6K7 plate is rather high so changing 29 to 82K might be a good idea. The other B+ voltages seem to be OK. Measurement devices in 1937 had a wide range versus today's high impedance devices and today's higher line voltages throw them off even more. My philosophy is the pwr transformer is 70-80 years old and you will want to be as gentle as possible with voltages. So I put a dropping resistor in the primary to bring the voltage down to about 115.
There are two major reasons for higher voltages (here I am not considering those that are results of circuits' malfunctions but only benign ones)
- Higher voltage in Mains nowadays, and
- Measurements are done with todays high impedance multimiters.
The latter you do not need to worry about, the former I do not address unless I intend to listen to a radio for prolonged periodes of time.
Resistor in the primary, although electrically it is ok, is not the preferred way, as it will dissipate power.
The preferred way with transformer radios is the bucking transformer.
You could find the circuit on the web, it is rather simple and includes a small (10-20W) 120/12V transformer and then you could either install it in the radio, or (I would rather do it this way) take an electric outlet metal box from Home Depot and assemble a "bucking outlet". Then you could plug your radio the way it is should be, in a 105-115V outlet.
It takes a tad more labor and costs a bit more, but won't dissipate anything other than normal transformer losses.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
(This post was last modified: 05-19-2020, 09:38 AM by morzh.)
Thank you so much for this information. Two things I never thought about: 1. Never heard about a bucking transformer, but will research that. 2. Voltage dropping resistor in the primary sounds swell. But, what does lowering the transformer voltage do to the voltages that are already too low? Like the plate voltages on the 6K7's or the 6A8?
First of, lowering the input AC Voltage does this: it brings the input within specifications.
While working about10V higher does not necessarily kill anything, it certainly makes tubes run hotter as the filament voltages increase proportionately, and increasing the voltage 10% raises 5he power 21%.
After that, if you have lower than needed voltages, you should figure out that one by troubleshooting, but one way of NOT fixing that is raising the input.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
Good information. But, now I have one more problem with that dadgum radio. Just checked the dc outputs from the power supply. Unit has 630 vac on the 12 mf cap and 235 vac on the 8 mf cap. What gives?? Somehow the power supply isn't providing filtered dc although I do have dc on both caps. Acts like the filters are not working. Any ideas?
So what happens if you select the appropriate DC range for your meter and measure the DC voltage from pin1 or 4 (either one of the fat ones) on the #80 tube + lead and connect the - lead to the chassis? If all is well should see about 325vdc there.
In order to have an AC volt present at the + leg of the filter caps your rectifier tube would have to be shorted. Does your 80 look angry?? Are there sparks flying around inside it??
GL
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
(This post was last modified: 05-21-2020, 05:58 PM by Radioroslyn.)
If you had 630V, let alone AC on a cap you would see a blowup very fast.
630AC is across the xfmr output.
Your cap will have DC half of that (roughly) figure as this is a centertapped rectifier.
Measure on DC setting across the caps.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
I had a Beckman that had a fluke. It didn't like rf mixed with the dc voltages. Wouldn't settle on any particular reading. It would have been erroneous anyway. Bart doesn't care. It laughs at rf!!! https://www.simpson260.com/260-6/simpson_260-6p-1.htm
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
When I got two Barts from Kirk for $25 and made one working out of the two, it works great.
I always wanted one. But don't get to use it much.
Found out I don't need it as much. Even for semiconductors where I prefer an analog meter. As I have just discovered.
Recently I found out all I need to fix just about anything is a Voltmeter and an Ohmmeter. Which can be digital. And since I have a Fluke, those are pretty good. I guess there is a reason a Fluke is $250 - $700 while those not-so-bad el cheapo ones are $25 - $60.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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