I just was given this Electrohome/Dominion radio, a Canadian radio, and it hasn’t been touched. All the caps and resisters are original and I haven’t changed anything yet, BUT, what I do need is a speaker. There isn’t even one there to repair. I was hoping to replace all the electrolytics, caps and out of range resistors and then fire it up on a dim bulb but I can’t proceed without a speaker.
It originally had a 10” dynamic speaker and I was hoping that I could get some advice as to my options on how to replace it. Getting an original may be impossible and if I replace it with a permanent magnet speaker how would I go about connecting it correctly. I have read up somewhat on that process but figuring out what value resisters or additional caps may be required and where they are connected is beyond my abilities at this time.
If there are other options other then a PM speaker please pass that along as well.
Any advice at all would be greatly appreciated.
The PM speaker will require a power resistor to substitute for the field coil. The OEM speaker is also equipped with a hum bucking coil, to use a PM an additional 20% in capacity should be added to the "filter output condenser". Same OEM field coil speaker also has the output transformer for the'42's. Even with a PM speaker the output transformer is required.
You may find the info here helpful to determine the correct output transformer...
Not sure I understood what humbucking coil has to do with extra 20% of the output capacitor, but I wholeheartedly support the outcome: yes, when using a [power resistor (which should be more or less the same as the DC resistance of the Field coil), the output capacitance should be increased compared to the sch value to compensate for the lack of the reactance.
What I do not understand is the values of those capacitors, the o\first C24 SAYS 400mfd, the second C25 says 500mfd. Considering the "mfd" at the time stood for Microfarad, this is a bit excessive. Or I am not reading it correctly. Given the 80 rectifier tube.
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.
I’m enclosing the service notes for this radio. I just figured out how to do that. It may have more pertinent information then the other schematic from Pacifictv.
In this design the AF circuits are supporting a certain amount of hum. That is negated with the hum bucking coil. Adding the additional capacitance as I suggested should abate the hum that would be present in a speaker designed without a hum bucking coil. Overall, removing the hum from the B+ removes a useless work load from the AF as a system.
Be aware adding additional capacitance to the 1st electrolytic from the rectifier is limited to the rectifier itself and any voltage increase that will occur.
You will need to find an output transformer. I would suggest a Hammond, a bit pricey but the modern design will be helpful. Be sure the output tubes are correct and in good condition, no substitutions.
GL
Chas
Pliny the younger
“nihil novum nihil varium nihil quod non semel spectasse sufficiat”
First off, it seems that Electrohome was very generous with the capacity of the filters to begin with, so I would not increase the capacity. Per the RCA Receiving Tube Manual, for a "Capacitor Input Filter" (of which this), the maximum value of the input filter cap is 40 uF (same as MFD). Per the schematic you provided, the filter "caps" are 40.0 and 50.0 uF. This as as much as or more than what was used in later model radios with a PM Dynamic speaker with no field coil or choke.
As for the resistor, start with about 1.2KOhm. the goal is to get about 250V at the center tap of the output transformer.
CAUTION!! Do not run this radio without the output transformer. The screen grids of the '42s will take all the current, glow white hot and burn out, causing shorts.
The Hammond 125B transformer should work. However, if you search "the auction site", you may find a used output transformer for less. I think that a plate to plate primary impedance of about 8 KOhm will do for 250V. The manual I am looking at says 10 KOhm but that is at 315V.
If using the Hammond 125B, use secondary taps 2 and 5 if using an 8 Ohm speaker. If using a 4 Ohm speaker, use taps 3 and 5.
Hope this helps.
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