Speaker help for a newbie.

[Clausenro]

Hello all, my very first post and I’m looking for guidance. I just was given this Electrohome/Dominion radio, a Canadian one, 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.

http://pacifictv.ca/schematics/phonola683sdata.pdf

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.

Any advice at all would be greatly appreciated.

[Phil]

Welcome to the forum!
Since you are missing the speaker I am operating on the assumption you are also missing the output transformer, which appears to have been part of the speaker assembly. You will need a transformer of about 10K plate to plate primary with a secondary to match the speaker you chose, probably 8 ohms. You can probably find something suitable on eBay or from Hammond. You will also need a choke to replace L1. It is probably not critical value wise, since, if I am reading the schematic correctly, this set utilizes a 400 mFd capacitor input filter (!?). I think this is probably an error since it would exceed the 80 rectifier tubes rating by a huge amount. 4 mfd would be more typical. I would look for something on the order of 20-40 henrys @ 30-50 ma. Someone else may have better information for you on this. If it were me I would put a 10K adjustable wire wound in series with the choke and adjust it for 220 volts on the plate of the 6C5.

[Phil]

No affiliation to the sellers.
This would be a good choice for the transformer:
https://www.ebay.com/itm/16573628858...Bk9SR5CEu6SZYQ

[Clausenro]

I just figured out how to attach a file so I’m enclosing the service notes for the radio which is more detailed information then the pacifictv schematic.

I believe that C24 is 40 mfd and C25 is 50 mfd.

[Electronic M]

Originally Posted by Phil

Welcome to the forum!
Since you are missing the speaker I am operating on the assumption you are also missing the output transformer, which appears to have been part of the speaker assembly. You will need a transformer of about 10K plate to plate primary with a secondary to match the speaker you chose, probably 8 ohms. You can probably find something suitable on eBay or from Hammond. You will also need a choke to replace L1. It is probably not critical value wise, since, if I am reading the schematic correctly, this set utilizes a 400 mFd capacitor input filter (!?). I think this is probably an error since it would exceed the 80 rectifier tubes rating by a huge amount. 4 mfd would be more typical. I would look for something on the order of 20-40 henrys @ 30-50 ma. Someone else may have better information for you on this. If it were me I would put a 10K adjustable wire wound in series with the choke and adjust it for 220 volts on the plate of the 6C5.

It looks like the audio output transformer is powered through the field coil... I'd probably look for a choke rated for 150mA or more. Don't want to smoke the choke with excessive current.

[old_coot88]

Originally Posted by Electronic M

... I'd probably look for a choke rated for 150mA or more. Don't want to smoke the choke with excessive current.

Actually, just to get the radio running and verify operation, he could temporarily sub a wirewound resistor of appropriate value (and wattage rating) for the choke.

[Electronic M]

Originally Posted by old_coot88

Actually, just to get the radio running and verify operation, he could temporarily sub a wirewound resistor of appropriate value (and wattage rating) for the choke.

Absolutely. I've seen power resistors even used as a permanent fix before. Heck he could probably jump the thing out if he's running it on a variac (so he can control the B+ to keep it from being excessive) and the only consequence would be increased hum while it was set up that way.

I just don't like advising someone to buy a part that might smoke in the application.

[old_coot88]

Increasing the filter caps' value a bit above original specs might just subdue hum acceptably (using the power resistor fix).

[Phil]

Perhaps I am reading the schematic incorrectly but it appears to me there is a jumper on the plug taking B+ to the Ct of the output transformer. If that is true then the current for the output stage is not going through the choke.

[Electronic M]

Originally Posted by Phil

Perhaps I am reading the schematic incorrectly but it appears to me there is a jumper on the plug taking B+ to the Ct of the output transformer. If that is true then the current for the output stage is not going through the choke.

It looks like it's on the output side (farthest end from the rectifier) of the choke so the audio trans current would be going through the choke... Unless I'm reading it wrong on my phone screen.

[old_coot88]

If current to the output xfmr were not going thru the choke (field coil), the field would be very weak. That's cuz the output draws more B+ current than (probably) all the other stages combined.. thus maximally energizing the speaker field. Tryin' to sound all fancy-shmancy.

[Phil]

Originally Posted by old_coot88

If current to the output xfmr were not going thru the choke (field coil), the field would be very weak. That's cuz the output draws more B+ current than (probably) all the other stages combined.. thus maximally energizing the speaker field. Tryin' to sound all fancy-shmancy.

Quite true! Obviously, I'm not reading the schematic correctly on my phone.

A choke of that current is going to be expensive so he might want to replace it with a resistor and put a larger cap after the resistor. I wouldn't increase the 40 mfd cap as it is much to large for the rectifier anyway (based on tube data sheets) and filtering will be better with the RC circuit.

[Clausenro]

Appreciate all the assistance. I ordered a push/ pull 125 bse output transformer from Hammond and will wait for it to come in before I proceed on the rest of the restoration. Thanks!