Admiral 20x122 - need help finding speaker - Videokarma.org TV - Video

dano · #1 08-01-2010, 08:58 PM

I recently got one of these off of Craigslist. I haven't done anything w/ the electronics yet but I know the first problem I'm going to have is the missing speaker. I have a few that might work in with my radio parts, but I don't have a schematic and have no idea what impedance the field coil should be.... I have never used a TV with an electrodynamic speaker, I'm guessing the extra circuitry that the TV has compaired to a radio would require a bigger or lower DC resistance coil? If anybody had a schematic or actual set handy that they could check, it would be a big help.

David Roper · #2 08-01-2010, 09:49 PM

If I'm looking at the right parts list, the DC resistance is 85 ohms, 2.5 henry inductance. The current is considerably more than a typical radio--200ma. If you have a PM speaker that fits the mounting, use it. I'll take a look at what Hammond has available that would work as a replacement choke.

David Roper · #3 08-01-2010, 10:20 PM

Radio Daze carries Hammond chokes and there are two available that would work:

193H is a 5 henry choke in an enclosed case; 157R is a 2 henry choke with an open bracket mount. Neither costs less than 20 nor more than 30 dollars.

ETA: It probably would be helpful to link you to this schematic assuming it is the one for your set.

dano · #4 08-01-2010, 11:12 PM

Thanks! One speaker that I thought might work looks to only have a 7 or 8 ohm resistance on the field (not voice coil), I suspect it's just shorted. I'll turn up a perminant magnet speaker for now, before I go any further I'll test the picture tube and make sure I'm not putting money into something I can't get working

Thanks again, this is a huge help!

bandersen · #5 08-01-2010, 11:25 PM

Here's what the genuine article looks like. I'm reading 92 ohms across the field coil. Let me know if you'd like any dimensions or other measurements.

Good luck,
Bob

wa2ise · #6 08-01-2010, 11:49 PM

Quote:

Originally Posted by David Roper

this schematic assuming it is the one for your set.

If that is the correct schematic, the speaker is a permanent magnet type, and you'd need an audio output transformer 4K primary to a secondary that would match the speaker you plan to use.

The filter choke specs out to be 2.5H and 200ma.

David Roper · #7 08-01-2010, 11:51 PM

Well shoot, you will need an audio transformer too. I hope you have one in your junk box, it seems the prices for the universal replacements have gone up...a lot....

David Roper · #8 08-01-2010, 11:55 PM

wa2ise, check the parts list. It clearly says the choke is 'part of the speaker' which is borne out by the picture bandersen posted.

bandersen · #9 08-02-2010, 12:12 AM

The parts list is a little confusing. I'm not sure why it states PM in the field resistance column when it has a field coil / choke

wa2ise · #10 08-02-2010, 12:20 AM

Quote:

Originally Posted by David Roper

wa2ise, check the parts list. It clearly says the choke is 'part of the speaker' which is borne out by the picture bandersen posted.

Looks like you're right, though under the Sam's listing for the speaker replacement, for "Field resistance" it says "PM". Source of my confusion. But further down it says that the L1 choke is part of the speaker assembly. And I don't see it on the TV chassis . Admiral may have changed, in later production, from the field coil speaker to a PM one, and used a regular choke for L1, mounted to the speaker frame. In any event, you'd be more likely to find separate speaker, audio transformer, and filter chokes, and you could build a replacement assembly with that, if it turns out that the picture tube, flyback and so on are in good condition. A temporary kludge would be to use a power resistor of about 85 ohms and some extra capacitance across cap C1B to make up some of the lost filtering the choke would have given you. A 100uF cap probably.

dano · #11 08-02-2010, 12:21 AM

Wow guys, it's only been a few hours and I already have all my questions answered

The schematic does look to be the correct one and the chassis seems to match at first glance. My set is wired with the speaker socket between the two sections of the filter cap, so a choke or resistor should connect to that socket. I went through my box of spare speakers in the attic, and found two with the correct mounting hole pattern and with output transformers attached. I hate to pay the price for a universal replacement so if I'm lucky one of these two will be close to a 4k impedance, otherwise I'll try yet another speaker and look for separate transformer that gets me about 4k total. Now off to measure turns ratios....

wa2ise · #12 08-02-2010, 12:26 AM

Quote:

Originally Posted by dano

I went through my box of spare speakers in the attic, and found two with the correct mounting hole pattern and with output transformers attached. I hate to pay the price for a universal replacement so if I'm lucky one of these two will be close to a 4k impedance, otherwise I'll try yet another speaker and look for separate transformer that gets me about 4k total.

Don't worry too much if the primary impedance isn't exactly correct. All that would happen is you get less audio output power (before distortion sets in) if the impedance is wrong, say by a factor of 2. After you get the rest of the set working well, then you can worry about it.

dano · #13 08-02-2010, 01:38 AM

Well, from what I measured, I have an 11k and an 18k speaker/transformer combo. I decided to set those aside and just use a plain old 3.2 ohm speaker, and I'll locate a transformer and choke later. Who knows, by the time I get ready to work on the set I may have found the perfect speaker for it! The next step is to check out the picture tube and see if it's any good. The TV came with several missing tubes and the picture tube unplugged, so I suspect that something was wrong when retired.