Pointers on repairing a tuner (SS)?

[_pmc]

I bought a DOA Realistic TM-70 at a flea market for $5 today. I'm handy with a soldering iron and I have a multimeter. But I don't know where to begin. Can someone point me to a general reference guide or get me going somehow? There sure are alot of components in that thing! I could hunt-n-peck forever.

TIA!

-pmc

[nashvillebil]

Does it power up any at all? If it doesn't then the first step is diagnosing the power supply.

[_pmc]

Does it power up any at all? If it doesn't then the first step is diagnosing the power supply.

It does not. I took a closer look at it and it looks like AC power runs through a transformer and then to a board with lots of capacitors on it. I'll start by testing that stuff with a multimeter.

Thanks.

-pmc

[Chazb11]

I'm a self taught hobbyist, not a technician so I don't know a whole lot about tuners either. I do know you're not going to accomplish much on a tuner unless you have a signal generator and an oscilloscope, and know how to use them.
If it's not powering up then you need to look toward the power supply. Use your meter to check for current into and out of the transformer. You'll need a schematic to know what the value at the transformer outputs is supposed to be.

[_pmc]

I'm a self taught hobbyist, not a technician so I don't know a whole lot about tuners either. I do know you're not going to accomplish much on a tuner unless you have a signal generator and an oscilloscope, and know how to use them.
If it's not powering up then you need to look toward the power supply. Use your meter to check for current into and out of the transformer. You'll need a schematic to know what the value at the transformer outputs is supposed to be.

I've got none of that stuff, but I'm still gonna try. :)

If I'm lucky, that transformer is bad and I can simply replace it. I'm not sure what makes a transformer die (isn't it just wire coils and metal plates?).

I've got a halogen desk lamp that's got the same problem and it's been learing at me (my wife tells me to toss it, but the circuit is so simple it seems like it has to be easily fixable).

At the very least, maybe I can learn something about power supplies. I wanna build an amp this fall so I have alot to learn.

So I'm looking for current on the other side of that transformer, right? Not voltage. OK.

....still hunting for that multimeter.....

BTW: someone else here bought one and took pics. This is what I'm looking at:

http://www.audiokarma.org/forums/attachment.php?attachmentid=77640&d=1202178823

-pmc

[outlawmws]

The first thing I'd be looking for is:

o Blown fuses
o Obviously blown/burned parts
o Corrosion from leaky caps
o Defective power switch

Also if you are new to electronics learn some safety rules. (One hand in your pocket whenever you are poking around where AC or High Voltage lives… Jump in here any time guys…)

If none of this does it, start with the voltmeter set for AC, and start verifying that you have voltage to the transformer.
Then check the outputs of the transformer.

[_pmc]

I'm not new to electronics. But I'm new to tuners and my troubleshooting skills will end shortly after this "does it get power" exercise. I was really hoping it'd have a fuse, but I don't see one (or anything burned or melted).

Anyway, I won't hurt myself. Promise. :)

-pmc

[_pmc]

Checked it last night. The transformer has two leads: one runs to that capacitor board; the other to the power switch. I have voltage at the power switch (tested across two of the connectors right on the switch). I looked for voltage between the two connections to/from that capacitor board and got nothing. I poked around randomly looking for voltage anywhere else and got nothing. So I'm guessing that the transformer is probably OK and something else upstream from there is busted.

I double-checked for fuses and burnt out components and found nothing. The whole unit is a bit dirty and much of the solder isn't very clean (i.e., not shiny). But none of the joints look bad enough that the unit wouldn't work. It looks alot like it was hand soldered. And there's evidence that someone has been in there before.

Any idea where I could find a schematic?

Oh... and I didn't electricute myself, but I did accidentally (and briefly) short one of the power connectors to the chassis and got a pretty arc. Thanks for the warning. I'll take greater care as I continue to poke around.

-pmc

[dr*audio]

At minimum the transformer has to have at least 4 wires in order to work. There will be 2 wires that go to the AC cord, one through the power switch and on possibly through a fuse. The secondary side of the transformer will have at least 2 wires, possibly 3. You should be able to measure an AC voltage across the secondary wires.

[_pmc]

At minimum the transformer has to have at least 4 wires in order to work. There will be 2 wires that go to the AC cord, one through the power switch and on possibly through a fuse. The secondary side of the transformer will have at least 2 wires, possibly 3. You should be able to measure an AC voltage across the secondary wires.

Yup. I counted 4 total wires but will check again in case there are more hiding undeneath. The two incoming wires obviously come from the AC cord. I didn't test them because the wiring looks perfect from the wall to the transformer. There are no loose connections. I have no reason to believe that the transformer is not getting power but I can cut off the plastic connector caps and test anyway. I don't really think it's necessary (famous last words) but I guess I should be thorough.

There are 2 wires that come from the transformer and then go into the tuner. Those are the two I chased. 1 goes to the power switch. There is no fuse. The other goes to the rear board that has capacitors on it. I had assumed that I'd be able to measure voltage across the two end-points of those two wires (one probe on the power switch, and the other where it solders to that capacitor board). I did not see any voltage across that span. So I tested across two pins on the power switch and that's where I saw voltage. I think that the power switch runs back to the other terminal on the capacitor board (i.e., so when the switch is on, the capacitor board is energized). However, I did not read voltage between the two solder points on that board.

To be clean about it, I can cut the two wires that leave the transformer and test for voltage across the wires. I'm assuming that I'll get a reading since I have power at the switch. I can also remove the power switch to test it. I think it had 3 or more terminals on it but need to double check.

I want to learn more about transformers. Especially if I end up building an amp (likely). Can anyone point me to a site that fully explains these types of transformers?

-pmc

[dr*audio]

If you have your meter on AC volts and you don't read any voltage on the wires going to the board, but you doread voltage across the switch, the switch is probably bad. You can connect a clip lead across the switch and see if the power then comes on. No need to cut wires and yank off the crimp connectors on the AC cord.

[_pmc]

OK. I'll try that.

One other thing I discovered while cleaning/inspecting further. See the attached photo. Is that cap burned out? There are two like that. The one to the left looks the same way, but it's brown on the other side. I didn't initially notice it even though it's obvious to me now. Can someone please confirm that's what a burned out cap looks like?

Also, I read that I should have infinite resistance through a cap, and I shouldn't get continuity through it either. Well, I have measurable resistance on each cap (including one that doesn't have those brown marks) and I also get faint continuity through them. That said, I did not disconnect them when I tested... so the rest of the circuit may interfere with the measurement.

I'll try that jumper test.

-pmc

[dr*audio]

Not burned, the elecrolyte leaked. It will have to be replaced, but I doubt that's the problem. You can't measure caps in circuit except to check for shorts or approximate value.

[_pmc]

If you have your meter on AC volts and you don't read any voltage on the wires going to the board, but you doread voltage across the switch, the switch is probably bad. You can connect a clip lead across the switch and see if the power then comes on.

You nailed it. I jumpered across the switch, plugged in, and it came up!

One thing I discovered when I removed the aluminum face is that there are two fuses hiding back there (one on each side). they are marked "8V ROYAL" but they are not marked with an amperage rating so I'm not sure what to replace them with. The glass looks blackened in the center but the filament doesn't look broken. The filament is fat on each end and very thin in the middle, but appears to be intact through-out. Given that I broke one of the fuses while removing, I plan to replace both anyway. I'm not sure where in the circuit they are and therefore what they are meant to protect.

The power switch is held in with tiny philips-head bolts and nuts. I couldn't get a screwdriver to fit without potentially stripping. I'm going to try my tiny torx set. Worst case, I'll drill it out I guess.

But where will I find a replacement rocker switch like that? Would I be better off soldering a permanent jumper on the switch and using the cord to plug/unplug? I'd like to keep the tuner stock if at all possible so an alternate power switch isn't something I really want to do.

I'll be at the electronics shop next weekend to get anything I need. We'll see what they have for replacement swtiches...

Thanks for your help!

-pmc

[Alaric]

Check fuses with an ohmmeter , visual inspections can be misleading.

[outlawmws]

Those "Fuses" are probably lamps, based on an 8V marking.

[dr*audio]

Those are 8V 150mA fuse - type lamps, for the dial lights. Get them from dgwojo, send him a PM. I don't think you are going to find a replacement for the rocker switch.

[_pmc]

Yup. You guys are right about the lamps. Learn something new everyday. :)

Can I use these? $1.09 each.

http://www.partsexpress.com/pe/showdetl.cfm?Partnumber=070-120

[3Guncolor]

Those "fuses" could be the dial lamps... 8V stands for 8 volts. Don't put a fuse in there until you are sure they are not the dial lamps..

[dr*audio]

Yup. You guys are right about the lamps. Learn something new everyday. :)

Can I use these? $1.09 each.

http://www.partsexpress.com/pe/showdetl.cfm?Partnumber=070-120

Yes, that is the correct part. Make sure to use the link to Parts Express from the AK website, I think AK gets a kickback.

[_pmc]

It cleaned up nice once I wiped it down and used some wood polish on it. Lamps look great. See attached pics. Sound is OK (not awesome). The stereo light is out so I need to open it again and replace that bulb. For $5, I'm happy as pie. Thanks for the help guys.

-pmc

[dr*audio]

Looks nice!

[Jon_Logan]

That "8V Royal" is probaly a fuse shaped lamp.

[Jon_Logan]

That "8V Royal" is probaly a fuse shaped lamp.

Oops, sorry wayyy late.