How would I test this transformer? Schematic included.

[John Henry]

I'm troubleshooting a Kenwood Basic C2 preamp. It just stopped turning on one day. No lights, no sounds... totally dead. It may have overheated for a period of time leading up to this. After giving it a thorough cleaning and testing that the power switch is making contact I'm now thinking I need to test the transformer. (It may have a thermal fuse in it, right?)

Any suggestions? I'm very new at this but know how to use my meter. If anyone can take a look at the schematic and tell me which leads to test, and for what, I'd appreciate it. Any other ideas are welcome as well. Thanks.

 

[Fred Longworth]

You hook an ohmmeter across the primary of the transformer. If you get a reading of, say, a couple dozen ohms, the transformer is fine. If you get infinity, the transformer primary is open. The thermal fuse is normally in the primary circuit.

NOTE: You probably know this, but I have to say it. Make sure that the set is unplugged when doing this measurement.

Fred

 

[ryuuoh]

See if you have voltage at the input and outputs of the transformer. that would be the 2 wires in, and the 4 wires out.
Or do as Fred suggests~

 

[John Henry]

You hook an ohmmeter across the primary of the transformer. If you get a reading of, say, a couple dozen ohms, the transformer is fine. If you get infinity, the transformer primary is open. The thermal fuse is normally in the primary circuit.

NOTE: You probably know this, but I have to say it. Make sure that the set is unplugged when doing this measurement.

Fred

Thanks, Fred. I tested resistance between the Red and Black wires leaving the transformer and got 4.8 ohms. But I think need a little help understanding which is the primary. The transformer has three wires going in (Red, Yellow, White) and four coming out (Black, Red, Yellow, Orange.) Is there a convention to this, or is it possible to tell by my schematic which is the primary?

After that do I test for resistance by putting the meter's red probe to the primary and the black probe to ground, or does the black probe go to the primary entering the transformer? Thanks again!

 

[petehall347]

primary is the voltage going in from the mains via the power switch ..secondary are the voltages for the various circuits .

 

[John Henry]

See if you have voltage at the input and outputs of the transformer. that would be the 2 wires in, and the 4 wires out.
Or do as Fred suggests~

Thank you. Here's what I found:

For the wires entering the transformer:
125V Between Red and White
125V Between Yellow and White
0V Between Yellow and Red

With the black probe connected to the mains neutral lead I get 19V by touching the red probe to each of the four wires leaving the transformer. It seems strange that they're all equal. What's also strange is I get 6-7V with the black probe on the mains neutral and the red probe not touching anything at all.

Any guidance greatly appreciated. Thanks.

 

[John Henry]

primary is the voltage going in from the mains via the power switch ..secondary are the voltages for the various circuits .

Thank you, sir.

So to "hook an ohmmeter across the primary of the transformer." I'd put one probe on the primary going into the transformer and the other on the primary going out?

 

[Livin4]

With the black probe connected to the mains neutral lead I get 19V by touching the red probe to each of the four wires leaving the transformer.

You do not measure the secondary from the primary.

Set you meter to resistance and measure the priamary side.

Between Red and White
Between Yellow and White
Between Yellow and Red

What are the readings?

From the first pic, you have 4 secondary wires, red, black, yellow, and ?orange? (can't really tell what the drawing says here).

From the drawing of the board the wires connect to, it looks like the red and black wires are 1 winding, and the yellow and ?orange? are another.

With the power connected, and on, measure between the red and black wires, then the yellow and orange wires. You should have readings, since I don't have a schematic, I don't know what they should be.

The second pic has nothing to do with the transformer.

 

[John Henry]

You do not measure the secondary from the primary. Thanks. I was confused about this.

Set you meter to resistance and measure the priamary side.

Between Red and White Zero
Between Yellow and White Zero
Between Yellow and Red Zero

From the first pic, you have 4 secondary wires, red, black, yellow, and ?orange? (can't really tell what the drawing says here). Yes, it's orange

From the drawing of the board the wires connect to, it looks like the red and black wires are 1 winding, and the yellow and ?orange? are another.
Here's a picture. It looks to me like they are paired as you said.

With the power connected, and on, measure between the red and black wires, then the yellow and orange wires. You should have readings, since I don't have a schematic, I don't know what they should be.
I get zero on both these pairs also.

The second pic has nothing to do with the transformer.

Here's a PDF of the service manual with schematics.

It seems like this transformer is dead. Is it worth it to try and replace the heat fuse (that it may or may not have)? I think I'd rather just replace the whole thing. Any thoughts on this or finding a replacement based off the schematic?

Thanks again. I've learned a great deal with this thread.

 

[allsmith]

Thank you. Here's what I found:

For the wires entering the transformer:
125V Between Red and White
125V Between Yellow and White
0V Between Yellow and Red

With the black probe connected to the mains neutral lead I get 19V by touching the red probe to each of the four wires leaving the transformer. It seems strange that they're all equal.

It sounds to me that your transformer is working,(but im a noob still trying to learn this stuff,so i very well could be wrong)

Could you get us some farther out clear pics so we can see all the wires and where they go?

Are you sure you have the right scematic? if so it looks to me that all 4 wires coming out all are the same(it shows one wire coming out splitting into 4)

I have a audio power meter that i have had since 1982 that got hit by a lightning strike power surge that wont turn on no more,and would love to fix it.

I tried testing the transformer a while back and wasnt getting anything,so i google every part number on it and couldnt find anything! tried and tried but those numbers didnt pulled anything up!(so im not sure what to do now)

But i need to test it again because i may have been testing wrong(I have lots to learn!)

 

[Livin4]

It seems like this transformer is dead. Is it worth it to try and replace the heat fuse (that it may or may not have)? I think I'd rather just replace the whole thing. Any thoughts on this or finding a replacement based off the schematic?

Sorry, I do not understand your readings, my mistake as it looks like I did not specify exactly what to look for.

If your meter is digital, and you are measuring resistance, it will read close to zero if the windings are good, and "OL" or something similar if it is bad.

From the manual, you have the multi-voltage (E-Type) transformer, that explains the 3 primary wires. As you are in Texas, the voltage select switch should be set to 120V, and the primary winding you will be using is the Yellow-White.

So, with the power disconnected, if you measure between the yellow-white wires do you get close to zero ohms? Or is it an open as if the meter leads were not connected to anything?

If it measures open, the transformer primary windings are bad, if you get a resistance reading, the primary winding is good.

If the primary winding is good, then test the secondary’s by connecting the power, turn the unit on, and measure AC VOLTAGE between the red and black wires, then the yellow and orange wires. You should have readings, if the windings are good. If you don't, disconnect the power. Disconnect the secondary red wire, (make sure you note where it connects, pictures are great here) and measure resistance between it and the black. Then do the same with the yellow and orange.

Here again, if the readings are close to zero ohms, the windings are good, if they are open, they are bad.

The thermal fuse you refer to will be on the primary winding if it is there. Replacing it will be a problem as they have never really been easy to get. You will need to know the temp and current ratings, or if you are lucky, there might be a # on it. Not a great hope of finding a replacement.

The transformer, a quick Google search for the part #L01-2776-05 pulled up

http://www.encompassparts.com/model/KENBASICC2/_/_/Kenwood/BASICC2/

Don't know if it is available, but maybe.

 

[John Henry]

Thanks again. It's a digital meter and it does read 0,L for the readings above where I said Zero. I guess this means that there is infinite resistance, or no connection.

What is the next step to getting this running? I'd think I just need to try a new transformer since I'm going on the assumption that this one overheated. The one you found on encompassparts is out of stock. Would it be difficult to find a generic replacement? This unit does have the 110/220 switch, but I'd be happy to bypass that as 110 is all I use.

 

[kfalls]

It is unusual that you would get the same voltage on all output wires. Normally different transformer outputs would be different for different voltages needed for the circuit. Of course the regulators may bring the voltage down enough since all I see needed for the circuits are +/- 12.3v (I think there's also 18.5v in one circuit, but can't read it very well). I can't tell from the connections to the board which leads go to which regulator ICs. Read across pairs of the outputs (red/black, org/yel) or possibly the black is the output neutral and the others the output voltages, so reading by putting the common lead on blk and the + lead on each of the others would provide their output voltages.

A bad transformer usually shorts when bad either due to heat melting the varnish insulating the windings causing a short (close to 0 resistance) or the wire opening (OL resistance). I'm really surprised not to see some type of fuse or circuit breaker in the incoming voltage, or what we can see of the power supply circuit.

 

[John Henry]

It is unusual that you would get the same voltage on all output wires. Normally different transformer outputs would be different for different voltages needed for the circuit.

Sorry, that was my mistake with those voltage readings above. They weren't between the output wires, but between each output wire and the main ground.

Read across pairs of the outputs (red/black, org/yel) or possibly the black is the output neutral and the others the output voltages, so reading by putting the common lead on blk and the + lead on each of the others would provide their output voltages.

I get zero V trying all of these pairs.

 

[bktheking]

You should get continuity with the switch set to 120v between the yellow and white wires, if you do not chances are (and i'm a betting man) the thermal fuse inside the transformer is open. If you can get to the internals of the transformer, carefully remove the fuse and replace it with the same rating.

 

[John Henry]

You should get continuity with the switch set to 120v between the yellow and white wires

Thanks. I don't get continuity here.

How do you go about finding a replacement transformer? This is no longer available as a part number. Is there any hope of finding an available transformer that could do the same job? I do not understand how transforms are spec'd so I do not know what to look for. If anyone can point me in the right direction I'd greatly appreciate it!

 

[djmtascam]

I would disconnect (unsolder etc.) the primary leads (WHT/YEL/RED) from that voltage selector switch first and then use your meter set to resistance (low scale or autorange) between WHT and the YEL and see if you get a reading of a few ohms. It won't be very high like 10-20ohms
Sometimes those voltage selector switches go bad.
Then check your power switch disconnected without power and make sure the yellow wire is getting continuity and being switched.
OL is overlimit which is "open"
If there is continuity you will get a reading of like .5ohms..

 

[John Henry]

I would disconnect (unsolder etc.) the primary leads (WHT/YEL/RED) from that voltage selector switch first and then use your meter set to resistance (low scale or autorange) between WHT and the YEL and see if you get a reading of a few ohms. It won't be very high like 10-20ohms

I tried this and get no reading.

Then check your power switch disconnected without power and make sure the yellow wire is getting continuity and being switched.

You mean the 110/220 power switch, right? Testing this it shows the switch does work. Yellow goes from no conductivity when in the 220 position to 0 ohms when in 110 position.

I still have the three wires disconnected as suggested above.

 

[Livin4]

How do you go about finding a replacement transformer? This is no longer available as a part number. Is there any hope of finding an available transformer that could do the same job? I do not understand how transforms are spec'd so I do not know what to look for. If anyone can point me in the right direction I'd greatly appreciate it!

There are 2 you could use L01-2771-05 (120V), and the L01-2776-05 (120V/240V), the only site I found that did not outright say NLA was the one I posted earlier. Other than that, I can't find it online, I'm sure there is one out there, but finding it...

If I read it correctly, the Specs show yours at 0.15A current draw, so at 120V I think that equates to ~28VA, these 2 from Mouser might work.

http://www.mouser.com/Power/Transfo...5?P=1yzsnbwZ1z0wczcZ1z0wd2b&Ns=Power+Rating|0

My math could be off though.

These are not exact drop in replacements, and would require some custom wiring to install. The hard part here is your transformer has 2 secondary windings, 12V/24V I believe. While these are available, they are not easy to find. Also, the exact amount of current draw through each winding on the original is unknown, and so if you get one too small, you have the chance of it smoking like yours did. To large will not hurt, but you have physical space issues to contend with.

Hope this helps.

 

[djmtascam]

I think your looking for a new transformer....
Is it really worth it?