Inverter problem

Thread Starter

twister007

Joined Feb 29, 2012
43
I have a Kenwood Ts 520 transceiver that the 12V inverter has stopped working. I tested everything and the only thing suspicious was a transistor. They both tested good with a ohm meter. So I tested the hfe. One tested 60 which is what the data says it should be, but the other one tested 506 gain! I did the test twice so I think it's correct. I priced transistors but they are $25 each! Can I substitute. 4 5305 mosfets in parallel?0205201143.jpg
 

Thread Starter

twister007

Joined Feb 29, 2012
43
I have a Kenwood Ts 520 transceiver that the 12V inverter has stopped working. I tested everything and the only thing suspicious was a transistor. They both tested good with a ohm meter. So I tested the hfe. One tested 60 which is what the data says it should be, but the other one tested 506 gain! I did the test twice so I think it's correct. I priced transistors but they are $25 each! Can I substitute. 4 5305 mosfets in parallel?View attachment 198358
I forgot to mention that the transistors are 2n4049
 

AlbertHall

Joined Jun 4, 2014
9,281
It isn't going to work with MOSFETs.
I found a datasheet which says that the gain is between 60 and 180 but it will vary a lot with the collector current at which you measure it.
The 2N4049 is a germanium transistor so finding a replacement that is still being manufactured will be difficult/impossible.

Are the two by the same manufacturer?
 

dendad

Joined Feb 20, 2016
3,316
You may be able to go to Silicon transistors and increase R2 a bit to change the bias. But before you do, have you checked the resistor values and C1 also?
 

Thread Starter

twister007

Joined Feb 29, 2012
43
It isn't going to work with MOSFETs.
I found a datasheet which says that the gain is between 60 and 180 but it will vary a lot with the collector current at which you measure it.
The 2N4049 is a germanium transistor so finding a replacement that is still being manufactured will be difficult/impossible.

Are the two by the same manufacturer?
Yes, they are identical.
 

dendad

Joined Feb 20, 2016
3,316
As I mentioned, check the capacitor. In fact, just replace it. If the cap has dried out, and that is quite likely as the unit sounds to be pretty old, the base bias voltage may have lots of spikes on it and that could have killed a transistor.
Then again, the transformers may have shorted turns and that will stop the supply operating to.
Have you measured the DC resistance of the windings to make sure each half is identical to the other?
 

Thread Starter

twister007

Joined Feb 29, 2012
43
You may be able to go to Silicon transistors and increase R2 a bit to change the bias. But before you do, have you checked the resistor values and C1 also?
The resistors check exactly right. The capacitor checked good but I changed it anyway, but no difference. The transformer windings were hard to check because they ohm so low. They were only 1 ohm or less. I could be wrong but I think the torid transformer is good.
 

Thread Starter

twister007

Joined Feb 29, 2012
43
As I mentioned, check the capacitor. In fact, just replace it. If the cap has dried out, and that is quite likely as the unit sounds to be pretty old, the base bias voltage may have lots of spikes on it and that could have killed a transistor.
Then again, the transformers may have shorted turns and that will stop the supply operating to.
Have you measured the DC resistance of the windings to make sure each half is identical to the other?
The cap was changed even though it seem good. The windings all checked aproximently one ohm or less. No sign of over heating on the wires.
 

dendad

Joined Feb 20, 2016
3,316
While taking the clothes off the line (it just started to rain) I had a thought....
Check the secondary of the power supply. Those self oscillating supplies will often not work with a short on the output.
Look for shorted diodes and caps.
 

Thread Starter

twister007

Joined Feb 29, 2012
43
While taking the clothes off the line (it just started to rain) I had a thought....
Check the secondary of the power supply. Those self oscillating supplies will often not work with a short on the output.
Look for shorted diodes and caps.
I thought that too. This transformer is also wired for 120V. I tried it on ACV and it works but I need it to work on 12V.
 

dendad

Joined Feb 20, 2016
3,316
Have you measured the volts around the transistors? And made sure the heater switch is ok to supply the bias to the transistors?
 

Thread Starter

twister007

Joined Feb 29, 2012
43
Have you measured the volts around the transistors? And made sure the heater switch is ok to supply the bias to the transistors?
Yes, the heater switch voltage was there. There was voltage on 11 and 12 but I can't remember exactly what the voltages were now. I thought I wrote them down somewhere. The wires to the transistors are all loose now because I was going to try mosfets that I have. I don't have any silicon PNPs. I don't think that they make any with this much power anyway. If I remember right, one transistor was a little warm, but not what you would call hot. I have never seen a transistor have that much gain before. I suspect that is the bad one.
 

AlbertHall

Joined Jun 4, 2014
9,281
I have never known a transistor failure to cause increased gain - reduced gain, yes.

[Edit] How did you measure the transistor gain?
 

Thread Starter

twister007

Joined Feb 29, 2012
43
I have never known a transistor failure to cause increased gain - reduced gain, yes.
I measured them with a harbour freight multimeter. I did it twice. Both times I got the same results. I was surprised too. Maybe tomorrow I will measure them again. They both test good with a simple resistance test.
[Edit] How did you measure the transistor gain?
 

AlbertHall

Joined Jun 4, 2014
9,281
Germanium power transistors were (are) much more leaky than their silicon counterparts. I wonder if the meter was confused by this leakage.
 

AlbertHall

Joined Jun 4, 2014
9,281
Maybe that's why the gain was so high. But if it is leaky, would that cause it to turn on and not want to turn off?
The meter would be checking at a low collector current where the leakage would be more significant. In the inverter, the collector current will be much greater and the leakage becomes insignificant.

This inverter is self-oscillating and if the load on it is excessive that can stop it oscillating. I would be checking what the inverter feeds to make sure it is not overloading the inverter.
 
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