I think I blew a 2sc4910 output transistor made by sanyo in my rc transmitter. My friend left the antenna collapsed and it probably overheated from the missmatch. Is there a way I could test or measure the trans if it is really blown without taking it out of the pcb?

It is AM 75MHz and in the specs sheet of the tx says 150mA for current draw. I cannot find any retailers for this transistor so if it is blown, I have to find a replacement that will work. My question is, what specs are the most important for these transistors and what to look for when choosing a substitude? I've seen simple FM circuits with BC547 and 2N3904 finals above 75mhz, will these work in my transmitter? Any suggestions?

2sc4910 datasheet:
http://www.datasheetcatalog.org/datasheet/sanyo/ds_pdf_e/2SC4910.pdf

Thanks a lot!

 

Your transistor runs 150 ma in the base for 1.2 amp in the collector. A 2N3904 would vanish in a puff of smoke.

This is a specialized transistor that includes an emitter ballast resistor in the package - looks like TO-220 without the tab. Could be interesting to replace. You could try a parametric search.

Have you tried to transmit yet? If the transistor is toast, it's not too likely that more harm will be done. Just applying power and seeing if the transistor heats should tell you something.

If you want to check it, do so out of circuit. The B-E junction will check funny because of the ballast resistor.

 

Thanks for the reply!

The 150mA I was referring to is the total current drain of the transmitter with 195.0Mw output power. I think this transistor is way overhead of what is needed. It is a 2ch JR XR2 pistol tx.

I tried to transmit but it's not working. Could be the receiver too, but I don't think so. The transistor is not heating up and I don't measure anywhere near 150mA from the battery either, which is why I suspect it's the transistor.

What is the purpose of the ballast resistor? Any chance with a transistor that can handle the 75Mhz?

 

Without the schematic, it's just guesswork. Perhaps someone who is more familiar with this transmitter will chime in.

 

if you should at be able to see DC wise if the part is still working. measure Vbe and Ic and that will give you some rough idea.

as to replacement, this looks like a fairly beefy part. But if your transmitter isn't tramitting lots of power, you can drop in a 2n5551 (.6w, 300mhz, 600ma). it should have no problem at 75mhz and is a penny each.

 

A quick circuit inspection and measurements:
Base is connected to ground by two paralell resistors and two parallel capacitors. It is also decoupled to collector by an other cap.
Collector is connected to Vcc straight with a 2.2uH inductor. The antenna is connected to the collector through a matching network which leaves the modulation hooked up to the emmiter. Tough to track all the traces, but I'll try to make a schematic if it helps.

Vcc and Vc: 9.71
Ve: 0.54V
Vb: 35mV
Vce: 9.14
Vbe: -0.51V

Thank you for your help!

 

the amp is reverse-biased, Vbe=-0.51v (this is a npn transistor). this by itself may not be a problem since most radio power amps are class C.

your hope is that the prior stage will pull the emitter low enough for the transistor to conduct some of the times at least. But with the base at 35mv, that's unlikely.

is there by any change the base has a bias scheme that is not working correctly now?

 

I might have switched the orientation of the transistor. Looking at the datasheet it might just be that E is at 35mV and B is the one modulated with a 0.5V bias.

The signal transmitted is a 1-2ms pulse about every 20ms per channel. A single MOD wire along with VCC and GND comes in from the digital circuitry to the RF stage. Is 0.5V at the base too low for biasing? I mean the baseband should be transmitted along with the pulses and the power amp is biased at cutoff. Am I measuring something wrong? I am familiar with analog audio AM signals but I'm not sure about digital RC signals.

 

Frakk, that makes sense. I wouldn't worry about 0.5v Vbe: it like runs in class C so that makes sense.

 

In the morning I got the TX and RX working together. I took a quick measurement and Ve was changing around 1.5V and Vb was around 0.5V and a bit up. I came home from work, it still worked when I turned it on. I screwed the RF board back in the TX and it stopped working and remained so. This is strange. I will keep trying it but something is definetly wrong.