irf520 upgrade

[instagater]

I have a galaxy 98vhp. The finals in the amp section has burned out twice. I was wondering if anyone has found an upgrade part for the irf520. I know these are fairly new for amps, but i just thought i would ask?

[preacherman]

There is no upgrade part that I am aware of. If your finals have blown more than once,
there are a couple of things to check out to ensure better longevity in the future.
1. SWR is very important with high powered radios and amps. Even at 1.3 to 1, you are
may be sending more RF back into the radio than it likes. Grounding is also very important.

2. Those radios will do over 300w pep, but will provide a shorter duty cycle between failures
at that level. Keep your dead key below 50w and the radio under 250w. Heat is what often
kills these radios.

3. If a "swing kit" has been installed, you will be running higher average power than the radio was
intended to make. this will increase heat build up ... as will long key down times.

4. Once in awhile, you just get one that gets hot faster than the rest. Make sure it has adequate
ventilation and if need be, add extra fans. Yeah...I know...a PITB...but it is what it is. I owned
three of these over the last several years and haven't encountered difficulties, but I know some who have.

Good luck....
Preacherman

[instagater]

Thanks preacherman for the reply. It does have some sort of swing kit in it. One that i am not farmiliar with. It has a diode ran from the right leg of Q53 to the middle leg of Q55. I guess that it is a swing kit, it does the same thing anyway. I noticed today that Q39 has been desoldered also. I like the swing kit, but am wondering if i should resolder Q39 back in. I think this is my mod. limiter. I have another amp board i plan to put in it, but am waiting on getting an alignment done first. It was swinging 325w before the last final meltdown. Now it does about 220w peak. One of the big resistors on the amp board got hot enough to leak out both ends. I don't want to run it that way. Any suggestions on the Q39? Will it interfear with the swing mod, or should i just leave it the way it is?

I was running 1.5 to 1 on swr's and a dead key of 100w. I think this was causing my problems. swr's are down now and have been running about 65w dead key. I may just turn that down to 50w like you said.

[preacherman]

Q39 is the mod limiter. The radio will do just as much output and sound
cleaner with the limiter in place. I'd put it back in if it were mine.
Preacherman

[TheCBDoctor]

Hi instagater

Absolutely get rid of that diode that is jumping the two transistors. It is causing the MIc gain from doing its job. You are disabling the Mic gain; it is no longer linear. It goes from OFF to ON. As for the IFR-520 can be replaced with the IFR-2010, but there really isn't much difference.

100 watts dead key is too much along with the "Swing Kit." There is no such thing as a swing kit. It is just a way to separate an operator from his money.

Respectfully,

[Texas Mobil 219]

There is some good info in this thread. Two thrumbs up. So what is being said on the 98 vhp it just as good as a performer stock then it would be with a peak n tune???????

[TheCBDoctor]

Hi Texas Mobil 219

Except for a few radio models that require an alignment right out of the box, such as the DX-959, there is no such thing as a peak and tune. Most Peak and tunes are nothing more than disabling the audio limiter and dropping the dead-key for use with an amp.

You can be just as loud using a power Mic. If the engineers thought it was a good idea to add a diode to make the radio swing to the point of distortion they would have added it. Even the illegal exports conform to the physics behind radio theory, although not the legal restrictions.

I would invest in a decent radio and concentrate on the best antenna system I can get. That is the trick to getting out besides raw power in watts.

Just my humble 2 cents,

Respectfully,

[instagater]

Hey cb doctor, thanks for the replies. You said that diode disables my mic gain knob. Could you explain that in a little more detail? I am not exactly sure what the diode is doing. I thought it was just a swing kit. Also do recommend anyone to do an alignment for me?

[sparky17]

To answer the question on IRF520 upgrades.

There's a 'mosfet upgrade' kit (listed on cbtricks), I think the EKL or something, that has a beefier IRF520 with better heatsinking.. I doubt they sell the mosfets by themselves however. I seem to recall it was 30-40$ for each 'kit'. soo..

Also, the IRF 5xx line goes through the whole range..

There's an IRF530, 540, etc.. The higher number ones handle more power and have less resistance.. I have no idea if it's a drop in replacement however (output transformers or whatever).

I would guess that the IRF530 is just a higher grade bin for the IRF520's.. (When they make chips, they test them, and good ones get sold as the premium model, and the crap ones go to the bargain bin model). Less resistance, less heat, more efficient, hence the higher amperage rating for a part that is mechanically identical..

[Night Crawler]

You said that diode disables my mic gain knob. Could you explain that in a little more detail?

I'd like to know that too.

[TheCBDoctor]

Hi

You said that diode disables my mic gain knob. Could you explain that in a little more detail?

I'd like to know that too.

The diode placed in the export radios at the Am modulation transistor prevents the Mic Gain from performing with a linear function. It basically makes the Mic Gain go from low to almost on once you get past 10 o'clock. Anything beyond that can not give you more power. It does make your Talk-Back get louder beyond that point, so the operator does think it is doing more, but it isn't.

In the sideband mode the circuit is disabled and holds the voltage at 13 volts at the Final rather than 6 volts found in the AM mode. The modulation is low level on SSB and is done at the balance modulator. In the AM mode the Modulation is done at the Final and Driver by modulating the carrier using the Modulation circuit. The diode screws up the circuit and makes the carrier look like crap.

Respectfully,