GP300 No Transmit

goldvetter · Post by **goldvetter** » Sat Dec 03, 2005 6:07 pm

I have a GP300 VHF model that recieves fine but no transmit. The transmit lights and with a scanner on, it can be heard within a few feet of the scanner. Also I know the battery is good because it works fine on other GP300 radios.
Any help is appreciated.

va3wxm · Post by **va3wxm** » Sun Dec 04, 2005 7:46 am

Sounds like the PA. What you're hearing is the "exciter" stage (10 mW or so). That's why you only hear it within a few feet.

goldvetter · Post by **goldvetter** » Sun Dec 04, 2005 9:20 am

well thats what I was thinking. Is this easy to replace and where can I get the PA.

k2hz · Post by **k2hz** » Sun Dec 04, 2005 12:39 pm

goldvetter wrote:well thats what I was thinking. Is this easy to replace and where can I get the PA.

It is not user replaceable. It is a major shop repair and realignment job.

va3wxm · Post by **va3wxm** » Mon Dec 05, 2005 5:27 am

Might be cheaper just to pick up another one here or on Fleabay.

Tony Soprano · Post by **Tony Soprano** » Mon Dec 05, 2005 5:54 am

Don't throw it out just yet, there are a number of good reasons why a GP300 won't TX that don't involve the PA...if the antenna contact spring isn't making contact with the with the housing, you will get the same result.

Take the chassis out of the housing and make sure the contact spring isn't bent out of shape, then carefully re-install it in the housing. VHF PA's don't fail very often, there could be other easier options here.

Tony

goldvetter · Post by **goldvetter** » Mon Dec 05, 2005 1:40 pm

Ok, thanks for the info. I have checked it and it seems to be ok. What are the other possibilities?

Post by **wavetar** » Mon Dec 05, 2005 5:33 pm

If there's any way you could test it on a 7.5 volt power supply with a current meter, it should draw close to 2 amps while transmitting if the power amplifier is working. It'll draw almost nothing if the PA is shot. Replacing them in the GP300 really isn't that big of a deal compared to other radios...I guess the trick would be to find one cheaply.

Todd

goldvetter · Post by **goldvetter** » Mon Dec 05, 2005 7:26 pm

It hardly pulls 500 mA on a meter. That must be whats wrong. Where can I find the part and how to install it. Thanks in advance.

Tony Soprano · Post by **Tony Soprano** » Tue Dec 06, 2005 5:45 am

One thing left that bears mentioning is the PA keying transistor, Q154. It turns on the 7.5 volts to the PA. There is also a chip capacitor that lives next to the heatsink on earlier models that cracks, killing signal to the PA. These are common failures, but if you don't have a schematic to know where to look for them the point is moot anyway. IMHO, this radio should visit a good radio shop. Even if the PA turns out bad, the radio should still be aligned.

Tony

Post by **wavetar** » Tue Dec 06, 2005 10:24 am

Tony Soprano wrote:One thing left that bears mentioning is the PA keying transistor, Q154. It turns on the 7.5 volts to the PA.

Tony

I am familiar with that keying transistor & have replaced a few over the years...but if it goes, does it not kill the TX light? I can't recall, but it seems to me there's no TX light when that transistor is bad...I haven't come across the capacitor problem yet.

Todd

Tony Soprano · Post by **Tony Soprano** » Tue Dec 06, 2005 11:00 am

When Q154 goes, the TX light will still come on.

The chip cap, C116 couples the RF output from the VCO to the PA input. It lives next to the bottom of the heatsink where it contacts the board. If the heatsink gets shifted to the left just enough, it will fracture the cap, therefore you are just running off the VCO signal. Later versions of the board have this cap in a different location.

One thing I have found is that, usually, if the PA is bad, it will still heat up but have no output, although this is not always a given. One thing to try maybe, is to TX for a while, maybe up until the time-out timer. If the PA gets warm but has no output signal, it's probably the PA. If it stays cold, it probably is fine.

Where I'm going with this is that there are too many variables at play to say "yeah, your PA is bad" without getting a good look at it.

Bat2way · Post by **Bat2way** » Tue Dec 06, 2005 11:30 am

It is not user replaceable. It is a major shop repair and realignment job.

Yes...It's easily replaceable! Just tired of seeing posts like this from people who don't know s from s. This comment is what the hourly tech said at an area MSS every time a radio came in for repair that didn't have a bad battery. They have recently closed. Suppose the $125.00 per hour rate or just technical incompetence the reason?

k2hz · Post by **k2hz** » Tue Dec 06, 2005 1:32 pm

Bat2way wrote:
It is not user replaceable. It is a major shop repair and realignment job.
Yes...It's easily replaceable! Just tired of seeing posts like this from people who don't know s from s. This comment is what the hourly tech said at an area MSS every time a radio came in for repair that didn't have a bad battery. They have recently closed. Suppose the $125.00 per hour rate or just technical incompetence the reason?

I have been involved with 2way service for over 40 years. Note I said it was not "user replaceable" responding to an inquiry from a person who did not even know what a "PA" is. Yes it is a straightforward repair for a competent technician with the proper tools and the equipment to perform the necessary test and adjustments after repalcement. I stand by my statement that it is "shop" bench job requiring test and retuning and not a "user" repair.

k2hz · Post by **k2hz** » Tue Dec 06, 2005 3:39 pm

A few more comments on why I believe the radio needs to go to a competent shop:

The reported fact the the receive is apparently OK and the transmit signal is so extremely weak would indicate it was not an antenna contact problem and this has been verified by inspection.

As Tony Soprano pointed out, there are too many variables to be sure that the PA itself is bad without tests that can only be done at a shop with the proper test equipment. It could be a less expensive component.

If proper procedures are not followed in replacing the PA, there is a good chance of destroying the new one and/or causing further damage to the radio.

I have no problem with helping a user do some simple troubleshooting to avoid an unnecessary expensive repair bill. However, I have seen too many cases where "do it yourself" repair or programming attempts have
turned what would have been a minimum charge repair into a destroyed radio.