Dead final, or something else?

[sjdev...@gmail.com]

My RS-HFIQ has started having problems. When I first turn it on, it transmits, but within a few seconds the power output starts oscillating and then drops to some very low level. It also gets very hot even after I turn the TX off again. At first I assumed it was a dead amplifier, but the fact it comes back to life briefly made me suspicious.

I checked the power draw on my supply. When I first turn on the switch, it's about 150 mA. Transmitting when it's working draws about 300 mA. When it malfunctions, it drops to about 200-250 mA and stays there even after TX is turned off. In fact, it stays at 200 mA even after flipping the on/off switch to off! I have to physically remove the power cord from the jack to return it to normal.

I also noticed the plastic case on L11 has cracked open, and the case on L4 is starting to crack. I doubt this affects anything.

Any ideas?

-Stephen DeVience

N8URE

[jcve...@gmail.com]

Interesting. Does it happen on every band? L4 is part of the 20/30 LP filter and L11 10/12M LP filter so if it behaves on other bands, you may have found your trouble.

Does it RX OK? If so that eliminates many sections of the circuit for potential issues. Also look at L32, L34, L35, L36 and L37 on the bottom of the board.

73,

Jim WA2EUJ

[sjdev...@gmail.com]

I just tried to RX on 80m and I'm not getting anything, so RX seems to be dead too.

TX on 17m has the same problem. I'll look at the chips again tomorrow. I didn't notice any other ones with problems when I looked earlier.

-Stephen

N8URE

[sjdev...@gmail.com]

I take back what I said about RX. It looks like RX is working, but last night something got stuck and didn't change the frequency.

[sjdev...@gmail.com]

I did some more testing, this time with TX on but no audio input. I found that even with no audio, there is a significant signal being transmitted at the tuned frequency (equivalent to 0 Hz audio frequency). On 80 m, it was around 15 dB smaller than a full power tone signal. 

I transmitted with no audio input on 80, 40, 20, 15, 12, and 10 m bands. In all cases, the spurious signal is transmitted for 30 seconds to 1 minute before it stops and the circuit goes into the problem state.

-Stephen

N8URE

[sjdev...@gmail.com]

I replaced the final and it's working again.

I figured out why it continued to pull current even after turning off the switch. The amplifier is connected directly to Vin, not +14V through the switch, so once it got into some broken mode it stayed there until the power cord was pulled. The switch had no effect.

I might have killed it because I was working satellites duplex through a transverter, so I left the TX on even when not talking into the mic. I was under the apparently mistaken impression that this wouldn't count toward the duty cycle if there is no other input, but I guess that's not true. I notice that just turning on TX even with no input heats up the amplifier regardless. 

There is still a signal at the VFO frequency even when there is no input, and I don't understand why. It's around 20 dB below the full output power. 

-Stephen

N8URE