Installed new VRF2933 - Can't adjust bias

[Mike Saeger]

Somehow I blew the tops off the original transistors. 

New Transistors arrived. I installed them and tried to set the bias.

No matter how much I turn the bias pots, the display remains at 30ma.

Bad transistors or did something else blow when the finals blew?

[Geert Jan PE1HZG]

Hi,

Sorry for responding after this time - I didn't see other responses, so perhaps this is still useful. For me, google groups is more difficult to use than some other forums but let's not discuss the lay of the land.

Probably like you, I am worried connecting a 60 volts, 20amps power supply to some expensive semiconductors "that can go pffft", even though they have survived all but one of my operator mistakes.

In my recovery (see my earlier mail), what I did, was:

1. Make sure both BIAS pots are fully CCW to make sure BIAS is set to MINIMUM
2. Disconnect the radio from the RF input of the amplifier. We are *not* providing RF drive here!
   
3. Disconnect the main power supply to the AN1819 board, and replace it with a lab power supply with current limiter. I found 15V enough, and the current limit is something you should be comfortable with - start with 100mA or so. Make sure the ends of the HR500 power supply you disconnected are isolated and don't touch anything - I put some tape over them, just to be safe.
4. Switch on the HR500 amplifier.
5. Switch on the lab power supply. There should not be any current flowing on the lab power supply.
6. Switch the "Set BIAS" on the LCD panel menu. There should still not be any current flowing on the lab power supply.
7. Gradually turn up one of the BIAS pots (turn CW,). At some point, the lab power supply will show current flowing. Don't worry - because of the current limit of the power supply, the most current that can flow is 100mA.
8. If you get current flowing, you know that this part of the BIAS circuit works, as well as the transistor being healthy. Put the BIAS pot CCW again
9. Gradually turn up the other BIAS pot (turn CW,. slowly). At some point, the lab power supply will show current flowing. Don't worry - because of the current limit of the power supply, the most current that can flow is 100mA.
10. If you get current flowing, you know that this part of the BIAS circuit works, as well as the other transistor being healthy too. Put the BIAS pot CCW again.
11. If you can't get BIAS flowing, check the circuit. "Set BIAS" should switch +12V to pin 2 of the JP2 connector of the AN1819 board. This comes from the controller board (Q2, Q3) which you can debug w/o the expensive AN1819 board connected to power at all. You should be able to switch a LED w/ 1k resistor on and off using the set BIAS menu thing
    
12. If both transistors have demonstrated to be able to draw current, then the circuits and the transistors are OK.
13. Switch off the lab power supply, as well as the HR500. Alllow the HR500 power supply to bleed down (measure!).
    
14. Check one more time that both BIAS pots are fully CCW.
    
15. Disconnect lab power supply and reconnect HR500 power supply to AN1819 board.
16. You should be able to do the bias adjust procedure now per section 15 of the assembly manual, after which the amp should be OK - it was in my case.

I guess the thing is checking out everything wile minimizing the chance of expensive damage - the lab power supply helps. BTW, I used the same method to check out the HR500 after I built it.

Let us know if you solved this.

Happy 4th july,

Geert Jan PE1HZG

[jcve...@gmail.com]

Hi Geert,

Were you able to set the bias with the procedure below?

Check to make sure that you have the proper drain voltage.

Check the voltage at the gates of the MOSFETs, you should be able to get over 5V when you turn the bias pot.

If you have 58V on the drain and 5V on the gate and no current is flowing then:

1. the current measurement system is faulty.

2. the MOSFET is faulty.

73,

Jim WA2EUJ

[Mike Saeger]

The Chinese VRF2933s that I ordered from eBay were cheap but shorted -- reason I couldn't adjust the bias. I returned them for full refund. I ordered new VRF2933s from Mouser and after installing them everything is back to normal. The amplifier is working GREAT again. I'm using it with a TAPR Hermes SDR transceiver installed in an Anan-10 enclosure. After installing the 5W HobbyPCB amplifier inside the Hermes, it drives the Hardrock-500 to full power.