UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Superstar 6900N V6

Mike. Watterson · 24th May 2020, 1:39 pm

Note to self:
Do not test bais power FETs with long leads and no decoupling and RF chokes at the pins! Even on a giant heatsink.

I have only IRF510 not IRF520, but principle is the same. These are designed for switching stuff on/off, or low voltage SMPSUs. Not simple to use linear.

I set PSU to 6V and current limit to 1A

Practically off to 2.5V
0.1A @ 2.5
0.1A @ 4.75
Between 5V and 5.1V, oscillates, harmonics interfere with nearby VHF radio
>3A @ 5.3V. First PSU current limiter dies.

PSU voltage regulator now dead, +32V out.
The FET is now short on all pins.

I'd a memory before that the bias on these was super critical for linear operation and needed changed for different FETs. The FET will be most unstable with no carrier (SSB, no Audio). With FM and AM, it's being driven off on every cycle.

I'd suspect you MUST realign the bias pots at the driver and output for whatever the quiescent current is supposed to be on AM, which can be calculated given TIP36 o/p voltages for low, med and high RF power, assuming a 50% efficiency and the stated AM power that those settings is supposed to have, must be about 4W to 5W AM RF at 6V approx (12V supply). Thus current per o/p FET must be about 750mA max (maybe 500mA to 1A is OK?). The driver FET can't be much less as it has to be in the linear region and the gain per FET stage must be poor due to the gate capacitance.

I'd imagine the service instructions for bias for replacing the IRF520 FETs (single or dual FET output) on ANY CB rig with AM will be the same. Basic maths of the standing current needed at 6V for AM for about 2W to 2.5W per device.

Well, I learnt something about ill advised use of 20cm test leads on a FET without decoupling at the pins and series chokes on supplies near the pins.

Now I need to repair my PSU's regulator. I may add an RF filter between the o/p +connector and the PCB. I already added reverse voltage protection diodes on all three outputs when I had to replace the +12V and +5V fixed regulator outputs. I've an IC regulator that does up to 40V in 32V out and 5A, so if I can't fix it, I'll fit that. I think there is a relay to use a lower input voltage at lower outputs to reduce cooling issues.

24th May 2020, 1:39 pm	#88
Mike. Watterson Heptode Join Date: Dec 2014 Location: Limerick, Ireland. Posts: 901	Re: Superstar 6900N V6 - Fault with SSB mode Note to self: Do not test bais power FETs with long leads and no decoupling and RF chokes at the pins! Even on a giant heatsink. I have only IRF510 not IRF520, but principle is the same. These are designed for switching stuff on/off, or low voltage SMPSUs. Not simple to use linear. I set PSU to 6V and current limit to 1A Practically off to 2.5V 0.1A @ 2.5 0.1A @ 4.75 Between 5V and 5.1V, oscillates, harmonics interfere with nearby VHF radio >3A @ 5.3V. First PSU current limiter dies. PSU voltage regulator now dead, +32V out. The FET is now short on all pins. I'd a memory before that the bias on these was super critical for linear operation and needed changed for different FETs. The FET will be most unstable with no carrier (SSB, no Audio). With FM and AM, it's being driven off on every cycle. I'd suspect you MUST realign the bias pots at the driver and output for whatever the quiescent current is supposed to be on AM, which can be calculated given TIP36 o/p voltages for low, med and high RF power, assuming a 50% efficiency and the stated AM power that those settings is supposed to have, must be about 4W to 5W AM RF at 6V approx (12V supply). Thus current per o/p FET must be about 750mA max (maybe 500mA to 1A is OK?). The driver FET can't be much less as it has to be in the linear region and the gain per FET stage must be poor due to the gate capacitance. I'd imagine the service instructions for bias for replacing the IRF520 FETs (single or dual FET output) on ANY CB rig with AM will be the same. Basic maths of the standing current needed at 6V for AM for about 2W to 2.5W per device. Well, I learnt something about ill advised use of 20cm test leads on a FET without decoupling at the pins and series chokes on supplies near the pins. Now I need to repair my PSU's regulator. I may add an RF filter between the o/p +connector and the PCB. I already added reverse voltage protection diodes on all three outputs when I had to replace the +12V and +5V fixed regulator outputs. I've an IC regulator that does up to 40V in 32V out and 5A, so if I can't fix it, I'll fit that. I think there is a relay to use a lower input voltage at lower outputs to reduce cooling issues.