RF Concepts RFC 2-117 Troubleshooting

Hello,

I recently purchased an RF Concepts RFC 2-117 amplifier. Upon receipt, I wired it to an Astron power supply (14.3 V). With nothing else connected (no antenna, no load, no keying line) and the amplifier power switch in the off position, I turned the power supply on and quickly noticed that current was being drawn, maybe around 10 Amps, so within a few seconds I turned the power supply off. Shortly after that an odor was noticeable.

So I opened up the amp and noted two visibly burnt resistors. There is one near the MRF248, which I believe is a 10 Ohm resistor, either R32 or R34. And the other is near the MRF450 and appears to be another 10 Ohm resistor, R107. As they sit in the circuit, the former currently reads ~150 Ohms and the latter reads about 6 Ohms. The power transistors do not appear to be matched, but I venture to guess this was repaired one or more times in the past.

I triple checked the input polarity for the power leads - red to (+), black to (-). I suppose it is possible there is a problem with the polarity of the leads within the Cinch Jones connector that came with the amp. Does anyone know what the polarity of each of the pins on the Cinch Jones should be?

The 35 Amp fuse on the back of the amplifier is still in-tact.

Can anyone comment on the 'draws current with the switch in the off position' condition?

Tips are welcome on where to look for broken solder joints or similar that could have been damaged in transit.

Any guidance on troubleshooting is appreciated.

Thanks in advance for your input.

- Patrick, KJ2G

 

With the power switch off, the only things that should draw current if defective are C53, C54, and the reverse polarity protection diode D19.
Check the Cinch connector with an ohmmeter to the power switch and chassis ground.

 

The Collector of the finals is wired direct to the incoming 13.6v after the fuse as is the driver and it looks like the driver and both finals need to be replaced.

 

Hi,

It looks like there have been some changes to the bias circuits on the Two MRF 248 (SRF3897) devices. There should be one (1) ten ohm resistor not the two seen in the picture on each transistor. Unless my eyes are bad, it looks like the two (2) of the three (3) 22 ohm resistors connected to the transistor bases have been changed to 10 ohms. There should be only two 10 ohm resistor in that area not four (4).

The bias circuit for the MRF 450 seems to be normal other than the burnt 10 ohm resistor. Would it be possible to get better pictures of the two pictures that have "burnt" in the title?

As for the pin assignments on the connector, you will need to follow the trace from F1 (35 amp) to the connector pins of J1P1 to see if the polarity is correct.
Use an ohm meter to verify positive is connected to the proper pin on the power cable.

 

Yep, I missed that.

 

Thanks for this note. For a current-drawn while off condition, I agree it makes sense to focus on routes that bypass the front panel power switch S2. Attached, I highlighted all of the +13.6V points that I could find on the schematic. Any thoughts on what would allow current to flow in such a way that R107 in front of the Q101 base and R34 on the Q10 feedback circuit could be burnt up? Is it common to have failed power transistors for both stages?

Upon further inspection, C58 or C59 is also burnt - if we allow Q10 to be the MRF248 device.

 

I verified the continuity of the power jumper's positive lead to the backside of F1 was less than 2 Ohms and the negative lead to the circuit board ground was less than 2 Ohms. The resistance between the positive and negative leads was 135-140 Ohms. More on this to follow.

 

If C58 or 59 is showing signs of heat, it has failed and that may be the cause of the 10 ohm resistor failing.
This amp has been modified by adding the 10 ohm resistors. It probably is not biased properly.
It would be very difficult to maintain balanced queiscent current through Q9 and Q10 because of the mismatched transistors.
Anyway, my two cents worth.

 

Attached are closer photos of each portion of the overall board. Please let me know if photos with different lighting or angles would be helpful.

I tried following portions of the schematic relative to what is present on the board. Deviations are listed below, i.e. if it was present and accounted for then I have left it off of this list.

NIS = not in schematic

First stage? Starting at C102, ending at T branch near C112:
- could not find where trace goes from C113 to R110 - maybe on backside of board?
- NIS: near C109, there is an additional capacitor to ground (10 uF electrolytic)
- C112 is present, but not as "27", instead a block & screw-style adjustable

Second stage? Starting at T branch near C112, ending at +13.6:
- C31 not found - no PCB through-holes nearby either
- C32 not found - no PCB through-holes nearby either
- R35 not found
- NIS: near C34, 1x"150" red ink capacitor to ground
- NIS: near C46, 1xcapacitor (one side "123 AHC", other side "104 C1K", I think - a little difficult to see)
- C40 not found

For the second stage, the lower branch (Q10) was noted in detail above, but the upper branch (Q9) is symmetric.

So I agree the schematic does not line up perfectly. The changes present seem to have generally been done with purpose - perhaps by the manufacturer. On the other hand, some of the deviations may have come about later, perhaps for compatibility between the MRF248 and SRF3897 for example.

Let me know if I can provide more details on any features of interest.

 

I will look at this and let you know what I think. It might take a day or two.