Here is a report of an unusual failure in a FlexRadio 5000A, which presented with apparently correct receiver operation but no transmit. The initial advice I received was that the radio was working when removed from service, and that PowerSDR was not properly communicating with the transmit system. However, that seemed unlikely given the correct receiver operation and testing the same instances of PowerSDR with a FlexRadio 1500. The worry turned to a potentially failed LSI chip, such as the main codec. The solution turned out to be simpler and much less expensive. Hopefully, this information might help others resurrect a seemingly dead radio.
Symptoms:
- Correct receiver operation in all modes and frequencies using all versions of PowerSDR
- No power output in any mode at any frequency.
- The local oscillator was operating at the frequency set on PowerSDR, as confirmed by an oscilloscope and hearing the signal in a nearby receiver.
- A tiny, unmodulated carrier signal reached the input of the PA board.
- Power controls on the PowerSDR do not affect any measurement in the transmit chain, including the residual signal reaching the PA driver input.
- The power amplifier board was operating correctly, with appropriate gain. No faults in the LPF, driver, or PA stages.
- No modulation of the carrier signal in any mode by any means.
- No CW output with the key connected to the radio.
- No output from the speaker or headphone jacks.
- No failures of the FireWire drivers or protocol were noted.
- PowerSDR was operating correctly in all other respects making a selective failure in all transmit modes unlikely.
All of these indicators point to a failure of operation of the codec CS42448 as the common element. Among other functions, the CS42448 drives the speakers, headphones, as well as providing the transmitter drive. The fear was that it had failed. However, this chip in this circuit has at least six separate 3.3V power supply connections to isolate the internal subsystems for noise reduction. The failure of any of these supplies would likely result in the observed symptoms. Basic service philosophy suggests checking al power sources before testing other functions.
Upon testing all the supply connections, it was found that the output of IC13, a TPS79333 3.3V LDO voltage regulator, was 1.5V at TP9 and on pin 5 of IC13. IC13 should have put out 3.3V regulated from a 5v rail. An in-circuit resistance check at the output of IC13 showed about 50k ohms to ground, suggesting that IC13 had not failed due to a load short. Removal of the TPS79333 revealed it had failed open, and the 1.5V measured in-circuit was leakage from the power supply connection to the CS42448, pins 53 and 44, which are connected in parallel. IC13 also supplies power to some other downstream amplification elements.
- HTRX B-0026 & 30 Rev F1R Schematic.jpg (840.7 KiB) Viewed 30128 times
Replacement of the TPS79333 immediately restored full operation, resolving all transmit issues. No cause for the failure could be determined. The relevant portion of the circuit diagram is below. IC13 is a small SMD device in a crowded area fed by very fragile traces. Removal and replacement require proper SMD rework equipment. Repair with standard soldering equipment might be possible, but risks PC board trace damage and displacement of nearby components.
James Todd
WB0KWJ