Sometimes it's okay to give up 😀 I don't know, maybe one pin of the pic needs a missing reference voltage or, as other people commented, the diodes are leaky. Good set of videos by the way.
I have a HP 8640B with no RF output. The Power meter light does not turn on. The unit turns on but no RF output and no counter output movement when the internal frequency dial is turned. The power supply LEDs are and no -5.2VDC. I removed the RF Divider Filter Assemble and the -5.2 VDC is correct. Can you please provide suggestions on what is drawing the voltage down to 0VDC? The rectifier boards and filter caps are good. Al
I’m sorry for the sad end…To add an idea, it looks to me like it’s an analog problem. You are not reading the right amount of power, this might be that the incoming signal it’s not going through as it should. This also could be the reason of failing frequency reading, being not enough to satisfy the threshold of frequency meter section. I’m not familiar with that unit, but usually there is a rectification by fast diodes that can go bad giving enough signal to woke up PIC but low for the measures. Also bad coupling caps in the signal path might fail, the input circuit is called to face high power after all. Interesting difficult repair attempt, thanks for sharing it. 👍
please re-check the wake-up signal. according to the schematic, it's a square signal, with the frequency identical to the input signal, not a DC signal. Also 100 mV for 100 watts input is low, check the SWR bridge. You are transmitting into the dummy load, SWR can't be 3 !!
Tnx for your comment. The wakeup signal IS a DC signal, because the driver for the PIC input pin 28 is Q33 which is an open collector circuit. The input of the PIC however is a TTL input which requires either a push-pull ("totem pole") configuration or at least a pull-up resistor (100 to 150Ohm) for 30MHz square wave. Both are not existing so the collector of Q33 goes down and doesn´t go up during the half wave of the input signal. Additionally the base of Q33 is driven with a serial resistor of 10k which is not good for HF. SWR bridge: I think that is the problem - more will follow.
Sehr ärgerlich Stefan, wenn man soviel Zeit umsonst verbraten hat. Hab mir mal das Schaltbild runtergeladen. Die 150 mV Vorwärtsspannung für die SWR Messung scheint mir sehr wenig zu sein. Kann es sein, dass irgendwas mit dem T1 f. SWR Brücke oder mit den Schottky Dioden D43 D44 ist ?. Vielleicht ist ja auch was mit der Kompensations- Schaltung um VC1 nicht OK. Was die Frequenz betrifft: Hast Du schonmal an U2 (74HC14) am Pin 2 oder Pin 6 gemessen ? Dort müsste doch eigentlich ein gutes sauberes Rechteck HF Signal zu messen sein. 73 de DL1LAJ Andreas
I agree that it could be the diodes d43/d44. I had same problem with an other unit and it where this diodes in that part of the circuit. They were leaking . I could not measure it but just replaced them an it was working again.
Sometimes it's okay to give up 😀 I don't know, maybe one pin of the pic needs a missing reference voltage or, as other people commented, the diodes are leaky. Good set of videos by the way.
There will be a part 4 - pse wait
Thanks for keeping it real.
Great job. 73!
I feel your pain Stefan...working on my R&S FS300 spectrum analyzer without shematics is becoming an exercise In futility!
你查看Fwd 电压是否正常。这个很关键。另外我查看我的pic182520第六脚波形也是乱的 但是他可以正常工作
I have a HP 8640B with no RF output. The Power meter light does not turn on. The unit turns on but no RF output and no counter output movement when the internal frequency dial is turned. The power supply LEDs are and no -5.2VDC. I removed the RF Divider Filter Assemble and the -5.2 VDC is correct. Can you please provide suggestions on what is drawing the voltage down to 0VDC? The rectifier boards and filter caps are good. Al
Maybe a short on the -5VDC rail?
I’m sorry for the sad end…To add an idea, it looks to me like it’s an analog problem. You are not reading the right amount of power, this might be that the incoming signal it’s not going through as it should. This also could be the reason of failing frequency reading, being not enough to satisfy the threshold of frequency meter section. I’m not familiar with that unit, but usually there is a rectification by fast diodes that can go bad giving enough signal to woke up PIC but low for the measures. Also bad coupling caps in the signal path might fail, the input circuit is called to face high power after all. Interesting difficult repair attempt, thanks for sharing it. 👍
That´s not the end - think i found the problem. SWR Bridge
@ that’s very nice! 👍
Hi. This is Monty W5DN. 40 years in service business. This happens to all techs. Good try.
Hello Stefan - that must be heart-breaking not to find the fault or collection of faults. However MFJ did not make good equipment.
please re-check the wake-up signal. according to the schematic, it's a square signal, with the frequency identical to the input signal, not a DC signal. Also 100 mV for 100 watts input is low, check the SWR bridge. You are transmitting into the dummy load, SWR can't be 3 !!
Tnx for your comment. The wakeup signal IS a DC signal, because the driver for the PIC input pin 28 is Q33 which is an open collector circuit. The input of the PIC however is a TTL input which requires either a push-pull ("totem pole") configuration or at least a pull-up resistor (100 to 150Ohm) for 30MHz square wave. Both are not existing so the collector of Q33 goes down and doesn´t go up during the half wave of the input signal. Additionally the base of Q33 is driven with a serial resistor of 10k which is not good for HF.
SWR bridge: I think that is the problem - more will follow.
Sehr ärgerlich Stefan, wenn man soviel Zeit umsonst verbraten hat. Hab mir mal das Schaltbild runtergeladen. Die 150 mV Vorwärtsspannung für die SWR Messung scheint mir sehr wenig zu sein. Kann es sein, dass irgendwas mit dem T1 f. SWR Brücke oder mit den Schottky Dioden D43 D44 ist ?. Vielleicht ist ja auch was mit der Kompensations- Schaltung um VC1 nicht OK. Was die Frequenz betrifft: Hast Du schonmal an U2 (74HC14) am Pin 2 oder Pin 6 gemessen ? Dort müsste doch eigentlich ein gutes sauberes Rechteck HF Signal zu messen sein. 73 de DL1LAJ Andreas
I agree that it could be the diodes d43/d44. I had same problem with an other unit and it where this diodes in that part of the circuit. They were leaking . I could not measure it but just replaced them an it was working again.
Die SWR Brücke ist vermutlich das Problem - Weiteres folgt.
U 6