Metcal RF soldering base unit playing up

[Chris Wilson]

After singing this RF soldering station's praises here and elsewhere it has decided after many years good service to play up. Whilst tips et hot, probably to full temp, they do not retain heat when touched to joints. I have tried several tips, one brand new and my RF tweezers on their own RF co-ax seem to do the same. I suspect whatever part of the circuitry is supposed to maintain heat when a junction is taking it from the tip is not working properly, but that's merely a guess. It's an earlier version than some I have seen mentioned in here, a
STSS-P2V-O2 but I would imagine the schematic of the later ones will be similar unless anyone has the correct diagram / service manual

http://www.chriswilson.tv/MX-500P-11.pdf

Any ideas where to look and has anyone had this issue themselves and fixed it? Thanks

[JFJ]

... I suspect whatever part of the circuitry is supposed to maintain heat when a junction is taking it from the tip is not working properly, but that's merely a guess...

The tip regulates its own temperature:


(Taken from an old copy of the MX-500 User Guide).

The tip's power uptake can be reduced when it's in the vicinity of a strong magnetic field (magnets are used in Metcal's "TipSaver" workstands to reduce the tip's absorption of RF energy). If a magnetic field is not the cause of the problem, then failing/aging power supply components may have significantly reduced the RF output.

Are you able to measure the output frequency and amplitude of your power unit - during the tip's initial heat-up? If so, then perhaps someone here will be willing provide comparative readings from a fully functioning unit.

[mikeselectricstuff]

From memory I think it regulates RF power by adjusting the DC supply to the RF PA, so looking at this supply would be the first step. I think it goes up to about 48V.

[mmagin]

After falling in love with Metcal stuff (though I use Thermaltronics tips now), I've bought a couple used STSS-001 models and preemptively replaced the electrolytic capacitors, but aside from the dead incandescent pilot lights, they've been no trouble.  Assuming you've tried different tips, it does sound like it's having trouble with the DC power supply.

[Chris Wilson]

From memory I think it regulates RF power by adjusting the DC supply to the RF PA, so looking at this supply would be the first step. I think it goes up to about 48V.

Thanks Mike, I am but a raw beginner but I thought the circuit had the means to control voltage to the output FET, yet all the Googling i have done seems to suggest it operates on full power all the time and the Curie effect of the tips controls the operating temperature. I will find my thermocouples later but I get the impression the tips initially heat up to full operating temps, but if touched to a hefty solder joint just conduct the heat stored away and for whatever reason the power to them is insufficient to bring them back up. Certainly failure to regulate the voltage on the output FET high enough would cause this! Thanks again for your input.

[Chris Wilson]

... I suspect whatever part of the circuitry is supposed to maintain heat when a junction is taking it from the tip is not working properly, but that's merely a guess...

The tip regulates its own temperature:


The tip's power uptake can be reduced when it's in the vicinity of a strong magnetic field (magnets are used in Metcal's "TipSaver" workstands to reduce the tip's absorption of RF energy). If a magnetic field is not the cause of the problem, then failing/aging power supply components may have significantly reduced the RF output.

Are you able to measure the output frequency and amplitude of your power unit - during the tip's initial heat-up? If so, then perhaps someone here will be willing provide comparative readings from a fully functioning unit.

Hi, thanks for the reply, this is what I have always thought to be the case, yet Mike and the schematic seem to suggest that there is a circuit controlling voltage to the output FET which WOULD control the power produced... Hmmm

[Chris Wilson]

After falling in love with Metcal stuff (though I use Thermaltronics tips now), I've bought a couple used STSS-001 models and preemptively replaced the electrolytic capacitors, but aside from the dead incandescent pilot lights, they've been no trouble.  Assuming you've tried different tips, it does sound like it's having trouble with the DC power supply.

Thanks for the reply! Ahh! Someone that has had a similar modelt to mine apart Can you recall please if it would be possible to power the thing up out of its case? The thing that comes to mind first is heat sinking any semiconductors that rely on the case itself for that function. Cheers.

[mikeselectricstuff]

If you look at that schematic you'll see there is feedback from an RF detector to the LM2576 regulator, so it's regulating for constant RF power, and as the tip absorbs more, it needs to increase the drive to maintain the same output. 

[JFJ]

... Mike and the schematic seem to suggest that there is a circuit controlling voltage to the output FET which WOULD control the power produced...

Metcal's 1986 US patent describes the voltages that are applied to the FET's drain, as follows: 16V is applied when the system is not loaded (the controller is not used), and a variable voltage of 16 to 32V (set by the controller) is applied when the system is loaded.

[macboy]

The heater power is "controlled" by the tip. The base station provides high power RF to the tip (at 13.56 MHz). This is absorbed by the tip, until it goes above the Curie temperature when its physical magnetic properties change, and it reflects most of the power back to the base.

I can see several possible causes of this issue:
- Faulty passives in the handpiece (see below).
- Bad connection at the RF F-connector.
- Break or near-break of the conductor within the cable.
- Faulty power supply not able to maintain the >40 W RF power output.

Note that the handpiece is more than just a coax cable with a handle at the end, there is an inductor and capacitor near the tip connector.  See this thread and especially this thread .

If possible, borrow another handpiece and/or power supply to test with. This will help narrow down the issue to the power brick or the handpiece.

[Chris Wilson]

Getting somewhere now, there's definitely one faulty tip, but the new one and the tweezers fire up to temp and maintain it if I wiggle the F type connector at the main unit about. I suspect something is up with the socket, so I will get inside and have a look see Many thanks for the help! It's great unit and i would miss it as it allows me to do heavy work and SMC quite easily.

[GreyWoolfe]

Hopefully, it's nothing more than reflowing solder joints.