PACE ST25 Soldering Iron Base - Repair

[southpacific]

PACE ST25 Soldering Iron Base - Repair.

R27 Failed

The fault was that the iron took longer and longer to get up to heat (whereby the green LED flashes). Sometimes 10 minutes or more.

Resistor R27 (270R) had burnt out and was at fault. This resistor appears to have been replaced before with a 1/4W leaded resistor soldered onto SMD PADS. Hence it stood out.

Interesting... I purchased this iron new - so it must be a factory mod (or more likely a local distributor mod)(Australia is 240V AC where-as other countries are 220V AC). I have another ST25 iron which has a 1206 or larger SMD resistor mounted to R27's SMD pads (as per normal).

This resistor must get hot!
 
So I replaced this resistor with a (R27) 1W 270R resistor. Sitting above the PCB for thermal dissipation.


Transformer replacement from 110V to 240V

A "pre-owned" ST25 was obtained from USA. It contained a 110V US mains Transformer - which was unsuitable for Australian 240V use.

Internet discussions were of it being a 30V 3A (90W) transformer.


It appears to be somewhere between 20-30V Centre Tapped transformer. It swings +15V to -15V AC.

It was replaced with a Jaycar MM2015 Dual Winding 12V/15V. The two 12V windings were used.

The use of a SMPS was pondered. However, it appears that the secondary AC voltage is used directly and gets switched in and out for heating. So an AC Mains transformer must be used.

The plot shows the original PACE transformer AC secondary as it connects to the PCB. Think of it as 15V-CT-15V (Centre Tap). The "sinusoidal" Waveform shows a 50Hz 0V to 15V excursion - both positive and negative.

Luckily, this plot captures a second, slightly higher amplitude waveform. When the Green LED flashes ON, the waveform is 15V-CT-15V. When the Green LED is extinguished,the waveform is 17V-CT-17V.   

Be aware that the Jaycar M2015 transformer secondary has two independent windings. The transformer does not have a centre-tap - so the 0V winding on one winding must connect to the 12V winding on the other winding.

12V - positive
0V connected to 12V - common 0V
0V - negative


Removal of the Solder Pencil Cable connector.

I had to de-solder the solder pencil cable PCB connector to allow removal of the PCB. This was easier done by rolling a ball of solder (using a wave tip) over all 6 pads - rather than individual pin de-soldering, that didn't appear to release the connector pins.

It might be interesting to know whether the inner cylinder section can be detached from the outer front panel mount surround - so de-soldering wouldn't be necessary.

I also noticed that years ago I had added stick-on rubber feet to my bases, as they'd slide around the bench.