TENMA 21-147A

[DIY KINDA GUY]

RE: TENMA 21-147A

Hi All,

I have the Tenma 21-147A soldering station. It is very much like the one shown for the TENMA 21-7930 https://www.electronicspoint.com/threads/tenma-21-7930.217198/ and the schematic looks exactly the same.

Mine had all the LED lights lit when turned on except for the HEATER LED. No turning of the knob or potentiometers helped. I got the base of the unit with some junk and had always wanted to repair it and get a soldering pen/tip for it to make it complete and working. I had opened it up and had made tests years ago and knew it had good voltage at the transformer but had never done extensive testing to the circuit board. The LM324 IC1 was already socketed, it seems someone in the past had repaired/replaced that IC. The LM339's were both still soldered to the board. Earlier this month I sent in an order to MPJA.com for some supplies and noticed they had solder pens to repair another solder station I purchased from them. I also noticed they had one that was a 24 volt pen and it had the 5 pin din plug like the Tenma 21-147A used, so I ordered one and decided to give repairing of the Tenma another try. I opened it up and began testing components on the board, starting with the triac, it was good. I sprayed all the potentiometers with cleaning spray and got them clean and tested them, all were in spec. I tested the diodes, I plucked two of them and although they tested good, I replaced them. One capacitor had pulled loose of the solder pad on the board so I resoldered it, but then added a wire to jumper it to the rest of the connections following the trace so it I would make sure of its connection. I tested all the resistors, desoldered the three C945 transistors and they all tested good, so I put them back. Everything on the board looked good and I narrowed things down to the three IC's IC1, IC2, and IC3. I went to radio shack, they happened to be having a 50% off sale on everything in the parts drawers, so I bought two of each of the LM324's and 4 of the LM339's in addition to other parts I felt I might need for other projects. I couldn't find any 14 pin sockets at the radio shack, but I knew I had some at home, so when i got home, I began desoldering the LM339 IC's, IC2 and IC3. I plucked them from the board and put 14 pin IC sockets in their place. After all that was done, I pulled the LM324 and replaced it, and replaced the LM339's with new IC's from my radio shack purchase and then plugged in the new soldering pen in the din plug, plugged in the station and turned it on. Immediately the bad symptoms were gone, the LED's weren't all lit. I turned the temp control knob and the heater LED came on and the fist green LED at 300F came on, as I turned it up, the LED's lit up and the iron began to get hot! YEAH!!! It was working! I had tinkered around with the setting from VR1, VR2 and VR3 on the board, so I set them close to the original settings and I plan on using a laser thermometer to check the temperature and calibrate the soldering station to the temp so when I dial it in on the base, it gives me that temperature at the tip. My base has the nylon board supports underneath the front so no issues with anything shorting out. Thanks to all that shared info and the originator of this post for sharing the schematic. My device is basically exactly the same in photos and schematic. So if you have a TENMA 21-147A, this info will help you service it.

 

[hevans1944]

Thank you for sharing.

 

[73's de Edd]

Sir DIY KINDA GUY. . . . . . . .

I also had some dealings wih MCM wherein I had caught one of their ultra deep throw 8 inch woofers on sale and was ordering one, but also caught some of those YELLOW handled replacement iron units for that series of station at a DEEP discount. of a mere $3.12 Amellicandollah per unit . . . just had to order 3 of them also.
When received, I figured that I could use some of the old Neanderthal-ic IBM female connectors to mate with them. Tested the 3 Units, and they worked fine.

Question . . . has your unit received long enough and hard enough use to confirm if there might potentially be a siezing /oxidation problem between the stainless steel clad heating element and the very long plated tip, which CLOSELY slips over and then telescopes around the heater for about 2 inches?
Then there is an additional outer SS sleeeve and its knurled screw, which holds the pair in place.
One might check that situation closely after 10 or so use cycles, inspecting stone cold, because a siezed pair ends it all.

Also while that unit is at hand and fresh on your mind . . .can you fill us in on some of the only data being left unshown . . .no, make that TWO shortcomings.
That would be the capacitance and voltage ratings of C1---C2---and C3.

AND the other troubleshooting tidbit that they did not consider at all, was the cold resistance of the heater element and the ambient temp resistance of the RES 2 thermistor inside the unit.
I will measure and provide that info from one of my units.


73's de Edd

 

[DIY KINDA GUY]

C1 is 470uf @25v, C2 is 47uf@25v ,C3 is 47uf@25v. I'm have a RCA voltohmyst I can pull out and double check that. I checked with the voltohmyst and on the x1 scale it shows 3.4 ohms on pins 3/5 the heater pins, and 2.1 ohms on pins 1/4, which is the resistor temperature sensor.