There is another failure mode with this supply. If there is something like a large capacitor on the output and you turn down the voltage, it applies a large reverse voltage on the base-emitter junction of Q2 and Q3. Q3 is large and robust, but Q2 tends to fail. I added a diode in series with the output to prevent this.
Note the schematic in the video shows reversed-biased 1N-4002's across Q3 and across (not in series with) the output. These protect against back-feeding. None of the units I've done, nor any of the original manuals, had these. So I added them to my rebuilds.
@@dtss_smudge Yes, I noticed them on the schematic you showed before you mentioned them. They are definitely a good idea. There are actually many things about the circuit that are flawed. I can imagine a large current spike in the base-emitter junction if the current sense transistor Q5 during the brief instant before the regulator reacts. A resistor on its base would easily prevent that, but they do live on all these years, so I can't fault them too much. Have you ever had transistor failures in yours?
I believe that blue cap was original since the one in front of me looks the same. Either that or its one of the wildest coincidences that the old owner and I used the same cap. This one doesnt work. Its completely dead. I thought its probably a fuse. I couldnt find one. I didnt see one on yours either then you said it doesn't have a fuse lol! The manual shows one in the schematic and a chassis photo by the power switch but I don't think they had them back then. Mine is serial # 06415. The blue tag is supposed to be inside but yours does look good still. Looking at it, its kind of a death trap lol. The puny wires with thin insulation connecting the power switch to the board are lying against the chassis. I'm pretty sure my power transformer is fried. This is one of three of these I have. I'm going to build a switchmode supply in the chassis which will put out some real power. Thinking 0-24V and 0-2A. Im not going to try to source a power transformer for this wimpy supply. Still, it'll be fun watching this! 👍
Some, presumably early versions of the IP-18, had no fuse, as didn't much of Heathkit's pre-1970s kits. Good luck with your switch-mode redesign. Keep an eye on the RFI..
Friendly notice : The standard voltage in USA and canada is 120 Vac not 110Vac. This thrue for more than 50 years now. ( See NEC and canadian code CEC).
I'll take your word regarding the NEC, but you got me curious. So I went back to the original manuals, and the specs for the IT-12 (dated 1962) called for 117VAC 50-60cps (!) and for the T-4 (dated 1958) was also 117 Vac 50-60 "cycle". Go figure.
...and I just realized you were commenting on the IP-18, not the T4/IT-12. The IP-18 (1968) called for "105 to 125 [assuming wired that way] 50-60 Hz" So it was more forgiving.
Not 100% sure what you're referring to, but if you mean the 4-position white screw terminal block I use to wire the AC side, search Amazon for "terminal block white screw terminal". Not really quick disconnect, you have to loosen or tighten the terminal screws. Thanks for watching and leaving a comment!
LOL they increased the space around the line traces significantly...but then bridged the gap by printing copper S's where the switch connects. I suspect those were added by someone who didn't understand the intention of increasing the space. 😂
This but one of the reasons I moved all line AC componentry (line cord, switch, fuse, neon, primary...) off the PCB and onto free space in the chassis. BTW thanks for all the comments!
@@dtss_smudge Yes, that's very wise. This was designed before safety, back in the days of toy injection molding presses producing lava hot snap together toys and power tools with metal housings and two prong plugs. It's amazing we survived! Thanks for your videos. I have enjoyed them a lot!
Thank you sir , I enjoyed the Rebuild . It was fun to see an oldie get a new life . Thank you , WA3FRD
You're welcome. Thanks F.LV. for commenting, and '73s from a fellow ham.
There is another failure mode with this supply. If there is something like a large capacitor on the output and you turn down the voltage, it applies a large reverse voltage on the base-emitter junction of Q2 and Q3. Q3 is large and robust, but Q2 tends to fail. I added a diode in series with the output to prevent this.
Note the schematic in the video shows reversed-biased 1N-4002's across Q3 and across (not in series with) the output. These protect against back-feeding. None of the units I've done, nor any of the original manuals, had these. So I added them to my rebuilds.
@@dtss_smudge Yes, I noticed them on the schematic you showed before you mentioned them. They are definitely a good idea. There are actually many things about the circuit that are flawed. I can imagine a large current spike in the base-emitter junction if the current sense transistor Q5 during the brief instant before the regulator reacts. A resistor on its base would easily prevent that, but they do live on all these years, so I can't fault them too much. Have you ever had transistor failures in yours?
I believe that blue cap was original since the one in front of me looks the same. Either that or its one of the wildest coincidences that the old owner and I used the same cap.
This one doesnt work. Its completely dead. I thought its probably a fuse. I couldnt find one. I didnt see one on yours either then you said it doesn't have a fuse lol! The manual shows one in the schematic and a chassis photo by the power switch but I don't think they had them back then. Mine is serial # 06415. The blue tag is supposed to be inside but yours does look good still.
Looking at it, its kind of a death trap lol. The puny wires with thin insulation connecting the power switch to the board are lying against the chassis. I'm pretty sure my power transformer is fried. This is one of three of these I have. I'm going to build a switchmode supply in the chassis which will put out some real power. Thinking 0-24V and 0-2A. Im not going to try to source a power transformer for this wimpy supply.
Still, it'll be fun watching this! 👍
Some, presumably early versions of the IP-18, had no fuse, as didn't much of Heathkit's pre-1970s kits. Good luck with your switch-mode redesign. Keep an eye on the RFI..
@@dtss_smudge Thank you!
Friendly notice : The standard voltage in USA and canada is 120 Vac not 110Vac. This thrue for more than 50 years now. ( See NEC and canadian code CEC).
I'll take your word regarding the NEC, but you got me curious. So I went back to the original manuals, and the specs for the IT-12 (dated 1962) called for 117VAC 50-60cps (!) and for the T-4 (dated 1958) was also 117 Vac 50-60 "cycle". Go figure.
...and I just realized you were commenting on the IP-18, not the T4/IT-12. The IP-18 (1968) called for "105 to 125 [assuming wired that way] 50-60 Hz" So it was more forgiving.
What is your ac safety power block device with the quick disconnect feature?
Not 100% sure what you're referring to, but if you mean the 4-position white screw terminal block I use to wire the AC side, search Amazon for "terminal block white screw terminal". Not really quick disconnect, you have to loosen or tighten the terminal screws. Thanks for watching and leaving a comment!
@@dtss_smudge Sorry, I meant the ac mains connection box with the lift up covers, bigclive has one like that also, but I can not find one.
Sorry, should have looked it up first, it is at 14:22 on the video. Thanks much!
@@dtss_smudge Sorry, should have looked it up first, it is at 14:22 on the video. Thanks much!
LOL they increased the space around the line traces significantly...but then bridged the gap by printing copper S's where the switch connects. I suspect those were added by someone who didn't understand the intention of increasing the space. 😂
This but one of the reasons I moved all line AC componentry (line cord, switch, fuse, neon, primary...) off the PCB and onto free space in the chassis. BTW thanks for all the comments!
@@dtss_smudge Yes, that's very wise. This was designed before safety, back in the days of toy injection molding presses producing lava hot snap together toys and power tools with metal housings and two prong plugs. It's amazing we survived!
Thanks for your videos. I have enjoyed them a lot!