Hi all! This is my very first post. I’m super thrilled to be learning this and to start engaging with all of you!
Apologies for the massive post but I figured this is a small group and it might be good to provide a little context about my level of expertise in electronics repair.
I got the vintage audio bug about a year ago but really, I’ve been interested in audio systems my entire life, even before I could drive. As soon as I would get a new car, I would tear it apart and put a new speakers, new stereos, amps, subs, etc… I had a large stereo system in my bedroom with speakers almost as tall as I was. So I’ve loved this hobby for a very long time. Once I heard the 70s receivers and compared that to my newish Yamaha receiver, I fell in love with the sound of a vintage unit, and just dove in headfirst. I started buying receivers that needed to be fixed so that I could learn how to fix them and learn electronics repair. A year ago I didn’t even know what ground did in an electronic circuit. So that was my level of expertise back then. I know a lot more now but I’m still a novice when it comes to electronics repair and circuits. I just felt like I needed to state that first before asking my question. One more thing to note, is that I have researched safety and I do have a dim bulb tester that I hook up these repair units to first. I also have an AC power supply by BK precision, model 1653, which has an isolation transformer in it as well. I also only use one hand while testing a plugged in unit. I’ve done this based on safety videos that I have watched and studied.
I have restored a couple of units (replaced caps and transistors) already and felt like I was finally ready for a challenge where I could try to diagnose what was wrong with one of the units that I had. I started poking around with my multimeter and checking for voltages, and I did find a transistor that was faulty, I replaced it, and it fixed the issue that this particular unit was having! I was thrilled! Once I fixed that, I moved on to start restoring the receiver, and I started at the power supply board.
I had recapped the power supply board and everything was going fine. Some of the voltages were a little on the lower side but they weren’t crazy. For instance, in the service manual, Q2, the service manual listed the collector as having a negative voltage value, -62 V and it was reading at -56. So nothing too off. I replaced all of the capacitors and moved onto the replace a few of the transistors and everything again was going fine. But then as I went to bring my soldering iron to the board, I saw a little zap between the board and the tip of my iron. It looked like static electricity but I’m not sure if it was. I checked voltages again on Q2, the collector was still reading -56, yet the emitter and base, which the service manual says I should be getting around -50 V, we’re now reading at -15. So the zap fried something. That’s what I’m working on now. Also I know this may sound odd but I’m a little excited that this happened because it’s giving me a challenge and an opportunity to learn more about how circuits work.
My question is… I’ve never had a shock like that happen to me before. I had the receiver turned off and no power supply to it either. It was plugged into my isolated AC power supply but I did have the power switch on my power supply turned off. So while it was plugged in, the receiver was not powered. Does anyone know why would I get a zap from my iron to the PCB? And also, how could I prevent this from happening in the future?
Thank you!
Adam
I too have a similar story and am new to vintage repair and have only been doing it for about two years. I’m no expert either but what it sounds like is a capacitor discharge. Capacitors will hold a charge sometimes long after units are unplugged. It is why it is a good idea to manually discharge capacitors before working on any circuit especially large filter caps. As for dim bulb testers they are easy to make check out Mr. Carlson’s Posts or just YouTube it. There are dozens of designs. As for the isolation transformer that is a good thing to use but that does not limit current so you will need a dim bulb unit too.
@john-barron Oh my gosh I completely forgot about the capacitor charge. Crikey! I bet you’re right. And what I was doing, because I was paranoid that I’d make a mistake was to remove one component, replace it, then test the receiver. So I was charging the caps each time! Thank you for the reply!
i will be taking care of this differently going forward.
A good place to start would be to build the MCL’s Capacitor Discharge device : Videos 37, 38 /Patreon. 😜 I’ve built one myself and find it really useful. I’ve built mine in a small Hammond metal box (point to point style) and it took me literally 1Hour ! 👍 . In the distant past I’ve got zapped many times until I started using a big chunky wire wound resistor to discharge large caps but MCL’s device is proven to be way better and safer than a simple resistor. Highly recommended. !
Another thing to think about, is your soldering iron. Is it a simple iron, or a soldering station? Soldering stations, at least here in North America, will carry the earth ground from the receptacle all the way to the soldering tip. They will usually state “ESD Safe” somewhere on the packaging, or manual. This can be checked by a continuity test between the iron’s tip, and the earth ground pin of the station’s plug. A simple iron usually has only a two wire plug, and isolates from that earth ground, so any static charge isn’t bled off to earth ground. In dry climates, especially during winter, your basic iron can become a static zapping tool under the right conditions. If the DUT were plugged into your isolation transformer, the earth ground “usually” isn’t switched, and carries through regardless of the switch being on, or off.
These are all things to think about when working in solid state devices.
Even though my soldering station, and desoldering gun station are “ESD Safe”, and I have verified this, when working on solid state stuff, I always try to remember to tap the case of the unit with the tips of my soldering tools to dissipate any charge that could be built up between the two.
Hope it helps!
Thank you very much everyone for the extremely valuable advice. I have a Heathkit capacitor checker which has discharge capabilities (FYI I am aware of the dangers with this tester and have labels on it to ensure I turn my voltage down and also to switch it back to discharge). I will use that from now on. Also I won’t turn the DUT on and off so often while replacing components! That unit is pretty big though maybe I’ll look at building the capacitor discharge device.
Also I have a Hakko charging station but I will tap the tip to the chassis ground from now on, even though it says ESD Safe.
Again, I very much appreciate the advice everyone.
I need to quit watching all the videos and start building…LOL
Hello Larry,
I had a good chuckle when I read that and thought I should quip back that I should be warming up the iron instead of heating the mouse via scroll-wheel friction. <grin>
I like sparks, so my cap discharge tools are gloves, googles, and long-bladed, insulated screwdriver. :)) (And, no, kids of any age, don’t try this at home!)
cheers & 73
-HP
Latest Post: hi all im new Our newest member: The Tow Rat Recent Posts Unread Posts
