Rant: The $200 power supply that smoked my bench because I skipped the isolation transformer

Working on a 1970s Sony receiver in my garage last Tuesday. I was on a roll after 2 days of tracing a dead channel. Hooked up my scope ground clip to what I thought was the chassis ground. One second later, pop and smoke. Fried the power supply on my scope. Turns out the transformer in that old Sony wasn't isolating the hot side. I was basically shorting the house line through my equipment. Cost me 200 bucks for a used replacement PSU board. Now I'm debating if isolation transformers are mandatory or if I just got unlucky. Half the guys on YouTube never use one. What do you do when you work on old hot-chassis stuff?

4 comments

4 Comments

the_evan1mo ago

The "half the guys on YouTube never use one" part is where I gotta push back a little. Those dudes are either editing out the smoke or they got lucky on a specific unit. You definitely weren't just unlucky, you ran into the classic hot chassis problem. Old Sony gear from the 70s is notorious for that - they used a two wire cord with the chassis directly tied to one side of the AC line. An isolation transformer would have broken that path and saved your scope 100%. Most of us who work on old radios and amps religiously use one for exactly this reason, it's a cheap insurance policy compared to replacing your bench gear.

angelar571mo ago

You really nailed it with "a cheap insurance policy compared to replacing your bench gear." I used to be one of those guys who thought isolation transformers were overkill, like something only the super paranoid used. I figured as long as I was careful and knew what I was doing, I'd be fine. But reading this and the other replies, I gotta admit I was wrong. It's not about being careful, it's about the fact that those old Sonys are literally wired so the chassis is hot. You can't be careful enough to avoid that physics problem. I'm definitely going to grab one of those eBay transformers you mentioned before I touch another old radio.

the_thomas1mo ago

Yeah, @the_evan nailed it. I've personally fried a cheap multimeter probing an old tube radio before I learned my lesson. It's not even about being unlucky, it's just physics. The chassis on those 70s Sonys is literally half the power cord. You hook your scope ground to that and you're basically shorting the line. I keep a little 1:1 isolation transformer I got off eBay for like 30 bucks hooked to my main bench outlet, never had an issue since. It's one of those things you don't think about until you smell that magic smoke, then you never forget it.

paigep201mo ago

Oh man, I felt that one in my wallet just reading it. I once shorted a vintage amp to my bench ground using a screwdriver with a chipped handle, like I was personally trying to find the fastest way to meet my breaker. I thought I was being careful too, you know, real smart guy with a multimeter and everything. Then I realized I was basically playing Russian roulette with my gear while wearing no gloves. I got a little isolation transformer off Craigslist for like 40 bucks and now I feel way less like a disaster waiting to happen.