Heathkit IO-4555 oscilloscope repair

Bringing my dad's old scope back to life.

[@youtuuba]

I DO remember that scope......after I built it and started to operate it in order to do calibration, I noticed those same two resistors got hot enough to result in their paint stripes and varnish to bubble away merrily. I called my designated Heathkit support technician, who advised that although they were quite hot, they were within their power dissipation specification, and everything was normal. This is not the only Heathkit design I encountered where one or more resistors, technically operating within specs, ran too hot to touch. But nevertheless, that equipment keeps working over the decades, so apparently Heathkit was correct.

[@frankpmarsala7682]

I still have some of my many Heathkits in storage, one being the IO-4550 dual trace. I pulled an all nighter assembling it and got a BIG surprise when I installed an electrolytic backwards!!!! Their kits were awesome . . . that is until Zenith took over. I built 4 of their TVs. The GR-2700 (a Zenith set) came with boards and harnesses assembled in Mexico. It kept blowing the circuit breaker until I saw that a power supply harness came wired backwards. The Tech at the store was pulling his hair out when he assembled their in store model. Seems a whole production run came out like that. "The Quality went out, when the Zenith name went on"!

[@swinde]

I know there were other factors, but I believe that Zenith's acquisition of Heathkit was a major cause of the failure of the company. When Schlumberger was their parent company they produced the most fantastic products ever. I have many of their kits. Zenith only wanted Heath as a stepping into the microcomputer market and were successful for a while with their laptops that were popular with government and some education institutions. They cared nothing about the line of kits that made the company famous.

[@ezl7052]

Well, a lot of the companies took shortcuts in their design to save money and to keep costs down for the kit builders. I've built a lot of their kits and one thing I can say for Heathkit, the quality of the components is real high compared to the Japanese counterparts at the time. The old Drake, Collins and Swan Amateur gear is a good example when compared to the Kenwood and Yaesu gear. The resistors and capacitors were rated conservatively and were better in quality but the Japanese equipment was sometimes more refined in the engineering.

[@youtuuba]

Looks like a dual trace scope (there is that extra BNC and vertical deflection knob).....I think I built one of that model once, looks familiar......

[@youtuuba]

SinuSOOdial? Not sure if you said that deliberately.....?

[@duffyb11]

I had built an IO-4555 back in 1984. The only problem I had with the scope was arcing in the power transformer from the outer most winding to the copper shield. This caused a resistor on the HV board to fail. I put a piece of silicon tape between the copper shield and the winding. Yes, it arced through the insulating paper. I would be careful with the replacement can electrolytic cap as the date code looks to be from the 13th week of 1978. I am currently recapping a IO-4235. I will be replacing the can cap with three radial lead caps that will fit in the same place on the LV board. Also, it looks like some of the other caps on the LV board are swelling. You may want to check the caps for leakage. The IO-4555 was a great scope. I did homework from my college days on it. Used it in a guitar factory for testing circuits.

[@loughkb]

+Gregory Burns Thanks for the tips Greg. I have checked those other caps with my ESR meter and they're all under 1 ohm. There's really no swelling, that's a slight optical distortion from the angle and cheap camera. I'll probably replace that can with modern caps eventually, but between jobs at the moment and money is too tight to mention. I'm going to get a digital scope eventually, but I'll finish the repair on dad's old scope for sentimental reasons and keep it around.

[@ronshaw80]

I would like to get a modern digital scope as well, but the 8 bit resolution is a big turn off for me, as I do mostly audio work. It seems DSO's are really aimed at digital types, who don't need good vertical resolution. I currently use a Goldstar 9020 20MHz, and my main scope, a Tek 2215A 60MHz. So far they both still work great.

[@loughkb]

Ron Shaw And there's something soothing about the smooth phosphor glow of a trace on a CRT. I did get a cheap digital scope, and have used it for various projects where I wanted the quick feedback of the displayed measurements on the screen. And one where I needed waveform graphics for a presentation I'm doing. But I still keep the heath around.

[@ronshaw80]

My first scope was an old Heathkit 5MHz tube type. Single trace. I used it for years, and it worked fine, even though I never replaced any tubes. It still worked when I gave it to a friend back in the mid 90's. I built a lot of Heathkit stuff back then. A TV, a couple of receivers, a VTVM, a powered breadboard and dummy load (which I still have), a function generator (still have that, too).

[@youtuuba]

@@ronshaw80 , you have to be thinking of a pretty crappy digital or DSO to think that 8 bit resolution is all you can get with them. That said, although I have a pretty high end 4 channel DSO with integral logic analyzer and all the bells and whistles, I rarely turn it on, favoring my old Tektronix 200MHz dual trace, delayed sweep analog scope.

[@ezl7052]

Good ole Heathkit equipment and they had good manuals to go with them for building and servicing. It's a shame that they don't have anymore kits available. I really miss the kits even though some of the engineering on some weren't designed good but that was only a few. What's bad about some of the equipment to day is that most of it is throwaway and not serviceable. Thanks for your video review.

[@loughkb]

+Dennis Petersen I built quite a few Heathkits when I was younger. BTW, in looking at the schematic, I discovered that the 150 and 170V supplies filtered by this cap are nothing more than a simple resistor divider network directly off the transformer with rectification. That 10 Ohm power resistor next to the two carbon resistors is reading just a tad over 9 ohms, which is why those two rails are a bit high. I'll probably replace that 10 ohm resistor to bring them down.

[@youtuuba]

@@loughkb , the voltages on the Heathkit schematics are approximate, and depend on the type of meter used to do the measuring. Since these particular supplies are not regulated (at least at that point in the circuit), those somewhat higher voltages might be due largely to your line voltage being a bit higher.

[@wireworks616]

Have the same scope, did the same thing. High voltage cap hard to find.

[@twobtwobtwo]

Sooner or later the several 10uf tantalum caps will dead-short (replace all of them). You will get the magic smoke to come out once again.

[@swinde]

Components fail when the smoke gets out.

[@gilbus4989]

Just replace all the electrolytics. They get dry or leak with that age of scope.