Terry, love the trouble shooting! A little tube amp tech diddy for ya. "Got one hand in my pocket the other on the probe. Cranking the varactor waiting for the glow. Seeking validation for my troubleshooting plan. If I can fix this baby I will be a happy man. The filaments are glowing as the heater current flows. Watching AC current while my dim bulb barely glows. Checking for high voltage, no flashes we are hot. Max the bias voltage with a twist of the right pot. Looking good so far, good luck we have had some. But lets apply the volume we've got signal but no hum. Victory attained fidelity restored it's time to start another one so that we don't get bored." Cheers!
Nice job Terry, it’s good to see you get involved in troubleshooting real electronics and taking us through the process. What gets me is you ALWAYS have the right part on hand regardless of how old, scarce, rare, obscure, or long since discontinued it might be! Keep it up!
If I remember from my 16 week digital class in 2002 ,7000 series logix chip is faster than 4000 series of chips. Maybe all obsolete/ Good video. One of my nine -16 week semesters for my electronic technology certificate,[ Night's part time after work - start 08/28/2000 finished 12/13/2003-Triton college,River Grove,IL we used BK scopes & Tektronix digital scopes. But you are refreshing my digital memory. Good video
Terry: The Heath 4550 that I'm working on uses the same UA760 for trigger comparator. I have not yet finished my repairs so the 760 in mine may still have an issue. I'll keep your stash in mind. Thanks for the video.
Hi, I am also restoring a 1981 vintage IO-4550. I have decided to try and upgrade the scope to try and achieve 50 Mhz bandwidth. I am replacing most of the chips with SN74AS type TTL type chips. The UA760 will be replaced with a Maxim MAX913CPA comparator. I will build a cheater socket to do this. This mod is not for the weak of heart but as I am a retired electrical engineer and a bit crazy I'm going for it! The parts are getting hard to find. If you buy any buy lots of spares. Don't forget to replace all those old electrolytic caps. A can type cap can be replaced with individual caps. Good luck!! Mike at arabimike999@gmail.com
If you do anything to your scope do this! Replace the .1Uf 1600 Volt caps on the High Voltage power supply. There made of paper & foil and will fail. They are undersized at 1600 Volts as the transformer puts out 1700 Volts. Use Digi-Key part number 338-3169-ND which are rated at 2000 Volts and are Polypropylene. They are slightly larger but will fit. They work fine and will last a long long time. About $6 each. The Voice of Experience!
Interesting comment about the chip being the culprit this time, but not so often in his experience. I'd been told long ago that chips were pretty reliable & thus replacing them & hoping that would fix a problem didn't pay off very often. Thanks for this video, Terry!
Chips are normally pretty reliable, and when these chips were new they would have been just as reliable. Now, with 40+ years under their belts, they are starting to fail. I am seeing it more and more.
Certain older chips turned out to be not reliable, because the early days of IC manufacture there were long term degradations that were not anticipated. Things like purple plague, which only show up long term, and also electromigration, which also are long term failures. As well IC's are often vunerable to failure if the power supply voltages are exceeded, or the inputs are powered up without the power rails being present. that uA760 probably did not like having a high voltage, and was rendered inoperative in the input stage. However it is relatively simple, and could be replicated with modern SMD technology onto a smallish board, probably small enough to fit into the existing circuitry with no problems, as it has only NPN transistors and resistors and diodes in it, all of which are easy to get as SMD parts. 30 identical SMD transistors, 2 diodes and 22 SMD resistors are low enough volume to hand place.
Wow those Heathkit manuals are incredibly detailed. They set the bar for HP and Tek. You can actually RTFM with these service manuals and repair instrumentation without having an annual tech support contract. How many times have you asked a newbie tech, did you RTFM?
I think you have a bit of a misunderstanding of the oscope trigger circuits and user operation. The trigger selector does not set the auto trigger mode. The selector only chooses the input. The auto trigger is set by the trigger level's potentiometer switch. So when you rotate that control fully counterclockwise and you hear the click of the switch then you are in auto trigger mode. In fact you can be in auto trigger mode for all the inputs (Line, Y1, and Y2). The labeling on the front panel with the red curved arrow the dot and AUTO is for the outer adjustable trigger level knob and not for the TRIG SELECT switch. The auto trigger circuits do not bypass the input comparator as mentioned in the video. The calibrator square wave signal, I believe from your video seems to be around the common 1 kHz frequency and is created by an isolated astable multivibrator. Therefore when you thought you were in auto trigger you were on +Line and that selection is usually used for viewing time related line frequency waveforms. If you check the schematic you will see the Line + and Line - are taken from the power supply and are 60 Hz related. That is why even with a replaced IC you saw it would still not sync, (1 kHz verses 60 or 120 Hz). Auto trigger's main function is to produce a trace in the absence of a signal and may set a trigger level. Glad you found the bad IC. Hope I didn't make any mistakes and this information was helpful. By the way you do a very nice job on your audio and focus/lighting of the video. I am also with you on Heathkit manuals being very good.
Might be time to be looking for a suitable replacement chip. Maybe a surface mount comparator mounted on a small custom pcb that plugs into that 14 pin socket. I've done that for other unobtainium chips.
Easy enough, almost the same size board as the original IC, with 30 identical SMD transistors, 2 diodes and 22 SMD resistors. A low enough volume to hand place. The original was a very early IC, and is large enough to emulate with little problems, just have to use close matched SMD transistors to copy the internal transistors, current mirror and the internal matched diodes and zener diodes, all using the same SMD transistor type. Pull them all off the same reel in sequence and they will probably match well enough, coming off the same wafer in close proximity.
Very good troubleshooting! I really like the format of your videos in that they cut to chase and explain all the important stuff clearly. Also great when you cut a video into part 1 part 2 etc. Weren't you having trouble with a Tektronix scope too?
At least Heath used off-the-shelf parts... Tek made their own custom chips for their scopes, so if one of those goes bad you basically have to cannibalize another dead scope. Maybe you'll get lucky and Tek has old parts available, but I wouldn't count on it.
Terry, love the trouble shooting!
A little tube amp tech diddy for ya.
"Got one hand in my pocket the other on the probe. Cranking the varactor waiting for the glow. Seeking validation for my troubleshooting plan. If I can fix this baby I will be a happy man.
The filaments are glowing as the heater current flows. Watching AC current while my dim bulb barely glows. Checking for high voltage, no flashes we are hot. Max the bias voltage with a twist of the right pot.
Looking good so far, good luck we have had some. But lets apply the volume we've got signal but no hum. Victory attained fidelity restored it's time to start another one so that we don't get bored."
Cheers!
I miss those days also.
Nice job Terry, it’s good to see you get involved in troubleshooting real electronics and taking us through the process. What gets me is you ALWAYS have the right part on hand regardless of how old, scarce, rare, obscure, or long since discontinued it might be! Keep it up!
Really nice job Terry... I wish all electronics had a Heathkit book... Life would be sweet !!!
Hi Terry, good deal OM and that IC didn't even have a filament or glow in the dark... You da man !!
Absolutely excellent video on o'scope repair and troubleshooting methodology !! Thanks a ton Terry !!!
As long as there is audio freq those old scopes will still be useful, good stuff.
If I remember from my 16 week digital class in 2002 ,7000 series logix chip is faster than 4000 series of chips. Maybe all obsolete/ Good video. One of my nine -16 week semesters for my electronic technology certificate,[ Night's part time after work - start 08/28/2000 finished 12/13/2003-Triton college,River Grove,IL we used BK scopes & Tektronix digital scopes. But you are refreshing my digital memory. Good video
2:47 Cornparitor? Great job, thumbs up to you and Heathkit engineers.
Terry: The Heath 4550 that I'm working on uses the same UA760 for trigger comparator. I have not yet finished my repairs so the 760 in mine may still have an issue. I'll keep your stash in mind. Thanks for the video.
Hi, I am also restoring a 1981 vintage IO-4550. I have decided to try and upgrade the scope to try and achieve 50 Mhz bandwidth. I am replacing most of the chips with SN74AS type TTL type chips. The UA760 will be replaced with a Maxim MAX913CPA comparator. I will build a cheater socket to do this. This mod is not for the weak of heart but as I am a retired electrical engineer and a bit crazy I'm going for it! The parts are getting hard to find. If you buy any buy lots of spares.
Don't forget to replace all those old electrolytic caps. A can type cap can be replaced with individual caps. Good luck!! Mike at arabimike999@gmail.com
If you do anything to your scope do this! Replace the .1Uf 1600 Volt caps on the High Voltage power supply. There made of paper & foil and will fail. They are undersized at 1600 Volts as the transformer puts out 1700 Volts. Use Digi-Key part number 338-3169-ND which are rated at 2000 Volts and are Polypropylene. They are slightly larger but will fit. They work fine and will last a long long time. About $6 each. The Voice of Experience!
Thanks Heathkit for chip sockets!!
Interesting comment about the chip being the culprit this time, but not so often in his experience. I'd been told long ago that chips were pretty reliable & thus replacing them & hoping that would fix a problem didn't pay off very often.
Thanks for this video, Terry!
Chips are normally pretty reliable, and when these chips were new they would have been just as reliable. Now, with 40+ years under their belts, they are starting to fail. I am seeing it more and more.
Certain older chips turned out to be not reliable, because the early days of IC manufacture there were long term degradations that were not anticipated. Things like purple plague, which only show up long term, and also electromigration, which also are long term failures. As well IC's are often vunerable to failure if the power supply voltages are exceeded, or the inputs are powered up without the power rails being present. that uA760 probably did not like having a high voltage, and was rendered inoperative in the input stage. However it is relatively simple, and could be replicated with modern SMD technology onto a smallish board, probably small enough to fit into the existing circuitry with no problems, as it has only NPN transistors and resistors and diodes in it, all of which are easy to get as SMD parts. 30 identical SMD transistors, 2 diodes and 22 SMD resistors are low enough volume to hand place.
Thanks for explaining your procedure to debug! Really nice video.
Wow those Heathkit manuals are incredibly detailed. They set the bar for HP and Tek. You can actually RTFM with these service manuals and repair instrumentation without having an annual tech support contract. How many times have you asked a newbie tech, did you RTFM?
I miss those days too.
Nice job, enjoying the repairs!
Yes indeed, those days are gone; but then again, in those days,
there was no Snow Pig. Something to consider.
I think you have a bit of a misunderstanding of the oscope trigger circuits and user operation. The trigger selector does not set the auto trigger mode. The selector only chooses the input. The auto trigger is set by the trigger level's potentiometer switch. So when you rotate that control fully counterclockwise and you hear the click of the switch then you are in auto trigger mode. In fact you can be in auto trigger mode for all the inputs (Line, Y1, and Y2). The labeling on the front panel with the red curved arrow the dot and AUTO is for the outer adjustable trigger level knob and not for the TRIG SELECT switch. The auto trigger circuits do not bypass the input comparator as mentioned in the video. The calibrator square wave signal, I believe from your video seems to be around the common 1 kHz frequency and is created by an isolated astable multivibrator. Therefore when you thought you were in auto trigger you were on +Line and that selection is usually used for viewing time related line frequency waveforms. If you check the schematic you will see the Line + and Line - are taken from the power supply and are 60 Hz related. That is why even with a replaced IC you saw it would still not sync, (1 kHz verses 60 or 120 Hz). Auto trigger's main function is to produce a trace in the absence of a signal and may set a trigger level. Glad you found the bad IC. Hope I didn't make any mistakes and this information was helpful. By the way you do a very nice job on your audio and focus/lighting of the video. I am also with you on Heathkit manuals being very good.
Might be time to be looking for a suitable replacement chip. Maybe a surface mount comparator mounted on a small custom pcb that plugs into that 14 pin socket. I've done that for other unobtainium chips.
Easy enough, almost the same size board as the original IC, with 30 identical SMD transistors, 2 diodes and 22 SMD resistors. A low enough volume to hand place. The original was a very early IC, and is large enough to emulate with little problems, just have to use close matched SMD transistors to copy the internal transistors, current mirror and the internal matched diodes and zener diodes, all using the same SMD transistor type. Pull them all off the same reel in sequence and they will probably match well enough, coming off the same wafer in close proximity.
Very good troubleshooting! I really like the format of your videos in that they cut to chase and explain all the important stuff clearly. Also great when you cut a video into part 1 part 2 etc. Weren't you having trouble with a Tektronix scope too?
Hello, Yes Sir, The Tek 935A. Will be featuring as soon as time allows
@@d-labelectronics Good I can't wait.
@@d-labelectronics I've seen you use that 'scope in past videos. Has it released some magic smoke lately?
Thanks for this video. Gracias.
At least Heath used off-the-shelf parts... Tek made their own custom chips for their scopes, so if one of those goes bad you basically have to cannibalize another dead scope. Maybe you'll get lucky and Tek has old parts available, but I wouldn't count on it.
Terry what wattage is your ungar iron???
You can get those chips NOS on eBay for less than $10 -- even less if you can tolerate a modern knockoff and wait for shipment from China.