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 Post subject: Re: Heathkit IO-4550 dual trace trigger problem
PostPosted: Nov Fri 15, 2019 10:34 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11812
Location: Powell River BC Canada
From what I read, both vertical channels can display a sin wave, reasonably well,
and the scope has two working channels that the traces will move apart from each other
if the position controls are moved.

But the waveform will not lock, or synchronize.

A suggestion, connect the signal to only one channel, and make sure that
while the other channel is flat lining, touching that channel's input creates a
signal of some sort.


Now connect the signal generator also to the external trigger input, and select external
trigger as synch source.

This will allow you to follow your external trigger source from to front jack up the chain
using the scope's working other channel.
Attachment:
I O 4550 FOLLOW ME.JPG
I O 4550 FOLLOW ME.JPG [ 89.67 KiB | Viewed 165 times ]


Have you done any TTL digital troubleshooting in thw dim dark past.. :shock:

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VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
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 Post subject: Re: Heathkit IO-4550 dual trace trigger problem
PostPosted: Nov Fri 15, 2019 1:11 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5393
Location: Sunbury, Ohio 43074
looks to me like a shorted out IC201 as bob said, or maybe both of the zener diodes shorted dead off pin 5. Or someone slipped with a probe in the past when troubleshooting the scope. Who knows at this point. But the only thing common to both channels, as far as triggering is concerned, is the trigger selector switch and following circuitry. With both Q117's pulled, try the above method and see if it'll trigger. If not, you have an input trigger (your sine wave) to use as a troubleshooting signal for the IC201 circuitry and beyond.

You could also try LINE TRIGGER, and scope the filament transformer. If it locks that way, your IC201 circuitry is all OK since it's locking to the AC line.

If that works, it's anyone's guess as to what could have blown both channel trigger amps.... other than, ahem, "external forces"....

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 Post subject: Re: Heathkit IO-4550 dual trace trigger problem
PostPosted: Nov Fri 15, 2019 2:10 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 14903
Location: Fernandina Beach, FL
At the risk of sounding rather simple, since this is a newly acquired scope, I wonder if it ever did work properly? Could it have been assembled incorrectly? Does it appear to have much or any use?

Why I say that is because over the years, I have purchased some items that were kits and they appear to have never been used. And I found out in the TT-1 that there was a wiring error which made it just about useless. Does your scope appear to have zero wear around the knobs etc.?

Just a thought...

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 Post subject: Re: Heathkit IO-4550 dual trace trigger problem
PostPosted: Nov Fri 15, 2019 2:20 pm 
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Joined: Nov Sun 10, 2019 10:04 pm
Posts: 15
Location: Wellsboro, PA
As mentioned earlier in this thread - I don't know the history of this scope, the interior was spotless and the CRT very bright, no burn spots. So after checking voltages around the trigger amplifier I did a complete inspection of component values.

However, I did find and replace "dome top" electrolytics in the LV supply.

Channel 2 vertical circuits had been adjusted and channel 1 circuits were way out of wack. HV supply was set very high. And for some reason the line voltage switch was set for 105VAC (or low).


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 Post subject: Re: Heathkit IO-4550 dual trace trigger problem
PostPosted: Nov Fri 15, 2019 3:19 pm 
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Joined: Nov Sun 10, 2019 10:04 pm
Posts: 15
Location: Wellsboro, PA
OK - To answer some questions. Early in my career I was a test technician at Hewlett Packard. Then went on to microprocessor based controller design, which morphed into C / C++ Software Development. Now in retirement I'm getting familiar with Arduino stuff and repairing old tube Radios.

Back to the scope - In checking voltages around IC201 I found a missing -5 supply. Checking on the LV board, emitter Q306 -5V was fine. Turned out to be a bad solder connection on a brown wire to the Horz brd.

Still not sure why Q117's are blown but this sure explains the lack of trigger.


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 Post subject: Re: Heathkit IO-4550 dual trace trigger problem
PostPosted: Nov Fri 15, 2019 3:51 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5393
Location: Sunbury, Ohio 43074
Nice find. You never can be sure about kits .... as far as the skill level of the original builder. Even the highly skilled and experienced can miss one occasionally, magnified by so many decades of a soldering joint going "cold". I've even seen this on commercial gear occasionally.

Thanks for the biography... it's always best to know at what level responses to technical questions should be answered. Then again, this exact thread might be read by a relative novice, and explaining in basically, "beginner detail" might be helpful to others at times. Speaking for myself alone, although echoed by many others, teaching is really never out of style. ;-) ... it's not intended as a slight on the poster of a question.

It is interesting that both channel trigger amps are damaged.... please update the thread when you get it working and see if there was some common cause that we've all missed so far.

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 Post subject: Re: Heathkit IO-4550 dual trace trigger problem
PostPosted: Nov Sat 16, 2019 12:50 am 
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Joined: Nov Sun 10, 2019 10:04 pm
Posts: 15
Location: Wellsboro, PA
The comparator, IC201 checks out OK. Pin 4 input will vary from +0.6 to -0.6 as you move the trigger level. Pin 5 input is currently 0V because I still have all the parts removed on the vertical board. So the pins 11 & 10 outputs will change as the trigger level is changed.

Back on the vertical board I noticed that R149 ZERO adjust trim pots were no where near 500 ohms. One read 290 and the other 310 (with both removed from the board). So I ordered and received a couple of new ones from Mouser. An exact replacement is P/N 531-PT15B-500. We'll they are exact replacements except they really are 500 ohms! Not sure how much of a difference this makes but it will provide a bit more range on the adjustments.

I'll also be posting the Heathkit part number cross reference for some of these parts.


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 Post subject: Re: Heathkit IO-4550 dual trace trigger problem
PostPosted: Nov Sat 16, 2019 6:19 am 
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Joined: Jun Sat 09, 2007 8:14 am
Posts: 3912
Location: Florida
I could tell from your posts that this wan't your first time touching test equipment. You'll find the problem.

RRM


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 Post subject: Re: Heathkit IO-4550 dual trace trigger problem
PostPosted: Nov Sat 16, 2019 4:35 pm 
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Joined: Nov Sun 10, 2019 10:04 pm
Posts: 15
Location: Wellsboro, PA
Here is the Heathkit cross reference for part numbers, for the 4550.


Attachments:
IO4550_Xref.pdf [4.83 MiB]
Downloaded 8 times
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 Post subject: Re: Heathkit IO-4550 dual trace trigger problem
PostPosted: Nov Sun 17, 2019 4:35 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11812
Location: Powell River BC Canada
Have you got a digital ic test board, like a Pencilbox or Digi Designer ?
to test the logic chips ?

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VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Heathkit IO-4550 dual trace trigger problem
PostPosted: Nov Sun 17, 2019 3:54 pm 
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Joined: Nov Sun 10, 2019 10:04 pm
Posts: 15
Location: Wellsboro, PA
No, I do not own any digital test gear. However, I do have an Arduino Nano plugged into a proto board right now for another project. So I could always plug in the chip to test, a few jumper wires, a short program (excuse me, a "sketch") and then look at the outputs.

But at this point I'm not suspecting problems with the IC's.


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 Post subject: Re: Heathkit IO-4550 dual trace trigger problem
PostPosted: Nov Sun 17, 2019 4:04 pm 
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Joined: Nov Wed 30, 2016 7:35 pm
Posts: 5393
Location: Sunbury, Ohio 43074
I've had IC testers of sorts on and off over the years, never owned one but some shops I worked for did. I never used them. Always found it much simpler just to trace logic voltages through a chip. It's either there, or it's not. Such is the case with digital logic. I also found a scope much easier in troubleshooting than any of the various logic probes that were available back in the day. Just me, perhaps, but watching a blinking LED instead of a scope screen I found to be much less revealing about what was really going on.

I may have been spoiled since I was always at a bench, not out in the field, for this kind of work. I suppose dragging a scope around is a lot more difficult than a pocket logic probe. :)

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 Post subject: Re: Heathkit IO-4550 dual trace trigger problem
PostPosted: Nov Mon 18, 2019 6:06 am 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11812
Location: Powell River BC Canada
I have a Heathkit IT 7400 which is good for RTL AND TTL, and as well has
neon lamp readouts so the high voltage ends can also be tested. The dust
on it probably has mites that are now extinct. :D

Logic probes are great. Logic pulsers can do major harm.

Attachment:
IT 7400.JPG
IT 7400.JPG [ 19.65 KiB | Viewed 95 times ]

_________________
de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Heathkit IO-4550 dual trace trigger problem
PostPosted: Nov Sat 23, 2019 7:16 pm 
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Joined: Nov Sun 10, 2019 10:04 pm
Posts: 15
Location: Wellsboro, PA
OK - I thought it was about time to update this thread. The trigger is now working on the 4550, both manual and auto.

The fix for this scope was:
    Replaced the caps on the LV PS board, especially the + - 15 volt caps
    Replaced Q117 on both channels, the EB junction was blown
    Fixed an intermittent -5volt wire connection to the Horz board
    Replaced R149

The other issue I found is that the Wiper of the trigger mode switch would occasionally get shorted to ground by touching a mounting screw (see attached pic). This lead I bent up was not used, there must be another connection between the switch and the board. It was cut off but too long. It was not cut as part of the assembly instruction. It must have been cut at Heath (?).

One last note about voltages marked on the schematic in the area of the trigger amplifier, they are mostly wrong!.
    On the base of Q115 and Q116 you should have about -5V, depending on the trace position control
    On the emitter of Q118 you should have around -12.5 to -13.5 depending on Zero control setting. This results in approx. 6ma of current sink
    On the collector of Q118 you will get about -6.7V
    R144 should be dropping around 10 volts
    The adjustment procedure is to set R149 so that you have zero volts on the emitter of Q117

R149 on this 4550 wound up at 250 ohms for zero volts on TP14 (the emitter of Q117).

I'm still not sure why Q117 was blown on both channels. I looked at the data sheet for IC201 the ua760. If it looses -5volts I don't see why it would send something "nasty" out pin 5. However, if it did send +5volts out on pin 5 that would exceed the max Vebo on Q117.

Another note for future readers of this thread. IC201 is very obsolete. If you find out that you need one, contact D-Lab electronics, Terry has a stash of these.

Thanks to everyone that contributed to this repair, I appreciated it.


Attachments:
TriggerShort_edited.jpg
TriggerShort_edited.jpg [ 461.75 KiB | Viewed 61 times ]
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 Post subject: Re: Heathkit IO-4550 dual trace trigger problem
PostPosted: Nov Sat 23, 2019 11:33 pm 
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Joined: Jun Sat 09, 2007 8:14 am
Posts: 3912
Location: Florida
Good work. As for Q117, the previous owner may have blown these. I was a repair guy in a Radio Shack store back in the dark ages and there were people who could figure out a way to destroy almost anything.

RRM


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