Heathkit IO-103 oscilloscope

[pchilson]

I would love to see one for grabs. Thanks

[Tom Phillips]

I would love to see one for grabs. Thanks

Load the web page at https://sc2.vom.com/d8/index.fwx?C=MANUALS and enter “IO-103” in the search box.

The home page of the site is Data Professionals Main
It is run by the guy who bought the rights to the Heathkit intellectual property a couple of years ago. I have never had any dealings with the person / company. The web page is not fancy and is the same site that hosts the parts cross reference that Enzo linked earlier in this thread.

The manual should contain lots of information including theory of operation, calibration and troubleshooting. I expect that you could get more info about the exact content of the specific manual from the seller before you buy anything. (Including verification that it is an original hard copy) Let me know how it works out.

Tom

[pchilson]

Thanks Tom.

[pchilson]

Picking back up on this.

I have the power supply sorted and now have the proper voltages on the + and - rails to the vertical amp board.
I have more or less the correct voltages through the Q transistors up until I get to the Q106 and Q107 pair. At that point is when it starts to fail.
The collector on Q106 shows a voltage drop from rail voltage by the R106 resistor. On Q107 collector it is showing full rail voltage but R109 is fine out of circuit.
On the Q108 and Q109 pnp pair the voltage on the emitters is at full rail voltage and not dropped by the R111 and R112 resistors.
I am shutdown at the collectors on this pair, 0vdc.
The Q111 and Q112 pair is at 120vdc on the collectors when it should run at 100vdc.

Q108 and Q109 have been replaced with 2N3906s, taken from the cross reference as the original X29A826s are nowhere to be found.
Q111 and Q112 have been replaced with NTE171s which is the replacement for the original D40N1s.

The trace is visible on screen.
It is centered vertically but there is no movement with the vertical position control knob.
The trace becomes visible at the leftmost edge of the screen but this is with the horizontal control jacked all the way to the right.

I have no way of knowing but my suspicion is someone touched a high voltage source with a probe not rated and took the power supply out for the vertical amp. If this is the case the signal would have entered on the Attenuator board which flows to the Vertical amp board which is connected to the Sweep generator/ Horizontal amp board from the emitters of Q111 and Q112, which are part of the problem described above.

My limited skill level has been reached. I need a couple of nudges.

Thanks guys.

[bob p]

I bought an old Heath O-10 oscilloscope for 99-cents on eBay and did a local pickup. I have a total of 99-cents plus some gas invested in an old, ~500 kHz tube-based scope. I thought it would be a fun repair project, in a masochistic sort of way.

The unit had actually been in use by an automotive repair shop for engine diagnostics, so it was in "working" condition. The problem was that when I fed it a sine wave input the scope traces were really distorted. There was a lot of non-linearity going on inside of the scope, in both the horizontal and vertical deflection circuits.

One thing that became immediately obvious is that it was never going to be cost effective to repair this unit, and if I wanted to repair it, it was going to be a money-sink. The only reason for even bothering would be to spend some money to have a really funky old retro scope as an amusement. I imagine that your transistor based scope is going to be a similar type of project.

Once I realized what a problem child I had on my hands, I decided to buy a used Hitachi solid state scope, and to make the Heath scope a repair project rather than relying on it as a piece of test gear.

At the risk of stating the obvious, these devices are old enough that searching for a defective part in a circuit is going to be a time consuming effort. Truth be told, there are probably a lot of subtle defects going on inside of this scope that are only going to be noticed as you repair the major defects, which then render the minor defects noticeable. Looking over my scope, I found lots of CC resistors that had drifted upwards, lots of electrolytic caps that were in need of replacement, lots of coupling caps that were leaky, and many of the tubes were going to need replacement. Rather than trying to debug each individual circuit, I decided that a better approach would be to gut the entire circuit and rebuild it, starting with the power supply.

So if I were in your shoes, I would just start by replacing all of the resistors and caps in the power supply, and then start to debug the individual circuits in the scope. Resistors and caps are not at all expensive, compared to the amount of time that you're going to spend trying to debug a circuit. I think the most efficient use of time and money, if you really want to fix the scope, is to gut the power supply and build a new one. I'd probably end up doing the same with the rest of the amplifier circuits as well.

I think that with this really old gear, rebuilding complete circuits makes more sense than trying to troubleshoot and repair.

I think having the assembly manual is worth every penny of $20.

YMMV.

[pchilson]

Hi Bob, thanks.

This is just hobby/entertainment for me. I am not trying to get anywhere with this scope other than further ahead in diagnostics and troubleshooting knowledge.
We all start somewhere and I'm sure most of us didn't learn it all by our lonesomes.

[Jazz P Bass]

You have stated the collector voltages of Q106 & 107.
But what are the base voltages. (are Q104/105 Jfets working?)
And Q103. What are the BCE voltages?

[pchilson]

You have stated the collector voltages of Q106 & 107.
But what are the base voltages. (are Q104/105 Jfets working?)
And Q103. What are the BCE voltages?

Hi Jazz, thanks.

A bit strange, I fired it up to take these measurements and did the calibration adjustments for the vertical amp to get voltages in place at the points instructed and now I'm getting output from Q109 collector as I should but the collector on Q108 is still dead.

Here is what I have just taken...

Q101
G -01.5vdc
D +9.2vdc
S -00.2vdc

Q102
B -7.2vdc
C -00.2vdc
E -7.9

Q103
B -7.2vdc
C +2.0vdc
E -7.9vdc

Q104
G -01.3vdc
D +9.2vdc
S +2.2vdc

Q105
G -00.2vdc
D +9.2vdc
S +2.9vdc

Q106
B +2.2vdc
C +9.2vdc
E +2.0vdc

Q107
B +2.9vdc
C +8.2vdc
E +2.2vdc

Q108
B +9.2vdc
C 00.0vdc
E +9.2vdc

Q109
B +8.2 vdc
C +2.3vdc
E +8.9vdc

Q111
B 00.0vdc
C +121.4vdc
E 00.0vdc

Q112
B +2.3vdc
C +94.2vdc
E +1.7vdc

[pchilson]

Could this be some kind of bias issue?
My understanding is on a pnp transistor (Q108, Q109) if the base voltage is lesser than the emitter voltage then the collector is open for flow. Is this correct?
If so, then the equal 9.2vdc on the base and emitter would shut off the flow to the collecter. Is this right? How to bring the voltage lower as it should be then?

I'm speculating. Thanks

[Jazz P Bass]

is Q106 new?
I ask because Q108 is acting goofy.

[pchilson]

Q106 is the original part at present.

New developements. I started going through the calibration procedures as they seemed to turn on the output on Q112 when I did it before. I wanted to see if going further would turn on Q111 and it did.
I need to go through the entire calibration procedure before I can know where I stand it seems.

More later.

Thanks

[Jazz P Bass]

Heathkit Crossreference

Here is a PDF file of the info.

[pchilson]

Hey guys,

I went through the calibrations for the vertical and horizontal amps and I can report tentative success.
There is still a couple of pages of the calibration process and I'm sure things will improve as these steps are run through.

One thing I'm wondering. There are many trimmer pots that have to be used to fine tune but they are so touchy that its near impossible to land on the exact voltage the calibration is calling for, eg... you need to dial in R104 to get exactly 9vdc on C104. You can get to 9.11 or 8.89 but the tiniest movement and you jump back and forth between these two readings. Are these close enough or am I splitting hairs? Shouldn't the trimmer be valued such that it would be a smooth adjustment?

Anyway, I'm happy to see it running.

[52 Bill]

Congrats! Glad to see it working.

The trimmers are just like larger pots, they can be dirty and oxidized. Sometime if you hit them with a little Deoxit they will be easier to adjust.

[The Dude]

Nice work! Agreed on the Deoxit. Also, most manuals give a plus or minus figure on the adjustment parameters. If not, I would think the voltages you stated would certainly be close enough.