Author Topic: HeathKit IO-104 Repair - No Trace  (Read 6493 times)

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Offline tautech

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Re: HeathKit IO-104 Repair - No Trace
« Reply #25 on: June 05, 2017, 04:50:55 am »
Sorry, didn't read my meter correctly. The first Zener in that chain gets 12V in and then produces negligable output. What I said was -125V is actually -.125 mV. The chain of zenner diodes is supposed to start off with 68V.

Q404 to A procused nearly +200V
Which I think is in the ballpark of where it needs to be BUT understand it's a 200V difference between those 2 points as A is linked to -1400V via R5 1M \$\Omega\$.
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Offline mentaldemise

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Re: HeathKit IO-104 Repair - No Trace
« Reply #26 on: June 05, 2017, 05:06:42 am »
Great. That's one thing down. Is it possible that it's blanking all the time? It looks like the input for the blanking circuit comes from MM which is Q314's output and uses the +5 that's currently dickered. The Schmitt Trigger goes to two NPN transistors. Following things around it looks like Z should have 1.2V on it from Q314 but it appears to have 3.7V on it instead(with stability in the center and Time/CM set to 1mSec.)
 

Offline tautech

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Re: HeathKit IO-104 Repair - No Trace
« Reply #27 on: June 05, 2017, 05:18:17 am »
Without the circuit description I'm not sure where blanking is performed but that's academic as until you get the +5V rail up it doesn't matter. All rails need be up and close to spec.
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Offline mentaldemise

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Re: HeathKit IO-104 Repair - No Trace
« Reply #28 on: June 05, 2017, 05:23:06 am »
Well, as it happens I have the original assembly book that came with it an includes those! https://goo.gl/photos/q3ajba1UpTk1go1D8
 

Offline mentaldemise

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Re: HeathKit IO-104 Repair - No Trace
« Reply #29 on: June 05, 2017, 05:34:43 am »
I want to repeat my appreciation for your help! Am I incorrect in thinking that disconnecting M will change the voltage that I should expect there? I
 

Offline mentaldemise

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Re: HeathKit IO-104 Repair - No Trace
« Reply #30 on: June 05, 2017, 05:43:54 am »
Might be way off here again. It looks like to drop 18V down to 5 with the 120 Ohm resistor they have then I'm looking at 100 mA and only need a 1.3Watt resistor.

 

Offline tautech

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Re: HeathKit IO-104 Repair - No Trace
« Reply #31 on: June 05, 2017, 05:49:36 am »
I want to repeat my appreciation for your help! Am I incorrect in thinking that disconnecting M will change the voltage that I should expect there? I
If M is +5V do not disconnect while the other rails are fired up, you will fry something.
Supplying +5 from another supply is the way to go, there might be mention of the expected current draw in the manual or circuit description.
Take a step back and re-read the thread, give some thought as to next steps which should really be focussed on why the +5V rail is low.
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Offline mentaldemise

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Re: HeathKit IO-104 Repair - No Trace
« Reply #32 on: June 05, 2017, 07:06:17 am »
I was using the circuit diagram that says R407 should have +18V on one side and then goes to HH which should have 5 and then a resistor divider calculator to figure out what amperage it would draw with a 120 Ohm resistor as is in place. That gave me the 100mA. I hooked up another DC supply limited to 100mA and it drew full current. I'm guessing I have a short to ground on the 5V rail, but with 100mA nothing is actually getting hot. To actually get to things I'm going to need to disassemble the time/cm selector and de-solder its 20+ connectors. :(
 

Offline tautech

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Re: HeathKit IO-104 Repair - No Trace
« Reply #33 on: June 05, 2017, 08:26:06 am »
Of course you need to check there's no bead tantalums on the PCB, I couldn't spot any in the schematic but if there is let them be #1 suspects.
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Offline mentaldemise

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Re: HeathKit IO-104 Repair - No Trace
« Reply #34 on: June 05, 2017, 11:01:19 am »
I don't see any either, of course searching google for "vintage tantalum capacitor" shows pictures similar to all the caps in the thing. I think I'm going to order some parts to remove some of the bodges that are in here as well. Instead of 2 X .1 @ 2000 V there are 4 .05s. There are 2 1W 500k resistors instead of a 2W 220K. I wish there was a way to get the CRT to turn on to know if I should even continue. I can't even find a replacement tube, so if that's what's wrong I'm pretty much screwed and it's a display piece.
 

Online james_s

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Re: HeathKit IO-104 Repair - No Trace
« Reply #35 on: June 05, 2017, 02:49:40 pm »
I'm sure the CRT is fine, they don't tend to fail suddenly, they just get dimmer and fuzzier gradually over time. Also they're not impossible to find, you can easily sub a similar electrostatic deflection CRT.
 


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