Heathkit 10mhz Oscilloscope

[tehjrow]

A friends dad gave me a Heathkit IO-4210 10mhz Dual Trace Oscilloscope.  I have no probes for it yet.  When I turn it on all I get is a green dot on the screen.  Shouldn't it show a straight line instead?  I found in the manual how to reset everything to default settings so I did that and still a dot.  I started going through the Trouble Locator Chart (this thing is amazing) and I got to the point where it says to turn the AUTO-NORM switch to AUTO and the trace should disappear.  Well, the dot just stayed there.  So I dug deeper.

After going through the +5v and -5v troubleshooting tests (test #1 and test #2, it passed them) I got to Test #3 and did this: 
Measure -15v on TP4: failed (got 0v)
Measure .6 volts DC voltage drop across R307: failed (got .12V)
Measure voltage of -18V on the emitter of Q302: failed (got -21V)
Measure voltage of -20V on the base of Q302: passed (got -21V so -1V higher)
Measure voltage of .6 volts DC between Base and Emitter of Q303(different transistor): passed

Then it just says Check Q302 so I got the voltage between the Base and Emitter of Q302 at .12 volts DC (can't tell if this failed because it doesn't tell me what it should be).

Does this mean that Q302 or R307 is bad?  Or will the -15V TP4 not show -15V if I have no probes connected?

EDIT:  Just noticed that IC301 gets to over 130F (degrees) in a couple seconds after turning the unit on.  Could this thing be bad (its part number is sg4501n)?

[PaulAm]

It helps to supply a diagram when you're asking questions like that.

The dot in auto tells you the sweep generator isn't working.

I found a schematic for a 4205, but I don't see tp4 listed.  There is a -15V section on the power supply.  If that corresponds to tp4, then that could be the root cause of your problem.

I'd check the voltages on the power supply and make sure they're correct (particularly the +9 and -9V).   Stay away from the high voltage, the dot tells you that's working.  The trouble chart was made to help you track down assembly errors (and there may be some), but it kind of assumes the parts are good.  Given the age, power supply filter caps are always a suspect.

[tehjrow]

I could try taking photos of the diagrams but I can't seem to find them online.  I'm getting some probes tomorrow so I can check and see if it works with probes on, although I kinda think it won't since there is no -15V on that Test Point.

[tehjrow]

Got a probe and tried CH1 and CH2 hooked to a 9V battery, nothing, still just a dot.  Will scan/photograph the diagrams next.

[PaulAm]

That's kind of expected.  If you're missing a power supply not much is going to work.

These might be close enough:
http://web.archive.org/web/20020201194744/http://www.circuitarchive.co.uk/heath.htm

Look for IO-4205 about halfway down.  Those might match up enough to start.  At this point, I'd only be interested in the low voltage power supplies.

[tehjrow]

Here's the schematic for the High and Low voltage boards and some readings below:

This is what I get coming from the transformer to the low voltage board:
Orange to Orange:  16VAC
Yellow to Yellow: 36VAC
Red to Red: 160VAC

On the points marked DC Out, I get:
Says +5V, Measured +5V
Says -5V, Measured -5V
Says +15V, Measured +4V
Says -15V, Measured 0V
Says +150V, Measured 202V
Says +170V, Measured 207V

[Andy Watson]

Either IC301 or Q302 is dead (could be both). Remove Q302 from the circuit and measure the resistance every-which-way between CB, CE, BC, etc.. If it's good you should be able to identify resistance measurements indicating a diode between base and emitter and a diode between base and collector. Any other conductivity means the transistor is dead.

[PaulAm]

OK, that helps.

Measure the voltage at the emitters of Q301 and Q302 to make sure things are fine there.  You should see the +/- 21V, give or take a little marked on the diagram.

If that's fine, Q301 and Q302, should be looked at and also IC301.  It's possible one of the output filter caps is shorted.  With the scope off, measure the resistance from +15 to ground and -15 to ground, it should not be zero. Check the voltages on the collectors and bases  of Q301 and 302 as well.


If your dmm has a diode range, you can check the junctions on Q301 and Q302.

If all those parts check out OK, it's probably the IC.

[tehjrow]

I can't seem to find this IC for sale anywhere:

sg4501n

[Deathwish]

I can't seem to find this IC for sale anywhere:

sg4501n

http://www.4starelectronics.com/part_detail/SG4501N.html

http://www.ebay.com/itm/DUAL-VOLTAGE-TRACKING-REGULATOR-IC-SG4501-SG4501J-NEW-/251481899589

[tehjrow]

I saw the SG4501J one, I can't find a datasheet to see if its the same as SG4501N

[PaulAm]

N is the plastic package, J is a ceramic package.  They're functionally identical.