Heathkit IG-42 Checkout – Part 2

 

Heathkit IG-42 Checkout – Part 2

Filed in Heathkit IG-42 on Mar.18, 2018

I put the IG-42 Signal Generator back on the bench to continue my initial inspection.  Today, I measured the voltages specified in the construction manual and measured the resistors (in circuit) to see if there were any obvious problems to address.

Some of the tube pins were very difficult to access from the bottom side, so I stuck a wire in the top side on those pins and re-inserted the tube to get the readings.  This allowed me to continue without tearing the unit apart.

 

 

Cheater Wire for measuring tube voltages

All measurements were made with the unit plugged into me variac transformer, with the voltage set to 115 vac, as originally specified for the IG-42.

 

115 vac input voltage

The results are listed in the chart below…

TUBE	Pin1 Expected	Pin1 Actual	Pin2 Expected	Pin2 Actual	Pin3 Expected	Pin3 Actual	Pin4 Expected	Pin4 Actual	Pin5 Expected	Pin5 Actual
6AF4	65	89	10 - 30 Neg.	-13.8	0	0	5 - 7 AC	5.9	0	0
6AV5	0	0	5 - 7 AC	5.9	12	14.9	0	0	100	113.7
12AU7	60	72.4	1.0 Neg.	-1.3	2	1.95	5 - 7 AC	5.9	5 - 7 AC	5.9
OB2	100	131.5	0	0	0	0	0	0	100	131.6
TUBE	Pin6 Expected	Pin6 Actual	Pin7 Expected	Pin7 Actual	Pin8 Expected	Pin8 Actual	Pin9 Expected	Pin9 Actual
6AF4	10 - 30 Neg.	-13.8	65	89
6AV5	0	0	0	0	100	131.5
12AU7	100	117	30	35.5	33	37.5	0	0
OB2	0	0	0	0

As you can see, the plate voltages are running quite a bit higher than they should be. Some of the other voltages are also moderately high.

I chased this down a bit further and found that the voltage output of the regulator tube 0B2 was sitting at about 131 vdc.  It should be regulating to about 108 vdc.  I understand that these tubes vary a bit from one to another, but 131 volts is quite a bit too high.  The input from the rectifier is only 142 vdc.

So I pulled another OB2 from my Heathkit HG-10B VFO, to try it here and see if it corrected the problem.  With the replacement tube in place, the regulated voltage is sitting at about 116 vdc.  Still a little high, but probably usable.  I’ll have to look through my boxes of spares to see if I can find another replacement.

For the second part of the testing today, I measured the resistors, without pulling them from the circuit.  In that situation, I would expect that they should all read at or below their specified values.  There were a few that were far out, and I found one that had the wrong size resistor installed.  I don’t know why, maybe it was a mod to improve something.  I’ll have to look into it further.

Here are the results of the resistance measurements…

Resistor	Tube Pin	Measured Value	Filter Cap	Meter Switch	Fine Attenuator	Attenuator	Cable Term
51 ohm							Not Present
56 ohm						Inaccessible
62 ohm						51.4 ohm
62 ohm						50.6 ohm
62 ohm						56.7 Ohm
62 ohm						Inaccessible
470 ohm	6AV5-3	458 ohm
510 ohm						91.8 Ohm
510 ohm						95.9 ohm
510 ohm						Inaccessible
510 ohm						Inaccessible
510 ohm					268 ohm
1K ohm	12AU7-3	1 K
1K ohm 2 watt			940 ohm
1K ohm 2 watt					932 ohm
2.2 K ohm	6AV5-1	2.3 K
4.7 K ohm			4.47 K
4.7 K ohm					2.2 K
10 K ohm	12AU7-8	10.7 K
10 K ohm				10.1 K
10 K ohm				9.17 K
10 K ohm	6AV5-8	9.9 K
15 K ohm	6AV5-1	14.4 K
22 K ohm	12AU7-1	23.76 K
47 K ohm	12AU7-2	74.6 K (68K installed here)
100 K ohm	6AF4-2	97 K
470 K ohm	12AU7-7	970 K
1 M ohm	12AU7-6	5.5 M

There are a couple that I will replace (in red). Some of the resistors in the attenuator section were not accessible without tearing the unit apart. Given that those that were accessible were good, I’ll go with the assumption that the others are also good, at least until I see problems with the calibration.

Stay tuned for more adventures!