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 Post subject: Hickok 750 - Voltages question
PostPosted: Jan Mon 15, 2018 12:46 am 
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Hello, all:

Doing the voltage check on my Hickok 750 on an on-going quest for "calibration", and here is what I am seeing:

All Switches and Pots are in 6L6 test position.

Using 250k shunt for DMM:

Plate, 3(+) & 8(-): 150 VDC (Pressing P4)
* Line adjust check - correct on the line adjust mark
Screen, 4(+) & 8(-): 130 VDC (pressing P4)

So far near perfect...

No shunt for DMM:

Signal, 5(+) & 8 (-), rotating the "mMho" selector switch through positions:
"A": 2.526 VAC
"B": 2.523 VAC
"C" & "3,000": 2.525 VAC

So far, so good... Only ~0.025 VAC higher than the nominal value expected...

And this is where it starts to get strange:

"D" & "6,000": 1.589 VAC (?) - isn't signal voltage supposed to be exactly half of the "3,000" range, i.e. closer to 1.25 VAC?
"E" & "15,000": 1.191 VAC
"F" & "30,000": 1.123 VAC
"G" & "1,500": 1.582 VAC (identical to "6,000"?)
"H": 1.236 VAC

Does this look correct to any of you? If not - any suspicions? Also, if someone has a 750, I would really appreciate if this person can post his voltages, for comparison.

Dmitri


Last edited by Fennec on Jan Sun 21, 2018 6:17 pm, edited 1 time in total.

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 Post subject: Re: Hickok 750 - Signal Voltages question
PostPosted: Jan Fri 19, 2018 4:15 am 
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Well, in absence of replies I think I have figured it out.

I have noticed that when taking measurements I have not turned down the Bias pot, i.e. on AC I was reading the signal + rectified sine wave form from Bias supply.

Now Signal readings are much closer to "nominal", albeit a bit lower than I would like to see:

"A", "B", "C": 2.38 VAC (vs. 2.5)
"D": 1.2 VAC (vs. 1.25)
"E": 0.46 VAC (vs. 0.5)
"F": 0.24 VAC (vs. 0.25)
"G": 1.2 VAC (vs. 1.25)
"H": 0.75 VAC (= nominal 0.75)

As a side note, for this test I have re-calibrated the "Line Adjust" to 6.3 VAC on the heater... Was it the wrong thing to do? Calibrating for 150 VDC Plate results in signal being even slightly lower, as well as lower heater voltages...

Would this be an indication of 5Y3 tube being weak? Or possibly some of the resistors have crept up in value?


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 Post subject: Re: Hickok 750 - Signal Voltages question
PostPosted: Jan Fri 19, 2018 7:02 am 
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You need to set the line adjust based on the input voltage to transformer set for 100V AC, you cannot go by the filament voltage nor the plate voltage. Alternative is to use a variac set to 100V AC and set the 750's rheostat to 100% full on, then adjust your line set. The plate voltage and heater voltage unloaded often read higher, so the plate voltage might be 155V or higher (and this needs to be with a properly loaded voltmeter 1000 Ohm/V), the heater voltage might read 6.4 or 6.5. Most likely when you do so, the signal voltages will be good enough. The signal is taken off of a 5V tap from the transformer, then goes through a voltage divider chain of resistors R29, R30, R31, R32 and R30. Sometimes these resistors can drift a bit in value, but if your signal voltages are within 5% I would leave them be. Otherwise you can detach one end of R29 and then sequentially measure each resistor. The 5Y3 is used for the bias and screen voltages, has no effect on the signal voltage.


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 Post subject: Re: Hickok 750 - Signal Voltages question
PostPosted: Jan Sun 21, 2018 1:53 am 
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mksj wrote:
You need to set the line adjust based on the input voltage to transformer set for 100V AC, you cannot go by the filament voltage nor the plate voltage. Alternative is to use a variac set to 100V AC and set the 750's rheostat to 100% full on, then adjust your line set. The plate voltage and heater voltage unloaded often read higher, so the plate voltage might be 155V or higher (and this needs to be with a properly loaded voltmeter 1000 Ohm/V), the heater voltage might read 6.4 or 6.5. Most likely when you do so, the signal voltages will be good enough. The signal is taken off of a 5V tap from the transformer, then goes through a voltage divider chain of resistors R29, R30, R31, R32 and R30. Sometimes these resistors can drift a bit in value, but if your signal voltages are within 5% I would leave them be. Otherwise you can detach one end of R29 and then sequentially measure each resistor. The 5Y3 is used for the bias and screen voltages, has no effect on the signal voltage.


OK, here is the result:

NOTE: My 750 is a version with a leakage lamp, so schematic is 862W

1. Trying to adjust the line for 100 VAC after the main rheostat I ran out of range on R32 pot
2. Added 100 Ohm resistor in series with the meter
3. Now adjustment is successful - 100 VAC on primary of the transformer, Line exactly on the mark
4. Plate voltage (with 250k shunt) - 170.1 VDC (vs. nominal 150 VDC)
5. Screen voltage (with 250k shunt) - 150.0 VDC (vs. 130 VDC)

Both Plate and Screen readings are taken in non-loaded condition (no tube in the socket), and both are exactly 20V above what I understand should be nominal voltages...

Anyone has any suggestions as to what is happening here?

In regards to the Signal voltage, 5V transformer winding terminals are P and Q (from 862W schematic). Following P line, it looks like divider resistors are R28, R25, R24, R19, and R20.


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 Post subject: Re: Hickok 750 - Voltages question
PostPosted: Jan Sun 21, 2018 6:28 pm 
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Decided to take the readings with a tube under test.

Picked a NOS off-brand 6L6, warmed it up for about an hour, and here is what I get:

Filament: 6.4 VAC (drops to 6.1 when under test)
Plate: 145 VDC (P4 pressed)
Screen: 135 VDC (P4 pressed)

The tube tests well into the green on range B (for reference - around number 82 on the meter MILS scale).
Mutual conductance reads around 4200, and is roughly the same on both the suggested 15,000 range (E) and 6,000 range (D).

Does all this look about right? One thing I noticed - during the P4 test a Line Fuse bulb lights up... Not burning bright, but a nice steady orange glow.. Is this common for this type of tube?


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 Post subject: Re: Hickok 750 - Voltages question
PostPosted: Jan Sun 21, 2018 11:45 pm 
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I do not follow the 100 ohm resister in series with the meter?
The plate should be around 150-160V unloaded (using a 250K resistor between the plate and the cathode and measuring on either side with a voltmeter), under test with a 6L6 the plate should be between 135-145V as a estimate. You also need to check the signal voltages. Your screen voltage sounds a bit high, should be closer to 130 under test. Gm say 5000 per the role chart for a 6L6. I do not have this tester, so my comments are generic relative to my 539B.

The voltage divider resisters vary depending on the schematic. stevenjohnson.com/hickok/data/hickok-750-manual.pdf


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 Post subject: Re: Hickok 750 - Voltages question
PostPosted: Jan Mon 22, 2018 1:06 am 
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Checking the various voltages in a Hickok is an exercise in futility, at least until you have verified that the 100v line setting is correct. And as mksj points out, that has to be done properly or no readings will be correct. Making your line meter read 100v with a resistor is only a solution if you are absolutely SURE that the line voltage reading is the issue, otherwise all you've done is throw off all the rest of the voltage readings. To calibrate the line voltage adjust setting, you need to do it properly. If you are using a digital meter to test your voltages, your readings are most likely way off.

If the line setting is off, everything is off. Again pointed out by mksj, testing voltages without any load will also lead to frustration. Which I'm sure you already have ;-)

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 Post subject: Re: Hickok 750 - Voltages question
PostPosted: Jan Mon 22, 2018 1:10 am 
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Thank you for your help so far.

mksj wrote:
I do not follow the 100 ohm resister in series with the meter?

This is strictly for the meter circuit - as mentioned, I have maxed out the Line adjustment pot when bringing voltage up to 100 VAC on the primary.

mksj wrote:
The plate should be around 150-160V unloaded (using a 250K resistor between the plate and the cathode and measuring on either side with a voltmeter)

I agree, it does seem high, but measuring with a 250k shunt between pins 3 and 8 this is what I see.

mksj wrote:
under test with a 6L6 the plate should be between 135-145V as a estimate.

This is what I see, albeit on the high end of this range. Again, measured with 250 shunt between 4 and 8.

mksj wrote:
Your screen voltage sounds a bit high, should be closer to 130 under test.

Again - agree, for some reason all voltages are higher than what one would reasonably expect...

I will run some eadditional tests during the week.

I wonder if the primary voltage on a 750 should be some other value, ex. 93 VAC like on I-177 series units...


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 Post subject: Re: Hickok 750 - Voltages question
PostPosted: Jan Mon 22, 2018 10:29 pm 
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Barry H Bennett wrote:
Checking the various voltages in a Hickok is an exercise in futility, at least until you have verified that the 100v line setting is correct. And as mksj points out, that has to be done properly or no readings will be correct. Making your line meter read 100v with a resistor is only a solution if you are absolutely SURE that the line voltage reading is the issue, otherwise all you've done is throw off all the rest of the voltage readings. To calibrate the line voltage adjust setting, you need to do it properly. If you are using a digital meter to test your voltages, your readings are most likely way off.

If the line setting is off, everything is off. Again pointed out by mksj, testing voltages without any load will also lead to frustration. Which I'm sure you already have ;-)


Thank you for the input, Barry. Just to make sure I understand - am I supposed to use a 1,000 Ohm/V shunt when adjusting for 100 VAC on the primary of the transformer as well? I thought shunting only applies to DC voltage...

I've read that when checking AC signal voltage no shunts are to be used, so I have automatically assumed that it applies to ALL AC voltages...


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 Post subject: Re: Hickok 750 - Voltages question
PostPosted: Jan Mon 22, 2018 10:49 pm 
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It sort of depends on the meter you are using to test the Hickok meter. You are correct that for this measurement, a KNOWN good AC meter can be used to check or calibrate the 100v. It need not be shunted, as it has internally "done the math" to measure AC volts properly, as long as it is a reasonably decent meter digital or otherwise. My comments on "a resistor" in the previous post referred to the possibility that someone had inserted one into the meter circuit in the past. If the Hickok voltmeter was reading high, that's a cure. If it reads low, there *may* be an internal resistor inside the meter movement but let's hope this one reads correctly as below:

Paraphrased from several competent Hickok scholars:
With power off, check the mechanical zero reading on the volts meter on the Hickok. Connect a KNOWN accurate AC voltmeter across the primary of the main power transformer, power on, and adjust the line adjust control on the tube tester so your voltmeter reads exactly 100V AC. The meter on the tube tester should now read the same 100V. (the line on the meter). If it's off, see above or if you are SURE of your external meter you can simply make a new "100v" mark on the Hickok's meter face.

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 Post subject: Re: Hickok 750 - Voltages question
PostPosted: Feb Sun 18, 2018 3:04 am 
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OK, first of all - thank you all who piped in and offered their take on this. Now, after a few tests, here is what I have discovered:

The voltage on the primary for my specific model of 750, s/n 149-10004, is NOT 100 VAC, but actually 93 VAC - same as on I-177 series testers. I am not sure why this is, but it might be an early production unit, and an older transformer has been used.

I have selectively checked a couple of secondary voltages, and here is the result:

P-Q: 5.01 VAC (vs. 5.00 on schematic)
O-N: 160.2 VAC (vs. 160 VAC on schematic)
N-M: 160.2 VAC (vs. 160 VAC on schematic)

I have re-calibrated the meter 0 for this voltage. The results are as follows:

In non-loaded condition, plate and screen checked at:

Plate: 160.6 VAC
Screen: 136.4 VAC

Signal voltages:

"A", "B", "C": 2.522 VAC (+0.022)
"D": 1.260 VAC (+0.010)
"E": 0.487 VAC (-0.013)
"F": 0.243 VAC (-0.007)
"G": 1.260 VAC (+0.010)
"H": 0.790 VAC (+0.040)

Filament voltages are within +/-0.2 VAC

uMho ranges check out, using a "simulated" AC between pins 3 and 8 (BIAS and RANGE set to 0)

I have also tested a couple of voltage regulator tubes, and meter is indicating them striking very close to nominal values, so I think I will leave the VR part of the circuit well alone.

Considering all of the above, would you be in agreement that the tester is now reasonably adjusted? Or is there something else that one needs to check before making this assertion?


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 Post subject: Re: Hickok 750 - Voltages question
PostPosted: Feb Sun 18, 2018 6:28 am 
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93 volts for the transformer is correct. Here is a link you may find useful.
http://www.jacmusic.com/Tube-testers/Hi ... ndex2.html

Dave


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 Post subject: Re: Hickok 750 - Voltages question
PostPosted: Feb Sun 18, 2018 8:21 pm 
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easyrider8 wrote:
93 volts for the transformer is correct. Here is a link you may find useful.
http://www.jacmusic.com/Tube-testers/Hi ... ndex2.html

Dave


:shock: Indeed, it is 93... And somehow I managed to overlook it when referencing this page... Oh, well, it is correct then :)

With this in mind, I think that this particular tester is now usable as an instrument again, and I can move on to the next project :wink:


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