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 Post subject: Hickok 752 reading low on x1 but OK on x2 x4 x10 etc
PostPosted: Jan Tue 10, 2017 6:18 pm 
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Joined: Mar Tue 31, 2009 2:59 pm
Posts: 53
Hi - Just started looking into a Hickok 752.
Tester gives sensible results on all multiplier scales except the x1 scale where it reads low by a factor of about 3. - ie a 1000 reading would come up as around 350 microhmos.
I thought it might be a resistor on the multiplier switch but I think there is more to it than that as they all look OK.
Any pointers as to what might be causing this and where else i might look?
TIA


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 Post subject: Re: Hickok 752 reading low on x1 but OK on x2 x4 x10 etc
PostPosted: Jan Wed 11, 2017 1:09 am 
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Joined: Apr Thu 11, 2013 11:03 pm
Posts: 467
Location: 649 Lacy Wilkerson Rd Roxboro, NC 27574
I have a 752. How are you determining the error? Are you testing a tube using X2 then switching to X1 to compare readings? Tell me your setup and I will try to duplicate it on mine.

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Joel Walker


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 Post subject: Re: Hickok 752 reading low on x1 but OK on x2 x4 x10 etc
PostPosted: Jan Wed 11, 2017 6:35 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 12118
Location: Mpls, Minnesota
Have you checked the multiplier resistors??

Dave


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 Post subject: Re: Hickok 752 reading low on x1 but OK on x2 x4 x10 etc
PostPosted: Jan Wed 11, 2017 10:06 am 
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Joined: Mar Tue 31, 2009 2:59 pm
Posts: 53
joelandjoyce wrote:
I have a 752. How are you determining the error? Are you testing a tube using X2 then switching to X1 to compare readings? Tell me your setup and I will try to duplicate it on mine.


I had it set up for a 12AX7 using the data on the roll chart which is suggested as a x4 on the multiplier. This gives only a very small movement on the scale so i changed multiplier to x2 to get better magnification and got double the reading of the x4 position but still not a huge movement of the scale so i tried multiplier at x1 and got a smaller reading.
Is there some reason why you cannot use the multiplier x1 switch setting with data for other multiplier switch positions?


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 Post subject: Re: Hickok 752 reading low on x1 but OK on x2 x4 x10 etc
PostPosted: Jan Wed 11, 2017 2:31 pm 
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Joined: Mar Sat 28, 2009 8:27 pm
Posts: 691
Location: Georgia
I had a 752 a few years back but sold it after I repaired it. The multiplier resistors which were "bobbin" or wire wound types were at fault. I ordered some precision ones from digi-key but as I recall I also had to use a parallel combination to solve one or more. The resistance wire those bobbin R's are wound with must be soldered with silver bearing solder so I just replaced them if they were out of spec. Good luck.


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 Post subject: Re: Hickok 752 reading low on x1 but OK on x2 x4 x10 etc
PostPosted: Jan Wed 11, 2017 2:32 pm 
Member

Joined: Mar Sat 28, 2009 8:27 pm
Posts: 691
Location: Georgia
I had a 752 a few years back but sold it after I repaired it. The multiplier resistors which were "bobbin" or wire wound types were at fault. I ordered some precision ones from digi-key but as I recall I also had to use a parallel combination to solve one or more. The resistance wire those bobbin R's are wound with must be soldered with silver bearing solder so I just replaced them if they were out of spec. Good luck.


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 Post subject: Re: Hickok 752 reading low on x1 but OK on x2 x4 x10 etc
PostPosted: Jan Wed 11, 2017 4:52 pm 
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Joined: Nov Sun 04, 2007 6:01 am
Posts: 3039
Location: Redding, CA
Busters wrote:
joelandjoyce wrote:
I have a 752. How are you determining the error? Are you testing a tube using X2 then switching to X1 to compare readings? Tell me your setup and I will try to duplicate it on mine.


I had it set up for a 12AX7 using the data on the roll chart which is suggested as a x4 on the multiplier. This gives only a very small movement on the scale so i changed multiplier to x2 to get better magnification and got double the reading of the x4 position but still not a huge movement of the scale so i tried multiplier at x1 and got a smaller reading.
Is there some reason why you cannot use the multiplier x1 switch setting with data for other multiplier switch positions?


Yes, there is a reason. The multiplier switch also changes the magnitude of the signal applied to the grid. In this tube tester low amplification tubes have a large magnitude signal applied and high amplification tubes have a small signal applied. Applying the large signal of the "x1" position to a 12AX7 likely exceeds the operating range of the 12AX7 hence what you are observing is an estimate of the operating range with respect to the high input signal rather than the mutual conductance within the operating range.

Norman

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"No home is complete without music" - Capehart Sound Amplifying and Distributing Systems


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 Post subject: Re: Hickok 752 reading low on x1 but OK on x2 x4 x10 etc
PostPosted: Jan Thu 12, 2017 12:25 pm 
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Joined: Mar Tue 31, 2009 2:59 pm
Posts: 53
Thank you for the explanation. It is a bit more than just a multiplier switch then on the 752.

My last tester was a 533A and the range switch on that model was purely that so you could magnify up on the meter. With a 12AX7 you are working in the first 10% or so of the meter scale on x4.

Also I think i prefer the 533A system of giving you the good figure rather than the bad figure.


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