I have started to resurrect a Hickok 534A tube tester. The first problem is that the meter is no good. I traced the problem to an open wire-wound resistor inside the meter housing. It is marked 1630 so I am assuming that is the resistance. It looks like a bear to remove it and attempt a repair without disturbing the delicate meter movement. I intend to just replace it with 1630 ohms of series resistance. The meter coil and movement seems fine and it tests as 200 ua FS. My dilemma is that I have located and downloaded a 534 schematic from the web and the meter is listed as 50 ua 7500 ohms. That is very different from what I have here. Does anyone have a 534A schematic? It seems odd that they should be that different. <P>The second issue is that the Bias Pot is missing so I am looking for the specs on that. Again, a schematic would be helpful. I am not sure what R designation the bias pot is on the 534 schematic I have.<P>Third, does anyone have calibration information on this unit.<P>Thanks in advance for any assistance.<P>------------------<BR>Tim

Tim, I've got a 534B with all the original documentation that came with it and the meter also shows 50ua and 7500 ohm. There is no breakdown on what each pot's function is. Nothing about calibration either. The pot values for the 534B are R4-R5, Dual 150 ohm....R6 8500 ohm.....R7 3000 ohm....R9 30 ohm.....R19 312 ohm and R20 80 ohm. RB<P>------------------<BR>KA0SCR

My 534A manual shows 200µA, 2365 ohms.<P>As for the bias pot, however, I'm afraid you're out of luck. You'd have to get that from a junker Hickok, as it is a special taper, 3000 ohms. There are plenty of later-model Hickok owners who are looking for the same pot.<P>------------------<BR>

Thanks Randy and Alan. Interesting that Hickok would begin with a 50ua meter then use a 200ua one and then switch back to a 50ua movement in the same basic model. As the meter is different that means that the component values used in this are not the same as my 534 schematic. I am sorry to learn that the bias pot is made from unobtanium. I will continue to check this unit out though while searching for that elusive "junker" Hickok. Was the same pot used in earlier models as well or just the 500 series and later? Alan, If there is any way you could copy or scan that schematic for me I would gladly cover any copying and shipping costs. Thanks again.<P>P.S. Alan, how did you type the correct "mu" symbol in uA?<P>------------------<BR>Tim

I can xerox the schematic/parts list, it's 11x17. Drop me an email: adouglas@gis.net<P>Hit numlock, then hold alt and type 0181.<P>The original 534 used an 0.9mA meter, before changing to 50µA. That would have been a 1000 ohms-per-volt VOM. Why Hickok kept changing, I don't know. Maybe they couldn't get alnico for the meters and had to reduce the sensitivity? Goofy.<P>------------------<BR>

Thanks Alan. Interesting about the meter history. In going through this thing I have found that most of the wire-wound bobbin resistors are open in multiple places where the copper oxidized and left green marks. I am replacing the ones in the tube tester section but I do not plan to repair the multitester section as my DMM will do all of that more accurately anyway. I am also contemplating removing the entire multitester panel and installing a plate milliameter instead as it would be more functional. Any thoughts on this and what would be a good choice for a full scale value? I think I can just put it in series with the plate circuit without affecting tube test accuracy. Is that true?<P>Also, I wonder if anyone has mapped out the bias pot numbers to voltage readings throughout the range of the bias pot? If I had that data I am thinking I could install a voltmeter and a 3K linear taper pot for bias setting. I have seen cal data that 22 is 3 volts and 100 is 40 volts. Is this a viable solution or am I smoking something?<P>Thanks in advance.<P>------------------<BR>Tim

I had only one of the bobbin resistors open and luckily it was less than one turn in.<P>BTW....the trick for the micro symbol won't work with linux....I just figured that out after trying about a half a dozen times.....RB<P>------------------<BR>KA0SCR

Hi Tim<BR>I have a 534 plain missing the case that i used for my 539A.All other parts are intack<BR>and apear to be in good clean condition no tarnish. We can get together on the parts you need by the way many many thanks to Alan Douglas for the help on the rool chart.<BR>George Kirk <BR>Caseyville Illinois<P>------------------<BR>

Hi Tim just checked and the bias pot on my 534 is defective. I have a bias pot from a 6000a i dont know if it will work.I have a 6000A i could let go it needs repairs.<BR>George Kirk P.S. sorry about the bias pot<BR><P>------------------<BR>

I guess you were lucky Randy. I am sure the storage conditions affect them a lot. The one I have looks like it was stored in a very damp cellar for many years. The covering was falling off of the case and the plywood is delaminated in places. I am in the process now of removing the covering completely and gluing the plywood laminates. I will then just apply some stain and poly to the plywood to finish it. Should come out very respectable considering how I found it. <P>Thanks for the offer George. Too bad your pot was bad also.<P>Anybody ever think of fusing the bias circuit? I wonder what the correct value fuse would be to protect the pot from burning up? <P><P>------------------<BR>Tim

Fusing of bias pots was done on most later Hickoks. A #49 lamp was placed in series with the wiper of the bias pot and called the "bias fuse". The method seems to work well, and I have added this to a number of testers that lacked it. Lamps are cheap--bias pots are not.<P>This thread illustrates the great variation in Hickoks, and how the schematic you have, seldom matches the tester on the bench. the only good thing about the bias pot thing is that they seem to interchange between most Hickok models OK. The exception may be the TV7--where the taper seems to have varied based upon what version of the TV7 you have.<P>------------------<BR>

Try alt-230<P>The bias pot I suppose would be an exponential taper: the wire is wound on a triangular form. But the dial itself is nonlinear, complicating matters. I've graphed a couple of them. You could make a custom dial for a linear pot but the scale would be very crowded.<P>A plate meter is useful; 50mA is good. You do need to keep in mind however that the plate current is a half-wave rectified sine, not DC, and needs a correction factor to make it agree with tube-manual specs. I think this is why Hickok never offered a plate meter. It's about a ten per cent difference (I have never really checked it). Still, a meter will tell you a lot about how the tube is behaving.<P>------------------<BR>

Tim....Thanks for mentioning the tip about fusing the bias pot, I'm going to use it with my 534B. I acquired it several years ago in what I'd have to say is mint condition, the corners on the black stag covering are not even frayed and it came with all the documentation, the previous owner even saved all the old roll charts after he had updated the tester, I'm old enough to remember updating to check all the new tube types coming out. I've got a 6000 at work but it's been years since we've had any tubes it would check, hard to get those 3CX1500A7's in any socket on the 6000 but it's going to stay in my shop at work at least till I retire. BTW has anybody had much experience with a Hickok 209A VTVM, I've got one that I have to rezero constantly it seems to take forever to settle down. So far I've subbed all the tubes and recapped it ....RB<P>------------------<BR>KA0SCR

Chris - thanks for the tip on the lamp fuse for the bias pot. I am looking forward to getting the correct schematic for this to see if the published values I am reading for some of the resistors that do not match the 534 schematic actually do match the 534A schematic.<P>Alan - Thank you for the answer about the plate current meter. That is an important detail there about the waveform shape. <P>Great find Randy getting a tester complete with docs. That is always a plus.<P>------------------<BR>Tim

<BR> Wouldn't it be possible, given the form for the resistance appears to establish the "taper" for this pot, -to find replacement nichrome wire of the proper gauge and resistance per length and rewind it?<P> I looked at a spare one here and don't see anything that seems too unusual.<P> Seems like there would be a cottage industry for someone set up to accomplish this.<P> On my few excursions into "Hickok World," I've noticed some of these pots had some latitude regarding their overall resistance<BR> values.<BR> On a couple of older units, (model 532 IIRC,) I've seen factory installed resistors in the bias circuit, (not on scheme,)to compensate for variables in either the pot or its circuit,I suppose.<P> Pitfalls??<P> Den<P>------------------<BR>

That is an interesting thought but I have no bias pot to rewind as it was missing entirely from this tester. I have temporarily installed a 3K linear pot for testing and when measuring voltage across it with 1000 ohms / volt meter I have 43 volts. Is this a problem that it is not 40 volts? <P>Line is adjusted correctly per Alan's documented procedure of checking for 5.00 vac at pins 5 and 8 of the octal socket when the controls are set for a 6L6 and no tube installed. The meter lines up perfectly at this setting. Bias control is set to 0. Filament voltage is 6.56v measured with a DMM.<P>Should I be concerned about the 43 volts? Dennis mentions different values exist. Was that to trim the circuit to 40 volts across the pot?<P>Also, there are a number of resistors that have a plastic covering over them and some green liquid goo under the covering. Is this normal? Liquid cooled? <IMG SRC="http://antiqueradios.com/forums/smile.gif"> I cannot read any of the printed values on these so I am not sure if my measured values are correct as I do not have the correct 534A schematic yet.<P>Thanks again in advance.<P>------------------<BR>Tim

<P> A little bit means a lot, in this case.<P> I just did a "stare and compare" on a few knobs here.<BR> <BR> One thing for sure. Though most of these pots may have an overall value of 3K, some tapers among models must be slightly different.<BR> Enough so that a large test error is possible if different tapers, other than the proper one for the particular model, is used.<P><BR> Fer instance: The graduations on a set of model 532 knobs vary --aside from different physical characteristics/shape,--<BR> from the 533/600 and 605 series knobs.<BR> 5 divisions at mid-scale for bias knobs.<P> Now, I don't know where your model 534A fits into the model lineup for Hickok, but just guessing, since it doesn't have the bias circuit fuse lamp, it might be earlier than 533/600 types.(?) (OK, I/m too lazy to check the Hickok sites right now.)<P> I suppose what I'm trying to state is that there seems to be some variation re: that taper among _some_ model series, so consider that when looking for a replacement part.<BR> Overall resistance, even though "Hickok tapered", isn't the final answer, without the corresponding knob.<P> I have the knobs from a 532 here. Excess to my needs. Since you stated your bias pot is "missing," I figured I'd offer a knob (if compatible w/ your mod.#,) if it too, is missing.<P> Den<P> <P> <P>------------------<BR>

Great, just what we need - more variables in the equation. Thanks very much Dennis for the research and the generous offer. Fortunately though, the knob was with this, just the pot is missing. <P>I suppose also that the taper of the pot could be the same but the knobs were graduated differently to make up for specific model circuit and loading differences. Possibly someone has parts lists and the part number of the pot could be compared between models. <P>I appreciate all the assistance and time spent with this.<P>------------------<BR>Tim

Interesting that the knobs would be different, though somehow with Hickok that doesn't surprise me.<P>There's a series resistor (8.5k?) to adjust the 40V across the pot.<P>Winding a pot is no fun at all, even if you could get the wire. I think the highest resistance I have is 400 ohms per foot and that's pretty small to deal with.<P>The green goo is plasticizer coming out of the Tygon vinyl tubing. People who use vinyl tubing should be shot. If you're careful you can slit it with a razor blade and remove it, then wipe off the goo with a suitable solvent (I've used 1-1-1 trichloroethane). It's only an insulator to keep the resistor from touching adjacent components. The plasticizer doesn't sem to attack the resistor however, so it's only a cosmetic problem.<P>------------------<BR>

I'll be sure to duck if I ever use vinyl tubing around you Alan. I did cut one open and found the lettering on the resistor to be illegible. I could just barely make out a 4 and that was it. Can I assume that these type resistors are stable and do not change value like carbon comp? What has been the group's experience with these? For instance, R24 measures 219,000 ohms and the schematic calls for 215,000. I cannot read the printing on it to see what value it is supposed to be and I am not sure if I can believe the 534 Schematic I have. <P>On my 534 schematic there is R6 (6K 5W? WW) in series with the pot. That is the same value that is pysically in the tester. (Can't wait to receive the 534A schematic) If 40 volts is the magic number then I will adjust the circuit to have exactly 40 volts across the pot.<P>One thing that is interesting about this unit is the complete absence of any internal alignment pots like other testers have. I guess they just selected the proper values during final test. Seems like an expensive method. This would mean that each tester may be a little different. <BR><P>------------------<BR>Tim