Hickok 752 was the FAA's choice for tube testing for years. This machine has been rebuilt to Hickok Specifications and Calibrated using Fluke 87 III and Sencore 3080 test gear. It includes the operation manual, extended settings for tubes thru 1975 and a copy of the existing roll chart as it is a bit fussy to turn. Both the #83 and 5Y3 rectifier power tubes are new old stock and were tested on a Triplett 3444, Heath TT1 and Sencore MU140. The tube readings showed that the outputs for each section of each tube were balanced. But are they in the actual Hickok circuit? Read on....
Two special circuit additions have been added to the original circuit to make measurements far more accurate that the unmodified version. What these two extra circuits do is provide balanced outputs from both the #83 and 5Y3 rectifiers (one for plate and one for screen and bias). If you look at the scope pictures you can see the before and after on the #83 and the 5y3, and both now have equal peaks (the close together trace is the 5y3).
Why is this important? Hickok determined late in production that unequal pulses from the rectifier tubes caused "offset" to the accuracy of the transconductance readings. Hickok corrected half the problem with the 539C top of the line tester, but still did not correct the 5y3 errors. If you have ever used an analog multimeter you know that there is a "ZEROING" screw or pot knob so the needle starts to measure from zero not at 10 or 20 or whatever. The same principal applies here. The two new circuits in effect Zero the reference bridge and bias circuits. With out this addition, the amplitude of the plate peaks you see for the #83 tube is read by the Hickok bridge circuit as an addition or subtraction TIMES the AC grid signal. Hickok based the chart gm readings on the assumption that the plate peaks were equal and thus no additional AC component would be added. If the unequal peaks are present, there is a constant amount of gm added or subtracted from the meter reading of the tube under test. It is that simple.
The 5y3 hold much the same problem if unequal peaks are present. Peaks add an AC component to the bias AC SIGNAL and the larger the bias the worse the problem. For example, the bias for -3 will add one half of the error of a setting for -6 volts will.
If in unmodified Hickoks both the #83 and 5Y3 are showing unequal peaks, the errors are significantly multiplied.
Without the addition of the new circuits even the best, most balance sides for any #83 and 5Y3 tubes will create meter gm error.
This tester will not test nuvistors, the 752A does ( the nuvistor socket can be added and the roll chart nembers and instructions are the same for both testers). Adding the 5 pin nuvisior socket is a modest charge if you must have it.
Email for better pictures, the deck is in good shape with wear on the roller wheel deck area, the case shows wear and tear, some big ones...but I hope you won't be putting this in the living room anyway since it is test gear ! Again email for those pictures if the case is an issue for you.
Price includes quality material and solid packing shipping but NOT shipping transport cost. If Paypal add 3% address must be confirmed or payment made as a person to person account cash transfer.
Tester is warranted for 6 months, does not include user abuse, misuse, etc, so please read the manual ( testers are usually damaged by testing for gm before testing for shorts) . Buyer assumes shipping costs. The tester should remain in calibration for 3,000 hours of testing use before any calibration check needs to be considered. Once a year, as some 'repair' stations recommend is simply not necessary.
Please also see our ebay feedback under budngreg. Joe at Tempo Electric is a customer and an Audiogon member.