I recently picked up an Eico 667 tube tester in good nick and have been checking it out. A few of the resistors had drifted and have been replaced, switches cleaned, and the unit calibrated. As I then went to measure the voltages being applied to the tubes I found that:
* The various heater voltages were about 5-10% low as was the leakage test supply. Not a huge deal – I simply adjusted the line voltage rheostat a bit and they came into spec, although the meter now shows me deflected slightly beyond the half-way point. I can run tests either way and certainly be within the precision the 667 is capable of.
* More interestingly, the AC plate voltage is consistently about 10-15% higher than the nominal expected for the various transformer taps. For example, when the manual says Vp should be 180VAC RMS during the merit test it is instead ~215VAC, 90V is ~105V, and 45V is ~53V - and this is consistent both with and without a tube being tested. Dialing the line adjust rheostat all the way down can’t correct this enough to bring them closer to their nominal values.
It’s the second set of results that is confusing me a bit. A 15% variance from the transformer taps seems a bit too large - and there’s not much in the circuit that could be wrong that would affect the results by such a significant amount. The plate adjustment pot has no real effect on the voltage, which is I believe to be expected.
I do understand the limits of the tester’s precision and that the merit testing certainly can do its job as-is. I’m more just curious whether the high plate voltages are common on this tester at this age or if there’s something I can do to correct it while I’ve got it open. If anyone has an Eico 667 and would be willing to post their no-load AC plate voltages in each of the four “V” switch settings I’d be much appreciative!
As a side note/bonus question: is there an easy way to know if your unit was factory-assembled or was sold as a kit?
* The various heater voltages were about 5-10% low as was the leakage test supply. Not a huge deal – I simply adjusted the line voltage rheostat a bit and they came into spec, although the meter now shows me deflected slightly beyond the half-way point. I can run tests either way and certainly be within the precision the 667 is capable of.
* More interestingly, the AC plate voltage is consistently about 10-15% higher than the nominal expected for the various transformer taps. For example, when the manual says Vp should be 180VAC RMS during the merit test it is instead ~215VAC, 90V is ~105V, and 45V is ~53V - and this is consistent both with and without a tube being tested. Dialing the line adjust rheostat all the way down can’t correct this enough to bring them closer to their nominal values.
It’s the second set of results that is confusing me a bit. A 15% variance from the transformer taps seems a bit too large - and there’s not much in the circuit that could be wrong that would affect the results by such a significant amount. The plate adjustment pot has no real effect on the voltage, which is I believe to be expected.
I do understand the limits of the tester’s precision and that the merit testing certainly can do its job as-is. I’m more just curious whether the high plate voltages are common on this tester at this age or if there’s something I can do to correct it while I’ve got it open. If anyone has an Eico 667 and would be willing to post their no-load AC plate voltages in each of the four “V” switch settings I’d be much appreciative!
As a side note/bonus question: is there an easy way to know if your unit was factory-assembled or was sold as a kit?
