that timer depends on capacitors... R/C circuit. more than likely, caps c3 c4 and c5 should be changed and that may solve the inaccuracy problem.
otherwise as long as its consistantly wrong, leave it.
that timer depends on capacitors... R/C circuit. more than likely, caps c3 c4 and c5 should be changed and that may solve the inaccuracy problem.
otherwise as long as its consistantly wrong, leave it.
Adjustment. Read The Fine Manual
https://www.vintage-radio.info/download.php?id=641
Page 15-16 the adjustment procedure is described (R4 and R5). I would bet that the range of adjustment is enough to cover 10-20% and get it down to 1% or better. Achieving this with capacitor substitutions would be a lot more difficult (there are no continuously variable capacitors in the µF range); the schematic seems to be designed thoughtfully.
Even if absolute accuracy is almost irrelevant, the adjustment is necessary to ensure at least the consistency of the x1 and x10 switches.
Ya dont need variable caps or make substitutions... replace them with the same values as in the diagram. Simple unsolder job. As caps get old they dry out n qct more like resistors n that causes them to be unstable, discharging too slow or too fast n not hold their charge properly as designed. Those caps are easy parts to find, standard value electrolytic. Heathkits are designed for easy access to the parts so not much experience in electronics is needed to build them.
I looked up in the Mouser catalog for the value 2µF (that is C3). In 2020, the vast majority of caps available for that value are film capacitors. Tolerances listed are 10% or 5%. That is not good enough (without trimming adjustment) because 10% of a step on the x10 switch is... 100% of a step on the x1 switch.Those caps are easy parts to find, standard value electrolytic.
Look at electrolytics as Paul Ron proposes. The typical tolerance for 2µF aluminium electrolytics is -10%+75%(!!). For Tantalum electrolytics, the tolerances are better, typically 10%...20%; except 2µF is not available, only 2.2µF (a truly standard value from the E5 series).
My recommendation: follow indeed the advice of Paul Ron to replace the capacitors, as the original ones are old aluminium electrolytics. Replace them with film capacitors of proper rating (at least 200VDC). Use the available adjustment resistors to bring the timings closest to nominal.
no need to get crazy with "better.".... all you need are the same or similar parts to replace them with to be as accurate as heathkit states in its specs for the unit... you arent building to mil spec. its a consumer unit and does have an adjustment resistor for each range. new caps will preform more consistantly than the old parts. the timing depends on how reliably the caps charge n discharge.
also clean the trimmers (adjustment pots) with some contact cleaner n excersize them a bit.
I think you are right. I think I am just trying to get perfection when it's not necessary. What you suggest (in using 'pseudo-seconds') works fine. I do have an old exposure computer, which gives a close estimate of the exposure time, which I can still use if I calculate the error into the mix, but other wise, the timer works fine by giving constant exposures, even if it's not using a standard constant.Originally Posted by Bernard_L
Update. I looked for a manual for your PT-1500, as distinct from the PT-15 around which much of the recent discussion revolved. Only found a picture.
which tells me it is a completely different animal. (1) All the discussion about RC time constants and aging capacitors is most probably void, and the available manual for the PT-15 is not relevant. (2) Contrary to what I stated (based on the PT-15 schematic) it is likely that the mains frequency is the basis for the timing, as you had stated; so the +20% is not a coincidence. The good news is that the consistency between the x1, x10, etc scales is a non-issue. Also the exposure computer: it must be calibrated before being used, so why not in pseudo-seconds.
If I were in your shoes, I would peek under and inside the box for a switch or jumper for a 60/50Hz change. Heath sold worldwide, and it would not be economic to make two versions of the circuit; the transformer being a different story, with significant economic saving in having a single-voltage component, either 110 or 220V.
As an aside, I have embarked into the construction of an Arduino based timer following the description given there: https://eliadarkroom.wordpress.com/
and there ya have it! its a newer model.
there must be a switch or jumpers inside for 120/220 operation.
but almost all electronis timers use either RC or crystal or some sort of frequency generated reference signal n not depend on mains frequency since a power supply is regulating circuit voltage, converted to dc for the descrete components filtered through a rectifier n diodes.. old mechanical clock motors depended on mains since they operated directly off mains ac voltage. your pt-1500 has an lcd.. and more than likely chip controled and that is not mains dependant.
still... old caps are the general cause of most irregularities in RC biasing regardless of what type of chip or crystal. but because it has calibration pots to take up the slack, it may still be within calibrating tolerances plus being newer, the caps may still be up to snuf.... consistancy is your clue.
the search for the manual continues... there has to be one floating around the net.
. can you get a picture of the circuit board?
edit...
microprocessor controled self test....
found this...
http://nostalgickitscentral.com/heat...t1500_cal.html
Hi Paul Ron,
I now have the complete manual for the PT-1500. I purchased one from England & it showed how to convert from 120v to 240v by switching some jumpers, but I couldn’t see anything about setting the Hz value. I am away from home for the next week, but I can scan the manual & post it here when I get home. I don’t know much about electronics. The voltage switch was obvious & easy
to see, but not so for the Hz issue. Perhaps someone who knows what they are looking at would know more.
I do have these photos on my phone of selected manual pages, but I’m not sure the quality will be good enough. I’ll post them anyway...
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