HeathKit PT-19 Photo Timer - troubleshooting

[Todd Barlow]

I have acquired a Heathkit Photo Timer including the assembly manual and original purchase receipt date March 1975.

It times to 99 seconds but all times starting with 10 second selection switch are off.

10 seconds are actually 9 but consistent.
20 are 18
30 are 28
40 are 38
50 are 47
60 are 56
70 are 65
80 are 75
90 are 85

I would like to take a crack at repairing the unit. I have the list of components and their specs. I was going to focus on replacing the capacitors.

Any thoughts or suggestions would be appreciated.

Thanks in advance.

Todd

 

[ic-racer]

Hard to know without seeing the schematic. 1975, so a 555 chip would be available. Otherwise probably capacitors and resistors.

 

[koraks]

I would like to take a crack at repairing the unit.

There's not all that much to repair. The timing likely relies on an R/C circuit and the tolerances in esp. capacitors will make a unit like this inherently inaccurate. The deviations you mention are relatively small. I'd accept the unit as is, and/or use a more modern timer for more critical work.

 

[Todd Barlow]

@ic-racer: thanks for the response! No IC’s, but it has a tube and a number of discrete components. I will attach a photo of the schematic.

@koraks: thanks for the response! I am okay for an accurate timer so this is more of a project of interest for me, no PCB’s so with my limited soldering skills I thought I would give it a go and keep it out of the recycle depot!

 

[Todd Barlow]

 

[koraks]

OK, cool.

Note that there's a calibration pot (R5); I'd start by seeing if you can fix it with that.

Even for 1975 this was a relatively old-fashioned circuit.
Be careful as you'll be working with 150VDC. That's a very nasty jolt indeed, and if you're (very) unfortunate, it can be lethal.

 

[Nicholas Lindan]

It looks like it is consistently 6% off.

Is there an adjustment trim-pot in the unit? If there is then adjusting the the trimmer may be all that's needed.

If they aren't leaking and the unit is performing consistently there is no reason to go in and replace all the capacitors. Chinese capacitors in some late '80's to late 90's consumer electronics were notoriously unreliable and wholesale replacement made some sense; otherwise wholesale replacement is not a good idea. "If you fix something for long enough you will really break it."

 

[Nicholas Lindan]

Calibration instructions are available on the internet archive https://archive.org/details/Heathkit_PT-15_Photo_Timer/page/n3/mode/2up?view=theater

Following the calibration steps should restore the timer's accuracy. The specified accuracy is 5%, so by rights it is working to spec already.

 

[Todd Barlow]

@koraks and @Nicholas Lindan: Yes, I completely missed the three holes on the bottom of the unit. They lead to the calibration screws. The assembly manual does include the steps to test / calibrate. A classic example of RTFM!

Update later today…..

 

[koraks]

That's great! Well, with a bit of luck, you can get this back to perfect accuracy again.

 

[F4U]

I would also replace all electrolytics, check all resistors.

 

[koraks]

I would also replace all electrolytics, check all resistors.

I agree with @Nicholas Lindan :

If they aren't leaking and the unit is performing consistently there is no reason to go in and replace all the capacitors.

There's no need. If the times fluctuate or any other indicators of an unstable B+ voltage, replace just C1 & C2 and leave the rest alone.

 

[ic-racer]

Looks like it fills up C3 or C4 slowly depending on the resistor bank selected. Usually resistors don't go out of spec, but you can check them all. Maybe replace C3 and C4 with low tolerance items.

As mentioned, there are trimmer pots, so maybe you can just adjust those and be done.

Personally, without a transformer, I'd not use that device in my darkroom. That is just me, though.

 

[Nicholas Lindan]

Personally, without a transformer, I'd not use that device in my darkroom. That is just me, though.

Well, millions of vacuum tube table radios and TV sets from that era had no transformers. Stay out of the innards when it's plugged in and you should be OK. I'm still alive even though I constantly went poking into such live electronics in my misspent youth (maybe I should light a candle and say a prayer of thanks).

But, it wouldn't be a bad idea to install a new 3-prong grounded AC cord in the timer. Attach the cord's ground wire to the timer's metal chassis. Don't connect the ground wire to any part of the circuitry.

 

[Todd Barlow]

Update:
Followed the calibration steps as outlined in the manual and it the timing has improved substantially..
I tested each major time step like I had done initially and only the 90 second setting consistently times at 89 seconds, all other times are running at the times as selected.
The unit built in 1975 will live on!

Out of interest, the initial invoice for the kit was included in the manual. It shows that it was purchased for $42.75 CAD and using the inflation calculator it calculates to $234.34 in 2025 CAD dollars.

Thanks to all who replied!

All the best