Lektra model TM560R timer

[Terrence Brennan]

Here’s a question for an electrical engineer.

I have an ancient Lektra timer, model TM560R, which I haven’t used in more than 20 years, it having been in storage, and now I am hoping to use it with an enlarger I managed to squeeze into my darkroom. This timer is so old, there is actually a tube in it, a 6V6.

When I tested the accuracy of the exposure times, no matter the time set on the dial, the timer runs about 20% longer than what is called for. I can’t see anything obviously wrong; no bulging or exploded capacitors, nothing burned or blackened.

While I recognize that it’s impossible to troubleshoot the problem as I have presented it, if anybody has a useful suggestion or suggestions, all responses will he gratefully received.

 

[MattKing]

If it is consistently wrong by the same percentage, you could always just work with it As Is.
The only down side to that approach would be if you work with more than one timer, or need to make use of times determined with other timers.
You would obviously have to convert existing times for things like film development times.

 

[Terrence Brennan]

If it is consistently wrong by the same percentage, you could always just work with it As Is.
The only down side to that approach would be if you work with more than one timer, or need to make use of times determined with other timers.
You would obviously have to convert existing times for things like film development times.

That’s probably what I will end up doing. I plan to use it only for timing print exposure, and if it print is too dark, cut the time back!

I just thought if any of the APUGgers had any kind of quick fix, I would try that.

 

[koraks]

When I tested the accuracy of the exposure times, no matter the time set on the dial, the timer runs about 20% longer than what is called for.

A quick look inside (courtesy of an eBay ad) suggests the timing is probably done with a simple RC network:

If the incorrect times bother you, you could try and calibrate the timer by trimming the capacitor that is responsible for setting the time base. I assume (haphazardly) that the double 1uF (for a total of 2uF) cap is the one of interest. Note it's likely a high-voltage type given the size.

In your place, I would just use a modern timer instead, or if I were bent on using this one, I'd just accept the times as they are as long as they are consistent. My first darkroom timer was also something like 10-20% off. Like you said, print too dark --> cut back exposure. It doesn't really matter if 1 second is 1.0 seconds or 1.2 seconds, after all.

 

[MattKing]

Looks like it would also contribute to keeping your darkroom warm!

 

[koraks]

Looks like it would also contribute to keeping your darkroom warm!

One of the last tube amplifiers I built was about a decade or so ago - perhaps a little shorter. The problem with that darn thing (I still have it somewhere) is that I could only realistically use it in winter. It was a bookshelf amp, but it used fairly big beam tetrode tubes that originally were made for use in large-screen televisions. I think the whole amp dissipated something like 300W (for a measly 10W RMS of audio output). That's a pretty decent heater if it's sitting right next to you on a desk!