Replace dial light in Honeywell Pentax 1/21 spotmeter?

Donald Qualls · Jun 1, 2020

I've got a pretty clean 1/21 Spotmeter that I used to run with a zinc-air hearing aid battery (and a 9V, of course). I just got a regulated lithium cell replacement for the original PX640 that promises long cell life and very stable voltage.

I discovered, however, that the internal dial light (useful when metering low light levels) isn't working at all, likely due to a dead bulb -- I think I recall this from the last time I used the meter, years ago. I took the objective lens off, and the bulb isn't there -- it appears to live inside the top of the housing, behind the mirror. I don't see any way to split the case or move the mirror, and don't want to risk breaking anything. Can anyone tell me how to get at the backlight bulb?

ic-racer · Jun 1, 2020

I have a lot of service manuals. I'm not on my computer right now but I"m pretty sure I have that one too. I'll send it to you.

Donald Qualls · Jun 1, 2020

Wow, you're a very helpful person! Thanks!

That image, though, may not be the right version -- mine has a small white button in the ring on the (user's) left side of the grip that turns the dial light on when pushed.

GRHazelton · Jun 1, 2020

My Pentax Spotmeter - analog - also needed the tiny light bulb replaced. I spoke on the phone with George at Quality Light Metrics in Hollywood. He has no web site. He replaced the bulb and calibrated the meter for, IIRC, around $100. Turnaround was perhaps one week. Here's a link with will give the phone number:
https://www.yelp.com/biz/quality-light-metric-hollywood Note the excellent reviews George has received. He is tha Man!

Donald Qualls · Jun 1, 2020

Thanks, @GRHazelton I'll keep that in mind.

AgX · Jun 1, 2020

But in general these lamps last long. Seen that such meter hardly is used compared for instance to a transistor-radio with its dial lamps I would not be surprised if the fault is somewhere else.

GRHazelton · Jun 1, 2020

Donald Qualls said:
Thanks, @GRHazelton I'll keep that in mind.

My Pleasure!

Donald Qualls · Jun 1, 2020

AgX said:
But in general these lamps last long. Seen that such meter hardly is used compared for instance to a transistor-radio with its dial lamps I would not be surprised if the fault is somewhere else.

Certainly possible -- the meter is decades old, after all, and there's a mechanical switch for its power. OTOH, it is an incandescent bulb. If/when I can open the device, I can check to see if I'm getting 9V to the bulb socket.

Philippe-Georges · Jun 1, 2020

AgX said:
But in general these lamps last long. Seen that such meter hardly is used compared for instance to a transistor-radio with its dial lamps I would not be surprised if the fault is somewhere else.

Yes, those bulbs are hardly burnt, but, if it is the switch, then some contact cleaning fluid might do.

eli griggs · Jun 5, 2020

I have the same Pentax shown and recently asked if the bulb, which is dim to me, could be changed out for an LED, but I am unsure what effect it will have on proper metering if a brighter lamp is used.

You might also firstffirst out if the Pentax meter is sealed, like the Minolta M and F spot meters were, before taking a crack at it yourself.

Quality light metric does great work, based on my experience.

IMO.

Donald Qualls · Jun 6, 2020

The dial light worked in my meter when I bought it, and was certainly bright enough that, had it been within the field of view of the meter cell (that's a 1 degree spot, remember), it would have affected the metering. At no time do I recall seeing the needle move when i activate the lamp.

And no, the meter body isn't sealed -- I've been sent a service manual for both this model and the one without the dial light, and both can be opened up. These were built in the days of discrete electronic components, and there was anticipation that it would be economically sensible to replace a single component (like the CdS meter cell) if it failed, vs. replacing an expensive meter -- so the device was built to be serviceable.

eli griggs · Jun 6, 2020

Cheers.

Donald Qualls · Jun 6, 2020

Quality Light Metric apparently has no web presence at all -- not only not a page of their own (even the most rudimentary), but no Facebook account, no published email address. There's a phone number on their Yelp listing (put there by a reviewer, I'm sure), along with their street adddress.

Makes it hard to find out if they still service this (old) meter model, and if so what standard service would cost. I gather it was $125 for someone else several years ago; for that kind of money, I'm pretty confident I can deal with changing a bulb myself.

MattKing · Jun 6, 2020

Phone George Milton at Quality Light Metric and he will either answer or return your message.
I had no trouble reaching him from Canada.
He scavenged a part for me and sent it by mail, all at an extremely reasonable price.
If he services your meter, he will calibrate it - to the standards that the movie industry require.

John Koehrer · Jun 6, 2020

There are/were grain of rice and grain of wheat sized bulbs used in model railroading
around 60 years ago. No idea if they're still available or V used.

eli griggs · Jun 6, 2020

John Koehrer said:
There are/were grain of rice and grain of wheat sized bulbs used in model railroading
around 60 years ago. No idea if they're still available or V used.

There are many shops on and off line for model railroad cars and supplies.

Here is one in business for more than a hundred years with a good selection of bits, but search for lights" instead of "bulbs" as it's search engine finds no results for the latter word.

https://www.modeltrainstuff.com/search?search_query_adv=lights

Be Safe, Be Happy and Godspeed to all,
Eli

Donald Qualls · Jun 7, 2020

Yes, model railroaders still use the tiny micro bulbs in their layouts. Many of their trains are "retro" steam replicas, and the more fanatical of them (the ones who build permanent layouts with lichens for shrubs and treetops, sand for ballast between the rails, and epoxy or resin water) will use these over LEDs because the color of the light is correct for street lights and such from, say, 1920 (or 1890) for a steam layout, vs. white LEDs that would be right(ish) in place of mercury vapor lamps from the 1970s.

Now, whether these are still made is another question, but I'll probably need one for this repair, and I'm likely to get a spare -- and that will last me the rest of my life.

eli griggs · Jun 7, 2020

Donald, from what I saw in a quick look at the site looked to be new manufactured lights.

Cheers

Donald Qualls · Jun 7, 2020

Looks like the train guys have gotten their toymakers to tune their LED colors to satisfy -- I went through six of the 14 pages at the link above and found not a single light I could identify as not being LED.

eli griggs · Jun 7, 2020

Interesting to know.

If you do decide on an LED, or find OEM type bulb, please post it, as I still find my meter too low light at times.

Cheers

John Koehrer · Jun 7, 2020

I suppose, maybe with a bit of luck you might find an old model railroader with a couple in a drawer somewhere.
They tend to be packrats like some other hobbyists.

There must be a model RR site on the WWW.

Donald Qualls · Jun 7, 2020

I'm certain I can get them at Batteries+Bulbs, and paying way too much doesn't bother me as much when I need one and a spare. They had the correct replacement for a dial light bulb in a 1960s vintage tube radio I bought off eBay a while back...

Christophe1 · Oct 30, 2021

Donald Qualls said:
I've got a pretty clean 1/21 Spotmeter that I used to run with a zinc-air hearing aid battery (and a 9V, of course). I just got a regulated lithium cell replacement for the original PX640 that promises long cell life and very stable voltage.

I discovered, however, that the internal dial light (useful when metering low light levels) isn't working at all, likely due to a dead bulb -- I think I recall this from the last time I used the meter, years ago. I took the objective lens off, and the bulb isn't there -- it appears to live inside the top of the housing, behind the mirror. I don't see any way to split the case or move the mirror, and don't want to risk breaking anything. Can anyone tell me how to get at the backlight bulb?

Sorry for intruding in this thread, but I'm experiencing the same problem with my Pentax Spotmeter V. Though the light bulb seems to work fine outside the circuit. I've tested it up to 4.5 V (to make sure I didn't blow it out). And btw, it seems these kind of sub miniature bulbs, with a fully threaded base aren't made anymore.As far as I know the moment, after many inquiries already.
But the whole problem with the illumination probably lies somewhere else. Perhaps an issue with the ground.

eli griggs · Oct 30, 2021

IIRC, I believe you can get a replacement bulb from Model Train Stores or online catalog supply houses.

My light is dead also, so I'll be interested in hearing what is used successfully and where a PDF schematic may be downloaded from, as well as the experiences other have in this vital operation which seems to be about the only hiccup in an otherwise flawless analog device on which so much depends.

Godspeed and Cheers,
Eli

Donald Qualls · Oct 30, 2021

Following up to this, I concluded that in my case the issue is that the regulator for the lithium cell won't pass enough current to light the dial lamp -- because I'm pretty sure that lamp worked the last time I had a zinc-air cell in the meter.

The obvious solution here, if one were disassembling the unit anyway, would be to switch the dial lamp to the 9V booster circuit, replace the incandescent bulb with a similar brightness white, amber, or red LED (good arguments to be made for each), and wire in a correct current limiting resistor.

Replace dial light in Honeywell Pentax 1/21 spotmeter?

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