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03-15-2018, 12:36 AM   #1
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Flashes safe to use with Pentax KM (SLR similar to K1000)

Hi all,

I somewhat recently bought a second hand Pentax KM (these look pretty identical to the K1000 but have a depth-of-field preview and self timer) which was in pretty great condition. The camera also came with a Vivitar 285HV Zoom Thyristor flash.

I shot about 3 rolls of film before using the flash, and as soon as I used the flash (I didn't even take the photo it was just a test flash whilst it was sitting in the hot-shoe) all the electronics in the camera stopped functioning.

I did some research and figured that given it is an old SLR and can take a bit of voltage, and also that the Vivitar 285HV models are supposed to not have a dangerously high trigger voltage, that the voltage likely wasn't the sole problem. I came across a vague post that mentioned something about some Pentax camera's having a reverse polarity on the hot-shoe and that even small voltages can damage it when wrong polarity flashes are used.

Anyways I got the camera fixed and bought a new flash, a Pentax 280T, which is a relatively old one so I figured that it would be made to be compatible with the Pentax KM given it is the same brand and rough time frame. However, I used a multimeter to check the polarity and found it had the same polarity as the Vivitar flash which fried my camera, meaning that either this too will not work with my camera, or it has nothing to do with the polarity. I can't measure the polarity of the camera because it's doesn't have active electronics in the hot-shoe when it's not connected to a flash.

So, my questions are, does anybody know much about the Pentax KM and what flashes you can use with it? And if anybody knows what the polarity of the camera is/if the polarity is even a possible issue?

I would have assumed given it's similarity to the K1000 that it would not be too hard to find a compatible flash, especially by the same brand in the same era.

Thanks,
Lachy

03-15-2018, 02:44 AM   #2
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check for yourself

If you are worried about high voltage, power up the flash and measure the voltage between the ball contact and the base. Usually around 12v. I use a very old vivitar 2500 with my modern pentax with no problems.

Last edited by fb_penpho; 03-15-2018 at 02:55 AM.
03-15-2018, 03:47 AM   #3
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Yeah I've already done that, it was only about 7 volts with the negative terminal at the base and the positive at the ball where it is for convention cameras. However I'm not sure if this old KM has a conventional polarity on it's hot-shoe (or if that is even a thing having different polarity).
03-15-2018, 04:33 AM   #4
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QuoteOriginally posted by dislikelychee Quote
Anyways I got the camera fixed and bought a new flash, a Pentax 280T,
So, my questions are, does anybody know much about the Pentax KM and what flashes you can use with it?
Lachy
Lachy,
I canʻt answer a lot of the specifics of your inquiry, other than to say I have years of experience with the K1000 and various flash units. Itʻs hard to know at this point if what happened to your KM was due to the KM, the Vivitar flash, or both. It was probably the KM, but it could have been both.

My first flash units back in the 70ʻs were all Vivitar because they were readily available and inexpensive. However, most were problematic over time, and I finally gave up on them and switched to Sunpak flash units. Metz have been the best third party flash units for me, but at a premium price.

The electronics in 40+ year old cameras and flash units over 20 years old are going to typically have issues. But now that youʻve gotten your KM fixed, it should be fine with the Pentax 280T. The safest alternative would be to use a handle mounted flash that connects to the camera via the PC sync port (X), but I donʻt think thatʻs really necessary.

When the Vivitar killed your KM, were you using lithium AA batteries? Although rated for nominal 1.5v, new lithium cells are more like 1.7v so with four, instead of 6v from the flash unit, it could be closer to 6.8v which for an older flash unit could have overloaded your KM. I would stick with alkaline cells with older flash units. Below are the data spec sheets for both lithium and alkaline and youʻll see the difference in voltage output, life, and amperage.

http://data.energizer.com/pdfs/l91.pdf
http://data.energizer.com/pdfs/e92.pdf

03-15-2018, 06:12 AM - 1 Like   #5
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I also have a KM. Bought this one new in the 70īs, and it still works. Over the years I have used various flashes with it. Mostly Metzes, but also other brands. Never had any problems with it. I suspect if there was something unusual with the camera I would have heard about it. The Pentax flash should definitely be safe to use with the KM.

Last edited by PePe; 03-15-2018 at 11:13 AM.
03-15-2018, 10:57 AM   #6
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You can use a cheap radio trigger+receiver so the flash voltage won't fry the camera.
03-15-2018, 01:25 PM   #7
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I've used 3 Vivitar 285 HV flashes powered by either NiCad or NiMh batteries with a variety of Pentax film and digital SLRs with no issues at all--your problem with the KM may simply be a coincidence. The electronics in a KM would only relate to the meter since it has a mechanical shutter. If there is a voltage issue, either the hot shoe or PC terminal connections would still be a problem. A radio transmitter/receiver or a Wein Safe Sync hot shoe adapter should guarantee no future problems.
03-15-2018, 04:10 PM   #8
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QuoteOriginally posted by dislikelychee Quote

Anyways I got the camera fixed
I am curious as to what was fixed on the camera.

Clarence

03-15-2018, 04:31 PM   #9
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Flash unit voltages here :- Photo Strobe Trigger Voltages

However there seems some ambiguity about the Vivitar 285 HV - anything from 6v to 12v, or even 350v on an older 285! Some of this variation is probably due to the voltmeter used in users' reports; thyristor trigger lines can have a very high impedance so a cheaper voltmeter (or an analog one) can pull the apparent voltage down.

The KM/K1000 have very simple electronics and no flash dedication, and I do not believe there is any connection between the metering and the flash triggering. The shutter is entirely mechanical. The hot shoe contacts and the flash PC X socket on the front are both outlets of the same internal circuit.

I'm pretty sure that all Pentax cameras from the K-series onwards at least, can take or are meant for positive polarity, and the KM should certainly tolerate trigger voltages up to 12v if not more. Back in its day there were some fearfully high voltages around on flash units. Did the repairer of your KM say what the fault was?

Last edited by Lord Lucan; 03-16-2018 at 10:41 AM. Reason: Removed speculative part
03-16-2018, 05:31 PM   #10
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QuoteOriginally posted by Alex645 Quote
When the Vivitar killed your KM, were you using lithium AA batteries? Although rated for nominal 1.5v, new lithium cells are more like 1.7v so with four, instead of 6v from the flash unit, it could be closer to 6.8v which for an older flash unit could have overloaded your KM. I would stick with alkaline cells with older flash units.
Thanks for the info Alex645. I was using normal alkaline batteries with the flash, and I remember measuring the voltage of the flash after and it wasn't anything surprisingly high.

---------- Post added 03-16-18 at 05:35 PM ----------

QuoteOriginally posted by c.perkins Quote
I am curious as to what was fixed on the camera.
The repair report was a bit vague, but it mentioned fixing the light meter control switch, along with other check up things such as light seals and shutter speed adjustment.

---------- Post added 03-16-18 at 05:40 PM ----------

QuoteOriginally posted by Lord Lucan Quote
Did the repairer of your KM say what the fault was?
Well I funnily enough have no actual connection with the guy who fixed it, as the camera store had to send it off to somebody who specialised in film cameras, so all I have to work with is the repair report, which said he fixed the light meter control switch.

---------- Post added 03-16-18 at 05:44 PM ----------

QuoteOriginally posted by PePe Quote
I also have a KM. Bought this one new in the 70īs, and it still works. Over the years I have used various flashes with it. Mostly Metzes, but also other brands. Never had any problems with it. I suspect if there was something unusual with the camera I would have heard about it. The Pentax flash should definitely be safe to use with the KM.
Ah ok thank you for the reassurance. I think I will try the Pentax flash with it soon, after a bit more thought, fingers crossed!

---------- Post added 03-16-18 at 05:48 PM ----------

QuoteOriginally posted by Cascadilla Quote
I've used 3 Vivitar 285 HV flashes powered by either NiCad or NiMh batteries with a variety of Pentax film and digital SLRs with no issues at all--your problem with the KM may simply be a coincidence. The electronics in a KM would only relate to the meter since it has a mechanical shutter. If there is a voltage issue, either the hot shoe or PC terminal connections would still be a problem. A radio transmitter/receiver or a Wein Safe Sync hot shoe adapter should guarantee no future problems.
Yes it very well might have been a coincidence. Thanks for the advice, that will certainly be my next port of call.
03-21-2018, 05:23 AM   #11
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For full disclosure, I don't have schematics for this camera but my observations are based on many years of electronics knowledge. The battery in the KM is strictly for the meter. With the battery removed, any center pinned manual flash will fire which is proof that there is only a normally open mechanical switch being closed to fire the flash. It is possible there could be a diode in the circuit to allow electricity to conduct only one way which would mean it has a polarity. However, I doubt that and think it has no polarity. Even though it is a mechanical switch to fire the flash, there maybe a maximum voltage limit dictated by spacing as opposed to sensitive components that may burn out due to it.

It is possible that a short could have developed shorting the flash voltage with the meter circuit. But this is only likely if someone else had worked on it an introduced it. Most likely it is coincidental that the meter failed at the same time as you tried the flash.
03-21-2018, 10:51 PM   #12
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QuoteOriginally posted by LesDMess Quote
For full disclosure, I don't have schematics for this camera but my observations are based on many years of electronics knowledge. The battery in the KM is strictly for the meter. With the battery removed, any center pinned manual flash will fire which is proof that there is only a normally open mechanical switch being closed to fire the flash. It is possible there could be a diode in the circuit to allow electricity to conduct only one way which would mean it has a polarity. However, I doubt that and think it has no polarity. Even though it is a mechanical switch to fire the flash, there maybe a maximum voltage limit dictated by spacing as opposed to sensitive components that may burn out due to it.

It is possible that a short could have developed shorting the flash voltage with the meter circuit. But this is only likely if someone else had worked on it an introduced it. Most likely it is coincidental that the meter failed at the same time as you tried the flash.
Thanks LesDMess. Yeah I am thinking now that maybe it was just a coincidence, but your information was very helpful.
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