david_robinson7

<p>I thought that I might help others having PraktikaTL/SUPER TL metering problems with the following.<br>

Thanks to Subbarayan (and his good Smaritan) for supplying the Praktica Meter Circuit, it helped me to trace and correct an annoying exposure problem on my Super TL1000. This proved to be due to the change in type from the original (now banned) mercury cell (PX625) to its alkaline replacement (PX625A). The original PX625 provided a 1.2V bridge supply source whereas its replacement provides 1.5V. In a perfectly balanced bridge, the actual bridge supply voltage is irrelevant because the algebraic sum of the currents flowing in the meter arm of the bridge matrix is zero. The battery voltage problem occurs because zero current flow in the Praktica meter will not result in a zero visual indication on the needle display. There is a mechanical offset on the meter pointer which requires that a positive standing current has to flow in the meter arm in order to achieve a visual indication of zero. This means that there needs to be a deliberate impedance imbalance in the bridge and it is this imbalance that renders the bridge susceptible to variation in supply voltage. Performing a Kirchoff analysis (I am an Electronics Design Engineer by profession) of the circuit provided revealed that the displayed error would be multiplied by the delta-V change in the supply voltage, hence a supply voltage difference of only 0.3V would result in a significant metering error. This was the root cause of my problem (and of anyone else using the PX625A). In the circuit supplied, VR2 and VR3 control the effective 'span' of the ASA/DIN potentiometer and VR1 is responsible for balancing the impedance of the LDR. From my circuit analysis, I reasoned that the (factory set) span impedance would not need readjustment and that all that would be required to accommodate a different supply voltage would be a simple readjustment of VR1. From the information supplied by SP and so many other contributors (my thanks to you all) I removed the camera base plate, selected 100ASA and 1/125 S, chose a suitably neutral evenly illuminated background and adjusted VR1 to achieve indicated f-stop/speed correspondence of the camera with a hand-held meter. It was a bit fiddly (the camera base plate is used to complete the battery circuit) but after about 3 iterations I had it spot on. Checking the metering at both ends of the speed scale showed 100% conformance with the hand-held meter which confirmed my reasoning regarding VR2/VR3 not requiring readjustment. What of the future? The PX625A is probably not the best choice for the metering battery as the voltage discharge characteristics of an alkaline cell are not particularly stable but as the meter is only switched on for short periods it is likely that in normal use, a standard cell will stay in range for years. Use of a S625PX (1.55V) silver oxide cell will eliminate any uncertainty with battery voltage, as the voltage discharge characteristics of this type of cell are very stable in use but it should be fitted before any readjustments are made. At least all the nonsense of zinc-air (Wein cell) batteries and hot carrier diode adaptors are simply avoided!<br>

I hope that others will find this of use and thanks again SP!</p>

<p>David, they are beautiful pictures.<br>

Martin, I have had exactly the same sliding catch problem with my Zenit B.<br>

I have contemplated making a minor mechanical modification to the sliding catch to correct this (i.e. grind down the upstand of the catch release to reduce its height to a level where it is flush with or below the top level of the catch sideplate). I calculate there will still be sufficient upstand remaining to release the catch with a fingernail. Right now I am testing an alternative approach of lining the catch side of the ER case with adhesive PTFE tape but it is early days yet in testing this. If only the comrade designer of the ZENIT had sited the tripod bush at the catch end of the camera base then the problem simply would not exist. From a COG point of view it would have been much better if the bush had been placed in the centre of the base and I think that this also would have solved the catch problem. </p>

 

<p>The manually preset aperture mechanism of the Helios 44 can give its user an almost unique advantage when determining the optimum aperture for a given DOF.<br>

The lens aperture preset selection ring on the Helios 44 carries an index mark which is in direct proximity to the aperture select ring and gives a continuous indication of effective aperture between fully open to the preset limit. If the preset end limit is initially set to minimum aperture, once the lens has been focussed at fully open to the principal object of interest, the aperture preset ring is then progressively rotated towards the minimum aperture setting until the required image DOF is observed in the viewfinder, at which point the effective aperture can be directly read from the aperture select ring using the preset selection index mark. At this point you only have to select the correct shutter speed for the indicated aperture and take the picture.<br>

If you think this is being cultist and silly, then tough - I call it thinking laterally.<br>

Have a nice day</p>

<p>The Helios 44 WILL provide DOF preview by simply moving the preset aperture select ring on the lens from the fully open (focussing) position to the closed (preset aperture select) position.<br>

Simple ain't it!</p>

<p> </p>

I use a Centon DF-300 Camera with the Centon 50mm f1.8 MD lens and I think that this lens is superb. It gives pin-sharp images, even at full aperture, is very light and easy to handle. It would be very interesting to compare it to the Minolta equivalent; my guess is that they would be identical, but not in price!