Beseler 23c Dual Dichro power supply troubleshooting

nice job! rebuilding that PC baoard, complete with ic sockets n all. YOu're very lucky to have such a clean board n also parts that are still available. Thats a brand new power supply now.

do you think that power supply can be adapted to the duel dicro?

not all power supplies are created equal. the older systems have spider webs of wires n point to point instead of ribbon cables. Unraveling the mess is very tedious work unlike your very organized pc board.

to make matters worse.. many old technology parts like that neon lamp fired vactrol optoisolator aren't available these days. substitution is slightly more complicated for a handy photographer DIYer. maybe not to a hobbyist or electronics buff.

looking at 40 n 50 year old circuit boards, there are parts that have been so burnt in, you cant tell what they are. chard epoxy cover traces that go under other parts making them dissapear. that brown burnt flux complicates connections as well. then the can of worms, disintegrating lead based solder powdering so even after wringing out the circuit, it still doesn't work even though you replaced the burnt out parts due to a cold joint somewhere you havent checked or was disturbed while checking.

once again, beautiful job on your rebuild. I wish all electronics were as clean.

ic-racer said:

Since the D5500 power supply also drives the closed-loop filter motors, I rebuilt them from scratch. Your power supplies might also be rebuildable. To power the lamp, frequently the supply waveform is broken up by the TRIAC to pulse the lamp with the appropriate voltage 82v in my case, sensed by a 'true RMS' chip.

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Nice! Did you replicate the original design (using new components and sockets) or duplicate the functionality? My D5500 dichroic head is packed away right now, awaiting replacement of the rubber covering to drive the filters (I plan to use latex tubing). I believe my current power supply is functional, but we'll see.
I remember when I received it a bit ago going back and finding some extensive posts on the d5500 on APUG by you.

mgb74 said:

Nice! Did you replicate the original design (using new components and sockets) or duplicate the functionality? My D5500 dichroic head is packed away right now, awaiting replacement of the rubber covering to drive the filters (I plan to use latex tubing). I believe my current power supply is functional, but we'll see.
I remember when I received it a bit ago going back and finding some extensive posts on the d5500 on APUG by you.

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I saved the transformers and PC board and connectors. Everything else was sourced as new or NOS. Surprising story on the motor driver ICs. They were unavailable, even from the 'oddball old IC chip' sites. It turned out that Omega still had some!!! I found that almost unbelievable. Not listed on their website, but customer service had them. I bought enough to populate two boards; maybe all they had.

You even exchanged the few resistors. One could argue on the necessity of that.
Of, course, once taken out the board, heated up desolder- and solder-iron it is not much more hassle to exchange them too.

AgX said:

You even exchanged the few resistors.

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I replace EVERY resistor and EVERY component. All the ICs and Capacitors and Resistors.
These power supply boards were available new when I set up my darkroom in 2001. I did not buy one. But in 2003 when I needed one they were no longer available. So I wanted to make a NEW power supply on the existing PC board, as if I had bought one of the new ones.

One reason for wanting all NEW parts is that this particular power supply sits up in the head of the enlarger and is exposed to all the heat of the 250W lamp. This is different from a power supply that sits on the baseboard.

I have four of these boards. Two with all new components and two originals that still work.

Guys, I have one of the external stabilized power supplies for the Dual Dichro head (non-S version). The "S" version uses a transformerless integrated power supply).

I think the previous discussion is confusing the two power supply circuits of S and non-S versions. The external stabilized supply is designed for a 24vac bulb, while the "S" version is for a much higher voltage 77vac bulb.

The schematic Paul referenced is for the "S" version. I have never found a schematic for the circuit board in the older stabilized supply. Looking inside mine, the circuit seems much simpler.

The component that is likely failure-prone in the external non-S unit is a 21L255 Vactrol. This part is an incandescent light bulb and a CdS variable resistance photocell sealed inside a metal can. Reduced line voltage was fed to the incandescent light bulb. Line voltage variations would cause corresponding brightness changes in the 21L255 internal bulb which produced resistance variations in the photocell. These were used to control the 24vac voltage from the massive transformer through a triac circuit, similar to an old incandescent bulb dimmer switch. Of course, if the internal Vactrol bulb finally burns out, the unit will stop regulating. The 21L255s are "unobtainium". All available Vactrol units seem to use neon bulbs or LEDs which are NOT suitable replacements for the non-S circuit.

I believe it might be possible to simply bypass the regulator board and use the 24vac output of the transformer directly, although the unit would now become functionally equivalent to the unregulated power supply.

The Dual Dichro "S" version seems to have eliminated the transformer completely in favor of a AC dimmer-type circuit that drops the voltage by chopping the 120 vac line voltage. The design of the regulator in the "S" version uses the same incandescent bulb 21L255 Vactrol, as well as a neon bulb-based unit, but is far more complicated. The neon unit is the one that seems to go out most often in the "S" head and is the one that causes the delayed turn-on issue when the neon bulb inside eventually goes bad. These bulbs can be replaced by prying open the can and soldering in a new one.

Has anyone ever managed to obtain a schematic for the non-S Dual Dichro external regulated power box? Beseler's phone number gives a continuous busy signal when called.

Regards,

Don

I measured the following voltages in my working Dual Dichro stabilized external power supply: Note that a "true RMS" meter must be used to measure the regulated "chopped" AC output of the regulator to the lamp head because the waveform is not a sine wave and will read incorrectly on cheaper meters.

AC Line Input voltage: 121.5 vac
Transformer unregulated output: 36.28 vac under load with lamp on
Regulated output at lamp connector with head and lamp connected and operating: 25.58 vac (EJL halogen bulb rated at 24vac). This is in a 38-year old unit with no adjustments or calibration. Not bad.

No power is applied to the transformer primary unless the main power and lamp switches are "on" and the timer cord has 120 vac applied, lamp door on head closed.
Regulation is done on the "low voltage" AC side as input to the lamp only. Unregulated line voltage is sent to the timer outlet and fan in the Dual Dichro head,

The rear 6-pin connector pinouts are (looking at back panel of power supply):

1 - 2
3 - 4
5 - 6

1 - Regulated AC output to lamp in head
2 - Regulated AC output to lamp in head
3 - Safety Interlock switch on head (neutral connection from head and power supply circuitry)
4 - 120 vac to fan and lamp relay in head
5 - Safety ground to case and ground prong on plug
6 - Safety Interlock switch on head (to neutral of AC line main power cord)

120 vac for the entire system is switched through the safety interlock switch in the head, which completes the circuit when the lamp cover is closed.

WARNING: It appears that the safety interlock switch at the enlarger head works by opening the neutral side of the AC line when the cover is open!!!! This will leave the hot side of all circuitry still connected to the 120 vac mains if the main power switch is still on. Never depend on the power interlock to make the system safe when working in the enlarger lamp housing. Always turn off the power supply and disconnect the main power cord!!!

For testing, shorting pins 3 and 6 at the power supply connector will allow the supply to operate (without lamp load) without connection to head.
A regular incandescent 120vac 100 watt light bulb could be connected to pins 1 and 2 for testing, but not for final voltage adjustment.
A potentiometer on the board adjusts the regulated voltage. Adjust only connected to the illuminated lamp head using a "true RMS" meter!

Best,

Don

Inside view of external stabilized power supply for the Dual Dichro (non-S version). The transformer/circuit board and rear view of enlarger head socket are shown. The two contacts on the top of the connector are where a true RMS AC voltmeter would be connected for adjusting the output voltage to the nominal 24 vac needed by the EJL halogen lamp. There is a pot on the board for voltage adjustment. The lamp must be connected and powered when adjusting the voltage.

Obviously, hazardous voltages are everywhere with the case open, power applied and interlocks bypassed. Be very careful!!

Don

Hi...i am a new user here. I will suggest you to switch on the apparatus, check the powerline for being broken or of bad contact. Switch off, then pull the cable off the mains, check lamp and fuse, and its rating. If both are not blown, check lamp contact. Switch on again and see for the lamp. If no result, switch off and pull cable again, open the casing further and have a look at the electronics.

I am certainly not an electronics tech, but.....wouldn't it be somewhat easy to build a modern day 24 VAC power supply to replace this thing.?
Does it even need to be regulated.?
Is the regulation, offered by the old power supply, any better than the small amount of fluctuation in your Mains.?
If it is just for the light bulb, could you just run it off of a 24 VDC regulated power supply that is so common these days.?

CMoore said:

I am certainly not an electronics tech, but.....wouldn't it be somewhat easy to build a modern day 24 VAC power supply to replace this thing.?
Does it even need to be regulated.?
Is the regulation, offered by the old power supply, any better than the small amount of fluctuation in your Mains.?
If it is just for the light bulb, could you just run it off of a 24 VDC regulated power supply that is so common these days.?

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Yes, quite possible! Just feed 24 volts AC RMS to pins 1 and 2 of the connector to light the lamp. The EJL bulb is rated at 200 watts, so the 24vac supply would need to supply 200/24 =8.3 Amps. The unloaded output straight out of the transformer secondary is 36 volts, way too high for the EJL. Unfortunately, the Beseler manual warns against using 24volts DC, saying it will damage the head. A Variac on the transformer primary would allow tweaking the transformer secondary voltage to the correct 24vac RMS value. allowing it to feed the bulb directly.

You would also need a relay arrangement to simultaneously apply power to the 120vac cooler fan in the head on pins 4 (hot) and 3 (neutral). Or, just add a separate switch to turn on 120vac to the fan, as with the Beseler stabilized supply wiring. The internal guts of the power supply could be rewired fairly easily to accommodate, as all the timer outlets and plugs are already there.

Best,

Don

That is great info.....Thanks.
I always forget, low voltage means high amps.
Wow...8.3 Amps just for the stupid lamp.
Is it hard to find bulbs that run at 120 or 220.....or is there a different reason the choose 24 Volts.?
Thank You

CMoore said:

That is great info.....Thanks.
I always forget, low voltage means high amps.
Wow...8.3 Amps just for the stupid lamp.
Is it hard to find bulbs that run at 120 or 220.....or is there a different reason the choose 24 Volts.?
Thank You

Click to expand...

You're welcome!

I believe the reason Beseler warns NOT to use a DC 24 volt supply to the lamp head is not because of the lamp itself. 24vdc is equivalent to 24vac-rms for a purely resistive load, like the lamp. I believe it is because of a relay in the head. I can hear it actuate whenever the timer switches the lamp on. I believe this is to prevent lamp operation unless the 120vac to the cooling blower motor is present. The coil of the relay probably is rated at 24vac. For an inductive load like a coil, AC-rms and DC are NOT the same. Applying 24vdc to a 24vac relay coil would probably burn up the coil as the inductive reactance will have no effect on the DC current. It effectively lowers the overall "resistance" (actually impedance) of the relay coil. Just a guess. It doesn't matter if AC or DC is used for the lamp itself, as it is essentially a purely resistive load.

EDIT: Actually, the relay is probably used to turn on the condenser/diffuser light on the head, or possibly used for the interlock used to prevent the head from turning on if the dichro filters are retracted via the side lever. The cooling motor runs whenever power is applied to pins 3 and 4 of the head, so no need for a relay there. The same reasoning on using 24vdc versus 24vac still applies though.

The later 23C heads chop 120VAC directly to feed a higher-voltage bulb. Much cheaper, as you said. I guess the Beseler engineers finally thought so too. It does away with the heavy transformer but is potentially more hazardous as isolation to the AC line from the transformer is lost.

Best,

Don

OK...Right, they are using a relay.
Thanks Again

CMoore said:

OK...Right, they are using a relay.
Thanks Again

Click to expand...

Another thought I had for repairing a stabilized supply on the older 23C dual dichro head (non-S version) was this. The 21L255 optoisolator is an incandescent bulb pointed at a CdS light-sensitive variable resistor in a can. The bulb of the 21L255 was connected to the unregulated line voltage through some resistor. Variations in the bulb intensity tracked line voltage variations and were translated into variations in resistance, which continuously adjusted the dimmer-like triac circuit that drives the enlarger EJL bulb. When the incandescent bulb fails, the resistance of the CdS cell in the 21L255 will go way high, causing the voltage to go way high or low (not sure which) and stop regulating. The EJL bulb might even blow if it failed the "wong" way.

If you replaced the variable resistance CdS photocell side of the circuit with a simple variable resistor pot, you could manually tweak the dimmer circuit to 24vac to the lamp. You would lose the stabilization feature, though. Like replacing an automatic dimmer circuit with a manual dimmer circuit with a knob, as used in a house ceiling fixture.

As there is already an adjustment pot in that part of the circuit, you might even be able to figure out a fixed resistor nominal value that would emulate normal operation of the Cds part of the 21L255, using the existing control to tweak the voltage to the EJL lamp. I may investigate further. Unfortunately, I was never able to find a spec sheet for a Vactrol 21L255. If there was one, figuring out the needed resistor would be pretty easy.

Best,

Don