Vintage Lighting Mk V Scrimmerette Information

[RoyZ]

I have two Mark V Scrimmerettes. First, I am looking for manuals covering these units. Second, one of them has a problem on the Phase-A dimmers: they only go to half power. The other half of the dimmers work properly. I suspect the control circuit power on that phase. Does anyone have schematics or other repair information that would help in troubleshooting the problem. I will be replacing the bridge rectifier packs (W02M) as I am suspicious of the resistance measurements across one pair of terminals compared to the others and the other device. I've traced the two circuits out on the board (670-9083-01) and there doesn't seem to be much to them. Any help or data will be appreciated. I have contacted Alan Child, but he can not locate a schematic for this board.
Roy Z

 

[JD]

I have two Mark V Scrimmerettes. First, I am looking for manuals covering these units. Second, one of them has a problem on the Phase-A dimmers: they only go to half power. The other half of the dimmers work properly. I suspect the control circuit power on that phase. Does anyone have schematics or other repair information that would help in troubleshooting the problem. I will be replacing the bridge rectifier packs (W02M) as I am suspicious of the resistance measurements across one pair of terminals compared to the others and the other device. I've traced the two circuits out on the board (670-9083-01) and there doesn't seem to be much to them. Any help or data will be appreciated. I have contacted Alan Child, but he can not locate a schematic for this board.
Roy Z

Some of the early Scrimmerettes were only single phase units. If you tried to run one of these off of 2 legs of three phase power, half the channels would only come up to 66%, while the other half would work ok. Simple check would be to TEMPORARILY connect both hots to one phase leg. If all the channels work, then that is the problem. Do NOT leave them connected this way as the neutral conductor will be WAY undersized.

The Scrimmerettes in question was the 6 x 1.2k models that were equipped with a 10/3 SO power cable built in the 80's.
They had only one ramp generator that only looked at one of the hots coming in. (only one small transformer in the unit, and a double 30 amp breaker on the front.)
They also were analog units with an 8 pin jones connector that accepted 2-7.6 volts to go from black to full.
These units can be reconfigured using a 6/3 power cable (H/N/G) to run off of one hot.

If they do not fit this description, then the problem is in your ramp generator. This may be as simple as bad or dirty trim pot. Beyond that, there are several capacitors involved in the circuit, and a significant value change may trash the ramp waveform enough to not allow the dimmers to go to 100%

If this unit does use the analog ramp generator, I have a drawing of the circuit on this post: http://www.controlbooth.com/forums/...rimmer-troubleshooting-repair.html#post213369
Although it is a different system, the generator circuit should be the same.

EDIT: Note, you may find that entire thread helpful in troubleshooting. Although the unit on that thread is a modular unit, the same circuit was used in that vintage scrimmerettes. Many of those used a back-to-back SCR module (not to be confused with a SSR) as compared to two discreet SCRs.

 

[RoyZ]

Thanks, John, for the information. Ours is a 2-phase unit - the board is even marked as such. The schematic is quite useful, so thanks for that, too! Your troubleshooting link was also helpful. I will let you know what I find.

The date under the logo on the board is 1979. Can I assume that is the mfg. date? I'm going to purchase a few spare parts to have on hand should the need arise and time is short for ordering. I'm not too worried about the two transistor types, but I'll try to pick up some op-amps and electrolytics. You mention opto-isolators; are they the 1-per-channel 6-pin devices near the edge connector? These devices are labeled:
GE
019
HI 1CX I think it is "HI". It's more like H with a vertical line.
560S

I haven't found a listing for these devices, but since I'm not sure what exactly I'm looking for, I'm not surprised.

Since I have the card out, any suggestions on setting the trimmers: HI, LO, 7.5V, and 2V? I should probably build an extender cable so I can actually probe the circuits in "real life".

One last question. For some reason, the chassis ground is actually floating in this installation. The power plug connector has two phases and a neutral (3-prong) so there is no fourth connection for ground. The cable has the green wire, but it is taped off and their is no ground connection on the chassis lug inside the unit. This bothers me. Is there any reason you can think of for letting the cabinets float?

Roy Z

 

[JD]

I am not sure why they cut the ground, but that is downright dangerous. It is one thing to float the control signal, but the frame of the unit, and the ground pins on the receptacles all have to be bonded or a fixture/wiring short could make the case hot. The power plug must be a 4 blade plug with a ground (H-H-N-G) At no point should ground and neutral be connected together. It is time to have the plug and its receptacle checked out by an electrician. Due to liability issues, any power related issues should be handled by one.

The opt-isolators on this unit are photo-scr devices that are no longer made. The can be replaced with photo-triacs with no problem although there is a minor circuit change. (That is on one of these threads.) Generally, they are very reliable. The trim pots are Low end set and High end set and there is some interaction. The low end from the board is 2 volts (black) and the high end is 7.6 volts (100%) Check the board voltages first, then you can calibrate the trimmers. There should be a total of four trimmers, two trimmers for each phase leg.

The usual disclaimer applies here: There is danger of electrocution and you should not be working on this unit unless you are a qualified repair technician.

Most of what we are talking about is control circuits, but post isolator, line voltage does exist.

1979 would put it in the range of some of those single ramp generator units. The tip off is there should be two small power transformers in the unit, not one.

The date would be that of the creation of the boards, which may have been in use for several years, so the pack may be newer.

 

[RoyZ]

I agree with your observations about grounding the unit. I just wanted to be sure there wasn't some legitimate reason for letting it float. To set your mind at ease, I am a electrical engineer and have worked in the field on multi-phase aircraft power. I work with a power utility engineer who was in charge of field maintenance. We also have another power utility person working with us. I think we meet the "qualified" hurdle.

Thanks again, John, for all of your help. The circuit card says 2 phase because it is designed to work with the two ac control power inputs that come from the two transformers you mention. There are two diode bridge rectifier circuits on the card and two, 220 uF caps. There are also two LO pots and 2 hi pots so there appear to be two ramp generators. However, there is only one 2 volt trimmer and one 7.5 volt trimmer. Did they buffer the 2 and 7.5 voltages through op amps? I haven't found those traces on the card yet. On your schematic, it appears I should try to monitor the Hi/Lo voltages at the inputs to the ramp generator op amp. Or is there a specific waveform that I should be trying achieve?

I should be ready to put power back on tomorrow. I'll let you know what I find.

Thanks again!
Roy Z

 

[JD]

I agree with your observations about grounding the unit. I just wanted to be sure there wasn't some legitimate reason for letting it float. To set your mind at ease, I am a electrical engineer and have worked in the field on multi-phase aircraft power. I work with a power utility engineer who was in charge of field maintenance. We also have another power utility person working with us. I think we meet the "qualified" hurdle.

Thanks again, John, for all of your help. The circuit card says 2 phase because it is designed to work with the two ac control power inputs that come from the two transformers you mention. There are two diode bridge rectifier circuits on the card and two, 220 uF caps. There are also two LO pots and 2 hi pots so there appear to be two ramp generators. However, there is only one 2 volt trimmer and one 7.5 volt trimmer. Did they buffer the 2 and 7.5 voltages through op amps? I haven't found those traces on the card yet. On your schematic, it appears I should try to monitor the Hi/Lo voltages at the inputs to the ramp generator op amp. Or is there a specific waveform that I should be trying achieve?

I should be ready to put power back on tomorrow. I'll let you know what I find.

Thanks again!
Roy Z

The 2 and 7.6 volt trim pots are only for the local controls on the front of the pack. They have no effect on the incoming analog signal from your light board, other than if the 2 volt pot is set too high, the lights will not go to black. I am assuming you are using an external light board. If not, then you want to measure that voltage at the test points on the board. (they should be labeled.) If not, the best place to measure is at the input to the comparator on the channel. Each channel has one, although the packages are combined as they use quad op-amps. Each one receives the DC input on one input and the ramp on the other. You are looking for the DC value.

The 220 mfd caps tend to dry out and that may throw the ramp. If you scope the caps, a dry one will show a very large AC ripple. From your symptoms, we can be pretty sure the ramp generator is the problem. Although the quad op-amps do fail, the more likely failure is the caps.

 

[RoyZ]

The 2 and 7.6 volt trim pots are only for the local controls on the front of the pack. They have no effect on the incoming analog signal from your light board, other than if the 2 volt pot is set too high, the lights will not go to black. I am assuming you are using an external light board. If not, then you want to measure that voltage at the test points on the board. (they should be labeled.) If not, the best place to measure is at the input to the comparator on the channel. Each channel has one, although the packages are combined as they use quad op-amps. Each one receives the DC input on one input and the ramp on the other. You are looking for the DC value.

The 220 mfd caps tend to dry out and that may throw the ramp. If you scope the caps, a dry one will show a very large AC ripple. From your symptoms, we can be pretty sure the ramp generator is the problem. Although the quad op-amps do fail, the more likely failure is the caps.

Sorry to be so long to get back on this. Here's where it stands. I replaced one of the diode rectifier packs and it appeared both sections of the unit are working about the same. I don't really know if both at full power or if both are at half power, but they were about the same on the test lamp. The main control power switch on the front panel then fell apart, the tabs holding it together gave way. The bronze contacts in the switch had sustained mechanical damage so I had to find a new switch to fit the panel. Standards have changed, so I had to do some mechanical mods to allow a modern switch to fit the old hole. As a final insult, the plastic knob on the 2 volt trim pot was dried out from the heat - it separated when I turned the pot to adjust the voltage. Same with the 7.6 volt trimmer. These have now all been replaced. I have not found any test points on the circuit card.

Tracing the circuits, and using the partial schematic you referenced in the other post, the 2vdc and 7.6 vdc trimmers are connected to the V+ (top of the filter cap) as follows:

V+ -- 5.6k Resist. -- 10k Trimmer (7.6 vdc) -- 10 Resist -- 10k Trimmer (2 vdc) -- Gnd.

I don't know the exact value of V+, but unless it is in the vicinity of 10.5 volts or so, I'm not sure you can get 7.6 volts out of the circuit. Have I mis-traced the ciruit?

We have also reworked the power to each of our two units to include the proper grounding required by code.

Re-reading your troubleshooting article, I'm not clear on how to be sure the ramp generators and thresholds are correct. I plan to proceed as follows:
1. Set the 2 volt trimmer to 2 volts on the output of the voltage follower op amp connected to the wiper of the pot.
2. Set the 7.6 volt trimmer to 7.6 volts on the output of the voltage follower op amp connected to its wiper.
3. Set the LO trimmer to move the base of the waveform from the ramp generator op amp to be 2 volts.
4. Set the HI trimmer to make the top of the waveform from the ramp generator op amp be 7.6 volts.
5. Repeat 3 and 4 for the other ramp generator.

Please let me know if you have any suggestions.

 

[JD]

Remember, the pack is actually both a dimmer and a control board. Adjust the dimmer section (ramp generator) first:

There is a Jones connector on the back of the unit for the board to plug into. Usually 8 pin or 15 pin depending on the unit. One pin is the circuit ground. Some also have a pin that provides dc power for a "passive" light board. The rest are the control channels. I would use this input and a dc supply to provide the 2 - 7.6 volts for setting the ramp generator.

Once this is done, we can move over and adjust the 2 and 7.6 volt pots for the control board section.

Adjust your 2 volt pot until the lights start to idle up, then back it off a bit so that the lamp extinguishes. Then move one of the dimmers and the master to full and adjust the 7.6 pot until the light crests on at full. It is sometimes easier to listen to a "drop light" plugged into the pack. You will hear the "lamp sing" at less than 100%, and the sound almost goes away as you hit 100%. If you set them correctly, the lamp will begin to sing as you move the fader down from 10, and the lamp will just extinguish as the fader hits 0.

In the board circuit you noted above, somewhere there is a LM7815 regulator that outputs 15vdc to the circuit you referred to. The high end trimmer goes from about 6 to 10 volts, and the low end trimmer goes from 0 to 4 volts.

If the 7.6 pot is set too high, the dimmer will top out before the control is all the way up. Too low and it will never hit full brightness.
If the 2 volt pot is set too high, the lights will not go completely off. Too low and the lights will go off before your fader is all the way down.

So, in summary:
1) Align the ramp generator with a known voltage from an external metered power supply. (some interaction between controls.)
2) Align the 2 and 7.6 volt board pots so that the faders give you the best overall control of the lights.

 

[RoyZ]

I have two Mark V Scrimmerettes. First, I am looking for manuals covering these units. Second, one of them has a problem on the Phase-A dimmers: they only go to half power. The other half of the dimmers work properly. I suspect the control circuit power on that phase. Does anyone have schematics or other repair information that would help in troubleshooting the problem. I will be replacing the bridge rectifier packs (W02M) as I am suspicious of the resistance measurements across one pair of terminals compared to the others and the other device. I've traced the two circuits out on the board (670-9083-01) and there doesn't seem to be much to them. Any help or data will be appreciated. I have contacted Alan Child, but he can not locate a schematic for this board.
Roy Z

Well, I've tried for 3 years to find replacement parts and fix this unit, but I know it's a dead end. Parts are obsolete and hard to find, not to mention finding or making an extender board to test and troubleshoot it. We're running on only 6 channels and we're dead if it goes out. It's time to look for better hardware. Suggestions for a solid, reliable, 6-channel, 2400 watt/ch dimmer pack? I probably have to go to DMX control so, what about analog to DMX conversion? What's your best recommendation: dimmer / control panel?
Roy Z

 

[Les]

Well, I've tried for 3 years to find replacement parts and fix this unit, but I know it's a dead end. Parts are obsolete and hard to find, not to mention finding or making an extender board to test and troubleshoot it. We're running on only 6 channels and we're dead if it goes out. It's time to look for better hardware. Suggestions for a solid, reliable, 6-channel, 2400 watt/ch dimmer pack? I probably have to go to DMX control so, what about analog to DMX conversion? What's your best recommendation: dimmer / control panel?
Roy Z

Have you reached out to Alan Child about your dimmers?

http://www.ediserviceandrepair.info/

If you're wanting to find a converter to drive DMX dimmers using an analog board, things can get a bit tricky (usually it's the other way 'round), but I remember Northlight DMX (http://northlightdmx.com/) and Pathway Connectivity (http://www.pathwayconnect.com/) having a lot of options.

As for dimmers, Lightronics makes a nice and fairly budget-friendly rackmount as does ETC (a bit more pricey). On the lower end there is Elation and Behringer which come to mind, but I don't know their capacities and can't speak for reliability.

 

[RoyZ]

Have you reached out to Alan Child about your dimmers?

http://www.ediserviceandrepair.info/

If you're wanting to find a converter to drive DMX dimmers using an analog board, things can get a bit tricky (usually it's the other way 'round), but I remember Northlight DMX (http://northlightdmx.com/) and Pathway Connectivity (http://www.pathwayconnect.com/) having a lot of options.

As for dimmers, Lightronics makes a nice and fairly budget-friendly rackmount as does ETC (a bit more pricey). On the lower end there is Elation and Behringer which come to mind, but I don't know their capacities and can't speak for reliability.

Alan was one of the first people I talked with when I started on this. I also got a LOT of help from John Dziel. Thanks for the steer to Lightronics and ETC. I'll start a conversation with them. You guys are great. Just knowing that when a tough problem comes my way, I've got someone to turn to, is a big help.

Thanks!!
Roy Z.
Phoenix

 

[derekleffew]

what about analog to DMX conversion?

For the price of a quality analog-to-DMX protocol converter (ex. http://www.dfd.com/anl2dmx.html ) one could probably purchase a 12ch. 2-scene preset console, 100' DMX cable (and 2' jumper), and two 6x2.4Kw dimmer pack s of Leprecon/NSI/Lightronics/etc. manufacture.

 

[RoyZ]

Thanks to all of you here who have been so helpful. These old Mk V Scrimmerettes are now history. I've tried in vain to trace and piece together a useful schematic and board layout, but cobble a schematic together. I have to say that heat and age have taken their toll. I've had to repair and eventually replace the power switch, for example, because the Bakelite on the back broke. Runs are beginning to lift despite using very low heat settings on the solder iron. Parts are just very difficult to find, if at all. So I have replaced the two units with ETC Smart Packs, and replaced the light control panel with a Jands Stage CL. In the process I've also expanded the control booth, added sound panels to the rear walls (what a great difference), new audio rack, and the like. Still stuck with the old light fixtures, however, which need some shutter work. But at least we're now forward compatible LED lighting if someone would only help us fund them. (I noted with some interest Fountain of Euph's post on "Why is it so hard to give away money". I'll have so if I can help lighten those pockets.) So now I have two Scrimmerettes (one partially disassembled and usable for spare parts, one still operating (!!!), and an analog control panel. I don't have enough posts to list it here, but I don't know if it would be worth the effort. If there is interest I will keep posting 'til I cross the threshold. Is there a theater museum that might be interested? :>) Along the same lines I have a Panasonic, 3-lense, video projector as well. Anyway, thanks for the help and advice. I'm happy I went with new modern equipment instead of trying to limp along.
Roy Z
Phoenix, AZ