Rackmaster 260 wrong dim curve

[Stratzzi]

Dear Zero88 Forum,

i need some help for my Rackmaster 260 dimmers.
It seems that some Channels are just starting at 50% to dim up and then they go really fast to full output. 
I attached a video that shows the diffcult channels, the both in the mid are dimming perfectly, the others not.
Maybe someone could help me.

Thanks
Stefan

[KWR88]

Hi Stefan,

that looks like possible preheat issue.

if the pot is adjusted too far either way it casue the ramp for the channel control to distort.

it can either leave channels on at a level, or as you have the output don't come on until you get higher up the curve.

to set the preheat we would normally put a 10% control signal in to all the channels then adjust the pot until the lamps are starting to glow then take the signal to 0 and the outputs should then be no longer visable.

 

 

[iank99]

Has that setup worked before?

The fader control looks like it may be of chinese origin & sometimes the DMX they output isn't 'standard' & can cause erratic behaviour. To check the dimmer properly you need a small analogue desk of some flavour i.e a Level 6 or Level 12.

[Stratzzi]

Thanks for your answers. 

I checked the preheat, but when i turned it up, it dims up like it should be. All pots are at zero.

This setup had worked before! I measured the analog voltage and its the same on all 6 channels. So it couldn't be the multiplexer or dmx controller. I got these problems at another setup too (grandMA2 and other multiplexer). 

Any other ideas?

[KWR88]

If the preheat are set correcty then the next thing to look at would be the supply rail or the vref tail not being correct.

If the v-ref rail is out this will cause issue with the ramp as well. given the age of the unti you may find the capacitors on the supply rail have dried out.

these are the 470uF electalitic next to the transformer.

[Sape]

Just read this topic with great interest. As a volunteer technician in our theater we have a similar problem. If we control 4 identical lights simultaneously, their intensity differs considerably at partial load. We have 6 Rackmasters 260 connected to a Showtec Multi-exchanger and a number of halogen lights ranging from 500 to 2000W per channel.  I haven´t tried to adjust the preheat, that will definitely be a next step. The reason of my response: last week we analyzed the output with an oscilloscope and found weird curves. At 50% DMX signal many channels had no output at all. We hope/expect to improve this with preheat adjustment.

At 75% we actually saw two different patterns: one decent,typical, leading edge dimmer curve, and another one jumping up and down to high peaks with a peak-to-peak voltage of 915V, considerably higher than about 615V from the power supply. The 100% curve was more or less as expected.

My question: what could be the reason for this different behavior @75%? Could it be dried up capacitors mentioned in the latest response? A few comments: In all cases we used the same 1000W lamp. For safety reasons, the oscilloscope signal was connected via a voltage divider consisting of 3x 10k resistors so you have to multiply the voltages in attached images by 3. We haven´t opened a unit yet, that will also be a next step. Your reply is highly appreciated.

[kgallen]

Interesting. I have no answer other than cogitation wondering if this is an issue on the control side - in which case probably this would probably manifest on pairs of channels (1/2, 3/4, 5/6), or the output inductor which controls the rate of current change and hence the rise and fall times. However since they are "simply" a coil of wire, are very reliable, don't deteriorate in value over time like caps do (yes, the enamel could deteriorate), and would manifest on one channel, not across a range of channels.

It's with questions like this that we'll really miss Keith and Ian... I hope you get an answer or with further debug identify what the issue is. I'm interested in the answer either way.

[Sape]

Dear kgallen,

Thanks for your reply. Reading your comments, I checked my measurement notes. Due to time constraints we could not test all 36 channels. On one unit, signals from 1/2 were nice, 3/4 and 5/6 were weird. However on two other units, 3 was nice while 4 was weird and 5 was nice while 6 was weird. So these results do not clearly point the finger to the control side. From what I learned about dimmers is that the triac cuts away a part of the sinus. The remainder should still match more or less the original AC sinus. And that is clearly not the case. Just found a circuit diagram for Rackmaster 260/660. Needs a little closer investigation. It'll take a while, but I'll certainly come back on this topic.

[kgallen]

10 hours ago, Sape said:

So these results do not clearly point the finger to the control side. From what I learned about dimmers is that the triac cuts away a part of the sinus.

The triac will trigger part way through the sine wave so effectively the first part of the rise is cut off. The output inductor controls the rise time of this potentially sharp switch-on. This assumes the triggering circuit, and the diac in-line with the triac gate are functioning correctly. So I wouldn't rule out the control side just yet, particularly with so many channels looking bad. If a one-off channel was bad then I'd immediately suspect the diac, triac or inductor. Multiple channels looking bad in similar ways points more likely to a common problem rather than faults of the same type in the discrete components of multiple independent output stages.

Probing this is dangerous because of the mains voltages, but where competence and safety allows I would take a look at the inputs to the diacs on each channel and compare good and bad channels. Then I would work backwards to the opto-isolator and then onto the low voltage side of the control circuit.

Unless you think there are several different faults you don't need to debug on all 36 channels, just take a Rackmaster which has a good and a bad channel and compare step by step back from the diac.

If you can upload the relevant page(s) of the schematic here that would be useful to give more specific guidance.

Let us know.

Regards, Kevin

[Sape]

Hello Kevin,

Thanks for reply again. I included the circuit diagram which I found in a Rackmaster 660 service manual, but interestingly this print also lists model 260. However, AFAIK the 260 cannot run on a 0-4.3 mA input signal, so may I assume that components related to this input are left out?

The inductor you referred to is probably shown as filter in this diagram. Correct? Studying simple household dimmer circuits I do not find these coils.

In the diagram I cannot find an adjustable component related to the pre-heat function. Am I overlooking something?

Next Saturday we will take one or more unit apart and do some measurement. I suggest checking the multi-exchanger's 0-10V output as a starting point, the TP test point, the point between IC2 and C8 (at least the diagram shows what to expect here), the diac's LED, its input and output. 

It could well be that the 6 units are too tight in the rack so that components suffered from the heat. I also suspect the units haven't been opened since the start of their operation, so we expect to find a lot of dust. I let you know the outcome.

Sape

zero88_rackmaster_schema.pdf

[kgallen]

Hi!

Thanks for the schematic. Your plan seems appropriate.

Yes Filter 1 and Filter 2 should be large (maybe 2" diameter or more) wire-wound inductors (you should find small versions inside a domestic thyristor dimmer).

The diac I mention is part of the opto-isolator, IC4, IC5. They are probably socketed.

Given these are two-channel modules I would trace back on them comparing channel-for-channel as you go. Also confirm the indicated waveform at the TP and the negative pulse output on pin 1 of IC2 as you propose.

If both of the channels of an output board are suspected then check same circuit points on a known good board.

This should be relatively easy to debug by doing channel-by-channel compare and identify if the issue is in the common control section IC1,2,3 and associated passives especially capacitors and diodes, or on the output side, IC4,5, D15-18.

The LEDs L1,2,3 should also give you some info, so again compare channel to channel.

Preheat is R11. I compared with a Betapack 2 schematic and it's largely an identical circuit.

Also make sure switches like S3, S4 aren't incorrectly set or oxidised. They look like channel test buttons. I'm not sure what S2 is for - it looks like a jumper from the PCB layout. Check these are all in the same position, especially on channels that are showing different behaviour.

I'm not familiar with the 0-4.3mA current drive signal, I've not come across this before. Probably R28,29,30 are not fitted if it's not supported. IC2 is there for other circuit uses (it's a quad op-amp, as is IC3).

Kevin

ps I found a manual here if of any use: https://www.theatrecrafts.com/archive/documents/Rack-660-Manual.pdf