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Request for help with Eltec equipment.


Mad Al

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Hello,

 

 

 

This is my first post on this forum, I am having trouble with some Eltec lighting equipment and wondered if anyone could help me.

 

A bit of background about me, I am not a professional or qualified lighting person, I just like and collect old electrical equipment. This old equipment is not for professional use, I just like it to work and intend at some point to make a display piece of it. I have used analogue lighting racks and desks in the 1990's, firstly at Senior school and later at an Amateur dramatics society. I have a lot of electrical knowledge, but no formal qualifications.

 

A while ago I obtained an Eltec 3000 12 channel lighting desk, in very good cosmetic condition. I had no racks to test it with, so it just sat in a storage box. I don't know the history of the desk, although inside it there is a test sticker bearing the date May 1987.

About a month ago, I obtained two Eltec Studio 80 racks, again I know nothing of the history of these as they were sold to me as untested.

 

I powered the racks up after fitting them with a 13A plug, and all seems well, the red indicator lights on the front of the unit lit up. When I connected the desk to the racks and switched on, the small chip in the desk (an Op-Amp) literally exploded and broke into two parts. I have salvaged the parts to get the code and bought a replacement, which has subsequently exploded again. When I checked the output voltage on the positive and zero volt pins of the racks, they read about 38v (with no load on anything and the desk not connected)

 

Could anyone advise me on why the chip exploded and if the output voltage is normally this value, or indeed if the rack and desk are even compatible? If I get a different desk am I likely to explode or damage it if I connect it to the racks?

 

Also any other help or advice would be appreciated.

 

Thankyou.

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When I checked the output voltage on the positive and zero volt pins of the racks, they read about 38v (with no load on anything and the desk not connected)

 

High but not unreasonable.

 

The chances of finding any schematics for these are quite small; they certainly pre-date the 'net. But all is not lost.

 

Could you take some photos of the inside of the desk and its circuit board and post a link to them? You can't directly upload photos here so you'll need one of the many free photo hosting sites and them a link in a post here.

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I see from the Theatrecrafts site that this was another of Philip Snowden's enterprises but I either never came across it or if I did I don't remember it and I was in the market for this sort of gear in 1977 when the one catalogue I can find on that site comes from.
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I powered the racks up after fitting them with a 13A plug, and all seems well, the red indicator lights on the front of the unit lit up. When I connected the desk to the racks and switched on, the small chip in the desk (an Op-Amp) literally exploded and broke into two parts. I have salvaged the parts to get the code and bought a replacement, which has subsequently exploded again. When I checked the output voltage on the positive and zero volt pins of the racks, they read about 38v (with no load on anything and the desk not connected)

 

I'd be pretty confident that the Op-Amp won't be designed to work on anything as high as 38V. The highest supply voltages for op-amps are typically +/- 17V (i.e. 34V total), and +/-15V is a more normal limit. So I'm not surprised by the failure!

Most dimmer racks are controlled by between 10V and 15V DC. Polarity of this control signal (whether it was + or - relative to earth) is not uniform - Strand kit and compatible is negative control (-12V supply, about -10V for full brightness, 0V for off). Most other manufacturers were positive control (15V supply, 10V for full brightness, 0V for off). Mixing these up generally destroyed the desk by connecting -12V where +15V was expected or vice versa.

I think Eltec kit would be positive control (most of the small makers were), but I would be expecting a much lower supply voltage than 38V, which makes me wonder if there is a power supply defect in the rack?

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Look at a BigClive video on Youtube. Look at the method of doing a photocopy of the boards and a reversed copy of the underside. This way you can draw out circuit diagrams with some ease. Then you can work out what each piece expects as input and expects to send as outputs. Do consider that some components may say a value on the case but have a different value due to component aging.
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Thank you for your replies, I didn't expect so many so quickly 0.0

 

I will get the desk from storage this weekend and photograph the insides. Also I will double check the output voltage on both of my lighting racks as well, then I can report back on what I find.

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  • 2 weeks later...

The positive desk supply from the Eltec packs from what I recall was about 21v. Control voltage 0 to 10v.

 

Eltec was good solid kit aimed mostly at schools. I still have some analogue racks and as far as I know they work.

 

Does anyone know what happened to Phil Snowden?

 

I recall visiting their base at Llanfyllin in Wales and later a farmhouse in Shropshire (I think).

 

Prior to Eltec Phil worked for Contel Thespak in Stroud who produced early thyristor dimmers and desks with a 1.2v control voltage.

 

Pre DMX/d54 Eltec made a memory console called the "Sceptre". This was designed by Peter Wiggins from Derby. It was advanced for it's time and used a bbc model B computer to do the work.

 

Anyone remember that?

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The positive desk supply from the Eltec packs from what I recall was about 21v. Control voltage 0 to 10v.

 

Eltec was good solid kit aimed mostly at schools. I still have some analogue racks and as far as I know they work.

 

Does anyone know what happened to Phil Snowden?

 

I recall visiting their base at Llanfyllin in Wales and later a farmhouse in Shropshire (I think).

 

Prior to Eltec Phil worked for Contel Thespak in Stroud who produced early thyristor dimmers and desks with a 1.2v control voltage.

 

Pre DMX/d54 Eltec made a memory console called the "Sceptre". This was designed by Peter Wiggins from Derby. It was advanced for it's time and used a bbc model B computer to do the work.

 

Anyone remember that?

 

I went to the premises in Stroud in June 1973 when they were called I seem to recall Thespak Major. They offered me a summer job but a local factory offered more money. All I recall are some PAR lamp floods of the type later marketed by Pulsar and a very posh sounding girl behind the desk. They didn't seem to be very busy.

Edited by Junior8
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Hi, thanks for your replies and interest.

I am sorry for not getting the pictures sooner, however I have today taken some pictures of the Eltec 3000 internal boards and layout. I have also taken a picture of the two Studio 80 racks which I connected the desk to.

 

Here is the link, https://drive.google.com/folderview?id=1-4ELyH-N04EfP8rB4LR3nZayGEaWilYJ

 

Thanks.

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Some first thoughts:

  1. Looks like the desk output cables may have been replaced at some point, given there is insulating tape around the place where they join the red internal cables. This may mean the pinout isn't what it should be on the end of the cables!
  2. Not sure if the pair of red/black single wires and the brown connector are original - the connector is on the wrong side of the board, as well as angled (because the case won't shut with it straight?).
  3. In these wires Black is wired to op-amp negative, Red to op-amp positive. The big track on the right hand side of 164435.jpg is therefore the +ve supply rail (slight surprise as I guess ground).
  4. The absolute maximum voltage across these is 30V, should be less than 15V for single rail. So a problem at the rack end I think ...
  5. The master faders are "upside down" (in "independent" mode), i.e. they have positive control voltage at the bottom, top ends connected to the little blue trimmer (which adjusts dead travel?).
  6. This is fed by a shut regulator (resistor plus zener), arranged so that there is a stable voltage between the +ve rail (bottom of the master faders) and the top of the faders. Remember this lot is all upside down!
  7. The two halves of the dual op-amp are configured as a pair of identical circuits, but not any of the standard amplifier configurations! It gets mixed up with the little black devices just below the switches, which don't have visible markings (they will be on the vertical face).

Overall, this looks like a fairly standard desk, so would operate from ~10-20V DC control voltage, so spotblaster is probably correct with 21V. I haven't got far enough to be sure about +ve or -ve control yet - would need to get far enough to predict voltages on the yellow and orange wires down at the bottom which link to the fader boards. Whatever comes out there at maximum master will also come out to the racks at full channel!

 

So back with the first post, the problem is somewhere in the dimmers, which are kicking out far too much control supply voltage at 38V.I hope if you were to power the desk separately (do you have a variable DC power supply?) then there is some chance of it working. The only thing which could be broken by miss-connection are the TO92 transistors, although that usually happened if someone plugged into a Strand Tempus! (-ve control on the same 8-pin DIN connector).

 

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The IC's are in sockets which is not good for long term reliability because the contacts do tarnish. Tarnished IC socket contacts cause all manner of weird and unexplainable faults. A lot of problems can be temporarily fixed by unplugging and re-plugging each IC several times. The long term fix is to remove the sockets and solder the IC's directly in. IC sockets are considerably less reliable than the ICs plugged into them. The very high control voltage is most likely to be caused by a short-circuited pass transistor in the voltage regulator circuit.
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