Repairing a Service Bench High Intensity Light Source

[nevetslab]

Having had this A.G. Heinze Microscope Light Source working for the past 25 yrs, with just routine replacement of its’ EJA 21V/150W Halogen Lamp, it finally took a dump recently, blowing the 3A Slo Blo fuse twice in a row after replacing the lamp, as well as with no lamp installed. As repairs on shop gear tend to take a back seat if not essential for daily use, it’s been waiting for a break in the daily work load to see what’s up.

I managed to break a portion of the ceramic housing of the Lamp Holder when the ejector lever wasn’t driving out the dead EJA bulb and got more aggressive with trying to extract it. Hate when that happens. I found a suitable replacement part that cost me $12, now in-bound. Yesterday I finally pulled the unit apart to see how it was wired, and found mounted to the underside of the lamp holder platform an Omnephase 602A Triac, with it’s gate wired to the front panel AC Power Switch/50k Linear pot wired as a Rheostat. I’ve attached the data sheet I found on line of that part, as well as Teccor’s Triacs Data sheet, and think I could replace the 602A with a Q4010L5 10A/400V Triac. Though I would appreciate a thumbs up on that selection. Insulated case, of course, and would use heat shrink tubing on the leads as well.

Here’s the teardown / look inside images:



I didn't label the wire colors or terminals on the schematic, so relative to the 602A device, Terminal 3 is on the Line side, Terminal 2 is connected to the high side of the power transformer (RED lead) & bottom/wiper of the pot, with Terminal 1 connected to the Gate of the Triac, via Rheostat wiring of the pot. With a new Triac, I think the Line side would be MT2, Load side would be MT1 (triggering signal normally applied between the Gate & MT1, according to Teccor's data sheet), and Gate would be from the top of the pot.

There's 40 ohms end resistance on the 50k Linear pot, cranked fully CW.

OmnephaseAC Triac.pdf
Teccor Triacs.pdf

[nevetslab]

[ATTACH=CONFIG]52144[/ATTACH]

I didn't label the wire colors or terminals on the schematic, so relative to the 602A device, Terminal 3 is on the Line side, Terminal 2 is connected to the high side of the power transformer (RED lead) & bottom/wiper of the pot, with Terminal 1 connected to the Gate of the Triac, via Rheostat wiring of the pot. With a new Triac, I think the Line side would be MT2, Load side would be MT1 (triggering signal normally applied between the Gate & MT1, according to Teccor's data sheet), and Gate would be from the top of the pot.

There's 40 ohms end resistance on the 50k Linear pot, cranked fully CW.

[ATTACH]52145[/ATTACH]
[ATTACH]52146[/ATTACH]

The replacement Triacs arrived, and I thought I'd try to do something 'simple' before tearing down the Ampeg SVT-VR. Wiring the Triac up the way I see the original wiring with the failed Omnephase AC Triac, it doesn't work. I kept looking at that circuit relative to the information on the Littelfuse Teccor Triac data sheet, and it just looked like MT1 and the wiper/CW terminal of the pot should be connected to Line, and MT2 by itself be connected to the primary of the 21VAC power xfmr, MT2 being the load side.

When I wasn't getting anything but the fan to run as I ran up the line voltage, I ended up replacing the lamp socket assembly, since I had managed to crack the ceramic housing of the original lamp holder. That hasn't changed anything...I've either killed the Triac, or....it being connected as my diagram shows (that being the original wiring for the other Triac), it isn't correct for this Teccor Q4010L part. Take a look at the notes on pages E2-5 & E2-6 of the Teccor data sheet as reference, where MT1 is shown as GND, the Gate pulse is of the same polarity as that of MT2. Do I indeed have this wired wrong (though was correct for the other part)? It's always the 'simple' stuff I can't figure out!

[drewl]

Is that carpet? On your bench?

No anti-static mat? With snap for anti-static wrist band?

Shame.

[Dave H]

The Omnephase looks like it's complete phase angle control IC rather than just a triac. A triac phase control circuit usually has the triac plus a diac, resistor and capacitor. The diac, resistor and capacitor must be integrated in the Omnephase package.

[Jazz P Bass]

There is a replacement for that unit: CSR604A
CSR XXX.pdf


Unfortunately it is available in the UK.
https://www.newark.com/united-automa...-6a/dp/96K8085

[nevetslab]

Is that carpet? On your bench?

No anti-static mat? With snap for anti-static wrist band?

Shame.

No, it's actually a towel. I do have an antistatic mat along with the wrist band. I have to find where it got misplaced, though.

[nevetslab]

The Omnephase looks like it's complete phase angle control IC rather than just a triac. A triac phase control circuit usually has the triac plus a diac, resistor and capacitor. The diac, resistor and capacitor must be integrated in the Omnephase package.

[ATTACH=CONFIG]52473[/ATTACH]

OK, that makes a lot more sense. I had a feeling it couldn't be as simple as I had drawn up from pulling it all apart, finding a part that has been discontinued. The CSR604A that Jazz P Bass suggested would be an expensive part to implement, so I think I'll have to cobble the circuit to make use of the Triac I picked up, and run that by ya all. Many thanks on your input!

[nickb]

OK, that makes a lot more sense. I had a feeling it couldn't be as simple as I had drawn up from pulling it all apart, finding a part that has been discontinued. The CSR604A that Jazz P Bass suggested would be an expensive part to implement, so I think I'll have to cobble the circuit to make use of the Triac I picked up, and run that by ya all. Many thanks on your input!

Or pick up a dimmer at Home Depot and gut it....

[nevetslab]

Or pick up a dimmer at Home Depot and gut it....

I like that suggestion! I pulled apart the one I had set up as an RFI device for dealing with guitar/bass pickups, opened the cover to see what was inside that wall plate. A much more complex circuit, while all the essential ingredients are present. I'll have to scrap the 50k linear pot/AC mains switch for a higher value.

[nevetslab]

In looking further, I found Littelfuse/Teccor also offer their Triacs with a Diac built in to the gate circuit, and have ordered some Q4010LTH parts, which is their 'Alterinistor Quadrac', made for driving inductive loads, as this application is (150W 21V Halogen lamp on the secondary of a 120V power xfmr). 100nF/125VAC rated cap for the series cap at the bottom of the rheostat-configured pot. All the circuits I've been seeing are using a 250k linear pot, with an added series resistor, around 3k for when the pot is at full CW. I was able to find an Allen Bradley 100k Linear mod pot with Pull Switch in my parts stash. I'll put the two switch sections in parallel, as they're only rated at 2A/125V.

Littelfuse_Thyristor_QxxxxLTx_Datasheet.pdf.pdf

[tedmich]

Thats one simple design! I might buy two of whatever triac/diac fixes it in case they stop making them!

Just out of curiosity, are the flexible metal goose necks grounded? I'd be tempted to put them at real wall ground.

[nevetslab]

Thats one simple design! I might buy two of whatever triac/diac fixes it in case they stop making them!

Just out of curiosity, are the flexible metal goose necks grounded? I'd be tempted to put them at real wall ground.

Yes, those ARE at ground potential. And, thinking ahead, I've ordered 3 of the 'Quadrac' parts from Littelfuse/Teccor. The gooseneck fiber optic necks were originally fitted with adjustable lenses, though the unit didn't come with them out on the swap meet grounds. I'm guessing they would spread the light out, as it is a fairly narrow beam as is.

[nevetslab]

In looking further, I found Littelfuse/Teccor also offer their Triacs with a Diac built in to the gate circuit, and have ordered some Q4010LTH parts, which is their 'Alterinistor Quadrac', made for driving inductive loads, as this application is (150W 21V Halogen lamp on the secondary of a 120V power xfmr). 100nF/125VAC rated cap for the series cap at the bottom of the rheostat-configured pot. All the circuits I've been seeing are using a 250k linear pot, with an added series resistor, around 3k for when the pot is at full CW. I was able to find an Allen Bradley 100k Linear mod pot with Pull Switch in my parts stash. I'll put the two switch sections in parallel, as they're only rated at 2A/125V.

[ATTACH]52497[/ATTACH]

Finally had some spare time, while waiting for a client to show up with his Ampeg SVT-CL that's in Protect Mode. I pulled the Lamp housing off, removed the gooseneck, removed the new lamp mounting assy, front panel, and drilled a couple holes in that lamp support shelf to mount a pair of insulated turrent terminals to mount the 100nF/125VAC Trigger Gate cap to, which connects between the MT1 terminal and the Gate terminal. I first installed a 2.7k resistor between the pot and the Gate as a end stop value to the original 50k Linear pot having the power switch. Everything works, only there's very little light intensity change between the two extremes. Removed the 2.7k resistor, so it's back to the 50k linear pot off the MT2 terminal and the Gate. Didn't make any difference. I"m thinking I need a much higher pot value, like 1M linear? I only looked in my CE catalog to see what was available in pot/rotary switch controls. Nothing but Audio Taper in that form factor. It's at least working again, but not like it used to....it just doesn't go into what I'd consider 'Dim light' mode at the min control setting (max resistance).

The revised circuit look like what Dave H posted in # 5, less the stopper resistor. The Diac is part of the Teccor Q4010LTH 'Alterinistor Quadrac', being built in.

[nevetslab]

I don't know why I didn't think about ordering both a CTS Push-Pull 1M Audio Pot/Switch and a CTS 500K linear pot on my last Antique Electronic Supply order. I can pull the bushing off on both, swap the resistance module and re-assemble them, so I'd have a 500k Linear Pot with the AC Mains switch. Ordered those this morning, so hopefully with the new pot/switch, I'll get the desired dim thru fully bright light range on this nice light source.

[nevetslab]

Swapping out pot resistance element

The two CTS pots came in today, and once I had them in hand, with the light source opened up, I found (as I had suspected) the original pot was also a CTS pot/switch assy. So, I just unsoldered the wires from the three pot leads, took the control housing off, leaving the switch assy as is, did the same with the 500k linear pot that just arrived, swapped resistance elements & put that back together, wired up the new pot and reassembled it.

The first portion of rotation, from CCW thru 1 o'clock position didn't do anything, then the light turned on, and increased steadily to full brightness. So, I missed a step that I could have taken.....that being to substitute the pot with a decade box to see what the max resistance / min resistance should be. Looks like 500k was too much, and 250k would probably be the one. Too late....it does what I need it to do, even though the rotation range is not what it was with the original Triac module.

I broke out a Gossen Luna Pro F light meter to see what the range was, in terms of stops. 5 stop range, where I had measured it. Getting photographs of that is another matter, but I got enough to illustrate result.



Using the EV scale on the bottom of the meter dial, the min/max range is from 9EV thru 14EV (meter preset to 50 ASA, Ambient Light Dome in place). I didn't bother with foot candles or lumens....mostly interested in the range.