Dolby CP200 PS1B power light bulb replacement?

[Tj Hopland]

Working on some PS1B's and was thinking it would be fun to get the power indicator lights that are housed within the power switch working again. Anyone know or remember how to even access the bulb and what sort of bulb it is?

I was expecting it to be like other similar switches were you can just kinda grip the edges of the button/lens and pull that out which then gives you access to the bulb. I didn't want to grip too tight or pull too hard without knowing that this is how they should come out. The body of the switch inside the chassis is covered with heat shrink so without removing that I can't get any numbers off the switch or see if perhaps its a stacked arrangement that can be disassembled.

If I know how its supposed to work I would be more willing to use more force or cut off heat shrink but I don't want to break an otherwise working switch just to possibly fix the light.

[Mark Gulbrandsen]

Dolby told me years ago to replace the entire switch. Since the lamp is incandescent, I decided to install a nice panel mount red LED next to the switch on the front panel. I did that to the main and back up supplies at the same time.

[Steve Guttag]

One has to have their pilot lamps lit!

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One does just grasp on the white lens and that will pull out. It does snap in so you will need to overcome the snaps. You shouldn't need tools, however. It is a T5.5 "slide-base" lamp (think old 6-line commercial telephone translucent buttons). Since only its head will be visible with the button/lens out, you will need to be very gentle with some needle nose pliers to pull it out. Dolby used to include a rubber lamp puller (just a tube that would fit snug over the lamp so when you pulled, in theory, it clung to the lamp better than the lamp clung to the socket. The lamp was driven off of the 21.5V AC (right off the transformer that makes the 24VDC power.

If you are familiar with the PS1B and the CP200...the voltages are VERY nominal, based on what you have on it and what your incoming power is. Considerable sag is common. The +/- 15V rails are only rated at +/-14 and, in real life, they are closer to +/- 12.5VDC. They sort of knew it was a weak supply since they used 12VDC relays throughout...which would be exceeded if the 14VDC rails actually produced that.

For some reason, I think they used an 18V lamp but I may be confusing that with Kinoton. It would explain the short life of the plot lamps. 24V would be safer.

When I would rebuild those power supplies with switch mode supplies instead of the linear parts, I would switch the lamp to an LED and drive it off of the 24VDC supply. I would also cap the output of my +/- "14" volt supplies to 13VDC to ensure that all of the 12VDC stuff, like the relays survived. When the CAT517 and CAT560 came out, knowing that the PS1B rails sagged, they use the +24 and -14 on the line amplifiers to get the maximum voltage swing on the outputs...which is another reason I didn't go for the full 14 (or even 15) volts. It would put the new output amplifiers right around their max.

If you are looking for an LED retrofit that absolutely looks correct (see the photo above), then MARL makes it (but it isn't cheap and there are minimums if you need to special order). The warm-white is Marl 209-993-23-38. The sometimes easier to find cool white is 209-997-23-38 but it is much less satisfying and definitely too bright and too white. However, if you are going to use them you will need to modify the wiring some. I would add a rectifier (diode) and capacitor.

If you haven't done so already, I would definitely consider replacing the the capacitors in there (that are notorious for loosening up) with slightly larger capacitance. I was not able to source exact replacement screw down versions, when I was doing that. Moving to switch mode supplies significantly reduced the noise level in the CP200 (lower hum). However, one has to be careful on the supply they use to avoid adding switching noise to the situation.

[Tj Hopland]

Good to know that's how they are supposed to come apart, I will have to go give it a try. In the past for stubborn lamps like that I had resorted to heat shrink but I'm also thinking maybe hot glue could work? Either a blob of glue to grip or just glue the tip of a screwdriver or other took to it to yank on.

One of the supplies I am working on actually has a factory defect, you think its still under warranty? Who ever assembled the LED board for the rail indicators assembled it upside down. Its a single sided board (with no VIA holes) and they put the LED's and wires on the foil side. I thought it was just a wacky design choice but while complaining about it to another tech it was he that suggested maybe its upside down so I looked at the other unit that I had open and even though its hard to see with the transformer in place the foil is on the bottom as you would expect.

I think due to being upside down they had to extend the LEDS further out than they were supposed to be so they could sort of get to the pads with solder. This meant the board wasn't fully seated on its standoffs and there was pressure on the LEDS. At some point in its life the toroid transformer shifted slightly into the board and smashed and shorted the middle LED which apparently then took out its resistor and the fuse, at least I assume that was the chain of events.

I don't know its history it was just found on a shelf. New resistor and LED for that one and its mate so they look alike this thing should be good to go again as soon as the new caps come in. This one did have visibly leaking caps in it. Perhaps why it was pulled at some point in the last 40 years and ended up on a shelf? I didn't turn the board around because my LED's are shorter so that should take care of the clearance issue plus I already had the new ones soldered on when it was pointed out that it was likely upside down.

I did notice the lack of regulation with no load on the bench not to mention no regulators in the chassis. I want to say its something like 15 and 27 volts but sticking about 1 amp on each got em to something that seemed more reasonable. With these things power demand was kinda a curve wasn't it? Initially I assume they had enough power but then as some modules got added and evolved demand went up? But then things like 560, 517, 150F was lower again? Just fully stuffing the ACC rack (which I assume at least a few people did) I would think would have added quite a bit of load. You would think maybe the ACC rack would have had its own power ports and they would have maybe split the loads a bit?


Steve, what is the unit between the control and processor?

[Steve Guttag]

It is the Studio Interface "SI-1." It provided transformer balanced line inputs right before the CAT117/517. At the AFI/Silver (the unit shown in the picture), its inputs are brought over to a patch bay so most anything could be patched in.

[Tj Hopland]

It is the Studio Interface "SI-1." It provided transformer balanced line inputs right before the CAT117/517. At the AFI/Silver (the unit shown in the picture), its inputs are brought over to a patch bay so most anything could be patched in.

Is that the same place a DA20 or DTS 'got in'? Now you got me trying to remember the signal flow, 64's are before the 17? So it would not be the same place? That option bypasses everything except the fader?

[Steve Guttag]

Yes, the SI-1 splices in on the JM11-JM21 ribbon cable loop as well as the 9-pin power loop. In fact, where I located it in that picture, the connectors are in-line with the CP200 connectors on the Processing and Control units. SI-1s were very low-quantity. I ended up putting two of them in. We modded them to allow the subwoofer to flow through instead of mono-surround.