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Texar Audio Prisms

R

RadioMan203

I picked up a pair of Texar Audio Prisms and need some info. It appears on the pre-400 serial number units I have there are a couple of factory mods, including a board containing a TL072 on the main board wired in with point to point wiring and a trimpots on an 8 pin DIP header located on the main board behind the presence band control card. Also, my units have the M102 cards instead of the m101's referenced in the Texar manual I have from 1988.

In addition, on one of the m102 processor cards the bias pots have the seal visibly broke and the adjustments don't look like any of the others. SHould I just leave them and hope they are close or should I measure the corresponding voltages on the other boards and set the questionable controls to match?

Lastly, while I know this has little bearing on the operation of the units I would like to find of covers to replace the missing ones on the pair. It seems so many of them have been misplaced over time - I would hope somebody has a pair of these in a junk drawer. Failing that, I would take a high-resolution scan of a cover and use it to perhaps fabricate replacements from plastics.
 
IIRC, the M102 was from when Gentner changed to the Prism-2. I believe it was backward compatible. Many people screwed with the bias pots to try to calibrate the displays. There are only a few people around who can properly calibrate the cards. Many of the first Gentner units were made using all Monroeville parts. I have a set in storage. One of the several DVE mods gave at least 5 dB more gain reduction by the use of an outboard piggyback. If you could ever get Ed Young and that bunch to talk you may get some answers. He said Glenn was paranoid about the factory approved mods that were out there. A few of those Pittsburgh guys still keep the mods secret to this day
 
Bill DeFelice said:
<snip>
on one of the m102 processor cards the bias pots have the seal visibly broke and the adjustments don't look like any of the others. SHould I just leave them and hope they are close or should I measure the corresponding voltages on the other boards and set the questionable controls to match?
<snip>
Click to expand...

Hi Bill!

The Texars use photo-resistors in a feedback configuration. The idiosyncracy is that no two photo-resistors are identical, so there are adjustments to attempt to make the compressors act more uniformly, from band-to-band.

From all this, I would conclude that the **performance**, rather than the voltages, should the guiding measurements to use when adjusting the Texar trimpots. Make all the compressor modules **act** the same, as opposed to setting their internal voltages to be the same.

You may want to measure all of them, and pick which one (or ones) you believe is acting properly, and use that as a guide to adjust the others. IIRC, most of the adjustments affect loop gain, either directly or as a bias, at some point.

To get all bands to be truly identical may be a bit like herding cats. :)


Kind Regards,
David
 
Hi David,

After settling down with the copy of the manual I was able to find online I discovered (and remembered) about the photocells. I hope to find some clearer schematics so I may be able to sort out what was factory and what may have mounted to in the field mods for these units (my scanned copy of the Prism II book has schematics, the book for the original doesn't, so I'm not sure if these units where produced during the transition from Texar to Gentner). FM-Engineer's post seems to point in that direction as both units contain the M102 cards in them.

They do seem to operate for the cursory testing I did but I need to thoroughly run them on the bench to be sure. With a school system budget I was happy to get anything that resenbled "decent" processing for their on-campus stations. which will be a step above the Audimax/Volumax I was able to first acquire at the beginning of the project.

In a way I was sort of perplexed, as I know many audio recording folks drool over the UREI gear that uses the same photo-resistor technology as the Prisms - why haven't they gone equally as mad over used Prisms? Perhaps they want the "old school" wideband pumping action?

Thanks for the insight, David. I think I will probably check the modules and then move the non-sealed unit into a couple of different positions and see how it acts. If I can get performance that's close to the other modules I will be a happy camper ... and hopefully a little easier than herding cats!
 
LA_Guy said:
The little board with the opamp on it behind the audo controls is supposed to be there...
Click to expand...

Thanks LA_Guy. Without any schematics it makes it hard to figure what's "stock" and what's a mod. The 8 pin DIP header with the two pcb mount pots also has me curious. I lucked out as there was a sticker on one of the part stating that the input gain resistors were changed from stock and there were enough notes in the Prism II manuals for me to locate them and figure out what the original values were.
 
Hi Bill,
I used to have a set of pre 500 serial prisms,the main difference was that the display is dot mode on pre 500 and bar graph post 500(ish).

The early prisms had the serials in felt tip too!

The problems with pre 500 serial were that the positive reg used to drop out of regulation due to transformers being inadequate.
The early prisms used LP30-400 transformers,the later Texar and Gentner prisms used LP34-340 transformers.
The LP30-400 transformers are 12va 30v centre tapped,whilst the LP34-400 transformers are 12va 34v centre tapped.

Also the early prisms had no phase rotator option,but later on they had a module on a ribbon for the phases rotator.
The later versions had the phase rotator components on the main board.

Be aware that you cannot simply put the pre 500 series dot mode display in to bar mode by changing a pin on the driver!

Somewhere i have the proper factory calibration procedure for the agc boards.
Also the control circuitry of the early M100 boards is a bit different than the M101 boards.

I also have the manuals for the prisms as well,i wrote down notes and diagram differences as well.

I assume the M102 agc boards are similar to the M101 boards.
 
Hi Broadcast,

Thanks for the great info. The display doesn't bother me as long as it works and indicates what the boxes are doing. While the units have serial number sticker plates on the outside, they are indeed written in marker on the inside chassis.

Great info on the power supply - I'm going to include this thread in the file along with the rest of the docs I have been able to cull together for the units. Obviously if the units don't have the phase rotator there's not much I can do, but if the processors do the job that works for me.

If you happen to come across the factory procedure for the agc boards I would love to grab a copy so I can perhaps check one board that may no longer be in proper calibration. I have had several others confirm that the M102 is indeed the newer or revised board that was used toward the transition to the Gentner branding of the Prisms. If you have some readable schematics I could use those as well.
 
Bill,

Here is the alignment for the M101 boards.....................


Gentner Audio Prism M-101 controller card

PREPARATIONS

A) Connect audio oscillator to test chassis input
B) Connect rear end of R347 on motherboard THROUGH 2.2K to +15V.
C) Connect rear end of R346 on motherboard THROUGH 2.2K to GND.
B) & C) defeat input buffer
D) Put M101 card for adjustment in MID-BAND position.

PROCEDURE
1) Remove U5 (CD4016)
2) Remove R3
3) Short pins 1 & 2 of the U5 socket (Compress).
4) Put 20K 10 turn pot in R3 position, with audio voltmeter connected to
right-hand end.
5) Turn R1 & R2 fully CCW.
6) Set oscillator to 1 kHz, 0 dBm, 600 ohm source impedance.
7) Measure level at right end of 20K ten turn pot and set INPUT pot to
get 0 dBm (re .775 volts)
8 ) Turn R1 fully CW.
9) Set 20K 10 turn pot to achieve 24 dB of attenuation.
10) Remove 20K 10 turn pot and measure its resistance. If it is within
the range from 7.5K to 20K solder in the next higher value of 5% resistor.
If it is outside this range, replace the VacTrol cell and return to step 3.
11) Back off R1 for -22 dB on voltmeter.
12) Move short on U5 socket to pins 3 & 4 (Expand).
13) Adjust R2 for -2 db on voltmeter.
14) Move short back to 1 & 2 (Compress), trim R1 for -22 dB on voltmeter.
15) Move short back to 3 & 4 (Expand), trim R2 for -2 dB on voltmeter.
16) Repeat 14) & 15) until both readings are within 0.1 dB of specification.
17) Move short to pins 10 & 11 (Quiescent).
18) Verify level of -dB, (+ or -) 1 dB.

Do you have the schematics in your manual for the m101 boards?
If you don't have the schematics i can scan and email them to you,if so send me your email address.

The problem with the vactrols is that they age,so they tend to go out of calibration.
The vactrols have a led light source and a photo resistor inside.

Paul.
 
Also i forgot,

I also have the rare schematic for the early potted phase rotator as well.

Also the Phoenix am processor is an audio prism with an additional board fitted to the underside of the top cover(amc board(amplitude modulation controller).
This extra board connects straight to the cx-2 board,this cx-2 board connects the 8 control pots for i/p gain/gate/density/o/p gain and mix levels.
If you look at this board it has a jumper soldered on it,this jumper is removed and the amc boards audio in and out is connected instead of this jumper,as well as power supply!

Incidentally i have a fully working spare amc board,nrsc version,i took it out of a phoenix prism to convert it back to a straight prism!
Sorry about the long info,it is turning in to a novel!

Paul.
 
Hi Paul,

Thanks for the wonderful detailed info. I just pulled apart the pre-500 S/N units I have and they have the updated power supply transformer as well as the CX-2 control boards so that's a couple of things I won't have to worry about.

I'll send you my email via PM, as I would probably have to compare the M101's to the M102's that are in my Prisms to see if I could use the calibration procedure successfully. I don't know how the update will affect the calibration procedure or performance. I don't know if the either of the two "extra" mods on the main board are the phase rotator or not but perhaps the schematic will help me identify that as well. Since my copy of the manual doesn't include any schematics I'm looking for all the info I can get my hands on.

That's interesting about the add-on board for the AM "Phoenix" unit. At least I can keep an eye out for an AM unit to add to the school's sister AM station in the future.
 
One of the biggest problems with the Phoenix was that all of the controls were only accessable from the top of the chassis. You had to pull it out of the rack to adjust and see the status lights. It was no 9100A but did not sound that bad, plus as mentioned you could always turn it back into a "FM" Prisim.

Lane
 
When using Texar in front of Omnia, please confirm if the phase rotators must be enabled on Texar and turned off on Omnia, or inversed. Thank you.
 
Lane_Lindstrom said:
One of the biggest problems with the Phoenix was that all of the controls were only accessable from the top of the chassis. You had to pull it out of the rack to adjust and see the status lights. It was no 9100A but did not sound that bad, plus as mentioned you could always turn it back into a "FM" Prisim.

Lane
Click to expand...

That was the one thing that turned me off the the AM version at the time I was doing quite a bit of contract work. A friend was engineering the AM with the Phoenix and he put a 4 space blank panel above it so he wouldn't have to pull it from the rack to adjust it.
 
If any of you folks happen to have a copy of the Gentner Prism II manual or the Texar manual that covers the M102 gain controller boards I'm trying to obtain the part number of the Vactec Vactol (optoisolator). I'm not entirely certain one of the boards is performing correctly. Since there has been discussion of this particular component aging and thus changing calibration I figured maybe I'd order a few for comparison and/or replacement if I can't find a way to get the questionable ones calibrated.
 
Vactecs are VTL5C3. You may also replace all the capacitors. I've had the same problem with my set of Texars. Contact me off list with your email address and I will send you a PDF copy of the Prism II manual. If you need the Texar schematics, you can get it at mooretronix.com
 
If you use M-102 processor cards into early( pre 500) prisms, please make sure, that the jumper on the M-102 processor is set to M-100 mode, to make a proper system ground for use with the MB-1 motherboard. Otherwise the Bias voltages will fool around.
A good help for the Allignment procedure, wich BROADCAST provided overhere is to make the shorts with a switch instead of a wire. Otherwise you would not be fast enough for proper callibration.
 
Thanks for the heads-up, Parasound. I did note that the jumpers are in the position you referenced. These are all going into the reference file for the units. It's great that everybody has been able to share their experience with this gear - I thank all of the contributors for your insight and feedback.
 
As stha mentioned, the single best thing you could do is shotgun all the caps on everything including the power supply. Chances are they need it in a bad way. Good luck.
 
fm-engineer said:
As stha mentioned, the single best thing you could do is shotgun all the caps on everything including the power supply. Chances are they need it in a bad way. Good luck.
Click to expand...

Probably a best practice for any gear more than 10 years old, and with the Prisms probably being 20+ years old it would probably be due. I got a set of Belar FMM-1/FMS-1 monitors for the school station I'm building and I did a blanket replace on it as far as caps go.

So far I was lucky enough to get readable schematics for most every board on the pre-500 serialed Prisms except the M102's, but I may have that shortly with the help of fellow poster stha.
 
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