I just finished refurbishing a pair of the Texar Audio Prisims that I've had "in waiting: since 2015.
Texar Audio Prisims on the workbench...
The first repair I made was back in 2015 to cure "Molexia" intermittents. https://proaudiodesignforum.com/forum/p ... 764&p=8965
Molexia are the solder ring cracks that form around round Molex pins.
The Texar had quite a few which caused many intermittents.
Solder connections to round Molex pins develop crystallized microscopic ring cracks around the pin over time that must be resoldered.
I revisted the project this week and, based on the sonic results, I'm glad I did. Fast-forward to September 2020:
I did a full electrolytic recap which involved replacing all the 4.7 µF axial caps on the M-101 boards, 4 220 µF caps on the main board and 2 2200 µF caps in the power supply.
It should be noted that the Texar has no electrolytics in the signal path - all of the 4.7 µF caps are in the sidechain and control sections.
The 220 and 2200 µF caps are power supply bypass and bulk filtering.
Most of the 4.7 µF removed were reading about 30-40% low.
The filter and bypass caps were actually in tolerance.
Not bad for a 1986 unit that runs hot.
I also replaced the whimpy 28G ribbon cable leads from the power supply output to the main board.
These lines carry the full operating current for the unit which includes all the LED current for 40 LEDs.
Texar Audio Prism Power Supply
One of the bulk filter caps, C401 had a 0.1 µF axial lead monolithic ceramic cap installed underneath it wrapped around the electrolytic's leads.
This was presumably to reduce the possibility of the regulator oscillating.
I installed a radial lead 100 nF monolythic ceramic at the regulator terminals on both the positive and negative regulators underneath the board.
On the M-101 boards I checked the sidechain/Vactrol calibration which involves removing a CD4016, shorting pins on the 4016's socket and reading levels at a resistor lead. (See: https://www.proaudiodesignforum.com/ima ... gnment.pdf
Note that step 18 doesn't give a dB value - I think it's -9 or -10 dBu.
All but one of the 8 boards passed calibration tests which is a good thing considering that the trims are glued in place and the Vactrols virtually unobtainable.
Fortunately I bought a stock of VTL5C3 while they were still available and used one to repair the board which had aged out of limit.
That board required trimmer replacement and adjustment went fairly easy.
Texar Audio Prism M101 Board
I was not the first person who apparently checked calibration and following the suggestion to use bus wire to link the socket pins is not a good idea.
About 3 of the 8 cards' IC sockets for the 4016s were intermittent so I replaced all 8.
The sockets seemed to have low contact force and were a single beam type.
I didn't have enough dual beam high insertion force sockets so I used some nice machine pin versions.
I'm going to make a test jig with a proper male 14 pin header to keep from damaging the new ones.
These units were operated as a stereo pair but I found one of them had a misadjusted bypass gain trimmer, R115, which took some effort to find.
It's hidden behind the front panel and is one of the few front panel trims for which there is no access hole.
Texar Audio Prism CX-1 Board.
R115 is hidden between the two red toggle switches.
The sub panel has an access hole for R115 but to get to it the front panel has to be removed.
The one thing I haven't done is replace the input differential stage with an OPA2134.
Both the input IC and 5532 output are socketed.
Glen Clarke was very concerned about lightning damage and made sure those exposed ICs were easily field replaceable.
Balancing the two units' Mix controls for the four bands is easily done using a scope in X-Y mode and inputting mono tones at the band centers.
I haven't tried using pink noise yet.
I tried using stereo coupling and decided to follow Glen's advice and ultimately removed it.
Elements panned center stay centered without it and the width enhancement you get for elements panned off-center is very pleasing.
Texar Audio Prism
I'm really happy with the sound of these units after the TLC refurb.
They deserve all the credit they've received over the years and do sound just like I remember them at KTXQ/Q102 in Dallas in the late 80's and 90's when they were running ahead of the Optimod 8100.
They do a great job at unintrusive level normalization for the music collection and add a lot of life to dull-sounding older material with their unique spectral loading/dynamic EQ effect.
They'll be running in my background music playback system around the house.