UREI 527A Replacement Slide Assembly

UREI 527A Replacement Slide Assembly

Replacement slide assembly pops right in

Just 2 PCBs, standoffs, trimmers and
breakable headers connect the boards

New slide pots have center dentent

Used CAD software to design the PCBs

Both PCBs have large ground planes
connected to chassis

The UREI 527A graphic eq is a great circuit. It's inductor capacitor design is accurate and very quiet. But after 20 years sitting in some dusty attic, it's slide pots almost always get horribly gummed up. Of 4 units that I have, only one was actually usable. I worked on one with Deoxit for hours without success. Even if a slider can move, often there are positions where the wiper loses contact such that the effect of that band is lost. So these units are routinely dumped for less than $100 USD.

But the circuit itself is one of the most elegant as analog circuits go because when a slider is in the center position it contributes very little noise or distortion to the output (this is why the new slide pots have center dentent) and a single op amp can handle multiple bands. The 527 uses only 4 op amps (each handling every 4th band) for all 27 bands. The resonant filter of each band is a simple passive RLC (resistor inductor capacitor) circuit connected to the wiper of each slide pot. The result is a simple circuit that is measurably quieter than most graphic eqs even by today's standards. With my QA400 audio analyzer (basically a codec in a box but with software specifically designed for analyzing audio signals) I measured an SNR of over 95 dB with the eq IN and with all sliders in the center position. I measured an SNR of over 100 dB with sliders around the test frequency up.

So to fix the slide pot problem and resurrect a pair of these awesome eqs, I have created a replacement slide assembly for the UREI 527A. It's just two PCBs connected by standoffs and uses the old pins to connect to the motherboard just like the old slide assembly. It just pops right in place of the old one. The build takes about 2 hours. I just unsoldered the old connector pins (taking care no to lose them, I could not find replacements), stuff the boards with trimmers (or resistors), new slide pots (but leave out pots where header pins go for later), break header pins and insert, join the boards with standoffs, solder in header pins (making sure they don't protrude above the board so that they cannot possibly contact the slide pot above), solder in remaining side pots, solder in the old pins using existing residual solder, insert and secure the assembly into the unit, flip it over and reheat the connector pins adding solder so that the pins reset into their preferred position.

Finally I used the QA400 software to view the frequency response in real-time and adjust the Q of each band by putting a slider up, adjusting it's trimpot to ~11 dB and then I did a second pass by putting two adjacent sliders up and adjusting so that the two peaks match. Adjusting the Q like this makes all of the peaks level and accurate.

Some people claim that the UREI 527A adds some distortion or grit or crunch. I have not experienced this. Naturally with enough signal and with the input level switch in the 0 dB position (which actually means that gain is applied at the input and a corresponding amount of attenuation is applied on the output) then with enough signal one could certainly overdrive the unit. But at normal signal levels the 527A has relatively low distortion. I always use the +20 dB level position so that there is no noise added by the gain / attenuation circuitry. Using the QA400 I measure ~0.006 % THD which is quite respectable (see graphic). The op amps in the two units pictured here are socketed LF356 which is a decent JFET / bipolar type of part. I'm not sure if someone upgraded the op amps in these particular units or if they're standard but I know some units have LM301 op amps which is a lesser part.

SNR 95.2 dB / THD 0.00582 %

The only "distortion" I have seen that is unexpected is when the sliders are positioned exactly as shown in the following pic, I get a strange 40 dB notch at 630 Hz. This only occurs with this particular slider configuration. If one of the high frequency sliders is moved down, the notch becomes 10 dB as expected. Also the notch does not occur if I supply a single tone at exactly 630 Hz. Strange.

Strange notch with sliders in
certain configuration

Screenshot of strange notch

Parts List

2 PCBs designed using CAD software and manufactured by Advanced Circuits using their $33 service.
4 6-32 1-1/8" pan head screw (McMaster Carr 91772A154)
4 6-32 7/8" standoff (McMaster Carr 91780A750)
4 6-32 5/16" standaoff (McMaster Carr 91780A725)
27 Bourns PTA4543-2215DPB202
1 row of 36 breakable header pins with overall length of at least 1" like 3M 929700-06-36-RK or 929834-07-36-RK
27 500 ohm trimmers like Multicomp MCWIW1012-1-501-LF (or you can just use resistors but Q of each band will not be adjustable)
27 NKK AT4003A bat lever switch caps

Alltogether, I got four sets of boards (must make 4 with $33 service) shipped for ~$290 USD and the other parts add up to about $70 for each unit. So 4 units could be converted for ~$150 USD each.

Gerber Files

There are two boards so you will need to submit two jobs:

Slide_AssemblyB1_v9.zip
Slide_AssemblyB2_v9.zip

The exact dimensions of both boards is 13.4" x 3" (34cm x 7.6cm). The boards pictured are 1.5mm thick and that might be rather important as the assembly must have a certain overall depth (if you use dirtypcbs.com choose the 1.6mm option).

Note: Some UREI gear have posts screwed directly into the front panel using tapped blind holes. Meaning the screws used to mount the assembly do not pass through to the panel. This solution will not work in such units.

Optional Parts

I also added a nice conductive plastic potentiometer and socket cap screws for a new look.

10K audio potentiometer like Bourns 51AAA-B28-D15L
6-32 3/8" socket cap screw (McMaster Carr 90585A212)
6-32 1/4" socket cap screw (McMaster Carr 90585A210)

Headers connect boards so that
there are no wires

Replacement slide assembly fits perfectly

Old pins used with new assembly

Adjust the Q of each band using trimmers

Obligatory gut shot

Two nice units converted

Very clean, no rack rash

Done and racked