Here we go again, Symetrix 528E mike Processors. Some noisy, some non-functional. Let’s pick one and see what we can find.
This one has all sorts of issues. The mike/line -15dB pad switch is crackly crunchy when pressed. Most of them are at this point, the question is whether it can be brought back with a good cleaning or needs replacement. There is no audio getting past the preamp stage so there are more failures elsewhere.
It is a piggyback switch. The upper switch controls the first three pairs of pins, the lower switch the rear three. Both E-switch and CIT made this switch but it is no longer a production item and Symetrix no longer offers parts support. They are becoming difficult to find. We have several dozen of these switches on hand.
This one just needed a good cleaning. This is still probably the best contact cleaner made, even though it was reformulated some years back. It also works well on noisy controls, which these units tend to develop, particularly the mike preamp gain control.
Now to disassemble the unit to replace the capacitors. If the back looks like this it’s a simple job. Just a handful of screws to remove.
If it looks like this, then you have an early unit. These have no SMD devices at all. The trick to disassembling these is to separate the XLR connectors. A small flat blade screwdriver fits in that small center hole to turn the lock and allow the XLR to slide out of the metal housing.
Once all the screws are removed the PC board with the front panel still attached, slips right out.
There is a mix of capacitors on these units. Some SMD, some through- hole. We’ll change them all. The SMD caps near the amplifier chips in particular tend to fail from long term exposure to heat. This one had several fail completely causing it to not pass audio.
The THAT 4301 chip is a dynamics processor (compressor). It’s one of several parts that are no longer available. They can be found on Ebay, but it’s a roll of the dice whether they are good. Hint: if they are shipping from China, they probably aren’t.
A hot air rework gun makes this a simple job.
Once the solder is heated to the melting point, remove the old cap and set the new in place, re-heating until the solder flows. I add a bit of flux to the new part before setting it, it’s easy to clean up after it cools with a q-tip and bit of alcohol.
Here are a few of the new caps going in. I tend to favor Nichicon, having had very good luck with quality and consistency.
105 C caps in the power supply, 85C is OK in the audio circuitry. If they get anywhere near 185 degrees F, then the thing is on fire and the caps will need to be replaced again anyway.
After soldering, snip the excess leads and clean up the flux with alcohol.
And there we are, new capacitors and clean, quiet switches and controls.
That row of TRS jacks on the back? Audio passes through every one of them.
A run through the audio test set (currently using PC- based Rightmark) shows 0.08% THD, -70dB noise and 0.5dB frequency response deviation. That noise level does not meet factory spec. I haven’t found one yet that does. The mike preamps are just plain noisy and the more gain they are asked to supply the noisier they get.
Everything gets scrubbed, knobs and case. Generally these stay pretty clean inside as they are sealed. That is both good and bad as it also means little ventilation.
Service sticker on the bottom with the date and work performed. Then back on the stack to return to the customer and on to the next!