Servicing a set of Neve 1084 Mic Amplifiers and EQ units
This racked set of 4 Neve 1084s have been in a studio for years and after being in a small fire within the studio
had suffered smoke damage throughout.
They are in need of checking as they are old and their functions are not equal to each other.
Faults: Well - Each module needed checking individually. Initially the main and sub switches were all set to 'Off' levels.
Module 3 showed that someone had re-aligned the setting knobs for the HF and Mid main equalisations 2 clicks above the STOP 'Off' positions.
The knobs were undone and re-aligned properly.
With everything set to the 'Off' positions each module was checked in the 'Line' setting using an Oscillator, DB meters, Osciloscope etc. at
a fix 2KHz frequency (module 1 was taken as the mean 0db level). Readings for EQ 'Off' and EQ 'On' were taken - quite dis-similar.
Each module was taken in turn - The HF main switch on module 1 showed that at the highest setting - without the calibration knob being involved -
the sound was accentuated - inspection showed its associated poly cap soldered to the main switch had an incomplete solder joint to the common rail -
once re-soldered - it operated fine.
Initially, all switches were cleaned and lightly re-oiled.
Decision Time - these modules are built as capacitor / resistor switching networks to alter the EQ throughout.
The majority of the powered cards have very old Electrolytic Capacitors and the dreaded Tantalum Capacitors as Input/Output de-couplers.
These all needed one to one replacement.
It was interesting to find that 'Someone' had changed some of these for "maybe" equivalents - but had forgotten to change the same items in the other cards.
What a mess!! Start again - find the circuit diagrams and replace all Electrolytics and Tants with proper items.
Firstly, Change all the Rack PSU Electrolytics - Nice smooth rails now!
Next change all the smoothing caps in each module - to save space - 4 x 470uf caps have been installed in parallel
within each module. These were replaced with modern units - including replacement of a seperate individual smoother feeding each Mike amp board.
Now for each of the B283 and B284 Cards. What discoveries: some Electrolytics soldered the wrong way round -
but still working as capacitors without blowing up their associated resistors.
The 20KHz mod for the B284 Cards was also implemented -
All of the Poly caps were tested - and were left on board as not needing replacement. Transformers checked for continuity - all OK.
Reconstruction of each module and re-installation - Let's take some measurements with everything set Flat:
Job Done! This multiple unit operates very nicely - Module 4 has some High HF problems which I will have investigate later -
as the Client wants the unit back for use this week -