NAIM NAC32.5 and NAIM NAP110 Servicing.

This intended 'servicing' of the above old NAIM products, is going to be in 'slow motion', in other words - a slow, but on going job, and one that will reveal some discoveries, and pending problems - no doubt!

Not used '32.5' and the '110' for several years, I decided to fix a broken stereo/mono toggle switch, which from start to finish took nearly three hours.

In order to get the board out, I decided to desolder the rear DIN and phono connections. The replacement mini toggle switch was a Dual-Throw, Double-Pole type 'DTDP' with no 'latch' effect.

Top Toggle Swich was broken - no easy direct replacement available.

Having got the NAC 32.5 working again, I was puzzled that the pre-amp in mute mode didn't null-out the sound completely - something that I'd not noticed before? 

Working the NAP110 again
Switching on the NAP110 Power Amp soon revealed that something wasn't right - the NAC32.5 was not receiving 24v from the NAP110. The only indication that some power was getting through was a faint light from the LED on the 32.5. Re-connecting the NAP110, the NAC32.5, and a SNAPS unit (to run the 32.5) lead me to believe that the power supply from within the NAP110 was faulty? Both pre-amp (32.5) and amp (110) played well once the SNAPS unit was in use - although on occasion, a sort of disconnecting noise could be hear randomly. Further investigation later revealed that the regulators were not functioning properly.

A quick look into the NAP110 revealed that the amplifier's in built low-current LM317 voltage regulator (and/or 3300uF cap?) was faulty - only showing +2.9v dc, as opposed to +24v dc. This commonly used regulator circuit appears to supply any associated NAC controller of that era - I'm thinking mainly of the NAC 42, and NAC 32/32..5 series? 

Obviously, a few or all of the components need to be replaced, then some re-testing......

NAIM NAP 110 Power Amp with regulator for external provision.

With LM317 based regulator removed.

NAIM 36v-to-24v Voltage Regulator
 
The standard NAIM regulator for 36v-to-24v conversion is a common circuit configuration, but with additional ripple effect reduction via C2. 

 

This is able to reduce ripple to around -80dB, according to Art of Electronics (Paul Horowitz/Winfield Hill). Their research suggests a 1v ripple reduced to a mere 0.1mV!, at 60/120Hz I assume?

Below, I've drawn out the circuit.

The Components of the LM317 Design

Flywheel BYF406 diode: 1.0A Iout, 800V Vrrm, Fast Recovery Rectifier Diode. Operational temperature range from 0^°C to 175^°C. If a replacement is required I'll use a 1N4006 diode, although I only have the 1N4004 (1.0A Iout, 400v Vrrm) in stock, so maybe I'll use this instead? 

BYF406 Purpose: probably to shunt any charge from C2 should either C3 or the load 'short'. And additionally to protect the LM317T from any back emf if the load become immediately disconnected.

Capacitors: both 10uF capacitors are tantalum (a transition metal) bead types.

Resistors: Are all 5% tolerances.

Regulated Voltage Output: The Formula for output voltage Vo of the regulator is: Vo=1.25(1+R2/R1). And here (without error tolerances taken into consideration) we obtain..

Vo=1.25(1+3980/220) = 23.86volts.

So I can conclude that the layout 'checks out' nicely with the theory, and with parts currently in the post, I can look forward to rebuilding the regulator.

NAIM SNAPS for the NAP 110 System

Coincidentally, the NAIM SNAPS (power supply) unit also employs the same LM317 based regulator circuit, but here there are two of them.

To my complete disbelief, one of the regulators in this unit was also faulty, delivering zero output, that is 0V! While the other was working nicely at 23.7v dc. 

 

Seems, that this unit also requires a complete service too. I did note that the unit soon got warm after switch-on, an indication that the LM317 was shorting via C3?

11/12/2019: With reference to the NAP110 Power Amplifier
Replaced unit with new regulator: the LM317T, 2 x 10uF tantalum capacitors (where one had initially 'shorted'), and the flywheel/protection diode was replaced with 1N4004 type. 

Since then, I have been testing the NAP110 with my NAC32.5 - so far all working well!

The meter was flickering between 23.8v and 23.9v. The calculation above was 23.86v, the target is 24v.

Testing the NAC32.5 and NAP110 after regulator circuit fix.

12/12/2019: NAIM SNAPS unit now has had all LM317T and output 10uF 35v tantalum capacitors replaced in the voltage regulator circuit. Unit working well.

Large 10,000uF Electrolytics:
(18/12/2019) I will eventually replace all 10,000uF electrolytic capacitors in time - have just ordered KEMET 10,000uF 63V soldered-tag type (35mmx55mm). This is an over-specification, but I prefer to over-engineer. The original voltage limit of the old capacitors was 40volts. 

So far all units are working well!

Power-on Diode: (13/12/2019) The NAP110 power-on diode failed a long time ago, so I decided to replace it. Not sure of the exact specifications so I decided to put my own circuit in place - a simple red LED, in series with (effectively) 30K ohms. The supplied voltage to the circuit is approx 36v. I wanted to try to match the brightness of this with the brightness of my NAC 32.5 contoller's LED. After a few investigations, even 30K (10K+20K) ohms was not high enough resistance to create a good match, but it's good enough for now - I'll change it again later to perhaps 51K ohms?

 

The circuit is shown exposed (ie un-isolated) for the purpose of the photograph. I used an old small circuit board and adapted it, but I will re-configure it again with some veroboard I recently bought.

27/12/2019: Finally settled for 2 x 100K ohm in-series resistors for the LED, which gives a good match to that of the NAC 32.5 and SNAPS LED intensity - not quite the same, but close enough.

(The circuit has been modified slightly since I took this photograph)

The revised circuit board is isolated underneath with a rubber mat, cut from an old bicycle inner tube, super-glued and shaped to fit. The LED is temporarily centred with Blu Tack. There's no danger of the assembly falling out - with or without the Blu Tack. The hole in the fascia, is actually threaded, I didn't realise this until re-assembly.

Later on, I'll re-cap the NAP 110, and the SNAPS with new 10,000uF KEMET capacitors. These arrived this morning...

Also of note - a heavier duty full-wave diode rectifier for the power amp, which I may or may not fit later?

20/12/2019: The SNAPS unit is now fitted with a 10,000uF 63v KEMET electrolytic capacitor. The 'spades' on these are not as easy to work with as the original ITT capacitors. I applied plenty of solder and secured the initial soldered connection, and later went further by wrapping the soldered joints with wire, then applying more solder so that the original joint could never seperate. The downside of this additional procedure is that the joint 'looks' untidy from the outside - this I find a little annoying if I am honest!

(The servicing of the NAP 110, NAC 32.5, and SNAPS unit is unfinished. All working fine so far. 18/12/2019)