EQUIPMENT HISTORY:
Everything I am describing below is IMPORTANT, read it please!
This amplifier is part of my "collection" of all things audio, both professional and consumer gear. I personally don't mind a decent NAD piece of gear and actually had a number of these 7150 series receivers, this particular unit was a "to be looked at" as it is NOT WORKING CORRECTLY - see the FULL details below.
Like many NAD amplifiers, these 7150 receivers surprise many. They use a design which results in an amplifier that easily produces far greater audio output than the listener could be expecting.
Another aspect of the 7150 which is often overlooked is the (then) newly developed Tuner system called Schotz Variable Bandwidth. This sophisticated FM detector system is described in more detail below, in the testing section.
I have over the years collected together a LOT of quality audio equipment due to my business activities in pro audio post production and general electronics but the day has finally arrived!
Well, my wife says "for goodness sake, get rid of that mountain of gear" so them's my orders ... out they all go.
This equipment does have issues, and it is extremely important that the complete description is read in full.
THE CONDITION OF THIS EQUIPMENT IS CLEARLY STATED BELOW, BOTH COSMETICALLY AND ELECTRICALLY
There is no warranty, there are no returns - but I am very thorough and do not "gloss over" any issues I might discover!
Even if the new owner doesn't feel up to repairing or having this equipment repaired, it's is still a great source of some very hard to find parts.
NAD Stereo Receiver
Model: 7150
Released from the factory 1983 (QC sticker on the base)
Serial # N7509592
Supplied in the ORIGINAL shipping carton (factory has attached identical serial number label to the carton) along with the correct styro foam end sections
Dimensions: 420mm x 96mm height x 310mm
Weight (unpacked): 9Kg (packed in the box) 11Kg
Australian version, fitted with Australian GPO plug (normal 3 pin with earth)
SPECIAL NOTE ABOUT AC INPUT:
Strange thing: The carton says E3 240V version but the unit has the rear labelled as 220V but the equipment and carton serial numbers match.
Before testing the equipment (see below) I traced the AC input and it is applied to Blue/Orange on the AC power transformer, this is correct for 240V operation
The AC transformers are 29-2072-7, 29-2072-7A, 28-2070-1 which are for the E2/E3/E4/U3/U4 versions, as they should be, so the AC transformers match up with the fitted power lead (E3)
No idea why the rear label is incorrect though!
BASIC SPECIFICATIONS:
Power output 60W RMS per channel into 8 Ohms
Now this can be very deceiving when it comes to NAD equipment! These amplifiers have very high dynamic-headroom ratings and are able to drive very low load impedances (such as 2 ohms) at surprisingly high power levels without clipping or other distortion.
The "real life" clipping-output ratings (across the audio spectrum 20Hz ~ 20Khz) are:
65 watts into 8 ohms
75 watts into 4 ohms
85 watts into 2 ohms
These are all your industry standard RMS ratings but if you look at the dynamic power ratings, they are:
100 watts into a 8 or 4 Ohm load
130 watts into a 2 Ohm load
In real life this means that if your music has a wide dynamic range and at times a heavy demand is placed on the audio output section to delivery much greater power, the 7150 easily takes it in it's stride without falling into a horrible distorted mess.
But wait! there's more ...
This 7150 has bridging capabilities (switch on the rear panel which allows you to create a HIGH OUTPUT MONO amplifier)
When bridged you can expect 125 watts RMS with up to 150W before clipping OR 225W Dynamic power into a standard 8 Ohm load.
Frequency response (3dB points) 10Hz ~ 35Khz
Damping factor: 100
Maximum Total Harmonic Distortion (THD) at full power into 8 Ohm load: 0.03%
Inputs: Phono (turntable), AUX, Tape 1, Tape 2
Phono inputs switchable between Moving Coil and Moving Magnet
Signal to Noise ratio: 68dB (worst case Phono MC and best case 95dB Line level inputs)
Preamplifier to Power Amplifier U links, meaning you can easily operate the two sections independently of each other.
2 section tone controls (Bass, Treble)
OFF, A or B or A+B speaker switching at the front panel
Speaker outputs are all spring loaded terminals.
Many other controls such as filters and switching which I feel don't need detailing, look at the photographs or you can see the full info online very easily.
COSMETIC CONDITION:
In a nutshell, Wow factor is 9/10!!!
I used this 7150 until about 10 years ago, then it failed me (see below for details) and I put it back in the box where it has stayed all that time since.
Never abused, always used.
I still use another 7150 series receiver everyday here in my little workshop/office and they are wonderful beasts!
The case is straight and true, no dropping has occurred (although it is quite heavy, carry with care!)
NO scratching or discolouration to the sides, front or rear panels.
I noticed when taking some photographs that the top has these strange little marks, not really scratches but little marks of some sort. They are not terrible but I don't want people to think it is perfect. NAD used a sort of "baked enamel look" finish when decorating these cases so perhaps it is something to do with that, but this only applied to the top - all other sides/front/back are fine.
Base panel is clean sheet steel, no corrosion signs at all (always look underneath an amp to get clues as to how it has been treated)
All knobs and buttons are correct and accounted for!
All rear panel sockets/switches are the same way.
Front panel display is clean, clear and unscratched.
Dare I say it, it is quite close to looking "as new"
TESTING:
Ah this is the part that really matters....
Gathered together some nice sound sources, Marantz CD player and a Technics turntable running with a SHURE VI cartridge. Multimeter at the ready!
Both the top and base covers are off, I want to look at the condition of the boards, any mods or repairs and watch out for failed or failing components.
Two large PCB's, the upper being the Tuner/Display etc and the lower is the preamplifier/power amplifier and masses of switching!
The three AC power transformers all sit tucked away on the left hand side of the amplifier chassis.
Visual inspection shows that the 7150 is clean and free of excessive dirt/dust both "on top" and underneath.
The PCB has not been "worked" so I conclude that since it left the factory, no-one has been playing around, servicing or modding this equipment - virginal.
No obvious electrolytic cap "popping/bulging" or overheated/burnt areas of the boards that are readily visible. No leakage from the electro's, just that horrible glue that NAD used to secure the main supply filter caps (4 of them)
Enough "looking", let's fire it up!
Testing undertaken at 240V AC input
Power ON switch working (locking) fine, green LED comes on.
FIRST ISSUE: the display does not "come alive" ... more about that further down.
Protection relay operates as normal after 1 ~ 2 seconds.
Prior to connecting my precious Vifa loudspeakers, I checked both outputs for the dreaded DC voltage being present, none at all thank goodness! Never ever should you see DC voltages at your amplifier speaker outputs because if you connect your prized speakers to such an amplifier, your speaker voice coils suddenly turn into little heaters and then go POOF! This is something you must prevent at all costs - ALWAYS check an unknown amplifier for DC at the speaker outputs before connecting your speakers.
No issues at all on either channel (no AC or DC of any significance), I connected my 8 Ohm test loudspeaker system.
I then wound up the volume to "10" the maximum level with NO audio inputs, listening very closely for background hum. Nothing of any significance heard so this indicates that at least the main filter caps/power supply is doing its job filtering the DC main rail.
I then wound back the volume, switched to AUX, TAPE 1 and TAPE 2 inputs successively and was greeted with wonderful "NAD sound" - so nice and clean, NO distortion and plenty of grunt! I wound up the system level and no signs of any problems at all.
This amplifier uses a pair of 2SC2921/2SA1215's (Sanken) in each channel output, these are originals and have never been changed (any service work, not matter how professional always leaves behind little tell-tale signs of work having been undertaken)
At this point I am relieved and delighted that the power amplifier is fully functioning - very nice indeed!
SECOND ISSUE: While rotating the Volume control I did hear a little "scratching" about mid-way, nothing serious but it does indicate a dirty "track" I decided not to proceed further to investigate this pot, the scratching is very very minor in one spot only (mid way) and I am under "time pressure" as my wife has instructed me not to spend too much time on each piece of equipment!. To access this pot, you have to remove the top board (not accessible underneath) and that means time, so I have left it as it is.
Actually, in "real time", with audio pumping through the preamp/power amplifier, I heard no issues with these scratching noises at all. So that means that in everyday use, these scratching noises are not an issue - I have just heard them when there is NO signal present with the Volume turned way up.
As the Balance/Volume are concentric controls, I also checked the Balance control for issues - NONE AT ALL.
THIRD ISSUE: switched to Phono and I am greeted by a decent level of 50Hz background hum (with NO connections to the phono input). These units use a plug-in Phono card (horrible design idea, I have had lots of issues with those cards over the years) mainly due to the NICKEL PLATED plug/socket arrangement (molex style connector) which has NO real securing mechanism, nor is it "keyed" - if removing, be careful to make sure all pins are aligned properly. The card just sits in the socket and is prone to movement causing issues. This seems like pretty poor engineering design, NAD, when you are dealing with extremely low level audio signals there is no way cards should be like this - BIG minus for effort NAD.
I have pulled out the Phono board and it is a AE3 type, with not only plug in connections but some hard wired cabling as well. Cleaned the connectors on the Phono board and the main PCB and then noticed a small insulating strip at the rear of the Phono connectors had "disintegrated" due to age - sort of a foam. I replaced this with a short strip of yellow insulating tape so that the Phono board doesn't short against the rear of the Phono connectors.
Powered up once again (always work on the 7150 with the AC power DISCONNECTED unless taking measurements!
Yeah, she's alive (although still some background 50Hz hum) - nice Phono audio now, but with low level hum still! I tried a few things here and it is as though there is a ground loop problem. If I remove the turntable inputs, the hum is much higher level than with the turntable plugged in and signal ground connected to earth. I get the feeling that something is amiss with the signal ground and I would be highly suspicious of that AE3 card!
But once everything is sorted out, the NAD Phono section of this amp is very very good - when they work properly!
So the third issue is partially resolved, the major 50Hz hum can be controlled but it remains at a much lower level.
AM-FM RECEIVER & DISPLAY BOARD:
The FM section of the NAD 7150 stereo receiver features the Schotz Variable Bandwidth PLL detector, which maintains the FM detector's bandwidth at the minimum value required by the reception conditions (signal strength, noise level, and amplitude modulation of the FM signal caused by multipath interference) in order to keep the noise level in the detected signal as low as possible.
The outstanding benefit of this detection system is greatly enhanced tuner sensitivity for both mono and stereo signals.
At moderately high signal levels the NAD 7150's tuner section offers performance comparable to that of top-quality FM tuners, but under weak signal conditions the Schotz circuit can deliver clean audio with markedly lower noise and distortion than conventional tuners while at the same time there is no sacrifice of other desirable tuner characteristics such as capture ratio, channel separation, selectivity, or frequency response.
This all contributes to making the 7150 FM Tuner section one of the most advanced FM tuners available to the consumer market.
The AM section is pretty "stock standard" and has the inbuilt swivel rod antenna at the rear. DO NOT USE THIS FOR A HANDLE!
Let's get back to ISSUE 1, lack of display.
This is all part of the top board, the tuner/display section - that is all it does.
Ok no display at all and when switching to TUNER, no audio either.
I took a closer look at the power rails going into this board, there are two power supplies on this top board and from past experience one of them does give problems.
First I checked the voltage out at the transistor Q902 and I have 30V, that looks pretty good. This is the power supply just behind and to the right of the display module.
It is the other power supply that can be a problem (near the heatsink, left hand side) so first I measured the AC from the transformer, all is fine here at 19V 0V 19V AC (it is a centre tapped winding from the transformer)
I then checked the supply rails to one of the main IC's, the Toshiba TC9137P - absolutely zilch! No supply at all.
Ok, I then went back to this power supply and checked the emitter of the TO220 transistor Q901 (2SD330) mounted on a heatsink (series pass) and again zilch. I then also noticed that in fact this area of the board is NOT pristine, someone has fitted a replacement transistor at some stage - those tell-tale white silicon grease marks! Then I also noticed one of the electrolytics is not looking good, while not bulging it is all "bumpy" underneath the outer plastic.
I double checked and the 2SD330 (genuine Sanyo) is still readily available online (around A$7)
So here is the problem, that supply is vital for the display and the Tuner board and will need service. With luck it will just be a matter of the electrolytic(s) around the power supply and possibly the transistor. As I have seen this sort of issue before, it's a good idea to upgrade the components in this area as well - if you can fit them, upgrade the working voltages of the caps in particular. Also there is a zener in there (going to the base of the series pass transistor), check that one out too.
All this will require a "board lift" from the chassis - an average job and it should be all done and dusted in a fairly short time - not a major job in my opinion.
I noticed a couple of other things while taking some measurements on this board. Two of the IC's are socketed (LA1235 & TL082)! Seems a bit weird to me, were NAD expecting the ordinary old TL082 to fail? and what about the LA1235 FM IF IC? surely they don't fail all the time, I have never experienced a failure here but there you go.
Another issue with the Tuner section only is the AM/FM front panel switch. It is supposed to lock (spring locking) but it does not! Push it in and when you release it, out it comes again. Bit limiting when you fix the Tuner board if you can only tune one band! This will need servicing.
While I rant about this power supply, it is a very good idea to keep in mind that ALL large electrolytics will in time fail, this is old gear and it is unrealistic to expect it to last forever with original components. Sometime in the future it would be a good idea to at the very least change those big main supply capacitors with a high quality, long life capacitor - some day.
SUMMARY:
Overall, electronically the 7150 Receiver is pretty good.
Big plus on the power amplifier and the preamplifier section.
A bit of investigative work to sort out that Phono 50Hz hum problem
Tuner/Display board is the biggest problem (as detailed above) None of the components are difficult or special but it will take a bit of time to resolve the power supply issue.
Looking around online, it seems the NAD 7150 commands anywhere between US$250 up to US$500 for a "fully working" unit.
I think that my asking price is fair, all things considered and I have provided plenty of detail so people know exactly what they are getting plus of course, the free shipping around Australia.
As I have the original shipping carton, I can easily ship this equipment anywhere around Australia and because it meets the conditions for FREE shipping, the cost of shipping will be zero, anywhere within Australia.
