Restoration of a Marantz PM450 vintage integrated amplifier

Last modified: Tue Jan 2 00:34:32 2007



The Marantz PM450 is a stereo 60 W/ch. integrated amplifier from the post-Classic era (beginning of the 80's), manufactured in Japan by Philips - Marantz Japan - Standard Radio Corporation and sold worldwide from 1981 to 1983.

The Marantz PM450

This amplifier does not belong strictly to the Marantz "Classic" period because Philips Electronics bought Marantz Japan in 1980. This is a textbook case of a big company buying a smaller one and trying to impose its (different) culture - and making a mess out of it.
Still, the PM450 is enough of a Marantz in terms of build quality, and the design is quite nice. Visually, it differs from the classic Marantz amplifiers in some details:
  1. The use of square buttons for various switches.
  2. Slider pots for tone controls.
  3. A basic LED VU-meter with rectangular red LEDs.
  4. Champagne-colored aluminium faceplate and cover.
The amplifier is rated (RMS) at 50W/ch. on 8 Ohms, 60W/ch. on 4 Ohms; however I assume these ratings are not as conservative as those for earlier Marantz amplifiers.

The Marantz PM450 - front view

The PM450 features:

Technical specifications

  • Output power @ 1kHz:
    4 ohm DIN   65 w
    4 ohm RMS   60 W
    8 ohm DIN   57 W
    8 ohm RMS   51 W
  • THD with 8 Ohm load: 0.05%.
  • Intermodulation distortion: 0.05%.
  • Damping factor: 75.
  • AUX input:
    Bandwidth (-3dB)   10-50000Hz
    Signal/Noise Ratio   96dB
    Impedance   28kOhm
    Sensitivity   150mV
  • Mic preamp input:
    Impedance   9kOhm
    Sensitivity   2.3mV
The front panel's nice clean look

Purchase and visual check

I bought my Marantz PM450 vintage integrated amplifier on eBay for 30 euros cash (roughly USD40) - no shipping, I had to go and pick it up. The ad stated that the amplifier was in working order, with minor noise issues with the volume control. Still, it looked like a good deal.

Brand, model and serial No.

It came naked: no wrapping material, no owner's manual, no nothing; I was just told by the seller that he himself had never opened it, and that the volume control was noisy, but after warming up for a few minutes, worked fine.
I had a quick look at the amplifier as he handed it to me and I handed him the cash. It seemed OK: no big scratches, no missing buttons, no big bumps on the metal cover, no signs of rust on the bottom, and the power cable looked like the original one.
I got back home in a hurry and set the amplifier on a bench. At this point I was slightly pessimistic: why would anybody want to get rid of an amplifier that looked OK and worked well? Something certainly was wrong with it, and I was soon going to find out what.

What's under the hood?

There are six screws holding the metal cover; once removed, the cover can be lifted and put aside.
I removed the cover to check the amplifier before turning it on. I would recommend that anyone buying a 25-year old piece of hardware do the same.
It was just as I expected: very, very dirty. But also: it was untouched!

PM450 Top view

That was the good news (also it didn't smell of cigarette smoke). The bad news is that one of the resistors in the power supply area seemed discolored from overheating; but testing it with a multimeter indicated that it was still good. I also checked for capacitor leakage, or other serious damage but could see nothing.
After cleaning the dust very quickly with a brush (the shot above was taken later, after extensive cleaning), taking care not to touch the trimpots, and having satisfied myself that no components were physically damaged, I turned on the amplifier and checked for DC at the speaker terminals: there wasn't any. Also the green LED in the middle of the VU-meters was on. So far so good. I turned it off, connected an old 6 Ohm Yamaha speaker to one of the speaker outputs and the phones output of a small stereo clock-radio to the Tuner input, turned the amplifier back on, set the volume at the 9 o'clock position, and proceeded to play some music, half expecting a puff of smoke to spoil my fun sooner or later.
Well, there was no puff of smoke, but there was no sound either. At this point I thought I had a dead amp on my bench. Merde!
I fiddled with the buttons, switches and volume control until at one point and much to my surprise, music started playing, with many loud noises and abrupt changes in volume level.
Quite obviously, dust had worked its way in the switches and potentiometers and these would require a thorough cleaning. But it was working!
It turned out after further investigation that the "TAPE 1 MONITOR" switch was the main culprit. The volume pot was fine, and the amplifier didn't require any "warming up", as stated by the previous owner.

Transformer and PS capacitors

Comparing the transformer and power supply capacitors of this 60W/channel amplifier to those of the classic Marantz 1060 (rated at "only" 30W/channel), we can begin to guess where the Marantz engineers were trying to cut costs: everywhere!

PS transformer and capacitors

The transformer is smaller, lighter and of lower quality than the transformer used in the 1060; it is shielded, but not completely.
The main electrolytic capacitors, at 6800uF/50V, are barely adequate for an amplifier with this rating, but they are quality components anyways.
Conclusion: Marantz engineers didn't skimp on the quality of the components used, but certainly were told to reduce margins compared to previous designs.
However after (visually) comparing the PM450 to the classic Marantz 1050 (also manufactured in Japan, circa 1979), I confess I find their transformer and filter caps of equivalent quality.
Anyway, this is a standard dual-rail power supply circuit topology with a center tapped transformer, and we have +/- 45V DC at the capacitors. The line voltage is selectable 110/220V. The four power diodes that make up the full-wave rectifier can be seen to the right of the filter caps.

Power supply - full view

Since the op-amps require regulated +/- 16VDC, the power supply includes a regulator circuit. This is a very simple zener and emitter-follower circuit with 400mW zeners and TO-220 power transistors. On the following picture one can see a 1/2W 270 Ohm resistor that overheated, as well as a region of the printed circuit that has turned a darker brown - probably also due to overheating. I have replaced the resistors with equivalent 2W parts.

A problem with the power supply circuit

Power output stage

I haven't had the time yet to check the power output stage in detail, but it seems to be a standard class AB amplifier, with what I believe is moderate biasing of the output stage, since the amplifier stays cool in normal use.
The Sanken complementary power transistors are shown below. They are mounted to the bottom of the aluminium heatsink and soldered on the opposite side to the other components; this keeps them clean and protected from dust buildup which could cause overheating problems. The solder may seem generous but actually power transistors should be soldered that way - without exaggerating, of course. Note the adequate use of thermal paste, and the neat and expensive-to-manufacture circuit board cutouts.
Macro shot of the Sanken complementary power transistors

Voltage selector and speaker connectors

There is a voltage selector switch to select between 110V and 220V 50Hz. The speaker connectors (for 2 pairs of speakers) are of the spring-loaded type, which were commonplace in the 80's, but nowadays are mostly found in entry-level equipment.

Voltage selector and speaker connectors

Volume and Balance coaxial potentiometers

Unfortunately somebody decided to shave an extra 20 cents off the cost of this amp and the Volume and Balance potentiometers are not sealed. In fact no effort was done to dust-seal any of the switches or potentiometers. Tsk tsk, not good!

Volume and Balance Potentiometer

VU-meter and LED driver board

The PM450 sports LED VU-meters that must have looked cool in the 80's but nowadays are just nice to look at.

The LED VU-meters

The VU-meter driver board uses two TL489C chips and one JRC 4558D dual op-amp. The TL 489C chips are 5-step analog level detectors and the 4558 op-amp is used to adjust the left and right channel levels for input to the TL489's (you can see the gain trimpots on the picture below). As far as I can tell, an identical or very similar circuit was used on the PM350, PM550 and PM750 integrated amplifiers.

The VU-meter driver board

Tone controls and tone control board

These early 1980's Marantz integrated amplifiers had sliding tone controls, instead of the traditional rotating knobs; note the PM450 has three frequencies, the PM550DC has 5 frequencies and the PM750DC has 10 frequencies i.e. a built-in equalizer.

Sliding tone controls for bass, mid and treble

Given the modern capabilities of on-board DSPs, analog tone controls are considered obsolete, but at the time they were considered a useful add-on.

Tone control circuit

The tone control board uses one NJM4558 and one NJM4560, perhaps because Marantz engineers decided that the NJM4560 provided a little more bandwidth.

NJM4560 (top) and NJM4558D (bottom)


Note the dual MM phono inputs.

RCA inputs

RIAA preamp board

The RIAA preamp board actually houses two complete stereo RIAA preamplifiers, since the PM450 has two turntable inputs and a fader/mixer to handle them. Marantz (or Philips?) engineers used two JRC4558DD (low-noise variants of the 4558) dual op-amps for the RIAA preamps and a third JRC4558DD to mix them.
The op-amps are fed regulated +/- 16.4 Volts from the power supply. On the picture below one can clearly see the three-conductor flat cable rising from the power supply to the RIAA preamp board. The preamp board is directly soldered to the RCA jacks on the back of the amplifier, to minimize noise pickup on the high-impedance turntable inputs.

RIAA preamp board

But how does it sound?

After listening to it for a few days, and despite all the obvious cost-saving and corner-cutting observed, all I can say is: it sounds pretty good to me. I'll refrain from any subjective audiophile statements such as "punchy bass" or "transparent highs" or anything of the kind. And I am sorry I cannot offer a full set of measurements, since I don't really have the instruments to measure power bandwidth, harmonic and I.M. distortion, signal/noise ratio and other parameters of the kind.

Some nice touches:

Replacing the electrolytic capacitors (re-cap)

Since I don't have the service manual or the schematics for this amplifier, the first step was to open it and check the ratings (capacitance, voltage and max. operating temperature if 105C) of all the electrolytic capacitors used, and their polarity.

Note: I now have a separate page for my Re-cap HOWTO. Check it out if you want to re-cap your vintage audio gear.

I prepared a list of the electrolytic capacitors found in the Marantz PM450 amplifier:
Quantity Capacitance (uF) Voltage (V) Temperature (C) Schematic ref. no.
2 100 50 85 CN03, C805
2 47 50 85 C807-C808
1 330    6.3 85 CN01
1 10 50 85 CS07
2 47 35 85 C713-C714
4 100 16 85 C421-C424
2 100 10 85 C705-C706
2 10 25 85 C811-C812
3 1 50 85 CN02, CX03-CX04
24 3.3 50 85 C401-404, C425-428,
CX01-CX02, CE01-CE04,
C701-C702, C707-C708,
CS10-CS12, CS15-CS16,
2 6800 50 85 C801-C802
Table 1: B.O.M. PM-450 electrolytic capacitors.

I am doing the re-cap a few capacitors per evening; like that if anything goes bad at any stage, I can easily back-track. Plus I can play some music in between re-cap sessions!
Here is a shot of the printed circuit board just after removing two old capacitors and cleaning the solder away, before soldering in the new replacements:

Detail shot showing clean solder pads

Op-amp replacement

Japanese Marantz/Philips engineers used op-amps in their integrated amplifiers after 1980 almost certainly to save costs; all hi-fi manufacturers did the same at the time. Nowadays it gives us an opportunity to upgrade the original circuit quite easily: unsolder the old op-amps, replace them with IC sockets and test various op-amps in their place until finding one that works well.
BTW the correct way to unsolder an op-amp in DIP format is to cut the leads and then remove them one by one, while heating up the solder on the opposite side of the printed circuit board. Or use a special desoldering tool for DIP packages. The final step is to remove the remaining solder, again using a special tool.
I'll post some pictures when I get around to doing it.
I intend to replace the JRC 4558DD op-amps in the tone controls and mixers circuits with modern OPA-2604's. The OPA-2604 is generally considered a very good replacement for the obsolete 4558 - as long as the circuit is well laid-out and won't oscillate.

Datasheets (PDF)

These are the Sanken complementary power transistors used in the power output stage: And these are the NEC transistors used to drive them: The TL489C is a 5-step analog comparator with digital outputs that drives the VU-meter LED's: The PM450 uses 6 NJM4558 and 1 NJM4560 op-amps. The JRC NJM4558D was a decent part in the early 80's, but modern op-amps perform a lot better. Just compare the datasheets:


A few websites which I have found helpful: