Marantz Model 1120 – amplifier adjustment and maintenance



A while ago we received an email from our reader, owner of a Marantz Model 1120 amplifier from 1975. The amplifier had a few minor but annoying symptoms, and local technicians were not willing to work on it. We, on the other hand, were quite happy to get our hands on (and into) this classic amp.

Marantz Model 1120 was manufactured in 1973-75 (from 1972 according to some sources); they unit that came to us was made in July 1975. It is a very interesting model: unlike most Marantz units from the mid 70s it was made not in Japan, but in the United States. This is a 60 W per channel direct coupling amplifier. It has a 4 line inputs and one phono input, separable preamplifier and power amplifier sections, posts for 2 pairs of speakers and a headphone socket tapping into speaker outputs. It lacks additional midrange potentiometer (present in many Marantz amplifiers from that period), but bass and treble controls are separate for each channel. The amp also has high, low and loudness filters, as well as the MUTING switch (-20 dB) and a complex mono switch that allows to listen to left channel only, right channel only, or both channels converted to mono. There are microphone inputs and special DUBBING input/output on the front panel (6.3 mm jacks), which doubles for the TAPE 2 IN/ OUT input/output on the rear panel. TAPE 1 and TAPE 2 loop sockets on the rear panel are additionally doubled – we have a choice of standard RCA jacks and DIN sockets. Interesting feature are the SCOPE sockets for connecting an oscilloscope, which could be found in top models of Marantz tuners and some receivers.

We wrote about the sound of this amp a few months ago when we compared my Marantz Model 1060 to the 1120. In a nutshell: Marantz 1120 is a very good amplifier, but the only aspect in which it clearly won with the 1060 was better controlled and lower bass; Marantz 1060 had better mids, highs and the overall sound (fullness, musicality). In this article we will focus on the amplifier maintenance.

The main problem was audible background noise. Noisy background is unfortunately an ailment that plagues many amplifiers, not just vintage ones. It can result from a failure or aging of the components, oxidized contacts in switches and potentiometers, dirty sockets – but also from the very design of the amplifier and the type of components used by the manufacturer. In the latter case, the noise can not always be removed, and if it is possible, it requires replacing components with something other than the manufacturer’s specifications. In other cases (dirty contacts, components’ age or failure), noise level often can be reduced, but finding the cause can be time consuming.

According to the owner, in the case of this Marantz noise could be heard from quite a distance (over 2 meters from speakers) even during the quieter moments in the music, and it was completely unaffected by the volume setting and all switches. It would largely disappear after removing the jumpers that connected the pre and power amp sections, so we knew that it the source was most likely somewhere in the preamp. This problem was largely solved by the preliminary cleaning of the amplifier immediately after we received it – the largest part of the noise was most likely caused by dirty or oxidized contacts or input jacks. But after a thorough testing of the amplifier we found a number of other issues. Switching high filter on caused additional noise. Channels were swapped (left input played from the right speaker and vice versa), control lamp was burned out, balance knob was missing, input jacks were heavily oxidized, potentiometers and switches were dirty. Then there were current adjustments general cleaning – dust removal, washing the face and knobs.

We came up with the following “to do” list:

1. Measurement and adjustment of quiescent current
2. Measurement and adjustment of DC offset
3. Correcting swapped channels
4. Replacing the control lamp (28V 40 mA)
5. Replacing the missing balance knob
6. Clean pots and switches
7. Cleaning input jacks – some were so dirty that they were intermittent
8. Interior cleaning: dust and dirt removal
9. External cleaning: washing the front panel and knobs
10. Attempt to further reduce the background noise/hum (already reduced after the initial cleaning)
11. Attempt to reduce additional noise caused by switching on the high filter

Current check and adjustment is one of the things you should do in every used amplifier, especially a 40 years old amplifier. There are usually two things to check and adjust, separately in each channel: DC offset (measured on speaker outputs) – this value should be as close as possible to 0 V, and quiescent current (DC bias, idling current – measured according to the service manual, often across emitter resistors) – the correct value depends on the amplifier. Service manual is usually necessary for these adjustments because you need to know which pot adjusts regulates which current, and where the measurement points are. The adjustment is usually quite simple because in many amp there is easy access to the measurement points and VRs – but in Marantz amps from the 70s (and this amp was unfortunately no exception) getting to the VRs and measurement points often requires at least partial disassembly. To make things even more fun, all measurements and adjustments have to be made on the amplifier that’s partially disassembled, but obviously powered on. Long story short – we managed, but it required some inventive positioning of the amp and its boards. You can see just how inventive in the pics in the gallery below, namely the ones in which the amp balances on a screwdriver. As per the service manual, we adjusted the quiescent current to 14 mV in each channel (the manufacturer recommends 11 to 18 mV); before adjustment it was about 8 mV. Too low quiescent current may increase distortion level, too high – unstable operation, overheating and damage to output transistors. DC offset was adjusted to 0.1 – 0.2 mV, before adjustment it was over 50 mV, which is not a dangerous value, but far from ideal.

On the other hand, correcting swapped channels turned out to be very easy. On the rear panel of Marantz 1120 there are preamplifier output jacks connected with wires to the preamp section. These wires are connected with plugs that were simply swapped, so we unplugged them and reconnected properly.

We had a small problem with lamp replacement, we couldn’t find such a small 28V bulb, and it turned out that the lamp either should be blue or with a blue filter – but luckily we could use a blue LED with a resistor instead and the owner luckily didn’t mind that it was much brighter than the original lamp.

Marantz knobs for switches and potentiometers are available, but relatively expensive. But I managed to find a non-Marantz knob that looked quite similar. The owner decided to go with this substitute.

Cleaning potentiometers, switches and sockets went quite fast, although access to most switches is not very easy due to the frame and housing design. Then we dusted the amplifier (small brush followed by Q-tips is the way to go) and gave the front panel and knobs a bath.

Background noise could be reduced a bit more by mowing wires inside the amp, but it was a slight change. We asked our technician to check the amp and try to find the cause of the additional noise that appeared when high filter was switched on. He checked all the possible culprits: the filter circuits, switches and power supply, but all components tested good. Because this function is currently not used in amplifiers and completely unnecessary – unless someone uses a tape player and has a very badly recorded cassettes – after consultation with the owner we left this filter as was. But the technician found and replaced a defective transistor in the power supply and a resistor in the filters circuits. These components were not causing any symptoms yet, but it would be a matter of time.

In summary, we were able to do everything except the high filter noise, which luckily was irrelevant for the owner. Tested, cleaned and adjusted amplifier went back to the owner. Tests in his system confirmed that the noise was significantly reduced. Here’s the feedback we received from the owner:

“Thank you for the delivery of my amplifier, it survived the trip and more importantly it works better. I do not know it was replacing some parts or improving connections, but the problem I reported has been removed. I have to admit that the noise is greatly reduced and in my opinion it is now at an acceptable level.”

We would like to thank the owner for trusting us to work on his amplifier, and the opportunity to do listening tests.

Marantz Model 1120 appears on the auction sites quite rarely. Too bad, because it is a very good amplifier – it looks good and sounds good, its strength is low and very well controlled bass. Like most Marantz amps from this period it is not cheap, but it holds its value.


Technical data:

Continuous power (declared by the manufacturer) – 2x60W into 8 ohms, 2x70W into 4 ohms, 2x30W into 16 ohms
Music Power (declared by the manufacturer) – 2x90W into 8 ohms, 2x105W into 4 ohms, 2x45W into 16 ohms
THD – less than 0.2%
IM – less than 0.2%
Frequency response – 10 Hz – 40 kHz (20 Hz – 20 kHz with a +/- 0.2 dB deviation)
Damping factor – better than 30 at 8 ohms
Input sensitivity: line 110 mV / 25k, phono 1.1 mV / 47k (to equal 1.1 V output from preamp)
Inputs/Outputs: Tape 1 (I/O), Tape 2 (I/O), Aux, Tuner, Phono (MM cartridge), 2 pairs of speakers, headphone jack, microphone jacks, dubbing jacks, Pre Out/In Main
Max. power consumption approx. 200W
Dimensions: 390 x 145 x 356 mm
Weight: 12.2 kg

Information page (specs differ slightly from those in the service manual):

Marantz Model 1120 – cleaning, maintenance, adjustment


Marantz Model 1120 – after cleaning and maintenance

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