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This Kenwood Supreme Series 700c was bought having one missing toggle lever and one loose lever cap. It did have serious distortion on both channels. The back is pushed in at a corner a bit. Further, it looks in pretty good shape.

It is a well regarded pre-amp from the seventies. When taking it apart, it is quite obvious it is of a neat mechanical design. All electronic functions have their own circuit boards. The layout of the boards is roomy, enabling service.

Although not necessary, and instead of only fixing what was wrong first, I decided to update it as a journey to gain experience how this all connects together, as I do not have an operations nor a service manual (which may not exist).

The schematic is available from different sources, like on Hifiengine and, without need of an account, on Elektrotanya.

As can be seen at the pictures, I use a lot of metal can old tantalum capacitors. They are better than the drop shape ones and used throughout avionics and military equipment. I also took advantage of the circuit boards being ready for bigger non-electrolytic capacitors. Apparently, Kenwood decided the film capacitors to be too expensive, using aluminium electrolytics instead.. They did put extra tracks and holes on the pcb's, and they added text, to cater for upgrade. This is why on my pictures, sometimes component relocations can be seen.

The following was done:

Replace old electrolytic capacitors (recap, although the need for this is always arguable), using new capacitors, film caps and tantalums.

Replace the NPN transistors of the types supposed to fail over time (2SC1451), based on forums information.

Replace burnt resistors (from some the paint was burnt off) and composite carbon resistors as they get unstable over time.

Clean boards and parts. Some components may be replaced just because of availability and cleanliness (looks).

Raise the power voltages for the headphone amplifier, as they are below the specified + / - 5 volts, making the amp performance more satisfactory using headphones having over 8 ohms impedance.

The Power supply block (X00-1470-10)

The power supply block supplies several voltages:

+ and - 50 volts adjustable, electronically stabilized, for buffer amp and equalizer amp

+ and - 24 volts, derived from the 50 volts and zenered to 24 volts, for mike amp, low filter board, tone amp.

+ and - 5 volts, stabilized by transistors, for the headphone amp.

+ 24 volts unregulated, for the relay board

AC 7.5 volts, for the power lamp.

The input of the 50 volts supplies is coming from smoothing capacitors of 2200uF rating. As I have no new or bigger ones available, they stayed as is, for now.

The input voltage appears almost 60VDC, with little less than 100mV ripple on the capacitors. The plus and minus 24 Volts lines appear to be almost + and - 25 volts

The DC voltages for the headset amplifier are 8.5 volts before and 4.5 volts after the smoothing transistors.

To have a little more voltage possible for headphones having higher than 8 ohms impedance, R15,16,17,18 have been changed, now the power supply voltage is + and - 5.7 volts. It also appeared to have lowered the DC offset on the headphone amplifier from 70 to 30 millivolts initially.

The wirewrap posts show some wear and tear, apparently, this unit has been worked on, before.

These were soldered out to be able have the whole board out.

Replaced parts:

Headphone amp (X13-1690-00

The headphone amp is a low power (20 milliwatt) single ended class A amplifier designed for 8 ohms headphones.

It takes the pre-amp outputs (from the Low filter block) as the source, it has an additional volume control on the back of the chassis.

It was found having the small plastic output transistors 2SC1213A and the current source transistors 2SA673A all having black corroded pins. One 2SA673 HFE was only 14, so it was defective. Note, they run hot and these were small plastic ones, which might have contributed to the failure of one.

They were replaced by metal can transistors, having a heatsink attached. The DC offset voltage appeared to be around 70 millivolts at no load.

Raising voltage to + / - 5.7 volts, it went down to 20 mV on one channel and 30 on the other. After replacing Q1,2,3,4 it went to 21 millivolts at both channels.

Because the amp did not have enough headroom to have a reasonable volume at higher impedance headphones, power was raised from the measured 4.5 volts to 5.7 volts

The pictures show the 4 small transistors not replaced, yet.

Replaced parts:

Low filter block (X12-1070-00)

The low filter block is actually the output amplifier. It has filter circuitry also, which is bypassed when not in use.

I did put in film capacitors instead of the 3.3 uF bipolar ones. Other film caps were exchanged, just because nice ones were available.

Replaced parts:

Tone amplifier block (X11-1220-00)

The tone amp block is in between the buffer amp and the filters+amp.

It appeared to have a faulty 2SA620 transistor, leading to audible distortion and a negative voltage on C14. The picture on the right still shows the original metal case PNP transistors.

Replaced parts:

Output switching relay block (X13-1870-10

The relay block switches on the outputs, after a turn-on delay. Although Q1 being a so-called "suspect" transistor type for noise, it stays in as it is doing low voltage level logic only.

C1 47 uF 25 volts was replaced by 47 uF 20V tantalum type 150D, and a 470 ohms composite resistor was replaced by a 2 watt inflammable one. It is in series with the relay and is missing in the schematic!

Buffer amplifier block (X13-1700-00)

The buffer amplifier buffers the inputs, microphone amp or equalizer amplifier. This card, employing + and - 50 volts, suffered from two resistors having the paint burnt off.

Because of the dissipation those 5k6 resistors were elevated a bit, the others were done the same way.

Replaced parts:

Microphone amplifier block (X08-1260-00)

The mike amp is a stereo one.

The 1 uF 50 volts and 2.2 uF 25 volts electrolicic capacitors were all changed for 2.7 uF 15 volts metal tantalums. Cm13 and Cm14, being the output capacitors, have their marking the other way around on the circuit card, versus the schematic. I mounted it as designated on the circuit card, theoretically this is the good side.

the NPN output transistors were exchanged for BC550C.

Equalization amplifier block (X08-1240-00)

The turntable amplifier board has resistors in its output heating up and losing paint, they are replaced.

The NPN output transistors were exchanged by selected BF258, its drivers by KSC1845.

At the bottom of the board two 10 nF bypass capacitors were mounted, Cd25 and Cd26, they were in the schematic, but not on the board physically, so why not...

Replaced parts:

A picture of the damaged switch cap.

The boards, partly repopulated and cleaned, but before troubleshooting.

Somebody not even knowing me was so kind making a new plastic switch cap for me! This was not easy since the cap is hollow!

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