Vintage Audio
this section covers some restoration work that I did on my vintage HI-FI gear
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Article #1 | 12:00 PM Sunday 02/10/2016

Sansui AU-7500: Introduction

I found this little gem in the local flea market. This was a lucky day. However the price was premium.

When I plugged the AU-7500 amplifier and played a few audio tracks, I instantly recognized the authoritative sound of the AU-x500 series filling the room. Naturally this amplifier replaced the dual amplification rig powered by AKAI AA-5210 units in my study.

This will be a long process but I document it well. Maybe you find my work inspiring or helpful. Or my descendants, during a distant future maintenance, should this amplifier survive the years to come...

Article #2 | 12:46 PM Monday 02/10/2016

Sansui AU-7500: Technical Data

This amplifier has the following technical characteristics.

TECHNICAL PARAMETERS
ParameterValue
Continuous RMS power (both channels)40 W + 40 W (8 Ω at 1,000 Hz)
Total Harmonic Distortion<0.1% (at rated output)
Intermodulation Distortion (70 Hz : 7,000 Hz = 4:1 SMPTE method)<0.1% (at rated output)
IHF power bandwidth (each channel driven at 8 Ω)5 Hz - 40,000 Hz
Frequency Response (at normal listening level)10 Hz - 30,000 Hz +0 dB / -1 dB
Load Impedance4 Ω - 16 Ω
Damping Factorapproximately 40 at 8 Ω load
SemiconductorsTransistors: 38 / Diodes: 15
Power Voltage100 V, 110 V, 117 V, 127 V, 220 V, 230 V, 240 V, 250 V at 50/60 Hz
Power Consumption100 W (max)
Dimensions / Weight140 mm (H), 440 mm (W), 322 mm (D) / 12.7 kg

Article #3 | 09:32 PM Monday 23/10/2017

Sansui AU-7500: Parts List

I have assembled a parts list for this amplifier. My restoration targets mainly the electrolytic capacitors. But some transistors are subject to change as well. Even though I consider them irreplaceable for an authentic Sansui sound, I am aiming for maximum reliability and low maintenance. Below you will find lists consisting of the various capacitors in this amplifier.

The schematic value corresponds to what normally can be found in the electrical schematics. The recommended value is what I replaced the former part with. The BP inscription signifies a bipolar capacitor. Where I found appropriate, I have chosen a film capacitor replacement instead of an electrolytic. My reasons are reliability in time. Less electrolytic capacitors, less time-ticking bombs.

Descriptions and Parts Listings

The capacitors mounted directly on the chassis are for decoupling purposes. They are located directly on the transistor sockets beneath the unit. These capacitors are directly soldered on the rightmost and leftmost edges of the power transistor socket array. For some reason, these capacitors don't appear on the schematic. But they appear at page 26 of the service manual designated as C823 and C824.

Chassis
IdentifierSchematicRecommendedMouser Number
C8233.3 uF / 50 V647-UFG1H3R3MDM
C8243.3 uF / 50 V647-UFG1H3R3MDM

The Power Supply Block is coded F-2013-1 and is located below the chassis, mounted in a centered position on a tilted printed circuit. You can quickly recognize it due to the stabilizer transistor cooled by a small aluminum radiator and the four small current diodes that form a rectifying bridge.

Power Supply Block
IdentifierSchematicRecommendedMouser Number
C0094.7 uF / 63 V10 uF / 100 V647-UKZ2A100MPM
C01010 uF / 50 V10 uF / 100 V647-UKZ2A100MPM
C01210 uF / 50 V10 uF / 100 V647-UKZ2A100MPM
C014220 uF / 50 V647-UKZ1H221MHM
R0041 kΩ ¼ W1 kΩ ½ W71-RN60D1001F/R
TR0022SA678KSA733512-KSA733CGBU
TR0032SA678KSA733512-KSA733CGBU
ZDZB-1-14BZV85-C13771-BZV85-C13,133
D00210D-11N5392625-1N5392-E3/54
D00310D-11N5392625-1N5392-E3/54
D00410D-11N5392625-1N5392-E3/54
D00510D-11N5392625-1N5392-E3/54

The Protector Block is coded F-2041 for stock no. 7591230 and F-1215-A for stock no. 7591300. That's right, two boards for one schematic. F-2041 is located on the left side of the chassis mounted directly on it. You can quickly recognize it due to the big speaker coupling relay. F-1215-A is located beneath the steel chassis directly under F-2041. There are three wires connecting the two boards. Both boards are easy to service.

Protector Block F-2041
IdentifierSchematicRecommendedMouser Number
C90147 uF / 10 V BP47 uF / 16 V BP647-UES1C470MPM
C902220 uF / 6.3 V220 uF / 25 V647-UKZ1E221MHM
C903100 uF / 50 V647-UKZ1H101MHM
C9061 uF / 50 V1 uF / 50 V FILMN/A
C9071 uF / 50 V BP647-UES1H010MDM
TR9012SC634AKSC1845512-KSC1845FTA
TR9022SC634AKSC1845512-KSC1845FTA
D00510D-11N5392625-1N5392-E3/54
D00610D-11N5392625-1N5392-E3/54
RL901DC24V MY-4 RelayOMRON MY4-02-DC24653-MY4-02DC24

F-1215-A is a little bit weird but beautiful at the same time. It does not have a green solder mask and the copper tracks are connecting the part terminals via rivets. Interesting.

Protector Block F-1215-A
IdentifierSchematicRecommendedMouser Number
C908220 uF / 35 V220 uF / 50 V647-UKZ1H221MHM
TR9032SC634AKSC1845512-KSC1845FTA

There is one combined audio driver board in this amplifier responsible for both left and right channels. It is coded F-2034 and can be found mounted on the chassis just between the power transistors array and the tone control board. In my unit there were two capacitors directly soldered on underneath this board between the emitter and collector of TR807 and TR808. For some reason, these don't appear on the schematic but are easy to change. I have marked them as N/A below.

Driver Block
IdentifierSchematicRecommendedMouser Number
N/A3.3 uF / 50 V647-UFG1H3R3MDM
N/A3.3 uF / 50 V647-UFG1H3R3MDM
C8014.7 uF / 50 V647-UFG1H4R7MDM
C8024.7 uF / 50 V647-UFG1H4R7MDM
C80547 uF / 50 V647-UKZ1H470MPM
C80647 uF / 50 V647-UKZ1H470MPM
C80747 uF / 10 V BP47 uF / 16 V BP647-UES1C470MPM
C80847 uF / 10 V BP47 uF / 16 V BP647-UES1C470MPM
C81147 uF / 50 V647-UKZ1H470MPM
C81247 uF / 50 V647-UKZ1H470MPM
C815100 uF / 50 V647-UKZ1H101MHM
C816100 uF / 50 V647-UKZ1H101MHM
C819220 uF / 6.3 V220 uF / 25 V647-UKZ1E221MHM
C820220 uF / 6.3 V220 uF / 25 V647-UKZ1E221MHM
C821100 uF / 50 V647-UKZ1H101MHM
C822100 uF / 50 V647-UKZ1H101MHM
VR8014.7 kΩ5 kΩ652-3306F-1-502
VR8024.7 kΩ5 kΩ652-3306F-1-502
VR8031 kΩ652-3306F-1-102
VR8041 kΩ652-3306F-1-102
R85110 Ω / 1 W10 Ω / 3 W71-CPF310R000FKEE6
R85210 Ω / 1 W10 Ω / 3 W71-CPF310R000FKEE6
TR8012SA726KSA992512-KSA992FTA
TR8022SA726KSA992512-KSA992FTA
TR8032SA726KSA992512-KSA992FTA
TR8042SA726KSA992512-KSA992FTA

The Equalizer Block is coded F-2028 and implements the Phono preamplifier and the RIAA curve corrector. This board is right positioned, under the large Faraday cage metallic shield. This board is socketed. Note that the TC notation signifies a tantalum capacitor.

Later edit: The capacitor marked with the * symbol is not on the original schematic nor on the circuit. I have explained in Article #6 below why I decided to mount it.

Equalizer Block
IdentifierSchematicRecommendedMouser Number
C6012.2 uF / 25 V TC2.2 uF / 50 V647-UFG1H2R2MDM
C6022.2 uF / 25 V TC2.2 uF / 50 V647-UFG1H2R2MDM
C60310 uF / 25 V10 uF / 16 V BP647-UES1C100MDM
C60410 uF / 25 V10 uF / 16 V BP647-UES1C100MDM
C60947 uF / 16 V47 uF / 25 V647-UKZ1E470MPM
C61047 uF / 16 V47 uF / 25 V647-UKZ1E470MPM
C61133 uF / 10 V33 uF / 25 V647-UKZ1E330MPM
C61233 uF / 10 V33 uF / 25 V647-UKZ1E330MPM
C62310 uF / 25 V1 uF / 50 V FILMN/A
C62410 uF / 25 V1 uF / 50 V FILMN/A
C6251 uF / 50 V1 uF / 50 V FILMN/A
C6261 uF / 50 V1 uF / 50 V FILMN/A
C*100 pF / 50 V MLCC80-C315C101J5G
C*100 pF / 50 V MLCC80-C315C101J5G
TR6012SA493KSA992512-KSA992FTA
TR6022SA493KSA992512-KSA992FTA
TR6032SC1000KSC1845512-KSC1845FTA
TR6042SC1000KSC1845512-KSC1845FTA
TR6052SA493KSA992512-KSA992FTA
TR6062SA493KSA992512-KSA992FTA

The Tone Control Block is coded F-2014 and is located on the steel chassis in the front part of this amplifier. Of them all, this board is the most complicated to work on because there are a lot of electrolytic capacitors and small signal transistors. Also a lot of wires on the solder side. I list the capacitors and transistors below.

Tone Control Block (F-2014)
IdentifierSchematicRecommendedMouser Number
C70347 uF / 16 V647-UFG1C470MEM
C70447 uF / 16 V647-UFG1C470MEM
C70733 uF / 16 V33 uF / 25 V647-UKZ1E330MPM
C70833 uF / 16 V33 uF / 25 V647-UKZ1E330MPM
C7094.7 uF / 50 V647-UFG1H4R7MDM
C7104.7 uF / 50 V647-UFG1H4R7MDM
C7191 uF / 50 V1 uF / 50 V FILMN/A
C7201 uF / 50 V1 uF / 50 V FILMN/A
C72110 uF / 16 V10 uF / 100 V647-UKZ2A100MPM
C72210 uF / 16 V10 uF / 100 V647-UKZ2A100MPM
C72347 uF / 16 V47 uF / 25 V647-UKZ1E470MPM
C72447 uF / 16 V47 uF / 25 V647-UKZ1E470MPM
C7291 uF / 50 V1 uF / 50 V FILMN/A
C7301 uF / 50 V1 uF / 50 V FILMN/A
C7391 uF / 50 V1 uF / 50 V FILMN/A
C7401 uF / 50 V1 uF / 50 V FILMN/A
C7414.7 uF / 50 V647-UFG1H4R7MDM
C7424.7 uF / 50 V647-UFG1H4R7MDM
C743220 uF / 50 V647-UKZ1H221MHM
TR7012SA493KSA992512-KSA992FTA
TR7022SA493KSA992512-KSA992FTA
TR7032SC1000KSC1845512-KSC1845FTA
TR7042SC1000KSC1845512-KSC1845FTA
TR7052SA493KSA992512-KSA992FTA
TR7062SA493KSA992512-KSA992FTA
TR7072SA493KSA992512-KSA992FTA
TR7082SA493KSA992512-KSA992FTA
TR7092SA493KSA992512-KSA992FTA
TR7102SA493KSA992512-KSA992FTA

As you can see, there are a lot of electrolytic capacitors to be replaced. While this amplifier desires a lot of respect, I cannot but advice you to take your time and observe the electrical schematics and my capacitor lists. You can stick to the original values and types or you can follow my list. It is up to you. But the results will be similar. Your amplifier will shine again and will produce that great Sansui sound. After all, this is a top performer and will surely reward your ears.

Do not hurry, take your time and do the job once. And do it well.

Article #4 | 10:00 AM Tuesday 24/10/2017

Sansui AU-7500: Restoration

As always before I start working on a vintage unit restoration, I organize all the parts that will be changed. Further more I am sorting transistors based on the hFE factor. This is a boring job but it pays for itself. Here I am sorting Fairchild KSA992 transistors. Careful with these transistors as their terminals are designated as ECB. The old 2SA726 are BCE. So the replacements must be mounted in reverse.

An array of Panasonic stacked film capacitors. Good quality.

This is one of the most easy to repair of them all amplifiers. I have seen quite a few units during my restorations but none like this. Nearly all boards can be serviced directly in place. Not a screw to remove. My way of working on this is to put it upside-down on the workbench. I can reach with my hand under the chassis all the components that I need to service. On the soldering side there is enough clearance to work in a clean manner. If you are careful enough (!) then you don't need to even desolder any wire. With a lot of patience, I managed to do a clean job without heat-damaging the various soldered wires. Overall it is a joy to work on.

General Considerations

I should begin by saying that working on this amplifier presents a very big health hazard. There are lethal voltages inside. Not knowing what you are doing might result in severe accidents and possibly death by electrocution. I am very skilled in electronics repair and I have been doing such repairs for almost 20 years. This qualifies me to work in this field. But if you do not have experience, please take this information just as a knowledge base. Do not attempt to repair something that you cannot handle as there is a big probability to severe it further while also suffering accidents.

Good working condition tools are also necessary for this restoration. I am using a quality temperature regulated soldering station with multiple tips for every situation that I could possibly encounter. Also I am using a good solder pump and quality desoldering wick in two sizes. Isopropyl alcohol is handy to decontaminate boards of old flux and other residues. I also use eutectic formulation quality solder. Every replacement part is brand new, from a reputable manufacturer, ordered from U.S.A., Japan, or Germany. Also I only use parts that are suitable as replacements in various sections of the amplifier, after inspecting and comprehending the original schematic. Last but not least I have the years of experience backing up every action that I take while working on this unit.

I have found out that working with a temperature of exactly 323 degrees Celsius is sane for these vintage printed circuit boards. I have never lifted a pad with this temperature. But it is also true that I never wait more than three seconds with the soldering tip on a pad. While working on the chassis, I am pumping up between 360 and 440 degrees Celsius in the soldering iron.

Flux fumes are extremely toxic and should be avoided at all costs. Especially toxic are the fumes released while working on these old Japanese electronic boards.

Chassis Restoration

The two decoupling capacitors, subject to change on the chassis are not figured in the original schematics. Or I could not find them. They are decoupling the power rails to the power transistors array. There is one on the leftmost side of the unit and another one on the rightmost side, directly under the fuse board. You cannot replace the one under the fuse board without removing this first. Disassembly is simple because only two screws are holding the board in its place. Changing these capacitors is very difficult due to the direct soldering to the steel chassis. This acts like a very large heatsink which gave a hard time to my 80 W soldering station. Even though the temperature was set to 450 degrees and I was using the widest tip I had available. But in the end I managed to solder the terminals. Here I show the original parts.

I have used Nichicon FG series replacements rated 3.3 uF / 50 V. Unfortunately I have not took before pictures. But this is after the replacement.

Very hard to spot the small electrolytic capacitor under the fuse board.

F-2013-1 Power Supply Board Restoration

Easy to work on; no need for disassembly.

Here it is in a before state.

A particularity is that C009 is rated 4.7 uF / 63 V on the schematic but in my unit it was 10 uF / 50 V. For C009, C010, C012 I have used Nichicon KZ MUSE series rated 10 uF / 100 V. C014 was replaced with a Nichicon KZ MUSE modern counterpart rated the same as the original.

This is after the restoration.

And on the track side I have re-soldered the terminals.

F-2041 / F-1215-A Protector Board Restoration

F-2041 is a joy to restore. A few capacitors and the coupling relay are targeted here.

This is before.

C901 was a 47 uF / 10 V bipolar capacitor and it was replaced with a Nichicon ES bipolar series modern part rated 47 uF / 16 V. C902 acts as the relay turn-on delay and it was rated 220 uF / 6.3 V. I replaced it with a modern part rated 220 uF / 25 V. C903 is a supply rail filter capacitor for this circuit. I replaced it with the same 100 uF / 50 V rating capacitor from the Nichicon KZ MUSE series. For C906 rated 1 uF / 50 V I have used a Panasonic stacked film capacitor of the same rating. C907 is a 1 uF / 50 V bipolar capacitor that was replaced with a Nichicon ES series counterpart of the same rating.

The relay can be ordered by the OMRON MY4-02-DC24 part number. This fits perfectly in place of the old relay. In fact it is identical despite the 40+ years that have passed.

Relays. Old and new OMRON parts.

And this is after.

F-1215-A is very easy to work on because there is exactly one capacitor to be replaced. Removal is easy as well. Only one screw to remove. Here is a picture before the job.

I have replaced the old 220 uF / 35 V capacitor with a modern Nichicon KZ MUSE series counterpart rated 220 uF / 50 V. And here is after the job.

While putting this board back, pay attention to the metal fixture. It has a tendency to run off and misalign with the screw hole. Nothing to worry about. With minimal care I have secured it back in place in no time.

F-2034 Driver Board Restoration

The driver board is just near the power transistors array. Very easy to service if you turn the unit upside down to have access to the soldering side while holding parts with your left hand from beneath the chassis.

This is the board prior to restoration.

Input coupling capacitors C801 and C802 were originally rated 4.7 uF / 50 V. I have used same rating Nichicon FG series replacements. C805, C806, C811, and C812 were rated 47 uF / 50 V and were replaced with same rating Nichicon KZ MUSE series modern capacitors. The bipolar capacitors in positions C807 and C807 were rated 47 uF / 10 V while in my unit were 68 uF / 10 V. I have replaced these with Nichicon ES bipolar series capacitors rated 47 uF / 16 V. Power supply rail decoupling capacitors C815, C816, C821, and C822 were rated 100 uF / 50 V and were replaced with the same type modern parts from the Nichicon KZ MUSE series. C819 and C820 are used for emitter decoupling of transistors TR805 and TR806 for static functioning point stabilization. These were 220 uF / 6.3 V and were replaced with Nichicon KZ MUSE series parts rated 220 uF / 25 V.

I have also replace all four 2SA726 transistors in positions TR801, TR802, TR803, and TR804. hFE matched Fairchild KSA992 transistors were used instead.

And here it is after.

I have reapplied solder on the component terminals. Here is a view on this.

On the underside of this board there are the two 3.3 uF / 50 V capacitors mounted directly on the emitter and collector terminals of each bias transistor. I have changed these with Nichicon FG series modern parts of the same rating.

This is a view before the replacement.

And after the restoration.

I really enjoyed working on this board as everything worked flawlessly and clean. If only there were more of these vintage units built like this. Normally with vintage amplifiers you face a deep forest of wires. Think about Pioneer for instance.

F-2028 Equalizer Board Restoration

The equalizer board is socketed and can be found in the rightmost side of this amplifier. It has a thick steel Faraday cage protecting the circuit of electromagnetic radiation. Removal of the shield is done by removing the two screws securing it in place.

Gently pull the board out of its socket presents you with this.

Input capacitors C601 and C602 were of tantalum type, rated 2.2 uF / 25 V. I have replaced these with Nichicon FG series modern parts rated 2.2 uF / 50 V. C603 and C604 in the RIAA correction loop were polarized parts rated 10 uF / 10 V. They were replaced with Nichicon ES series bipolar parts rated 10 uF / 16 V. C609 and C610 are rated 47 uF / 16 V on the schematic while on the board were 47 uF / 50 V. I have measured the voltage drop on their terminals and it is about 7 V. So I have replaced these with Nichicon KZ MUSE series capacitors rated 47 uF / 25 V. C611 and C612 were rated 33 uF / 10 V and were replaced with Nichicon KZ MUSE series modern parts rated 33 uF / 25 V. C623 and C624 were marked as 10 uF / 50 V on the schematic while on the printed circuit board were 1 uF / 50 V. Output coupling is done through C625 and C626, rated 1 uF / 50 V. I replaced all these with Panasonic stacked film capacitors rated 1 uF / 50 V. All fault-prone transistors were replaced with modern Fairchild parts, hFE matched.

Here is the board after the restoration.

I have reapplied solder on the component joints.

F-2014 Tone Control Board Restoration

On this board things are more complicated than on the other ones. This is because of the wires soldered directly on the solder-side of the board. Nothing to worry about as I could work my way on avoiding damaging or desoldering them. However extra care should be taken. There are a lot of capacitors and a handful of transistors subject to replace. In particular there is the big local filtering capacitor that is very hard to remove due to the fact that it has some ground wires soldered directly onto its terminals. These are the wires that I had to desolder.

Here is a picture of this board prior to servicing.

C703 and C704 were rated 47 uF / 16 V and were replaced with Nichicon FG series parts rated 47 uF / 16 V. C707 and C708, rated 33 uF / 16 V were replaced with Nichicon KZ MUSE series capacitors rated the same as the originals. Coupling capacitors C709, C710, C741, and C742 were rated 4.7 uF / 50 V and were replaced with same rating Nichicon FG series parts. C723 and C724 which are emitter decoupler were rated 47 uF / 16 V and I have replaced them with Nichicon KZ MUSE parts rated 47 uF / 25 V. C719, C720, C729, C730, C739, and C740 were 1 uF / 50 V polarized capacitors. I have replaced these with Panasonic stacked film 1 uF / 50 V capacitors. C721 and C722 were 10 uF / 16 V and were replaced with 10 uF / 100 V Nichicon KZ MUSE series parts. Why the high voltage? Because this is what I had available at the moment. Finally C743 which is also the hardest to remove due to the fact that two wires are soldered directly on one of its terminals, was originally rated 220 uF / 50 V. It was replaced with the same rating Nichicon KZ MUSE series modern part. Faulty transistors were replaced with modern Fairchild parts, hFE matched.

Here is the board after the restoration.

This is the hardest part to work on. You can see various soldered wires that I didn't want to mess around with. So I carefully worked my way through them. Keep in mind that the notion of easy is relative however to your experience. What I could find easy to service, you could find hard. And recto-verso.

An overview of the soldering side shows that I have applied new solder over the component terminals.

Aftermath

Old parts.

Electrical Settings

The restoration cannot be complete without performing the electrical settings as illustrated in the service manual. Thus there are two main settings that I need to touch before putting the case back on.

  • Output of Power Amplifier Section
  • Current Alignment of Power Amplifier Section

The procedure for setting the output of the power amplifier DC voltage is very simple. First commute the Speakers selector to SYSTEM-A position. Then connect a DC voltmeter to the SYSTEM-A left terminals. Set the scale of the DC voltmeter to 20 mV. Adjust VR801 up until the voltmeter reads 0 mV ±10 mV. Repeat this step for the right channel. This time adjust VR802.

Adjusting the idling current of the power amplifier is done by removing the speaker fuse cover on the back of the unit. Next step is to remove fuse F002. Turn Speakers selector to OFF position. Connect DC ammeter on the terminals of F002 fuse. Adjust VR803 for a reading of 30 mA ±2 mA. Repeat the procedure for the right channel. This time remove fuse F003 and adjust trimmer VR804.

Needless to say that you need to power the unit off each time you remove the fuses. It can be done with the unit powered but there could be a risk to short-circuit terminals and blow off the nice output power transistors.

On my unit, there is a slight variation of the DC output voltage, particularly on the left channel. It is close to 0 mV but it fluctuates repetitively around ±15 mV. This is subject for another further investigation.

Cleaning the Front Panel

Dismantling the front panel requires first removal of all knobs and switch caps. It is fairly easy but you need to be careful in order not to damage the front aluminum mask. I cleaned everything with window cleaner based on ammonia.

Knobs and felt between switches and the frontal mask come next.

There was a lot of dirt and I used about twenty cotton sticks for the buttons alone. The frontal mask came up pretty clean but there are some oily stains deep within the aluminum ridges that I cannot remove no matter how hard I tried. But I am happy the dirt is now a matter of the past.

Article #5 | 10:35 AM Sunday 29/10/2017

Sansui AU-7500: Impressions

So how does it sound like? Long story short, I cannot stop listening to music through it.

Here is the amplifier temporarily positioned on my left speaker. I have used the wood chunks because the amplifier feet are further spaced than the width of the speaker top cabinet. It looks nice, however it does have a dent on the top aluminum bevel. Even though I cleaned the top cover as best as I could, there are some scratches that I really don't have any means to deal with. But anyway, if you want perfection then buy new units. Old ones have their own drawbacks.

It doesn't sound like an AU-9500 but it is similar in a good way. The sound is non-fatiguing, laid-back, and sweet. Bass is deep and well contoured. Midrange adjustment is a little bit harsh for my ears. But I have found a sweet spot at around +1 gradation on the knob. The high frequencies are rendered in a clear manner.

This is my main amplifier for PC usage. This means that I listen to music through it every time when I work on the computer -- or when I play games. It doesn't heat up, it is reliable, and fulfills my needs in this area. I like this amplifier better than the dual AKAI AA-5210 rig it replaced.

This restoration was a success. Until the next maintenance, this topic is closed.

Article #6 | 10:24 AM Sunday 10/02/2019

Sansui AU-7500: High Frequency Oscillations

Let me discuss an interesting and annoying behavior that I have just spotted a few days ago. A year and almost a half before, when I was dealing with the restoration of this unit, I spotted fluctuating DC offset most notably on the left channel. While the variation was really minor, I decided to tackle with it.

So dismantling the unit and connecting the oscilloscope at the speaker output revealed a HF -- mostly RF -- oscillation. Interesting enough, I said. I have measured with the oscilloscope some key parts of the output amplifier section and the RF signal was there. Clear and repetitive. If you have ever heard a low frequency audio sweeping oscillator, then imagine the same but in RF spectrum. The speakers were mute, they couldn't reproduce such high frequency. But the Sansui circuits were clearly able to generate and maintain the oscillation.

Well, I moved on to the preamplifier then. The same oscillation was present everywhere bar the entry point, before the 220 nF coupling capacitor to the input of the preamplifier. What? So the oscillation was not coming from anywhere before the preamplifier.

Then I put the scope on the F-2013-1 power supply section. At the output capacitor, and at TR003 base there was this oscillation present. Very unlikely but I decided to test whether TR003 was oscillating. It was not, and as I said it really was unlikely. That circuit cannot oscillate in that configuration. However I was seeing the sweep signal with the scope. I added a low capacity ceramic capacitor between the base and collector of TR003 and the only thing that happened was that the oscillation signal quickly changed shape and then came back steady and repetitive as it originally was. Back to step 0. The problem is not originating here.

Then in a curiosity access I have commuted the sources selector on PHONO 1 input. The oscillations disappeared completely. Hmmm, so there it is. The equalizer board is the source of these RF oscillations. But why it is propagating when the equalizer stage is disconnected from the main signal line? And why through the power supply line? For sure there is no electrical contact -- besides the ground signal -- between the equalizer board and the input of the preamplifier, if the sources selector is on AUX for instance. But the oscillations are there.

In order to be convinced that the oscillation comes from the equalizer circuit, I have removed it altogether from its place and oscillations disappeared completely.

Let's debug this circuit then. Start by looking at the schematic and using the brain first. The first thing to come in mind was replacement of the previous transistors with modern ones. Let's start with position TR601 and TR602, formerly 2SA493. Transition frequency is 100 MHz and collector capacity is 10 pF. I have replaced these with modern Fairchild KSA992 transistors. These have the transition frequency set at 50 MHz and collector capacity is 3 pF. What if the difference in collector capacity triggers these oscillations? For a circuit to auto-oscillate you need an RC group. In this section, while the R is external, the C is internal. I didn't have a 7 pF capacitor in my spare parts box. The smallest one I could find was a C0G 100 pF MLCC capacitor. I gave it a try as per the altered schematic below.

I performed the modification on the tracks side as there was no apparent space on the parts side. It doesn't look obtrusive and Sansui did it before. Think AU-6500, driver board. Take a look on the tracks side and you will see a similar alteration of the circuit.

What do you know, the circuit is now not oscillating anymore. I have tested it with the oscilloscope and my ears. Dead silent. I then tried to use the turntable to check for audible problems caused by mounting of this new capacitor in such a low signal high gain stage. Everything seems good.

Now if only I could have installed a 7-10 pF capacitor in place, it would've been close to perfect I think. But the problem is solved and I am pleased by this resolution.

It is however very interesting to note the fact that the RF oscillation was propagating through the ground signal to the power supply circuit and from there it was distributed to all the other circuits in this amplifier. So should you ever find yourself in a similar situation like me, then you might have an investigation starting point.

In the end, I have redid the DC offset adjustment and now it is a steady 0.1 mV on each channel.

I have also updated the table with the parts list for the equalizer circuit at the beginning of the page. Now it includes the 100 pF capacitor as addenda.

Article #7 | 11:58 AM Sunday 10/02/2019

Sansui AU-7500: Small Upgrades

Working on a similar unit -- the AU-6500 -- revealed that some parts could have been improved from ground zero in order to prolong the life of the AU-7500 unit. I am now retroactively using the knowledge I got from previous restorations to overhaul the AU-7500 unit.

Let's take the circuit sections one by one.

F-2013-1 Power Supply Board

Resistor R004 was replaced inherently with a Dale precision metal film resistor. While on this unit it appeared to be in top condition, in the AU-6500 that I recently restored, this resistor was scorched. So why take the risk?

Transistors TR002 and TR003, originally 2SA678 were replaced with Fairchild KSA733 parts. Be sure to order the transistors with the C suffix. This means center collector.

Zener diode ZD was replaced with a Nexperia 14.1 V / 1.3 W modern part. The original was a 14 V / 1.5 W part. Diodes D002 through D005 were replaced with 1N5392 parts. These appear to be more of an equivalent to the original 10D-1 parts. Diodes received glass tubes as separators. Why? Rectifier diodes produce heat. While it is true that in this case, the heat is not sufficient to damage the printed circuit board, good engineering practices say to use separators.

Power supply section should be on the safe side now. The old diodes and transistors had corroded legs and who knows for how long they would have remained functional?

F-2041 / F-1215-A Protector Board Restoration

Transistors TR901 and TR902 were originally 2SC634A and I have replaced them with KSC1845 modern parts.

Diodes D905 and D906 were originally 10D-1. I have changed these with 1N5392 parts.

On F-1215-A, the single 2SC634A transistor with corroded legs was replaced with KSC1845.

F-2034 Driver Board

On the driver circuit I have replaced all variable resistors with new Bourns cermet parts. R851 and R852 are 10 ohm / 1 W resistors which I have replaced with Dale precision metal film resistors rated at 3 W and 1 % tolerance.

Aftermath

Old parts. Bar the resistors, all of them have corroded legs.

While mostly unnecessary, these small updates will increase the MTBF rate of this amplifier. Which is exactly what I originally wanted.

If you will

Please note that all the work presented herein this site is non-commercial. This is my hobby and I am doing this in my spare time. Through this page I freely share my knowledge with you. But if you like my work, please consider helping me buy a transistor or a capacitor for my projects.

Thank you!

Copyright © 1998- Alexandru Groza