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Technics SU7700 poor performance

Hi everyone,

My name is Francisco, and I am new in this forum, hope I can learn and help in my short possibilities! (I am new in the vintage and Hifi world)

I have purchased a 1976 Technics SU7700 which seems to be in good conditions, esthetic is perfect, but once I begin to play music there are some issues I want to know if can be solved with a recap of the 36 electrolytic capacitors, (I found a Nichicon kit for this amp). The ratings of this amp in internet make me think that it should sound quite better! My problem is to know if it is a good investment to recap or it needs other major electronic reparations that could rise the money I will need to put on it.

Before testing I have cleaned all the PCB, I have changed a 500 mA fuse that protect the vumeters lamp and I have tried to lubricate/clean the volume, balance, bass and treble knobs, but I think the job done isn´t enough. Should I dissamble all the cable connections from each potentiometer and extract them from the main aluminiun front cover for clean them properly?

1) The first test have been done using a mobile phone as source, connected to AUX, TUNER, PHONO and TAPE (playback). The loudspeakers have been connected biwired to main and remote connections. The problem, apart from the poor performance, is that the R channel sound very weak, and if you turn the balance to force this channel it doesn´t get better. This happens using all the sources, unless I connect the mobile to PHONO, where it sounds higher but without good quality. In TAPE the sound has a little more power and in TUNER and AUX there isnt´t any sound in R channel. I have not tested with other source, no CD player nor tuner.

2) If I check the headphones connection now it sounds in stereo, but again, the sound quality is bad, and if the volume knob is turned, it scratches a lot. So I know it needs a good potentiometers cleaning.

3)Other problem is the noise that produces the system all the time, which can be well checked with the headphones. Maybe I need to connect the ground of the amp to the ground of the house, as the 230 V cable of the amp doesn´t have ground line? The noise depends of the position of the minijack connected to the mobile phone.

4)I have noted that in the Technics manual there is an explanation for alignment, that can be done testing with a voltimeter each channel. If the measurement is over 25 mV it needs a bridge connection to be cutted, the same process for each channel. In my amp, this bridge connection has been cutted only in one of the channels, so I think that someone did this checking before. This can be the cause of the poor performance?

5) Finally, I have noticed that the thermal paste used for the connection of the power transistors to the cooling fins is not dried, once you touch you can remove it.

Best regards and thanks !!!



Re: Technics SU7700 poor performance

Thanks Jhonnysan and analogcd!

I have used WD40 Specialist, which says it is a contact cleaner.I have sprayed it inside the potentiometers, but I think that the liquid doesn´t reach the contacts.

I have tested again with a ground connection to the home electrical line using the PC as a source, change the old biwried JVC speakers for 2 home cinema Sony little front speakers, normal wired, and played some music for 1 hour. I have noticed that the amp begin to sound better, 2 channels, and also the headphones works better now, (but with them only get movement in the right vumeter?¿).

The heat exchanger becomes hot and the heat can be smelled once I play music at normal volume. So maybe the problem is that the amp has not played music for some years!

The only problem now is that the amp breaks the vumeters lamp constantly. The manual says it need to be 6,3V - 0,25 A. Haven´t got it so all the stuff I find is 4,8 V -0,3 A.

I have purchased a Technics ST7300 tuner as a source, and will look for a good CD player of this line. Any recommendation? Has Technic a good CD player of the same line that the amp and the tuner in the 70's?

And finally: Is the option of biwiring and 90's speakers a good choice for my amp or is better to select normal bookshelf speakers from the 70's? I think that 70-100 W is enough for my system, as it produces 50 W/ch, but will accept your goods advices!

Thanks and have a nice week!

Re: Technics SU7700 restoration suggestions

WD40 Specialist Contact Cleaner is not the proper chemical to address your unit's issues. WD40 CC is simply that: a cleaner. It does not have any lubricants nor deoxidizing elements. Both of these are needed when restoring vintage electronics. Best option would be to purchase both CAIG DeoxIT D5 and CAIG DeoxIT FaderLube. The former will remove corrosion and oxidation as well as enhance metal-to-metal contact. FaderLube is specifically formulated to clean and lubricate potentiometers (pots). While many folks use DeoxIT D5 to clean pots, it is not the preferred chemical. It will likely not damage controls, but using FaderLube is definitely better option.

When I service vintage electronics, I use DeoxIt in both spray and 100% liquid to get controls functioning. Once they operate reasonably well, I use FaderLube aerosol then follow up with 100% liquid. The latter provides lubricant to assure smooth, long-lasting, noise-free operation. The array is costly, but worth every penny once the results are witnessed. Realize too that some of the controls installed in vintage electronics will never be produced again and should be treated as delicate, irreplaceable components.

Heat exchanger, a k a heat sink, will become warm during use, this is normal. Overheating is a topic for a separate discussion.

6.3 volt incandescent lamps are readily available. Your rig can likely utilize something between 100 mA and 250 mA (0.1 - 0.25A). Do not exceed the 0.25A rating specified in service documentation. Digikey, Mouser, Newark and other vendors stock these lamps. Match mounting type (threads, pins, wires, etc.) to assure correct fitment.

Technics made many fine CD players and changers. Depending on budget and features, you may find something at a thrift store. Check out one of the local places as well as web for used item listings. There are tons of good, used CD players available, as this format is readily becoming obsolete. Since CD players only became mainstream in the mid-80s, matching your 1976 '7700 will be an approximation at best.

Bi-wiring is typically unnecessary and provides only minimal improvements for the rare situations where it is justified. Use 16 gauge stranded wire up to 50 feet, 14 gauge for greater distances. Don't waste money on boutique loudspeaker cables, just purchase good quality copper alloy. Get a decent pair of two-way loudspeakers and consider the sensitivity to be one of the more important specifications. This parameter indicates the amplitude a particular loudspeaker will produce sound with a given amount of power (input signal). The specification is stated in dB measured at one watt input from a distance of one meter. Typical values range from upper 80s to mid 90s. The higher the dB rating, the greater the volume produced all other factors being equal.

From a quick web search, it appears the companion tuner for your '7700 is model ST-7700, or the mark two variant ST-7700II. There are a few models within the range. ST-7300 seems pretty common on used market, '7600 appears now and then. A decent match to your integrated amp should be available.

Best of luck with your project.

Re: Technics SU7700 poor performance

As mentioned, do not use a phone when high-fidelity is necessary. Using a CD or CD/DVD player will provide true test of sonic performance. You need to specify brand and formula of "cleaner" used. DeoxIT is very common, other very good options exist. Not all chemicals address corrosion and lubrication, both aspects are very important considerations. With a unit this old, one must remove corrosion to avoid intermittent contacts and related failures. Is it worth replacing capacitors? That question can only be answered realistically by looking at the net gain versus costs and use. If one plans to keep such a legacy piece for a long time (more than a couple years), then of course such expense is justified. For any scenario requiring high-fidelity and reliability, capacitor replacements is wise choice. Should a user simply want to "rock out" in his garage, then connecting an old receiver and letting it run as-is would be fine. To summarize: Test again with CD source. What cleaner did you use? Have caps replaced if you deem it cost effective.

Re: Technics SU7700 poor performance

My 1st advice would be to stop using a phone as an audio source.
2nd would be to clean all controls and switches; you might have a signal problem through the source switch. You should be able to spray the cleaner through the holes in the tops of the switches.
Replacing capacitors may bring back the weak channel, if that is the problem; unfortunately it could be many other things also.
You say that it produces noise all the time. The real test of this is to short the input that is selected (like aux) then listen--should be very little noise. If not, it could be in the power supply or the amplifier. May be bad capacitors, or other bad components.
If this amp has been worked on it could have many problems.