Technics SU 7700 performance

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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 !!!
 
1) Definitely an issue if one channel is down like that - might take some troubleshooting to find it
2) Pots also wear out and might need replacing - a good deoxidant cleaner and lots of twisting the knob to clear off crud is the first step.
3) Can you explain "sound quality is bad" more specifically - is it more than just the scratchiness?
4) Yes, if the bias is right out that would explain poor sound. Have you rechecked this?
5) Its a paste, not a solid, its not designed to dry out (though some kinds do).


BTW don't put a high level signal into a phono input, that can blow out your speakers (and ears). Phono levels are a few millivolts.
 

PRR

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...I connect the mobile to PHONO, where it sounds higher but without good quality. ...

PHONO is a "special" input, ONLY for magnetic phono cartridges. It has LOTS of gain in bass, low gain in treble, to correct the natural high-boost of a phono cartridge. Do NOT connect "line level" signals like your cell-phone to a Phono input. There is some chance you will blow it up. (Most cell-phones won't, but it is a bad habit to put anything except a phono to a Phono input.)

Your bad balance may be jacks, switches, pots, or coupling capacitors. Since you have played with the jacks, I would find what is a good electronics cleaner today, and carefully spray switches and pots (unless someone jumps in and say that is a bad idea for this amplifier).
 
My first (obvious) thought is that 40 year old electrolytic capacitors are likely to be in poor condition. Also, a scratchy volume control makes me think the potentiometer might need replacing rather than cleaning. I had a Technics SU-7300 of about the same age, and it was the pots and switches that started failing first - scratchy pots, loud clicks from switches. Replacing mechanical components plus all the caps could be expensive.
 
Thanks everyone!


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.


The only scratchy noise now is with the volume knob, as it can be hear as I touch it, once I stop the turn the noise stops. There is also other little noises that can be checked with the headphones when I push some filters button or touch the bass-treble knobs, but there are despreciable once the loudspeakers are connected.


Mark Tillotson: I want to take an osciloscope to measure the mV of each channel, as my multimeter doesn´t have this mini scale. In case that the channels are over 25 mV, I can cut the bridge of one channel, but the other has been already cut. What is the solution in these cases? Change the main power transformer? Change the amplifiers coupled to the heat exchanger? Can I put any AC-DC power transformer or it need to be toroidal or any other type?


Thanks and have a nice week!
 
Hi - I have LOTS of vintage Technics gear and like all old stuff - many problems can be traced to dirty contacts. On some of the old Technics stuff, like the big SA-818 and SA-616 receivers, several of the switches controlled by front panel knobs are actually located well back in the chassis. You have to open the unit to properly access and clean them. Do any of that at your own risk and if you treasure this unit - practice on some gear you don't care about first. Let us know how it goes!

RK

PS. I'm not a fan of any WD-xx anything - I'd get real, good contact cleaner from a supplier here or an electronics supply store to be safe.
 
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?¿).
That is just about impossible, if you hear something you should also see the VU meter move (the VU meter connection is literally tapped off from the output, then things are attenuated and rectified) - maybe the meter range switch has an issue.

Speaking of contact issues, there are three potential trouble spots the signal has to pass on its way to the speakers:
1. Protection relay
2. 4 A fuse holder
3. Main or Remote speaker switch

One or more of these is likely to be in need of cleaning.
The only scratchy noise now is with the volume knob, as it can be hear as I touch it, once I stop the turn the noise stops. There is also other little noises that can be checked with the headphones when I push some filters button or touch the bass-treble knobs, but there are despreciable once the loudspeakers are connected.
I'm thinking you've got some current on the pots and switches, which I'd be inclined to blame on 40-year-old electrolytic coupling caps.

Looking at the schematic, the basic circuit is quite similar to more modern budget integrated amps - all-in-one pre + power amp with added tone control duties, opamp based RIAA, and that's it for active components. It's about as lightweight as a budget Pioneer of similar power rating from around 1990, too. Of course, there's a '70s twist to it with high impedances around the power amp (no idea how they even got the specified 97 dB SNR out of it) and the dreaded TA7136P. It's probably OK as-is but getting seriously good performance out of it would require a lot of work (new pots, recalculating component values for the pre/power/tone stage and changing them, getting a better opamp adapted into the RIAA stage) which, to me, wouldn't seem to be worth it, even though it's quite a nice-looking little amp.
 
My first (obvious) thought is that 40 year old electrolytic capacitors are likely to be in poor condition. Also, a scratchy volume control makes me think the potentiometer might need replacing rather than cleaning. I had a Technics SU-7300 of about the same age, and it was the pots and switches that started failing first - scratchy pots, loud clicks from switches. Replacing mechanical components plus all the caps could be expensive.

Thank for the info!

Is replacing mechanical components like potenciometers and switches possible with actual electronics replacement components or I need to find the same component from the date when this amp was built?

I mean: As spare parts I have only found the relay , which I will get and replace.Also, I have found a kit for the replacement of all electrolitycs with new Nichicon Audio capacitors, (except the 10.000 uF 2 power capacitors units, which seems to be dummy leg type), but I haven´t found any potenciometer or switch.
 
That is just about impossible, if you hear something you should also see the VU meter move (the VU meter connection is literally tapped off from the output, then things are attenuated and rectified) - maybe the meter range switch has an issue.

Speaking of contact issues, there are three potential trouble spots the signal has to pass on its way to the speakers:
1. Protection relay
2. 4 A fuse holder
3. Main or Remote speaker switch

One or more of these is likely to be in need of cleaning.

I'm thinking you've got some current on the pots and switches, which I'd be inclined to blame on 40-year-old electrolytic coupling caps.

Looking at the schematic, the basic circuit is quite similar to more modern budget integrated amps - all-in-one pre + power amp with added tone control duties, opamp based RIAA, and that's it for active components. It's about as lightweight as a budget Pioneer of similar power rating from around 1990, too. Of course, there's a '70s twist to it with high impedances around the power amp (no idea how they even got the specified 97 dB SNR out of it) and the dreaded TA7136P. It's probably OK as-is but getting seriously good performance out of it would require a lot of work (new pots, recalculating component values for the pre/power/tone stage and changing them, getting a better opamp adapted into the RIAA stage) which, to me, wouldn't seem to be worth it, even though it's quite a nice-looking little amp.


That is just about impossible, if you hear something you should also see the VU meter move

This is the strange thing! This only happens when I connect the jack to the phone out, and I listen with the headphones, only the right channel vumeters is moving. If I push the button for main speakers, both vumeters move as the sound is coming from the two speakers.


One new event is that the lamps don´t get totally broken, they get smoky inside and stop lighting or make intermitent light, so the problem can be the current passing through them.
Exist different E10 lamps for AC or DC?

I think that, as incandescence type, they can work with both type of circuits, but I´m wondering if I can change it for a LED unit, maybe I will need to correct the signal if the lamp are designed for AC working.


Anyway, I will learn a lot with this stuff!!:rolleyes:
 
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