Technics SA-TX50 Hissing

James298

Member
2020-03-11 1:37 pm
I just got my hands on a beautiful Technics SA-TX50. I've been slowly building my setup, and while this is not the king of all amps, it's the best I've had.

My issue is that it has a constant hiss! It doesn't matter which input is selected or what power outlet I'm plugged in to, it constantly hisses. The hiss gets louder as I increase the volume and can be heard faintly behind the music when it's playing. I have tried reseting, plugging in to non-grounded power bars, plugging in different inputs, no inputs plugged in at all, it always hisses!

Please Help!!
 

James298

Member
2020-03-11 1:37 pm
How many years old is the set? Amps generally have intrinsic noise levels, but the increased hiss can be caused by high frequency oscillations because a set of out-of-live capacitors inside it. If more than 15/20 years without maintenance, the caps may be leaked or degraded to the point of be useless.
The receiver is from 1998. I doubt it has ever been serviced much. I imagine replacing the capacitors only involves some soldering? Can the capacitors be purchased online fairly easily?
 
Surely. If you have minimum skills, you will do it pretty fine. I work in an industrial repairement job, so several thousands of caps replaced in my 50 years. Some suggestions from my experience:

1.- Take a photo of the original positions and type of caps, so if any doubt arises, you have a backup image.
2.- Order caps as you can in your place. Here there are several places for getting parts, but I don't know if it happens around the world.
3.- Prepare a test setup like this one. Do not work with the mains directly, as an accidentally reversed cap may explode causing personal injury and with disastrous consequences for the amp.
4.- Use good light and glasses if needed.
5.- Maintain a hand at the pocket while seeking in high voltages places, near the power cord/trafo/fuse holder.
6.- Take a cup or a magnet or similar to reserve the screws.
Keep us posted.

Note: the recapping doesn't warranty the amp will stop hissing, it is only one alternative between several, but one of the most common.

Good luck.
 
I would isolate the electronic volume controls as there is no issue after that section so changing lots of innocent capacitors is not the way to go.
Follow the signal back into the buffer amplifiers IC501/2 & 3, that will tell you of the hiss is before that point.
Use the service manual to follow the circuit back through from vol control to the input switching.
 

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This unit is rated at an S/N of 70 dB at rated power. Ouch. This probably refers to 300 mV of input sensitivity, so for 2 Vrms we may be able to expect 85 dB. Still no great shakes. This, I assume, is largely the fault of the non-bypassable DSP section, with ADC and DAC dynamic range rated at 100 dB each (so that's 97 dB under ideal conditions and with a following wind). There may be further screwups.

I think you may be best off skipping the preamp section altogether (ProLogic is long outdated anyway) and using the provided power amp inputs with something else. That something needs to deliver a maximum output level of ca. 820 mVrms, and depending on your speakers' sensitivity, should have no more than anywhere from 3 to 8 µV of output noise across the audio band (that's a dynamic range of 116.5 to 108 dB ref: 2 Vrms), with no tendencies towards random high-amplitude noises for obvious reasons.

Typical candidates for somethings may include MP3 players (a Clip+ may well do the trick), soundcards (or even onboard audio) of the better kind, headphone amplifiers of the less noisy persuasion or good preamplifiers, even a little compact mixer perhaps. (You could also use the pre-outs of another, more modern A/V receiver with a better processor section, but that sort of defeats the purpose. A/V preceivers or processors are neither common nor cheap, unfortunately.) Anything that has both too much noise but also way more signal than needed may be tamed by either buying or making a line-level attenuator. With ideal translation, dynamic range requirements would drop to about 100-108.5 dB, a very common range these days.

A Behringer UCA204 would get pretty close, too.
 
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UserAbuser

Member
2019-09-27 11:18 pm
"I imagine replacing the capacitors only involves some soldering? Can the capacitors be purchased online fairly easily?"

If you have to ask then I politely suggest this is not something you should think of doing.
Please enlist the services of an experience repairer before you destroy it or yourself.
 

James298

Member
2020-03-11 1:37 pm
Is it possible that the hissing is solely because of the relatively low S/N ratio of 70dB? My speakers are 93dB/W @ 1m....so fairly sensitive.

I am only using this receiver to listen to music from my phone with an RCA cable. I am looking at getting a preamp so I can hookup my turntable. Would a good preamp be able to take both the phone and the turntable inputs and reduce noise from the receiver?
 
Is it possible that the hissing is solely because of the relatively low S/N ratio of 70dB? My speakers are 93dB/W @ 1m....so fairly sensitive.
That's not going to help either, but it seems your noise is mostly coming from before the volume control at this point.
I am only using this receiver to listen to music from my phone with an RCA cable. I am looking at getting a preamp so I can hookup my turntable. Would a good preamp be able to take both the phone and the turntable inputs and reduce noise from the receiver?
Absolutely.

I strongly question the economics of the whole thing though - hi-fi preamps have never been too cheap (separates always were an upgrade from an integrated amp, though the ones Technics offered in the '90s were cheap and not anything special) and as such are neither super plentiful nor do they tend to sell for pennies.

You might get a whole decent integrated amp or another receiver (probably more modern, too) for substantially less than what a decent preamp goes for!
I might look up what a used Yamaha A-S700 costs these days, or an AX-497 or AX-596, or the R-S700 receiver perhaps (the 500 and 300 models seem to use the same source selector / volume control IC so should be similar in noise). (The older AX-750, 1050, 1070 or perhaps 870 would be very good amps as well, alas they and other models from the era are often plagued with severe input selector problems and need the better replacement part fitted if that hasn't been done already.)
If you want to stick with Technics, look for an SU-VX800 or SU-VX920.
Basically all of these have better power amplifiers than your existing receiver as well (which aside from being a rather neat-looking piece is quite powerful but its IMD spec really is not great).

Modern-day surround receivers can be very good in terms of output noise, too (with quoted full-power SNRs of 106-110 dB) - unfortunately a lot from the HDMI era seem to be afflicted with faulty TI DSPs, like basically anything Onkyo between about 2007 and 2012 (but e.g. Yamaha used these, too). Onkyo had a long-running repair program, so any unit going through that hopefully would not be affected again.

If you want a point of comparison the NAD3020 has 40dB better SNR than that amp...
110 dB, that old thing? I'll have to look that up.

Yep, apparently so. This shrinks to 104 dB for the 3020B, and 100 dB for the 3020i. I might have a go at simulating the preamp section to see whether the claim may be realistic. Seeing a 20k volume pot and 10k pots in the Baxandall makes me relatively hopeful that it could be better than average (i.e. 100 dB) at least. Besides, it only being a 25 wpc amp, voltage gain would be lower than usual anyway (power amp gain comes out to +23.7 dB), so 104 dB would still make for a very good output noise spec (89 µV).
 
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James298

Member
2020-03-11 1:37 pm
Hi Folks - An update! I had a very respected audio repairman in Winnipeg, Columbus Radio, look at the unit. They replaced 20 capacitors in the digital board and the unit sounds way better now. No audible noise between songs until the volume know is near 3/4 of max (generally to loud for even loud listening).