Odd problem with Sony VFET power amp

[Tillerman]

This is my first post on this forum.
I have a technical question about my Sony TAN 5550 VFET power amp. I live in a rural area that experiences sags and surges on our incoming electrical power. When there is a sag (brownout), the TAN 5550's protection circuit (relay) clicks off, then on, which has the audible effect of killing the sound for a second or two to the speakers.
The curious thing is that my Sony TA 4650 VFET integrated amp does not do this during a power sag. My other audio components also do not do this.
Any ideas?

Thanks,
Scott VFET Fan

 

[EchoWars]

Yep.

 

[EchoWars]

Oh...I get it. You wanted to know what the problem might be?

(Sorry, I'm a smart-ass early in the morning )

1. Your other amps don't have output protection relays, therfore they don't kick in.

2. The relay protection in the Sony you speak of is designed to kick in at a somewhat more sensitive level than the others.

3. You may have excessive DC-offset (have you checked it?) and the surge/brownout is enough to upset the input stage just to the point where the relay kicks in, since with the excessive offset it's knockin' on the door anyway.

4. The caps in the protection circuit are leaky/weak.

5. The driver transistor for the relay is leaky/weak.

6. Martians...

7. Nemo has been found!!! He's in your amp!!!

8..............................................

How much more of this can you take? I can go on and on, but if offset looks OK, then there may well be a problem. Of course, this is all just WAGging (Wild Ass Guessing, if you are unfamiliar with the term).

I really would think it ought to take a pretty hefty brownout to activate the relay. I'd like to say that the problem is with your power, but a gut instinct says that the protection circuit is weak and failing.

(Promise next time my nephew rolls one up, I'll Just Say No....)

:nutz:

BTW, welcome!!

 

[Tillerman]

Hey, I am OK with smart asses! You are referring to burritos with your nephew, eh? What, are you on a diet?

Thanks for the string of possibilities. I am not a person with electrical skills, such as yourself, so I have not measured the DC offset. However, I do like to try and diagnose problems before hiring someone. I understand you take in work from members. Where are you located?

Regarding the brownout, would you consider the house lights' noticeably dimming when the amp's relay cycles off/on a significant brownout?

BTW My TA 4650 also has a protection (muting) relay, but as I mentioned previously, it does not cycle off/on during power events.

Scott VFET Fan

 

[EchoWars]

The lights dim? Wow... That can't be good for gear.

You have one obvious problem, and one that may or may not be a problem....the AC power brownouts, and the amp that seems to be overly sensitive.

Regarding the first...

Is the building wiring OK? If its all coming from somewhere else (i.e., you know your wiring is OK), then your choices are pretty slim and all expensive. You need a UPS (uninterruptable power supply) for your gear. A unit which simply protects the gear by shutting everything down (like the Panamax 1500 or the newer 5000 series and Monster power stuff) won't do the job, as they will simply be turning the gear on and off. Seems you need something that will provide power during the brownouts, and a UPS big enough for stereo gear isn't cheap. Something like the Back-UPS RS 1500VA would be a minimum, and a 2000VA would be better. The advantage of the bigger ones would be that you could protect your computer also. Before I went this route, I'd call APC and tell them what I had in mind. The output of these should be a nice clean 60Hz sine wave, but I'd want to confirm that since these are designed for computers and servers rather than stereo gear.

The Sony...

I suspect a problem in the relay circuitry. There usually is at least one cap in this circuit that is designed to discharge very quickly on power down to disengage the relay and prevent turn off thumps. Thie cap is generally connected to the transformer output via a single diode and a low value resistor, and it takes a heavy beating in normal use. It is probably weak.

My only real gripe with vintage Sony gear is related to the size of the company. Sony actually owned their own semiconductor fabs back then (perhaps they still do) and of course many rare and nearly irreplaceable transistors went into their equipment. Your 5550 is one of those. The VFets on the output are uber-rare, and if damaged or destroyed your probably have a paperweight on your hands.

If it were my amp, I'd restore it just to protect the output devices. Repair the relay with whatever it requires, and replace all the electrolytic capacitors with nice tough new Panasonic FC caps. The big expensive power supply caps can probably remain as long as the rail volages look OK and the amp is capable or reaching its rated output power, but I'd put a good film cap bypass cap on them to compensate for the increased ESR due to age.

Servicemanuals.net has a manual for that unit...one of the few places that actually has it.

No magic to measuring the offset. Get a decent voltmeter, set to read DC voltage on a low scale, and connect to the amp output. You should see something under 100mV...the lower the better. Read here for more details.

I'm in Lee's Summit, MO, a surburb of Kansas City. If you were interested in pursuing this, by all means shoot me an email...you can do so through my profile.

 

[Tillerman]

I live in a rural area with some factory farms nearby on the grid. It seems when they came, we started having more problems with brownouts.
The only thing in my house that triggers a similar condition is when our new (last year) central AC unit kicks in. We eventually will upgrade service to 200 amps. The AC unit has a 30 amp fuse and is rated for 18-20 amps.
I am aware of the rarity of the SONY VFETs and would like to preserve the output devices. I ordered a service manual for the 5550 last week from a different source; should be here soon.

I have read the post on measuring DC offset. I have a couple of questions about doing the measurement.
- since I have a power amp, would it be preferable to disconnect the preamp before measuring?
- if the amp is on and the speaker cables are disconnected, do you just touch the leads of the DMM to the +/- speaker terminals on the amp of say, the left speaker and take a measurement? I just want to be sure whether you can touch the leads to the terminals when the amps is on or if you have to connect the leads to the terminal before powering on the amp.

Scott

 

[EchoWars]

You can leave the inputs to the amp connected, just turn off the source equipment. If you totally disconnect the source interconnects, you may get hum that affects your readings.

The impedence of a meter is very high (just make sure it is set to measure volts...not amps ) so the amp does not see it as a load. Connect right to the output terminals. Do not short the terminals together in any way, shape or form, either intentionally or by slipping with your probes.

Let us know what you find..

 

[Tillerman]

I just got done testing the TAN-5550 after 20 minutes of warm up.
Left = 7.4 mv
Right = 12.3 mv

Thank you EW for all your guidance on this matter!

Based on the results, what would you recommend? The amp sounds good to me, other than the behavior already mentioned.

BTW, I got my copy of the service manual.

Scott VFET Fan

 

[EchoWars]

The offset looks peachy, so the amplifier is pretty well balanced. The problem then is the relay circuitry. The driver transistor for the relay, or weak caps in the circuit are causing it to be too touchy. It needs attention, based on your description.

The UPS is still not a horrible idea, but because of the rarity of some parts in that amp, I'd go on a cap replacing spree, and give it a tune-up. Get that puppy set to last another 25 years...