Yamaha Natural Sound Stereo Amplifier A-7

[ChaseK]

Hi Hi update on the amp! I managed to score a 100v 600W step down transformer.



Opened it up also and found the issue for the phono dying a bit on me, it was a little oxidised! Spammed some wd40 Contact Cleaner (Deoxit is expensive here), did the rest of the (important) components and it's all working good - except that I'm unable to tell if the wattage selector is working all too well cause the light doesn't switch on and sliding it doesn't really make any hearable difference, maybe it's cause I don't play it loud.

The part of the amp with the inputs board and etc. is still getting TOASTY (45c from my thermometer, I'm unsure what class this is and thus can't really make a super informed decision if the heat is okay) so I'm buying some aftermarket fans and attaching them on in a real jank way, I'll post when they're in and installed and report my temperature findings

Also came to realise that I can only run a pair of 8ohm speakers at a time on this, so I will be doing that from now on instead of running four speakers in my really small room, I have too many speakers lying about...




Thanks all for the help!

 

[Martin.Ca]

That sliding monitor control is weird and counterintuitive. I think you set it to the level where you want the led to go off at.

45°C, Shouldn't be that hot.

 

[ChaseK]

Hi all! the amp has been working great except for one issue: The phono selector. I have to fiddle around with the switch for a while before the left or right channels play properly - not doing so results in only one channel playing.


I believe that the flexible switch selector (the internal mechanism) has dust or is not making full connections and I believe it needs to be cleaned for it to work properly. Is there any way go to about cleaning this?
Thanks

 

[Martin.Ca]

Spray it with contact cleaner, and rotate the control through all settings first. That usually works (I have a a760).

 

[avionic]

Is there any way go to about cleaning this?

To do it right.. Requires removal and disassembly of the slide switch. Trying to spray contact cleaner into it is a short term band aid.

 

[SingHifi]

[QUOTE="Martin.Ca,

Transformer are very simple thing but i used Soundtech Model AT-1000 (1KVA) 240VAC to 110VAC step down transformer to drive my Yamaha A-6 is no problem. These step down nowaday uses Toroidal Transformer. In Alibaba you buy direct from the Mfg in house online shop and the price are normal prices (like USD$ 20 for a 1.5KVA) where the Mfg has the reputation to uphold (not those fly by night shop) .

The A-6 or A-7 uses Magnetic Field Power supply (aka Carver Magnetic Field Power supply) you will really needed a scope to troubleshoot this if this is the problem., As stated in above message reply the +B and -B DC Power will be 10% higher due to the 110VC instead of 100V Step down transformer but it is no problem really but it is of course best to be able to find a 100VAC output step down transformer (literally 240VAC to 100VAC does not exists as a standard product since 240VAC is only unique to British). To check this desolder the output of the magnetic output transformer to the Main Amplifier board and see if the fuse blow.
The correct way should be use a variable transformer to slowly increase the AC Power voltage from 40VAC slowly and check the Magnetic Transfomer primary input waveform to check that the Triac is still working (should NOT be a pure sinewave as there comes a voltage where the waveform will starts switching). Remember to turn the amplifier volume all the way down

The Fuse of the The step down transformer is 10A, and the Fuse iside the Yamaha A-6 or A-7 will be 15A. So ensure the Step down transformer fuse rating is smaller than Amplifier fuse so it pop there.

Mine also trip the house RCCB and the fuse literally blew up onside (instead of a nice break). Amplifier volume is all the way down. The culprit was the very large 10,000uF 100VC Power Supply Filter capacitor [One of it budge and cap blew off) .

I replace all 4 pcs of 10,000uF ECAP with 100V rating it played for a few hours and then fuse start blew again intermittent (either blew instantly when power on or play a few hours before the trademark sizzling sound prior to fuse blew) The sizzling sound is like frying fish and is different from transformer sound. It is always preceded by sizzling sound (different from transformer Hum Buzzling sound, that occurs occasionally when the amplifier is turn on and the Carver Magnetic field circuit just started charging the board main Power filter capacitor)

. When i checked at the time while amplifier was able to power up (base on luck, it was 70% chance), i had checked the +b and-B power supply is 50VDC, so the Carver Magnetic circuit on the C Board must be working. but that 2 smaller 100uF 50V ECAP was really very hot so this is a sympton it is failing But the cap did not budge or show any oil or any blacken, it simply looks as good as new. Desolder both and check with component checker and the value and ESR and DC loss looks respectable. So it was an unusual failure mode. But the Temperature is DEFINITELY not OK. I replace these 2 ECAP and the new ECAP is only hardly noticeable warm. My experience if amp plays awhile and then blew the fuse usually it is ECAP issue. so the good old way of feeling with hand (but be careful as AC 100V is present on the board).

I checked the DC Voltage across this capacitor it was 33VDC so it was quite close to the rated 50VDC rating (i saw it was previously replaced so someone had worked on it). Also with the volume turned all the way down, these 2 capacitor SHOULD NOT be hot. So i replace it to 100uF 100V which runs only very slightly warm (hardly noticeable) > Just for the fun i checked these 2 capacitors with LCR Meter the reading is respectably good (but the DC Bias voltage is only 3V). HOpe it helps you.

 

[SingHifi]

My problem is the same as you both of us are in 240VAC country. I had gotten a 240VAC to 100VAC step down transformer. The cavar transformer will hum very loud at 50% volume and i dont know what is causing this to vibrate at 50Hz