Yamaha A760 amplifier
Mike North
I am trying to repair a yamaha a760 amp. This unit includes a strange
circuit with what appears to be a couple of SCR's controlling the mains
supply 240V to the power transformer, apart from a burned out input filter
capacitor (replaced) I can't find anything wrong with it and am considering
bypassing the main SCR to feed power to the primary of the transformer (but
of course I am concerned I may blow the thing !) Does anyone have any
experience of these 80's type yamaha amplifiers primary power circuitry and
what it is for ?
Many thanks
Mike
Loren Coe
maybe it is a circuit to limit input voltage(?), maybe to let a 120vac
device run on 240? you could control the on-time, just like a dimmer
for lamps.
anyhoo, i wouldn't bypass it. --Loren
Mike North
Thank you for your reply. I think you could be right, the amp is a 240V
only unit, Yamaha engineers in their wisdom may have been arm twisted into
saving money by using a common transformer and sticking this circuit in for
the 240V version (?)
I have been looking further and tracing out the circuit, it has two SCR
devices (numbered SCR501 "MOR56" (a TO92 package) and SCR502 "NEC AC16D" (a
TO220 package)) which look like driver and power SCRs (pinouts make sense)
and there is also an interesting 3 legged Yamaha custom hybrid package
"IG-04080" which appears to drive the smaller SCR and also the AC inputs of
a small bridge rectifier which appears to send a DC signal to the main power
amp board (which is adjustable with a preset pot) ... it almost looks like
it might be some sort of cleaver sensing device, either for the input
voltage or even somehow of the power amp state (the output transistors are
not shorted and test fine) ?
I've not tried giving this thing an input and connecting speakers (because
the power lamp does not even come on), maybe I'll give that a go or try
applying 110V to the primary transformer input. Another option is to power
the main amp from another transformer to check the power amp is ok.
Any further thoughts would be ost welcome.
Regards
Mike
Loren Coe <lo...@netnews.attbi.com> wrote in message
news:YWNha.269401$3D1.148015@sccrnsc01...
Loren Coe
> Hi Loren,
> Thank you for your reply. I think you could be right, the amp is a 240V
> only unit, Yamaha engineers in their wisdom may have been arm twisted into
> saving money by using a common transformer and sticking this circuit in for
> the 240V version (?)
the arm-twisting has to do with economics. the most keyboards probably
run on 120, and the xfmr is an expensive item to spec/stock so they went
for as much commonality as practical. used to be, when tv's had xfmrs,
the xfmr was the 2nd most expensive part, behind the picture tube.
>
> I have been looking further and tracing out the circuit, it has two SCR
> devices (numbered SCR501 "MOR56" (a TO92 package) and SCR502 "NEC AC16D" (a
> TO220 package)) which look like driver and power SCRs (pinouts make sense)
> and there is also an interesting 3 legged Yamaha custom hybrid package
> "IG-04080" which appears to drive the smaller SCR and also the AC inputs of
> a small bridge rectifier which appears to send a DC signal to the main power
> amp board (which is adjustable with a preset pot) ... it almost looks like
> it might be some sort of cleaver sensing device, either for the input
> voltage or even somehow of the power amp state (the output transistors are
> not shorted and test fine) ?
this sounds like a bias circuit for the main scr's and the pot would allow
some range of compensation for hi or low mains voltages. hell, maybe it
even can allow 120v operation(?). that would indeed be taking all economic
advantage. doubtful, tho, you would normally just select a fixed resistor.
> .... > Any further thoughts would be ost welcome. > Regards > Mike
good luck, --Loren
Stephen Sank
switchmode power supply, much like Carver's "magnetic field" nonsense. It
runs the transformer at a high frequency, so it runs on a 50 percent duty
cycle, thereby getting more power out of a smaller transformer. Carver
always did this to be a cheap bastard, adding much to the flammability of
his already "quality challenged" amp circuits. Yamaha tried it for only
this one model year(1981-ish?), then thought better of it. The only way to
fix the amp, in practical terms, is to replace the transformer with a normal
one of appropriate voltages & power.
--
Stephen Sank, Owner & Ribbon Mic Restorer
Talking Dog Transducer Company
http://stephensank.com
5517 Carmelita Drive N.E.
Albuquerque, New Mexico [87111]
505-332-0336
Auth. Nakamichi & McIntosh servicer
Payments preferred through Paypal.com
"Mike North" <morp...@bigpond.net.au> wrote in message
news:TTSha.3887$ft3....@news-server.bigpond.net.au...
N. Thornton
> No, the circuit on the transformer primary side is actually a sort of
> switchmode power supply, much like Carver's "magnetic field" nonsense. It
> runs the transformer at a high frequency, so it runs on a 50 percent duty
> cycle, thereby getting more power out of a smaller transformer. Carver
> always did this to be a cheap bastard, adding much to the flammability of
> his already "quality challenged" amp circuits. Yamaha tried it for only
> this one model year(1981-ish?), then thought better of it. The only way to
> fix the amp, in practical terms, is to replace the transformer with a normal
> one of appropriate voltages & power.
>
> --
> Stephen Sank, Owner & Ribbon Mic Restorer
Hi
So what exactly are the problems with this arrangement? Why did it
turn out to be not so hot? It sounds like a good idea at first sight.
Regards, NT
Stephen Sank
hashy, grainy & even vicious treble.
--
Stephen Sank, Owner & Ribbon Mic Restorer
Talking Dog Transducer Company
http://stephensank.com
5517 Carmelita Drive N.E.
Albuquerque, New Mexico [87111]
505-332-0336
Auth. Nakamichi & McIntosh servicer
Payments preferred through Paypal.com
"N. Thornton" <big...@meeow.co.uk> wrote in message
news:a7076635.03040...@posting.google.com...
Leonard G. Caillouet
completely filtered dc, how could it cause problems in the treble? No doubt
there have been some pretty lousy implementations of switching supplies in
amps, but I don't understand how your statement makes sense. The A760 was
not the best sounding amp that Yamaha ever made but it did not sound "hashy,
grainy & even vicious" if I recall correctly. It was mostly mushy and
lacked great detail compared to more high end products.
BTW, I never saw the initial problem with the amp in this thread, but wasn't
this one of those Yamaha designs that would not work if the front panel
lamps open. Why did they do that? Anyone recall?
Leonard G. Caillouet
Service Manager
Electronics World
Gainesville, FL
352-332-5608
352-332-5668 (fax)
"Stephen Sank" <bk...@thuntek.net> wrote in message
news:b6irsp$2njd$1...@reader.nmix.net...
Mark D. Zacharias
while others used the lamps to help with the sharp cut-off of the speaker
relay at power-off, IIRC.
Mark Z.
"Leonard G. Caillouet" <lcail...@devoynet.com> wrote in message
news:0skja.52522$fO.3...@fe03.atl2.webusenet.com...
Mike North
Many thanks for the interesting discussion on this, especially Stephen, I
have not heard of this before. Leonard did not see my original question (so
I have summarised latest position below):
- amp dead on power up (fuse intact), no lamps light - transformer output
measures 5V AC, and after a few seconds the relay protection relay clicks on
!
- a mains filter capacitor directly across the Live and Neutral input had
burnt out (? it is close to the heatsink on the power SCR, so maybe it
overheated and then shorted - enough for smoke but not enough to take out
the 6.3A fuse) - replaced
- mains power clearly getting to an unusual circuit consisting of a driver
SCR, a higher powered SCR on large heatsink, a few more caps, a large
inductor, a custom yamaha controller IC (3 pins) IG-04080) and a small
bridge rectifier which I assume sends the output DC signal to the power amp
(some sort of feedback or power on indicator - again only a few volts here).
The SCR circuit feeds power to the primary of the transformer (actually to
two of the three primary windings).
- I have checked all the lamps, they are not broken and work fine under
separate power.
- Checked the resistors in the odd circuit, they all seem fine, unable to
check the capacitors as yet
- Resistance of transformer primary seems normal for this size of amp -
about 2 ohms (at DC)
I am perplexed on how to fix, it looks as if nothing is actually wrong, but
it's just missing a signal or something to kick start the circuit (maybe I
could replace all the resistors/capacitors in the unusual circuit ?) - I
would prefer not to replace the transformer at this stage, it hasn't totally
got me yet ...
Regards
Mike
Mark D. Zacharias <mzach...@yis.us> wrote in message
news:b6mf54$6rajl$1...@ID-180484.news.dfncis.de...