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Yamaha A1 restoration 2021

Continuity / resistance with resistance of leads deducted.
CE to +E 0.1 ohm
AE to -E 0.1 ohm
As suggested,
- "try to set bias (7.4mV) when connected to CE & LO."
- then " Check/reflow solder connections on jumper J123
(red), CE and AE wires and also at left channel TP's."
At this point see if adjustment has become possible via TP's in the way prescribed by the SM?
 
- "try to set bias (7.4mV) when connected to CE & LO."
Stays at 0 mV and won't adjust via Bourns VR.
Will discharge smoothing caps and then
" Check/reflow solder connections on jumper J123
(red), CE and AE wires and also at left channel TP's."
 
O dear.
Had just done the reflow of solder.
Tried to adjust bias at TP1 and TP3. Still stayed at a reading of 0.00mV and wouldn't change with VR adjustment. However, turning the volume control would register an increase.
Was trying to do adjustment via CE and LO and the probe probably also touched CC, the amp was off whilst I was trying to make the connection but the big caps hadn't been discharged. There was a spark..
Tried powering on the amp but the power switch light came on and then went off. So I've powered it off at the amp switch and the mains for now.
What to do next?
 
Discharge main caps (100-1k resistor) then check all fuses. Shorting CE and CC should mean the surge current didn't flow through the output transistor and simply blew a fuse.
 
Hifibythekg said:
So, today's initial measurements are..
CE -13.5mV
AE -13.4mV
+E -13.3mV
-E -13.4mV
these are all moving a bit, measurements taken about 5 mins after power on.
Click to expand...
The numbers keep on changing, makes it hard to nail down. Above indicates near zero bias.

Hifibythekg said:
taken about 5 mins after power on
Click to expand...
AE,CE are equal, at 3-4 seconds after power up and a few minutes later was AE and CE unequal, looking for 3-15mV
difference, not interested in 0.xmV. Expect there may be a surge/flash at power on, give it a few seconds to settle...
 
Changing the main fuse, which is wahta blew. The solo one at the back of the unit. Other fuses are ok.
SM says 7 amps, T but mine had a T2.5 fitted
 
After you get the fuses sorted out, need to go back and get left channel voltages LO, +/-I... however need the case where CE, AE, +/-E are all aout the same giving 0mV bias.
 
Thanks Athanasios. Actually my choice 3.15A or 4.0A (with T or L prefixes) but I take your point.
Will probably go for the 4 amp for now, especially with all these typos in the SM.
 
Thanks again MBZ. I'll try to do all your suggestions tomorrow.
"need the case where CE, AE, +/-E are all aout the same giving 0mV bias"
If this is achieved, ("about the same giving 0mV bias) what then?
Just adjust the Bourns VR via CE and LO until 7.4mV is achieved?
And is that fine, see it as idling current having been satisfactorily adjusted?
Just accept that, for whatever reason, this amp won't let one adjust idling current at TP1 and TP3 on the left channel?
Not complaining, just wondering..
 
Hifibythekg said:
"need the case where CE, AE, +/-E are all aout the same giving 0mV bias"
If this is achieved, ("about the same giving 0mV bias) what then?
Click to expand...
This is the fault situation, I can then look at the other voltages +/-I, +/-O,,, and try and workout what's going on.

Hifibythekg said:
Just adjust the Bourns VR via CE and LO until 7.4mV is achieved?
And is that fine, see it as idling current having been satisfactorily adjusted?
Click to expand...
The previous set of voltages showed +/-E and TP's where not ok confirming 0mV bias. However CE, LO, AE voltages showed a 12mV bias, how could this be since CE and LO are connected to the TP's, I concluded there must be a bad solder connection or similiar to the TP's so I was wondering if the amp behaved normally if you attemped bias using the "other" TP's CE, LO. You can forget this line of investigation since they continuity checked ok. I won't ask you to explain why those voltage measurements differed by so much.

Hifibythekg said:
Just accept that, for whatever reason, this amp won't let one adjust idling current at TP1 and TP3 on the left channel?
Click to expand...
No, as above.
 
Thanks MBZ, much appreciated.
Changed the fuse to a T3.15 amp.
Connected black test lead to left speaker negative terminal.
Connected red test lead to -O and powered on.
I had meant to connect it to -E and commence taking the readings of +/-E, CE, AE.
My mistake and feel like this is turning into a comedy of errors.
Anyway, on power up there was a brief hum, fizz, and smoke from the left channel area of the amp.
Have switched off mains power and drained the big caps.
What should I do now? (Frankly I can imagine a series of witty one liners in response to my stupid error) but maybe there's a way back to a fully functioning amp..
 
Hifibythekg said:
Thanks MBZ, much appreciated.
Changed the fuse to a T3.15 amp.
Connected black test lead to left speaker negative terminal.
Connected red test lead to -O and powered on.
I had meant to connect it to -E and commence taking the readings of +/-E, CE, AE.
My mistake and feel like this is turning into a comedy of errors.
Anyway, on power up there was a brief hum, fizz, and smoke from the left channel area of the amp.
Have switched off mains power and drained the big caps.
What should I do now? (Frankly I can imagine a series of witty one liners in response to my stupid error) but maybe there's a way back to a fully functioning amp..
Click to expand...

I don't think that T3.15 amp fuse was the right one to choose, its a slow blow fuse. T is the prefix part number for slow blow I believe . I think you may have done some more damage by using it.

Athanasios
 
Hifibythekg said:
Or rather a ‘T’ suffix in this piece of text.
Click to expand...


Hmm, I usually never seen slow blow fuse for electronics. I may be wrong or just never paid close attention.

So many manufactures use different Prefix or suffix for time delay. Some use T some use D some use S . its a mystery lol

nashou
 
Thanks Nashou. Yes, its a long learning curve. Or maybe I especially feel that way as I've made some really silly mistakes recently.
I'd presumed that T was for Time and was slow blow, S was for Speed and wasn't slow blow.. but truthfully don't really know.
 
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