Yamaha P4500 - Page 2

toprepairman · 5th December 2012, 06:31 PM

Hi Tarzan.
Yes agreed the caps are 160V, but given that manufacture provides for 100 or 160V then that's what they need to be.
I've taken a further look at the unit and have since decided that 140V is probably correct, the 24V olt rails are about right.
The problem is the PNP output devices keep failing and I suspect it's due to exceeding the Vce max of the devices. The ramp up waveform from the EEEngine looks correct when compared to the other channel.
If run at reduced mains Volts then all is well. if the supply is taken to full mains then all still Ok. But if the input signal is taken up to max with the output swinging rail to rail then another PNP device shorts out (due as far as I can see to excess Vce).
I have 56R resistors in the rails to limit current so don't get catastrophic failure and running into a 100R load.
Any ideas?

toprepairman · 5th December 2012, 06:49 PM

Looking at the specs, it only claims 460 Watts into 8 Ohms. I'm assuming that's RMS, though it doesn't actually say so.
Doing the maths the peak output volts is 86 volts. lets say we lose 10 Volts across the devices (it should be less then that) so thats 96 volt rails.
The transformer is very big so regulation should be in the 5% regeion but even if we allow 10%, then our unloaded rail doesn't need to be above 110 Volts. Ok be generous and a high mains voltage and we're looking at a max of 120 Volts. Hence my suspition of the measured 140 Volts.

Tarzan · 6th December 2012, 06:16 PM

are you in need of the service manual?
There is not much in it but it can help with the biasing.
If the pnp's are failing all the time; then there must be another problem with the drivers probably?
I doubt very much that Yamaha will deliver an amp with a problematic final stage.
And the other channel is working fine?
Must be someting else...

toprepairman · 7th December 2012, 07:44 AM

A service manual would help possibly, I only have a copy of the circuit.
Agreed these amps have a very good reputaion for reliability and there's no way Yamaha could have got away with selling so many of them if there was any question there.
No there's definitly a fault but it's hiding itself well.
The other channel is fine, and the owner has had this unit from new.
It just died on one channel and he continued to use the other channel with no problems.

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5th December 2012, 06:31 PM	#11
toprepairman diyAudio Member Join Date: Nov 2010	Hi Tarzan. Yes agreed the caps are 160V, but given that manufacture provides for 100 or 160V then that's what they need to be. I've taken a further look at the unit and have since decided that 140V is probably correct, the 24V olt rails are about right. The problem is the PNP output devices keep failing and I suspect it's due to exceeding the Vce max of the devices. The ramp up waveform from the EEEngine looks correct when compared to the other channel. If run at reduced mains Volts then all is well. if the supply is taken to full mains then all still Ok. But if the input signal is taken up to max with the output swinging rail to rail then another PNP device shorts out (due as far as I can see to excess Vce). I have 56R resistors in the rails to limit current so don't get catastrophic failure and running into a 100R load. Any ideas?

5th December 2012, 06:49 PM	#12
toprepairman diyAudio Member Join Date: Nov 2010	Looking at the specs, it only claims 460 Watts into 8 Ohms. I'm assuming that's RMS, though it doesn't actually say so. Doing the maths the peak output volts is 86 volts. lets say we lose 10 Volts across the devices (it should be less then that) so thats 96 volt rails. The transformer is very big so regulation should be in the 5% regeion but even if we allow 10%, then our unloaded rail doesn't need to be above 110 Volts. Ok be generous and a high mains voltage and we're looking at a max of 120 Volts. Hence my suspition of the measured 140 Volts.

6th December 2012, 06:16 PM	#13
Tarzan diyAudio Member Join Date: May 2004 Location: Genk	are you in need of the service manual? There is not much in it but it can help with the biasing. If the pnp's are failing all the time; then there must be another problem with the drivers probably? I doubt very much that Yamaha will deliver an amp with a problematic final stage. And the other channel is working fine? Must be someting else...

7th December 2012, 07:44 AM	#14
toprepairman diyAudio Member Join Date: Nov 2010	A service manual would help possibly, I only have a copy of the circuit. Agreed these amps have a very good reputaion for reliability and there's no way Yamaha could have got away with selling so many of them if there was any question there. No there's definitly a fault but it's hiding itself well. The other channel is fine, and the owner has had this unit from new. It just died on one channel and he continued to use the other channel with no problems.