Help with Yamaha p7000s low ouput no distortion *SOLVED!*

[jwhmca]

This is driving my NUTS!!!!!
I happen to have 2 of these amps so I'm able to A-B them to try an lock down what is going on.

On the schematic you will find C211. I have set up a signal Gen to make 1vpp at this point. It is the same on both the good amp and bad. Eliminating all the input and volume controls.

At point "+" top right corner of the schematic... where I have written 5vpp etc...

The 5vpp is the bad amp the 30vpp is the good amp. Obviously there is at least a symptom of why it's so much lower. I can't seem to make heads or tails of the circuit in-between C211 and this + point. Th dumb thing is it is happening on both channels!!! L+R! I'm pretty sure the amp got damaged so how as there was a speaker blown as well. But it isn't distorted, just really low.

I will paypal the Guinness to those who help me solve this! and can email the full service manual or upload more pictures.

 

[PRR]

> I can't seem to make heads or tails of the circuit in-between

Then get Doug Self's power amp book.

It's a very common topology, fancied-up in a Japanese way. Q210 Q213 are a differential pair. Q209 Q214 are a current-mirror to combine the two sides. Q216 is what Doug calls "Vas", the main voltage amplifier. Q215 is an added cascode so Q216 does not feel full voltage (DC or swing). Q217 is the load for the lower transistors. Q218 sets a few-diode-drop difference between Q215 and Q217 collectors. Jensen 990 works very much the same, tho simpler, and NPN input.

The voltages observed at your test point *should* depend on what is to the right. Off-screen. Typically this will be a unity gain power buffer, a giant emitter follower. If it is working, the output swing will be 44 times (R230+R229/R224) your input, or 44Vpp. If the output stage is blown open, any input signal will slam those points fully to rails. Therefore we know the output stage is *short* to the load.

You got blown-short output transistors. BOTH channels. If the output stage (not shown) is as fancy as the part you show, it's a brain-buster. I used to get paid for such pain, in days when I was worth less than a transistor. But the value of transistors has declined. These days you should consider just replacing ALL the output devices and examining all resistors and caps for burn, cracked joints, solder-splash.

 

[jwhmca]

Thank you so much PRR!

Here is the stuff "to the right" the power amp section.

That seems so strange that a output could be shorted and the unit not blow a fuse or something.

Why do you say it's a "brain buster" to find a shorted output? It seems like it will be easy (which I'm doing right now) to go around and test the outputs for shorts? Could they be leaky or something? Something not obvious?

 

[JohnRoberts]

+1 to what PRR said... pretty common for output devices to fail as short circuit.

Besides measuring AC volts, look at DC volts. That point you are looking at is at pre-driver to output stage so should be close to 0V DC  with the top side a couple volts higher, the low side of driver a couple volts lower.

If output is pegged hi or low that will suggest which devices are suspect. With power off look for short circuit collector to emitter of power devices.

JR

[edit if your VOM has a diode scale that will help troubleshooting devices. There should be a diode drop from base to emitter and from base to collector. Direction of diode reverses between NPN and PNP devices. 

Before they had diode scales on VOM we would check for junctions with low ohms scale, but modern meters are too clever to trust doing that with.
/edit

 

[jwhmca]

Could I desolder the collector(or something) of each output one at a time until the "voltage went back up" or would that damage something...?

Edit** That "+" point is at 2.5VDC under no load no signal

 

[jwhmca]

I have tested all the output devices. No shorts. Are you guys sure that that test point will reflect what the power stage is doing? As I look at the schematic I can't see the path back or how that would be affected by the power stage.

 

[JohnRoberts]

I have tested all the output devices. No shorts. Are you guys sure that that test point will reflect what the power stage is doing? As I look at the schematic I can't see the path back or how that would be affected by the power stage.

It was a logical guess...

Don't be scared off by the number of components, it is a fairly simple design to trouble shoot.

We know that when it is operating correctly the output DC voltage should be close to 0V. Also if you have a good working channel to compare it to you can see approximately what it should be.

With no signal input, working backwards from the output you should measure roughly symmetrical voltages above and below 0V at the bases of the output devices, and then up another voltage step to the drivers.  If the output devices are all good and the stage is working properly both sets of output devices will be turned slightly on. That is the class A part of class AB topology.

Of course the problem can be anywhere in the entire signal chain, and the overall negative feedback can cause a problem early to show up late, and vice versa.

I would troubleshoot with no audio signal in, and just probe around for DC voltages. If the output stage is not well centered around 0V, figure out why... Comparing the good channel to the bad for DC voltages could reveal where the circuit goes off the reservation.

Of course be careful when probing around that you don't accidentally short something out and cause a new fault.

JR

JR

 

[jwhmca]

*Solved*

C213 100@16v

High ESR. I found it by finding the the collector voltage was slowly rising when the amp was powered on (thermal)

Took the "freeze spray" around and nailed it.

 

[JohnRoberts]

Good job... subtle fail... mostly just low AC gain...

JR

 

[bruce0]

+1 ... nice job