Yamaha P2200 with large amount of DC on outputs

[Ewan Penkey]

Hi all,I'm hoping someone can help me diagnose a fault with a friends P2200 I'm trying to fix for him.

So the short back-story is that he's had the amp for years, one day it stopped working, he can't remember if there was a bang or smoke, and it's been in the loft for a few years now. He wants to resurrect it, and I foolishly agreed to do the work. I'm more of a valve guy, but thought I could work out enough to be able to fix his amp - seems I was wrong!

So there was DC on both speaker outs when I first plugged it in (via a current limiter). I presumed that some of the output transistors had shorted, so I pulled them all out, and more than half were dud, so I've bought all new transistors. I replaced all the 0.47 emitter resistors as some of them were open. I put a pair of the new transistors in each channel to test it for DC, but was disappointed to find the DC still there. It's not full B+, but a bit lower. On the current limiter, the B+ is about 71V and I'm getting around 65V on the outputs.

BTW, the power supply seems fine - a steady +and - 79V with no ripple to speak of, when plugged straight into the wall.

I then pulled the amp modules out and tested the drivers and the transistors which form the bias circuit. Most of these were faulty too, so I've replaced the lot. I'm still getting 65V on the speaker outputs.

I've been through the preamp transistors and checked them all - they're fine. I've checked every capacitor which could possibly leak DC into the wrong place, and every diode, (including replacing the bias reference zeners), and all the wirewound resistors, and other resistors feeding the bias circuit.

It's hard (seemingly impossible so far) to trace where this stray DC is coming from, as nearly every component leg you can measure, measures between 65 and 71 volts. If I pull out the positive driver TR111, the voltage on the output drops to about +40V - that's about the only clue I have.

I'm really hoping someone on this forum has worked on one of these amps, or a similar one, and can point me in the right direction, I know I could be in for a long haul, but also feel I've put a lot into this amp already, and success is just out of reach.

Thanks in advance - I know it's quite a read for my first post!

Ewan


I know I'm probably

 

[ivandezande]

Gotta just keep testing. Resistors too, any of them, sounds like this thing really blew up so anything is suspect even if physically it looks fine.
Wish there was a better or easier answer.

 

[Skywatcher]

Hi all,I'm hoping someone can help me diagnose a fault with a friends P2200 I'm trying to fix for him.

So the short back-story is that he's had the amp for years, one day it stopped working, he can't remember if there was a bang or smoke, and it's been in the loft for a few years now. He wants to resurrect it, and I foolishly agreed to do the work. I'm more of a valve guy, but thought I could work out enough to be able to fix his amp - seems I was wrong!

So there was DC on both speaker outs when I first plugged it in (via a current limiter). I presumed that some of the output transistors had shorted, so I pulled them all out, and more than half were dud, so I've bought all new transistors. I replaced all the 0.47 emitter resistors as some of them were open. I put a pair of the new transistors in each channel to test it for DC, but was disappointed to find the DC still there. It's not full B+, but a bit lower. On the current limiter, the B+ is about 71V and I'm getting around 65V on the outputs.

BTW, the power supply seems fine - a steady +and - 79V with no ripple to speak of, when plugged straight into the wall.

I then pulled the amp modules out and tested the drivers and the transistors which form the bias circuit. Most of these were faulty too, so I've replaced the lot. I'm still getting 65V on the speaker outputs.

I've been through the preamp transistors and checked them all - they're fine. I've checked every capacitor which could possibly leak DC into the wrong place, and every diode, (including replacing the bias reference zeners), and all the wirewound resistors, and other resistors feeding the bias circuit.

It's hard (seemingly impossible so far) to trace where this stray DC is coming from, as nearly every component leg you can measure, measures between 65 and 71 volts. If I pull out the positive driver TR111, the voltage on the output drops to about +40V - that's about the only clue I have.

I'm really hoping someone on this forum has worked on one of these amps, or a similar one, and can point me in the right direction, I know I could be in for a long haul, but also feel I've put a lot into this amp already, and success is just out of reach.

Thanks in advance - I know it's quite a read for my first post!

Ewan


I know I'm probably

Post this in the Yamaha forum and you'll get more answers.

 

[Ewan Penkey]

Post this in the Yamaha forum and you'll get more answers.

Thanks, I didn't realise there was a Yamaha forum - I've reposted it as you suggest.

Ewan

 

[Fabrifabri]

Se stai testando l'amplificatore all'esterno del suo chassis prova a fare uno ponticello con un cavetto tra il polo comune dell'alimentazione (E) e la massa (E) dell'ingresso di massa del segnale audio. vedrai che il problema quello Scusami per la risosta in ritardo di 4 anni!!!