Shutoff of a Yamaha RX-V1500 receiver

[flambart]

Hi, I'm trying to repair a RX-V1500 Yamaha. When I put it on, after 3 seconds the protection relay shut it off. The previous owner told me that power transistors have been already replaced, with no results at all.

It seems that the fault is extremely common for this model, but I did not found any detailed solution. I've understood that the problem could be inside the voltage regulator section, but I'm not sure about

Anyone has an idea about? Any schematics/service manual available ?

As usual, thanks so much to everyone, Paolo

[flambart]

Any idea ?

Thanx, Paolo

[Shaun]

You can probably find an online schematic of RX-V1400, which is similar.
Your problem is likely counterfeit transistors. They don't last very long, so chances are you are still sitting with a defective output stage.

My RX-V1400 had this problem. A local repair shop could not fix it; they put in counterfeits. I had to bear the risk, but was finally able to find genuine Sankens.

[flambart]

Thank you very much, Shaun

In the meantime, I've progressed a little bit. Now I'm able to start a kind of diagnostic menu, and I've got the following message on the display:

DC PRT : 0000 E

I suppose it can be an useful info, but which is the meaning?

thanks in advance to everyone, Paolo

[flambart]

Some further steps: finally I've got the service manual, so now I know what the message means.

Herebelow, the explanation of the message "DC PRT : 000 E" :

"Cause: DC output of the power amplifier is abnormal.
Supplementary information: The abnormal voltage is
displayed in % based on 5V as 100%.
Turning on the power without correcting the abnormality
will cause the protection function to work 3 seconds later
and the power supply will be shut off."

Through the other diagnostic facilities, I've been able to test manually all the output channel, by a pink noise. Every channel works perfectly, so I think that there's something wrong about the DC voltage detection.

Where have I to look for ?

Thanks, Paolo

P.S.: ask if you need the manual

[Shaun]

Paolo, yes, thanks! I would love a copy of the manual. I'll PM you my email address.

As for DC offset: were you able to measure it yourself for each channel?

[flambart]

Finally, with the vital help of the service manual, I've found the problem. Between the ground and the "PRD" pin of the CPU, there are three diodes: D503, D504 and D505. The first two seem to be useful to protect the CPU itself against any over-current. In case of short circuit or similar, these 2 diodes can easily blow out, and force to "0" volt the pin 139 of the CPU ("PRD" pin, dedicated to detect any abnormal DC voltage from output transistors)
This is the reason why the protection circuit shuts-off the unit, even if the faulty power transistors have been replaced

Diodes are normal surface-mounted 1SS355: I've replaced them with two 1N4146, and everything works perfectly now.
Diodes are located in the "Function" board, at the right side of the unit: they are easily accessible

I've realized that the problem seems to be extremely common for this model, so I hope this hint will be useful in several cases

Thanks to everyone for any kind reply I've received

Ciao, Paolo

[rayneaven]

Paolo, I am having similar problem with my unit shutting off. However, I am not a techniciam as you seem to be but am willing to try and make the repair myself since nearest service center is over 45 miles away and it sounds like they may not repair it perfectly anyway. Is this an easy enough repair for a novice?

[nitrion]

flambart: Good job troubleshooting the problem. Likely the problem when all channels are at fault.

I had a similiar problem, however it was only the Surround R channel. I believe my issue is with a leaking 47uF (C43) cap in the pre-amp section. It was reading 100 ohms where the other channels were somewhere > 10k. The output was -5V biased.

Unfortunetly I can't directly test my theory since I accidently slipped with the DMM leads and shorted the base and collector of the npn transistor before the power transistors. Opps! I smoked 5 transistors which are on order now.

Anyways, future advice for anyone is that the bias issue can be on the pre-amp board as well. Sorry for bringing up an old post, but might help users googling this post.