Yamaha P2150, Output DC level near max V+, no shorted xstrs or open resistors

[nevetslab]

Having a Yamaha P2150 Power Amp on the bench, whose left channel had it's output latched up near full V+ voltage, but not drawing current, I pulled the module after disconnecting the Hi Current supply to the output stage, the front end bpiolar HV & LV connector and the support connections, and had a look at it. I'd previously serviced the same amp last year, where the servo-loop IC had failed, driving the output north.

After going thru all of the semiconductors on the module, I didn't find any semi's that were bad, nor any open resistors or connections that would explain the level shift. I moved an HP 6227B Dual Tracking supply to the bench to supplement a Tek PS503A dual tracking supply to run the servo-loop IC & level shift xstr Q208. The front end can run by itself without the power amp current gain stages energized, like a high voltage op amp circuit, so that's convenient.

After connecting the two sets of power supplies to the circuits, getting the grounds common, I found the upper zener diode D205, which provides bias to the base of Q210 would suddenly drop to 0.7V across it while slowly advancing the main bipolar supplies (which would occur at around +/-6VDC). I was finding both Q207 and Q209 going into hard saturation, explaining that 0.7V across the zener D205 to be the Vbe of Q210. Advancing the supplies further, the output also went north when Q207/Q209 saturated. I had no current flowing thru Q211 current mirror nor thru Q212/Q213. I could get variable Vce voltage across Q208. Nothing pulling high current. I ended up pulling Q208 level shift xstr, Q211 current mirror, and both Q210 and Q213 voltage gain stage xstrs, just in case, and replaced them.

That restored full operation to the circuit. I still didn't know who was the culprit, whether it was a leaky xstr or something else. In checking the current flow thru the current mirror stage and following voltage gain stage, now I was seeing equal currents flowing all the way thru each string, where before, there was no current flow thru the negative side of those stages.

This morning, after firing up my Hameg curve tracer, I found Q208 had nearly no hfe, while the other three parts looked normal. All measured fine using the DMM's Diode Test function.

Usually when you find that sort of level shift, there's high current draw associated with the problem, pointing at an output stage failure. This one took a bit more digging to find it.



All the test leads have EZ hook connectors on them, which make it relatively easy to clip onto the circuits. I had removed the four power xstrs, and initially had replaced the zener diode D205, as well as lifted the current limiter coupling diodes D212 & D213, just to be sure.

Once all was back up and running, I re-set the bias on both channels after it had been running at idle for a while.

P2150 Schematic.pdf

[Enzo]

If you have hard DC rail on the output, either the outputs are shorted or it is driven there. Shorted outputs will draw current load or not, but typically, with no load amps can sit there with DC on the outs.

In my mind DC driven output means the power stage is probably good back as far as the bias string.

Plus I found more often than not, the TO220 transistors on the individual U-shape heat sinks tended to have cracked solder.

[nevetslab]

If you have hard DC rail on the output, either the outputs are shorted or it is driven there. Shorted outputs will draw current load or not, but typically, with no load amps can sit there with DC on the outs.

In my mind DC driven output means the power stage is probably good back as far as the bias string.

Plus I found more often than not, the TO220 transistors on the individual U-shape heat sinks tended to have cracked solder.

The output protection relay circuit (schematic on a different page) opened, so there wasn't DC on the output. Those TO126 parts that were on the individual U-shaped heat sinks were the pair I had replaced in the front end. The Output stage survived just fine, perhaps due to the protection circuit opening the relay.

When I had first powered up the amp, there wasn't any DC offset, but after maybe a minute, it went DC. I didn't actually look to see if there was any difference between the output rails and the DC level that was present on the output buss........but it went there none the less.