QX-949A Stays in Protect

[JJCalvillo]

I refurbed this 949A, going through the Amp, Tone, Power, SQ/RM/CD-4 boards. It plays great no problems, until a couple of weeks ago it stayed in protect for about 10 seconds. Did that one other time but would come out and play normally, and usually came out of protect in 3 or 4 seconds when I powered it up.

Today it didn't pop outta protect. Dial lights, meter moves, stereo light comes on, but the amp doesn't want to kick in. I pulled the speaker connections and turned it on and that didn't make any difference.

Just about to put it on the bench, but I was wondering if anyone has run into this.

 

[JJCalvillo]

Guess what happened when I got it on the bench... it came outta protect and played normally once I got my bench speakers hooked up.
Looked normal on the DBT, so I plugged it in to the wall and it clicked out of protect.

Gonna leave it on the bench for a few hours to see if cold starting has something to do with it.

 

[JJCalvillo]

So, after a week of taking about three seconds to come out of protect, today it took a couple of minutes. Then plays fine, and comes right out of protect once it's been playing.

I left the skylight and window upstairs open last night so it probably got pretty close to 53º. It seems like it stays in protect when it's been cool or hasn't been powered on in a few days.

I know transistors will get warm and fail but what, if anything, would cause a problem starting cold?

Could a failing transistor delay turning on, or producing a proper voltage to turn on another transistor? One board I didn't mess with on this 949A was the Protection Board.

 

[EastPoint]

You’re having protection issues, I would rebuild the protection board.

 

[mbz]

Don't know the amp, thinking maybe a bad solder joint?
Hi-res pix of solder side of power amp and protect circuit?

Suggest measure the dc offset either
- between emitter resistors
or
- at input to speaker relay
or
- at bias test points (close enough to amp out)

Measure 5-8 seconds after power on when the amp doesn't come out of protect.
Don't measure offset at spkr connectors since relay is open.

 

[JJCalvillo]

Don't know the amp, thinking maybe a bad solder joint?
Hi-res pix of solder side of power amp and protect circuit?

Suggest measure the dc offset either
- between emitter resistors
or
- at input to speaker relay
or
- at bias test points (close enough to amp out)

Measure 5-8 seconds after power on when the amp doesn't come out of protect.
Don't measure offset at spkr connectors since relay is open.

Gonna go through the protection board first. It's the only board I didn't refurb when I went through this.
If the fault isn't on the protection board, at least I won't have to worry about it becoming a problem later.

 

[JJCalvillo]

So far it's working as it should, but going to test it for awhile before I put it back in place. Shoulda done the Protection board when I did the rest of it.
Lotsa stuff to replace on such a small board.

 

[JJCalvillo]

Put the 949A back into play and it worked fine, until I didn't play it for a couple of days. Yesterday it took a couple of minutes to come out of protect, then played fine for hours. It's always after a layoff of a day or more that it is slow coming out of protect.

Yesterday after it played for awhile, I'd turn it off and then turn it back on and it was 3 seconds until the relay clicked. Did the restart two or three times during the day, between sides or albums and never any delay of the relay. Today I turned it on and, again, 3 seconds until click.

I just replaced all the caps and transistors on the protection board a couple of weeks ago, but not the relay. Looks like that's the next step.

 

[mbz]

An amp will go into protect
- due to excessive Vdc at L/R amp output
- failure in the protect circuit
- power supply failure to protect or amp circuits
- should also include mechanical things like bad solder joints on wires/pins/...

So what do we know about your situation?

Yesterday it took a couple of minutes to come out of protect

Would have been great to have measured dc offset in this state, just to rule out(in?) some possibilities.
Likewise checking protection circuit voltages.

 

[JJCalvillo]

I’ve got a QRX -7001 on the bench and it will trade places with the 949A as soon as I get the 7001 running.

Been through the whole 949A, like every board, replacing caps and transistors as Imma keepin’ disone! Almost total refurb. Adjusted DC and bias when I went through it.

Seems like the big clue is it only delays after a 36 - 48 hour or more break. If I shut it down in the afternoon and turn it on the next day, it’s out of protect in 3 seconds. Once it starts playing there are no problems.

Why would leaving it off for a day or more make it slow to come out of protect? Seems like the only thing left would be the original relay.

 

[JJCalvillo]

Also, the refurb minus the Protection Board was done months ago. Did the protection board a few weeks ago when the delayed starts began. So I don't think it's something I did, more like a part slowly going south or some other reason voltages in the protection circuit are taking their time going to spec on startups after a day or more off.

 

[mbz]

Why would leaving it off for a day or more make it slow to come out of protect? Seems like the only thing left would be the original relay.

Yeah, might be the relay or maybe a dud solder joint or ...

voltages in the protection circuit are taking their time going to spec on startups after a day or more off.

Check DC voltages (in the protect state) on the protect board pins 13,14,15,16 one for each power amp stage. Alternatively and easier monitor voltage at Q6base where all 4 channels "merge". Anything more than 100mV is a concern but will probably need 300-400mV for protection to kick in.

Looks like C7(220uf/10V) is for the power on mute timing

Also record voltages on Q8b and Q9b in the extended protect state, ie wait 6-8 seconds after power on.

 

[eiraved]

Put the 949A back into play and it worked fine, until I didn't play it for a couple of days. Yesterday it took a couple of minutes to come out of protect, then played fine for hours. It's always after a layoff of a day or more that it is slow coming out of protect.

Yesterday after it played for awhile, I'd turn it off and then turn it back on and it was 3 seconds until the relay clicked. Did the restart two or three times during the day, between sides or albums and never any delay of the relay. Today I turned it on and, again, 3 seconds until click.

I just replaced all the caps and transistors on the protection board a couple of weeks ago, but not the relay. Looks like that's the next step.

This may be normal. Yes it's hard to believe, but the extended warm-up time after sitting unused for extended intervals is normal for certain amps from the same time period. Here what the the Sony owner's manual says about their "new" TA-3120 amp:

 

[JJCalvillo]

@eiraved Thing is the receiver didn't do this for months after I refurbed it, and never before I worked on it. Just suddenly started about two weeks ago. Also, when I read the notice from Sony, I get the impression "...has been in stock or has not been used for a long time..." is a longer period than a one or two days. It seems like they're saying the delay is one time, "...it takes several minutes to start operation after the Power Switch is set on for the first time."

That said, I wonder which is the corresponding cap in the 949A? Also, wondering about the filter caps.

 

[JJCalvillo]

@mbz thanks amigo, when I get the 7001 running and off the bench, I will do those measurements. Thanks for pointing out the cap responsible for mute timing!

 

[eiraved]

@eiraved Thing is the receiver didn't do this for months after I refurbed it, and never before I worked on it. Just suddenly started about two weeks ago. Also, when I read the notice from Sony, I get the impression "...has been in stock or has not been used for a long time..." is a longer period than a one or two days. It seems like they're saying the delay is one time, "...it takes several minutes to start operation after the Power Switch is set on for the first time."

I thought the same but the manual does mention, "...or not used for a long time". That being said, it may not be relevant to your issue, just thought I'd mention it.

 

[JJCalvillo]

Finally got this back on the bench, been busy w/"other peeps stuff".

Was thinking I believe the power switch might be a two part deal where the lights are on one arm and the amps are on a 2nd. If the arm to the amps or it's contact gets fried, it could look like the relay or protection is the problem?

We'll soon see if I'm full of it.

 

[JJCalvillo]

Yeah, might be the relay or maybe a dud solder joint or ...


Check DC voltages (in the protect state) on the protect board pins 13,14,15,16 one for each power amp stage. Alternatively and easier monitor voltage at Q6base where all 4 channels "merge". Anything more than 100mV is a concern but will probably need 300-400mV for protection to kick in.

Looks like C7(220uf/10V) is for the power on mute timing

Also record voltages on Q8b and Q9b in the extended protect state, ie wait 6-8 seconds after power on.

To recap, this board was redone with new transistors and caps quite awhile ago, months before the protection problem started. So C7 is nearly brand new.

Ain't the switch and everything is a wrong number on the protection board.

Pins #13-16 show the same weird cycle. Power on shows an immediate 27 Vdc flash up, then drops to 3.3 Vdc which counts down to 1. 6 Vdc in two or three seconds. When it gets to about 1.6 Vdc it flashes 26 or 27 Vdc again, immediately falls back to 3.3 and counts down again. It repeats this cycle.

Q6b = 2.6 Vdc on power up, counts down to 180mV, then pops back up to 2.6. This cycle repeats as well.

Q8b = 000
Q9b = 74 mV

Edit: All the other boards were refurbed months ago, the protection board was done after the delayed relay releases started. So C7 is a month old.

 

[mbz]

Pins #13-16 show the same weird cycle. Power on shows an immediate 27 Vdc flash up, then drops to 3.3 Vdc

On the protection board? Pins 13 and 16 are power amp outputs. Candidates are power amp, preamp, power supply. The protect circuit is doing it's job.

Would be good to split the pre/power amp connection, pin 17 on power amp board, I assume these are wire wrap (do not unwrap). Maybe worthwhile lifting that input/coupling cap C1, C2 to help identify in pre/power issue. Also usefult to look at power supply rails when it's spiking to 27Vdc. Need to review history, maybe done some of this...

 

[JJCalvillo]

Yes, measuremts from the protection board.

Haven't done the measurements you're suggesting previously. C1/2 on the amp boards? The pins are all wire wrap, and I have a tool for that.

Thanks for your help!