Replacing a Power On indicator lamp in a Pioneer SA7500 II should be straight forward - right?

[62vauxhall]

I am hoping someone can provide insight into this. I have replaced lamp bulbs before and it was a fairly cut and dried process.

Yesterday I visited someone who recently purchased a record store. Because I got some of her 2nd hand vintage turntables to work, she handed me a Pioneer SA700 II. It was the amp she got with the store that was used to play music in the shop. She said it would make spurious noises from time to time so swapped it out with something else. I think I discovered what that problem could be - 2SA798 twin transistors with black legs.

But first, I thought I'd fix the power on indicator lamp. It was apparently working when the amp was pulled from duty but it no longer does.

I removed the lamp bulb which is identified on the schematic as PL1 - 8 volt 50ma. There is no continuity so I declared the bulb dead. Then I measured the voltage present at the two points the bulb was wired to, pin 52 and ground. I measured 42.6 volts AC. My thought was no wonder the bulb was dead.

As an experiment. I temporarily wired a 12 volt 30ma bulb in place. It did not illuminate. I then tried an 8 volt 50ma bulb but it too did not light up. Am I wrong to assume that if the required or even greater voltage is present, that a bulb will at least glow?

Apart from that, I'd also like to learn what the segmented line indicates that passes through pin 52, pin 55, pin 54, etc, etc. I have not found an online explanation.

 

[laatsch55]

The 2SA798's can be replaced with 2 KSC 1825's....IIRC...I'll have to check. Emitters go in the common hole...transistor bodies glued together to even the heat..

 

[markthefixer]

The segmented, dotted line on the drawing is the boundary of the board that those circuits reside on.

The lamp has a series resistor that eats up the extra ~40 volts or so - it really depends upon the lamp current. See the 680 ohm 2 watt resistor R91 ?
It will be a different value with different lamp currents.
It's not lighting up with 12v 30mA or 8v 50mA says more about the resistor - probably burned out. 2 watts right next to the 2 fuses.

BUT BUT BUT you say, I measured 42.6v AC at the burned out lamp connections!!!!!

You measured them with a 10 megohm DMM, so the damaged resistor is not open circuit, but rather burned to a higher resistance, which is no sweat for the DMM to measure through. Won't carry enough current to light a lamp, or even a LED (3v 10mA).

It's the same thing as troubleshooting an automotive electrical circuit -
one uses a (grounded) probe with a test light that pulls current through it, because if the circuit can't deliver current, it ain't a hot line. DMM'll fool ya.

42.6v- 8v (lamp) = 34.6v excess at 50 ma is 34.6v/ 0.050a = 692 ohms and 34.6 v X 0.050A = 1.73 watts
That's pretty close to 680 ohms 2 watts..... R91

Want a different lamp / led? pick the current, lets say 10mA for a ~3v led.
42.6v - 3v = 39.6v / 0.010A = 3, 960 ohms 39.6v x 0.01A = 0.396 watt.

The 2SA798 gets replaced with a pair of KSA992 transistors. Both emitters go in one hole. BCEECB.
Lee - I've recently done it too!! mixing positive (PNP, A) with negative (NPN, C) - at least in instructions.
He / we / I burned up a perfectly good 470 ohm 1/2 watt resistor swapping polarities on some TO-220 drivers just this last week.
BTW that's ksc1845...

 

[62vauxhall]

Thank you Lee, thank you Mark.

A few years ago I replaced twin transistors in a small Sansui amplifier but can't remember if they were also 2SA798. Since I've run into them again, I'm prone to think they were. I successfully replaced them with back to back TO92's that were likely KSA992 as I have a quantity of them. So I shall do likewise with this Pioneer.

Thanks, Mark, for pointing out that resistor and why I was still able to see voltage where the lamp connects. I don't have a correct replacement resistor, not do I have an appropriate lamp bulb. I'll order a few 2 or 3 watt 680 ohm resistors and I've already emailed WJOE for a quote on a couple of bulbs. The amp's owner has OK'd my ordering the parts.

 

[laatsch55]

The segmented, dotted line on the drawing is the boundary of the board that those circuits reside on.

The lamp has a series resistor that eats up the extra ~40 volts or so - it really depends upon the lamp current. See the 680 ohm 2 watt resistor R91 ?
It will be a different value with different lamp currents.
It's not lighting up with 12v 30mA or 8v 50mA says more about the resistor - probably burned out. 2 watts right next to the 2 fuses.

BUT BUT BUT you say, I measured 42.6v AC at the burned out lamp connections!!!!!

You measured them with a 10 megohm DMM, so the damaged resistor is not open circuit, but rather burned to a higher resistance, which is no sweat for the DMM to measure through. Won't carry enough current to light a lamp, or even a LED (3v 10mA).

It's the same thing as troubleshooting an automotive electrical circuit -
one uses a (grounded) probe with a test light that pulls current through it, because if the circuit can't deliver current, it ain't a hot line. DMM'll fool ya.

42.6v- 8v (lamp) = 34.6v excess at 50 ma is 34.6v/ 0.050a = 692 ohms and 34.6 v X 0.050A = 1.73 watts
That's pretty close to 680 ohms 2 watts..... R91

Want a different lamp / led? pick the current, lets say 10mA for a ~3v led.
42.6v - 3v = 39.6v / 0.010A = 3, 960 ohms 39.6v x 0.01A = 0.396 watt.

The 2SA798 gets replaced with a pair of KSA992 transistors. Both emitters go in one hole. BCEECB.
Lee - I've recently done it too!! mixing positive (PNP, A) with negative (NPN, C) - at least in instructions.
He / we / I burned up a perfectly good 470 ohm 1/2 watt resistor swapping polarities on some TO-220 drivers just this last week.
BTW that's ksc1845...

Amen Mark. I'll check before I post next time..

 

[62vauxhall]

What are the chances of that 2W 680 ohm resistor exhibiting a high resistance only when under load? I removed it from the circuit board and saw the number of the beast - it measured 666 ohms.

 

[WOPL Sniffer]

Highly unlikely. These passive components are good or bad (Usually). That one has drifted a bit probably from getting warm but you probably have another issue hidden in there.

 

[62vauxhall]

The only thing I know for sure is that the lamp bulb inside the amp is dead. If characteristics of the 12 volt and 8 volt bulbs I experimented with, prevented them from illuminating when power is switched on, could be then that the only solution is a new bulb with exactly the same specs as the original bulb.

Maybe it is just a case of a burned out bulb.

Still waiting to hear from WJOE Radio whether he can supply or not.

EDIT: Assuming that resistor was faulty, before pulling and checking it, I put in a Digikey order this morning for some 3W 680 ohm wire wounds.

 

[grapplesaw]

The only thing I know for sure is that the lamp bulb inside the amp is dead. If characteristics of the 12 volt and 8 volt bulbs I experimented with, prevented them from illuminating when power is switched on, could be then that the only solution is a new bulb with exactly the same specs as the original bulb.

Maybe it is just a case of a burned out bulb.

Still waiting to hear from WJOE Radio whether he can supply or not.

EDIT: Assuming that resistor was faulty, before pulling and checking it, I put in a Digikey order this morning for some 3W 680 ohm wire wounds.

Garry look at this list. Mouser have 8 volt 50ma
part # 560-DW50

 

[WOPL Sniffer]

You never did say that the receiver actually works even though the indicator light doesn't. I think you said it worked at the store.

 

[62vauxhall]

You never did say that the receiver actually works even though the indicator light doesn't. I think you said it worked at the store.

It's an integrated amp and yes, it does work.

When I got it home, I checked for DC before connecting speakers. That was when I noticed the indicator lamp was not lighting up. There was some offset, less than 50mv per channel, so I hooked up speakers and an AUX source. It played for an hour or so then idled for a couple of hours at half volume. The intent was to determine if the intermittent "popping" sound that caused the shop owner to pull the amp from service, was on one channel or both. No such noise was heard.

When I took off the cover, I noticed the black legs of the 2SA798's and then removed the indicator lamp bulb and that 680 ohm resistor.

That is where I am at now.

 

[MusicSteve]

side note - I have some of the Pioneer SA intergraded amps and SX receiver
and the cracking noise is caused by dirty pot and switches.
since its not yours (might want to just clean the volume pot since it can be time consuming) and the Sa would need to clean all the pots and switches.
if you don't use fader cleaner and lube for the pots,,,, you will have to do it again in 6 plus months.

 

[62vauxhall]

I can't totally rule out anything but the noise was not described as cracking. It was said to be a "pop" which I was told happened intermittently during the day without laying a finger on the amp. The shop owner explained it was her concern about speaker damage that prompted swapping the Pioneer amp out for something else.

During the time I was "auditioning" the Pioneer amp at home, I repeatedly operated the controls and switches, attempting to induce some sort of noise but was unsuccessful. At all times the controls and switches operated completely noise free - no crackling, no static. In my presence, the amp exhibited no noise of any kind while in operation but such is the nature of intermittence.

I experienced something similar several years ago with a pre-amp. "Popping" sound and in that case, accompanied by a low frequency rumble. When it first manifested, I was not in within line of site. From where I was, it sounded to me like a thunder storm. It was when I went to close the windows that I discovered the sound was generated by the pre-amp. The problem turned out to be IC's which I overlooked as I was sure it was a problematic transistor.

 

[MusicSteve]

Good luck
That will be harder to fix if it comes back.

 

[George S.]

Have to wonder if it's a worn out wall outlet or power strip at the record shop making and breaking contact with the power cord plug.

 

[62vauxhall]

I’m fairly optimistic about one or both of the 2SA798 being the noise source. From what I’ve read, they have somewhat of a reputation for degrading and possibly becoming noise producers. I am compelled to think that their black legs are a strong indicator.

Glen was kind enough to provide a link to a Mouser page which have what might well be the exact, drop in replacement indicator bulb this thing needs. Those have been ordered. WJOE Radio have still not responded.

I’ve been delaying doing a re & re because I messed up my shoulder recently. Either aggravated an old injury or caused a new one. Won’t know until it’s ultrasounded on Jan 26.

At present, use of my left arm is quite compromised and two hands are better than one for unsoldering and resoldering.