Marantz Pm 493 amp issue - power cycles on/off - repeat

[mbz]

The regulator I pulled says 79M15A on it (0008A JRC)

I think you've pulled out Q803. The "79" series are for negative volt regulators, the "15" refers to the output voltage.
I would choose MC7915CTG from RS, next day delivery
https://au.rs-online.com/web/p/linear-voltage-regulators/5166228/
The jcar item is only slightly cheaper

Q809 should be for positive 18 Volts output, ie, 78 series XX"18"

 

[mbz]

The Jcar part should also do the job, rated for 1A, -15V. Note outer case is hot/live. Maybe original was mounted to heatsink?

 

[Homer4beer]

The Jcar part should also do the job, rated for 1A, -15V. Note outer case is hot/live. Maybe original was mounted to heatsink?

Yea it is, and you are correct, I did pull Q803. No wonder I was pretty confused when looking at the schematics!

 

[Hamish119]

+5.9V...JS07 pin 4 = +4.2V then -0.7V when it powers off
.

R801 +4.2V then -0.7V when it powers off

Q804 pin 1 14V then drops to nothing when it shuts itself off

Great and insightful video! Many thanks...



You've found the area of the original/pre-existing fault. 78xx series regulators have built-in protection;

"Each type employs internal current limiting, thermal shut down and safe operating area protection, making it essentially indestructible."

Assuming (dangerous - there are obvious errors) that the circuit diagram is correct, your last statement ("Q804 pin 1 14V then drops to nothing when it shuts itself off") is of concern.
If correct, check Q801 (pins 1 & 3) & Q802 (Emitter, Collector & Base) voltages during the fault condition. Check hard (thoroughly)!

N.B. Q802 has no protection mechanism. Its Emitter voltage cannot be 0V if its Base voltage is +15V.

There may be more than 1 x duff part.

Re-check the voltages with the cassette deck disconnected.

Good Luck!

 

[Homer4beer]

Ok I've replaced the part I fried and powered it up. Initially I heard the click as it powered up but no lights on the front. So I double checked all the cables and pushed down on the main board with the unit powering up.

There has got to be a bad joint somewhere as I heard a small electrical "crack" as a circuit closed during power up a couple of times.

I did pull the board out to replace the component and to take the relay out so I could use a very very fine sand paper to clean the contracts.

So I don't know where the dry joint would be. Good news is it is so far staying on and playing tapes without turning itself off.

Will test further.

 

[Homer4beer]

Hmm I found what my electrical "crack" was! The voltage regulator I replaced was sparking upon powering up where it touches the heat sink. I do believe this should be insulated like the one I pulled off? If I pry it away from the heat sink so it doesn't touch it I don't have any issues. What should I insulate it with?

 

[Homer4beer]

And an update on the turning off while playing tapes. It does it still. If I'm listening through the CD input no dramas but as soon as I push play on the tape deck.....zoooot, it turns off.

 

[mbz]

If I'm listening through the CD input no dramas but as soon as I push play on the tape deck.....zoooot, it turns off.

Very strange, difficult to reconcile with voltage measurements, maybe the tape deck is causing some sort of short causing the volt reg to shut down.

Has it ever shut down while playing CDP/AUX?

You could try connecting CDP to the TAPE Play Back

I would be checking the solder joints/connections for each of the regulators.

Did you replace all 3 suspect parts?

 

[Homer4beer]

I think the motors turning on the tape deck draw enough voltage to send the unit into meltdown.

I haven't replaced the other suspect parts as I haven't found current equivalents to use. I'm just not sure.

What about the component I replaced being metal on the back causing problems on the heat sink?

 

[Homer4beer]

This fault is quite problematic to find. Today I sat down to test further and played a tape for 5 mins without the power cutting in and out while checking the base voltage of Q802 which fluctuated between +14.5V and +15.8V for about 30 secs before settling down to a rock solid +15V.

 

[petehall347]

hope transistor is touching heat sink . will get too hot otherwise. might need mica .

 

[Homer4beer]

hope transistor is touching heat sink . will get too hot otherwise. might need mica .

Yea i'm sure it should be touching but it sparks when it does. Is mica readily available from local stores (Jaycar etc)?

Edit: Yes they do have it although I think I'll use silicone pads

 

[Homer4beer]

Ok This is really odd! I've now been playing the tape deck non stop for 1.5 hours without it turning off. I still have a hook test lead attached to the base of Q802 waiting for it to fail so I can measure however so far it just isn't?

 

[Hamish119]

This fault is quite problematic to find. Today I sat down to test further and played a tape for 5 mins without the power cutting in and out while checking the base voltage of Q802 which fluctuated between +14.5V and +15.8V for about 30 secs before settling down to a rock solid +15V.

Also check Q802 Emitter voltage - it should be 0.666V less than the Base voltage (but may not be...).

Do you receive 3rd degree burns when you touch Q802?

What about the component I replaced being metal on the back causing problems on the heat sink?

The Q803 TAB (the 7915...-15V regulator) is @ -28V and must be electrically isolated from the heatsink.

Do you receive frostbite when you touch Q809?

Do you receive frostbite when you touch Q801?

Do you receive 3rd degree burns when you touch Q804?

Test the above voltages and temperatures with the cassette deck, tuner etc disconnected.



EDIT: Corrected regulator voltage - Q803, 7915, -15V

 

[Homer4beer]

Ok it's playing up again after working without failing for hours. Frustrating stuff!

 

[Homer4beer]

Also check Q802 Emitter voltage - it should be 0.666V less than the Base voltage (but may not be...).

Do you receive 3rd degree burns when you touch Q802?


The Q803 TAB (the 7915...-15V regulator) is @ -28V and must be electrically isolated from the heatsink.

Do you receive frostbite when you touch Q809?

Do you receive frostbite when you touch Q801?

Do you receive 3rd degree burns when you touch Q804?

Test the above voltages and temperatures with the cassette deck, tuner etc disconnected.

View attachment 1057663

EDIT: Corrected regulator voltage - Q803, 7915, -15V

View attachment 1057693

Q809

output pin 1 +17.6V
Input pin 3 +40V

 

[Homer4beer]

I've found the problem! I pulled the board again and had a closer look at the solder joints of the power regulators and transistors. Now I know why I was getting such sporadic results and why when I had a probe attached to the leg of Q802 it worked perfectly without stopping. I must have pulled the legs (or on another regulator close by) back into contact with the circuit. The solder joints had broken away and cracked, probably due age over the years (heat/contracting) and moving the amy around by the previous owner.

So I re-soldered all the legs and so far so good!

 

[petehall347]

next one along looks bad too ..i would give it all a close look .

 

[Homer4beer]

next one along looks bad too ..i would give it all a close look .

Yep I re did all 4 of them (the 5th was the new one I put in).