Bias pots keep rising?

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giro1991

giro1991

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Joined 2013
#1
Hi
So left channel in a particular amp is heavily distorted, (spitting sound).
I opened it up, a resistor leg appeared to be touching heatsink of power regulator. Moved it away.
Distortion still there. Some browning underneath but this is normal I've been told. (The person who sold me it).
I've put mulitmeter across test points, set pots to 7mV / 19.4 mA (as directed in shemeatic manual) but they keep rising!
They will not stay.
tried replacing power regulator.
Original Part is 7806A
Replacement is 7806CV (both are 6v, 1.5a assumed should be ok) - even though in schematic it says NJM78M12FA (which is a 12v, 0.5a). Slight discrepancy there.
Employed replacement, hasn't fixed anything, infact the test points now read 0v, so nothing getting through that one. Really odd.

So regarding rising pot readings, any ideas on cause, caps perhaps?

Any help would be great
Thanks

edit; going to get freeze spray at some point, will post pictures of results.
This amp is fairly old 1989 also if that helps anything.
 
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#3
giro1991 said:
.....I've put mulitmeter across test points, set pots to 7mV / 19.4 mA (as directed in shemeatic manual) but they keep rising!.....
This amp is fairly old 1989 also if that helps anything.
Click to expand...
As you have a schematic and presumably a service manual, it would help greatly to at least share references like the make and model, if not the details.

That way we might recognize what you are referring to and check whether it's relevant to the problem.
 
#5
yes, the heat sinks need to reach running temp and the bias spreader needs a good connection to the heat sink, usually, if its BJT output devices. Can you show us a schematic?

And that resistor short to heat sink probably destroyed something, check the circuit diagram to see what that resistor is attached to.
 
#6
Hello,
Here's the schematic
http://www.google.co.uk/url?sa=t&rc...hHlzrp4_yoO30kHFIw8JFxg&bvm=bv.65788261,d.ZWU

I left it running for a while before I took readings.
Uhm, as for regulator reading... I have no idea what I've done but I think I've melted my dads multimeter... The diode test funtion piezo just stays on now.

I'll need to buy a new one before I can get back to you on the readings. -.-

When I swapped 7806A with 7806CV, the solder joints were definately intact. (tested with beeper) but upon turning it on, something defo wasnt right, 0v across test points and leds on the front wernt on.

As I said I'll need to get a new mulitmeter. Will get back to you asap when I do!
Cheers
 
#7
You have a lot of voltages to look to see if they are stable. Yes, all the electro caps will be leaky, but that may not be the specific problem you are fighting. C721 could be an issue. I usually don;t see a cap that large or an electro in that position. Curious, TP1 and TP2 have different voltages. What lead you to fool with the low voltage regulator if you were having output bias stability questions?
 
#9
I think we have a gentleman who is game to give it a try, but lacking in electronic theory. I hope when he gets a new meter, he will ask for some more focused advice on what to measure.

OP: All freeze spray will tell you is things do change with temp. We can tell you that.
 
#10
tvrgeek said:
What lead you to fool with the low voltage regulator if you were having output bias stability questions?
Click to expand...

My knowledge is limited yes,
I actually refered to a 'diagnose PNP/NPN transistor' tutorial video on YT.
With unit off I probed different legs with ohm setting on multimeter, got readings both ways like in the video, so I thought it was broken.
I was also in correspondance with the seller who mentioned testing 78xxx in his reply so I assumed this component was the culprit. Obviously not.

Hopefully it's just the battery in the meter, if not I'll need to source a new meter.
Perhaps I used an incorrect setting on the meter, too senstive for the reading, though it seems strange considering were only talking a few volts?

Ok so no freeze spray, just in time saved me from buying some.
 
#11
Meters are hard to ruin. The most common way is to measure a voltage with the probes in the current input. Your shorting the test point to ground and can get currents that are to large. Sometimes this blows a fuse in the meter. So make sure your probe is in the voltage input.
 
#13
giro1991 said:
My knowledge is limited yes,
I actually refered to a 'diagnose PNP/NPN transistor' tutorial video on YT.
With unit off I probed different legs with ohm setting on multimeter, got readings both ways like in the video, so I thought it was broken.
I was also in correspondance with the seller who mentioned testing 78xxx in his reply so I assumed this component was the culprit. Obviously not.

Hopefully it's just the battery in the meter, if not I'll need to source a new meter.
Perhaps I used an incorrect setting on the meter, too senstive for the reading, though it seems strange considering were only talking a few volts?

Ok so no freeze spray, just in time saved me from buying some.
Click to expand...

Victim of u-tube! You are not the first. Even though the 78xxx looks like a transistor, it is not. Sounds like the seller did not understand electronics. So, get your meter working and we will try to suggest toe more appropriate places to probe and what to look for.

We know due to the age, all the electrolytic caps will be dried out and leak DC voltages as well as being a lower than desired value.
We also all know there is no such thing as a good pot. If dirty ( it is) it may not hold the value.
We know that very frequently the thermal bonding to the heat sink may have degraded.
We know many older amps did not have very stable bias, so they drift anyway.

This may be a surprise to many out there, but electronics generically has a life span of about 7 to 10 years. Many thing contribute to this, but that is the way it is. Any electronics over 10 years old you should consider to be free and paid for. When we try to put vintage equipment back in service, it is often an expensive proposition.
 
#14
Trust your meter, and forget about bad caps to start with

giro1991 said:
tried replacing power regulator.
Original Part is 7806A
Replacement is 7806CV (both are 6v, 1.5a assumed should be ok) - even though in schematic it says NJM78M12FA (which is a 12v, 0.5a). Slight discrepancy there.
Click to expand...

This is a big deal. You really need to be sure if it should be a 6V or 12V regulator. Is there a similar part in the correct working channel? If yes - use the same kind as that one.
giro1991 said:
Employed replacement, hasn't fixed anything, infact the test points now read 0v, so nothing getting through that one. Really odd.
Click to expand...
The output of the 78XXX is shortcircuit protected, which is probably why it will read 0V on the output : It is shortcircuited!
Indicates that something else is really wrong
giro1991 said:
I've put mulitmeter across test points, set pots to 7mV / 19.4 mA (as directed in shemeatic manual) but they keep rising!
They will not stay.
Click to expand...
...this also indicates that something else is wrong. Pull out the output transistors, check them and check the voltages in the rest of the circuit.

A schematic ( sounds like you have it at hand) would really help us help you to diagnose it and make suggestions.
 
#15
...found your link to the service manual in post #6 :headbash:

I did see the pin-drawing indicating 7812, but I cant find any 7812 on the schematic or in the component lists, only 78LO5 (should probably be 78L05), 7806 and 79L05, so it seems to be fine with 7806.

Is it Q810? (should be printed on the PCB)
That one feeds some other circuit boards. It seems strange that you can get sound through the good channel without voltage to the smaller circuits. ( unless it is feeding the DAC section and you are testing through the analog section.)

However the circuitry around the Q801 is the +- 17Volt regulators which feeds the opamp in the poweramp sections. If the regulator has taken out some stuff here, that could be the reason for your poweramp being drifting.
(Don't assume the 'burnt' part is normal just because your mate tells it is)

Please check that your have proper + /- 17.2V on one of the legs of R715, R716, R717 and R718.
If not ( like if any of them sits at 23 volt) then it needs to be fixed.
 
#18
An externally hosted image should be here but it was not working when we last tested it.

The lower two heatsinks, are getting abnormally hot when power is on.
I will probably have to buy replacements, cuz i mean HOT.
When switched on, leds on volume knob and on/off button light up, but nothing else.
Everything used to light up before I swapped regulator (doh). I have swapped it back to original part.
So this nil voltage development is related to my regulator swap... :|

I measured those legs and the regulator - all read 0v - am I doing it right?...
The multimeter i'm using
DC setting, 20v setting (0.00), putting the black probe (plugged in the middle) to a ground on the amp, and red probe (plugged into the right) to what I want to measure to get potential difference.
Doing it this way I'm getting 0v.
Perhaps the regulator is not soldered correctly, I will double check tonight /resolder if I need to, using the 'diode test beeper' setting to test if the connections are indeed intact.

This is a nice amp, one is current going for around 240 gbp on ebay so fixing it would be great, though I don't intend to sell it once fixed.
Thanks
 
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#19
Dearest ...please seek expert help to solve your trouble
It seems that you don't have what it takes to fix this amp
Many people will run in to help you but it will not help since you will not be able to understand what are they talking about ..

Forum is populated by very willing people but you need to be able to implement the advices ..

Kind regards
Sakis
 
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