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oof.lucasmicrophone

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Posts: 90 Member Since: 24/01/2014

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May 18 17 5:32 PM

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There's an LM309 transistor (U67) in my primetime 93 that runs super hot.  after about 3 minutes you can't touch it.  I measured one leg at 5v, which is correct.  the other leg should be 8.5v but measures 10.4v.  The same transistor in my other primetime gets only warm to the touch.  I'll try to post the schematic (pg.98 in the service manual) but if i can't,... there is a 4.7uf 16v cap off each of the outside legs of the transistor. (C198 & C199). The voltages are correct at the various test points I've measured, as indicated in the manual.  

I am wondering if the transistor could be going bad, or if a nearby component may have drifted in value enough to elevate the voltage going into, and then out of the transistor.

The unti was acting up- all headroom lights lit- which of course was fixed by reseating all boards.

Just wondering if I've got an inevitable failure here.  thanks very much,
Davidimageimage

Last Edited By: oof . Edited 1 time

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scullyfan

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May 18 17 5:44 PM

The LM309 regulator in my Prime Time 93 was also VERY hot just before the power transformer experienced a meltdown. I measured it with a DVM and it was putting out the proper 5 volts. I'm not exactly sure why the PT went out because I hadn't done enough troubleshooting before it died on me. The unit wasn't passing audio when I put it on the bench. Is your unit fully operational, or did you have it on the bench for repairs?

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John Eppstein

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May 18 17 5:50 PM

oof wrote:
There's an LM309 transistor (U67) in my primetime 93 that runs super hot.  after about 3 minutes you can't touch it.  I measured one leg at 5v, which is correct.  the other leg should be 8.5v but measures 10.4v.  The same transistor in my other primetime gets only warm to the touch.  I'll try to post the schematic (pg.98 in the service manual) but if i can't,... there is a 4.7uf 16v cap off each of the outside legs of the transistor. (C198 & C199). The voltages are correct at the various test points I've measured, as indicated in the manual.  

I am wondering if the transistor could be going bad, or if a nearby component may have drifted in value enough to elevate the voltage going into, and then out of the transistor.

The unti was acting up- all headroom lights lit- which of course was fixed by reseating all boards.

Just wondering if I've got an inevitable failure here.  thanks very much,
David

You definitely have a problem. Do you know which leg of the transistor is at elevated voltage (Emitter, base, collector?)

I'd look for a leaky cap or another semiconductor throwing off the voltage. Could also be a resistor gone off tolerance (or a solder joint gone flaky), perhaps the lower leg of a voltage divider that's increased in resistance, thereby raising the voltage at that point. Can't say more without seeing a schematic.

At this point I would NOT run the unit, as if the transistor goes it could cause a cascade effect and blow others.

EDIT: Scratch all that except the last sentence. An LM309 is a VOLTAGE REGULATOR, not a transistor. It outputs 5 volts, so it is working as it should - HOWEVER the power rail that's feeding it is way, way off spec. You need to look earlier in the circuit to find out what is feeding in excessive voltage. Look at the power supply. If you can post a more extensive schematic it would help. Worst case would be a power transformer going out, but hopefully it's something cheaper and easier. Did you check to see if there's an AC component to the excessively high voltage?

Last Edited By: John Eppstein May 18 17 6:12 PM. Edited 1 time.

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oof.lucasmicrophone

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Posts: 90 Member Since:24/01/2014

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Thanks John, Scully & CrazyDoc for your replies. Here is the schematic.
i did check the voltages at u68 and u69, the regulators at the psu, and they were correct. i will check the other points listed in the manual this weekend. i'll try to just leave it on for short periods, in hopes of sparing the PT. although, this being a favorite piece, i may just bring it up the street to my tech. Just hate to be w/o it for a month....
thanks again to the tips-- John, I will investigate the voltage from the psu. and try to see where the regulator is being hit with too much juice.
btw- it does work fine as it is, but i understand the risk of blowing the regulator- thanks for the warning!

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waltzingbear

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The 8.5V supply is the raw supply for both the 5V regulated supplies, it's value is a function of the wall voltage and the load on the supply transformer. At this point I wouldn't put too much effort in trying to change that.

Typically a hot regulator is caused by pulling too much current thru it, often in this case with the PT it is a leaky decoupling cap across the supply. If you have an ohmmeter you could try to measure the resistance across the 5V regulated rail. If it is very low, you probably have a shorted cap, it must be removed post haste. I have seen cascade failures in these boxes before, you can end up with a parts donor machine.

Do not operate this unit except to troubleshoot and only then for very short periods of time until the problem is fixed. Keep the heat generated on the regulator low.

Alan

Alan Garren
Waltzing Bear Audio

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John Eppstein

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waltzingbear wrote:
The 8.5V supply is the raw supply for both the 5V regulated supplies, it's value is a function of the wall voltage and the load on the supply transformer. At this point I wouldn't put too much effort in trying to change that.

Yes, but he's getting 10.4 at that point, not 8.5. So something's wrong at the supply side.

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zmix

Aqua Marine

Posts: 4,060 Member Since:20/01/2011

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David,

Have you measured the line voltage there? 7 blocks south we have ~125V, this could account for the slightly higher unregulated voltage at the input of the LM309 regulator.



The LM309 regulator can handle a maximum of 35V at it's input, so I wouldn't be too concerned.

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oof.lucasmicrophone

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Posts: 90 Member Since:24/01/2014

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Hey Chuck, thanks very much for chiming in-- we have the same 125-126v here, although your neighborhood certainly is hotter.
What concerns me is that the same regulator on my other Primetime stays just warm, not burning hot as this one gets.

on Wednesday, after I had reseated the boards to try to clear up the "all headroom lights on" issue (which it did )I did have the suspect unit on for about an hour when i needed both units. Worked fine. But I am concerned that i may trigger a greater failure if i do that again.

Alan had an interesting thought with regards to a bad coupling cap on the rail, and I may pop it open again this weekend to measure more thoroughly, but as usual, I'm in over my head and really don't want to hurt this fella, so I may just bring her up to Dr. Z again.

And Alan- thank you for that advice too. Geez, it must be almost 20 years since you rebuilt my BX20 electronics. Still sounding beautiful-- great job!

Thanks again for the advice.

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waltzingbear

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ah, yes, I remember that. You're welcome.

That's DE-coupling caps, those are the ones that go between the rail and ground or rail to rail. Their job is to "de-couple" or remove fluctuations in the power supply in local positions. As you move away from the output of the regulator you have resistance and inductance that accumulate between the actual load and the regulator output. Adding capacitance to ground is a balancing act to neutralize these effects so different loads (usually ICs) don't "talk" to each other thru the power supply.

Again, its the current thru the regulator, not the voltage on it you should be concerned about.

Cheers
Alan

Alan Garren
Waltzing Bear Audio

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John Eppstein

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Coulod you post the schematic at higher resolution? as posted it's so tiny that it's completely illegible and downloading it and magnifying the screen view just  devolves into an unreadeable blur.

A format that is not PNG would be nice, too, preferably JPEG or even GIF. (It's not like the additional color resolution of PNG is needed on a black and white schematic.)

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oof.lucasmicrophone

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Posts: 90 Member Since:24/01/2014

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Well, I checked all the voltages, at numerous points on the board, according to the service manual and they all are correct. I had the unit on for 10 seconds max, to check each point. BUT i still blew the fuse that comes off the PT secondary, for the 2nd time. So... since these are dear to me, I'm bringing her in to be serviced. I wish I had a better grasp of this, since it would appear to be a simple fix, but I think if i continue, there's a chance i could fry the PT or more.

So thanks everyone for the great advice- I'll post back when i find out what the issue was. Bummer, since this fella had a nice spa day complete with Ultrasonic bath and pedicure, just a few years ago. Got to keep the robe at least.

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zmix

Aqua Marine

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David,
Keep us posted as to what the issue was.. I've been working with an artist who has a pristine 93 and it's so much fun to have those things around...!!

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oof.lucasmicrophone

Silverado

Posts: 90 Member Since:24/01/2014

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will do. she's with the good doctor now. yeah, i've had these 2 for probably 20 yrs now. one has never had a problem, and this one has twice been in for repair. i guess 2x in 20 yrs is still pretty good for these old fellas. Nothing like my old Varispeech that took years to get repaired. Worth the wait though.
Will report back.....

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