Re: Onkyo A-8250 50V on DC Offset transistor
This model and some other Onkyo amplifiers have an unusual architecture for the output stage.
There are 3 low value emitter resistors connected to the output transistors. One or more of these often fail open-circuit after an output transistor shorts out. These resistors are both “flameproof” and “fusible.” They rarely show any visible signs of overheating. Because these 3 resistors are in a series-parallel arrangement it is necessary to disconnect one end of at least one of these 3 resistors in order to check that...
electricboyo
Member
Last Activity: 04-12-2023, 01:26 PM
Joined: 03-03-2013
Location: Urbana Illinois
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Re: Electron Kinetics Eagle 2C amplifier problem
After looking at your photos of the PC board again up-close I am thinking perhaps some of the power transistors for the bad channel aren't actually soldered to the PC board. Or rather, their solder connections broke off. As far as I can tell there isn't much of anything mechanically holding the PC board except for the soldered pins of the large power transistors and the speaker output binding posts. I don't see any bolts or screws holding the PC board.
So, seriously, as suggested by an earlier post, it is really important...Leave a comment:
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Re: Onkyo A-8250 50V on DC Offset transistor
When an output transistor has failed I temporarily remove all of the main output transistors from the faulty channel. By this I am referring to the large output transistors bolted to the heat sink. Then I connect a 100 ohm 2 watt resistor from B to E terminal pads for each spot on the PC board where an output transistor was originally connected. Do not connect anything to the C terminal pads on PC board. The 100 ohm resistors provide the continuity that is required to get the rest of the amplifier circuit working. And it should be possible...Leave a comment:
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Re: Infinity PS-12 Powered Subwoofer Fired Amplifier
Always use a “dim bulb tester” to apply mains power to these types of amplifiers. 60W or 100W conventional incandescent light bulb is about the right size. You need to get the amplifier to the point where it can play sound at low to moderate volume before applying “full” mains power to it.Leave a comment:
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Re: Pioneer QX-949 in protect mode / no audio
There are some batches of the small TO-92 case transistors in 1970's era audio gear that develop intermittent and sometimes persistent failures after 40 years. Intermittent crackling, popping, and hissing noise is frequently a symptom.
If you identify one such small transistor that failed then it would be wise to search every PC board for other transistors of the same part number and physical appearance.
With electrolytics in 40+ year old audio gear it is a good idea to measure both capacitance and ESR for...Leave a comment:
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Re: Electron Kinetics Eagle 2C amplifier problem
Based on your previous voltage measurements I suggest checking all 4 of the large power resistors connected between the emitters of the channel B output transistors. One or more of these resistors is open. Or else there is a bad solder connection or broken foil on the PC board.
Under normal conditions the emitters of all 4 large power transistors should be within +/- 50 mV of each other, and this should also be very similar to the DC voltage appearing at the red output terminal post on the rear of the panel for this...Leave a comment:
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Re: Electron Kinetics Eagle 2C amplifier problem
I suggest checking the value of all resistors on the PC board. Do this with power off and with an ohmmeter. Resistors can fail in the "open circuit" state without looking burnt or damaged.
Because there are unusual DC voltages on the faulty channel I recommend that you should only apply AC mains power to the amplifier through a "dim bulb" tester. This is a conventional incandescent 120V light bulb either 60W or 100W which is wired in series with the AC power source. This can be made with a light...Leave a comment:
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Re: Electron Kinetics Eagle 2C amplifier problem
What we need are very sharp clear photos of both side of the PC board. If you remove the mounting screws from all the power transistors and the input and output jacks then you should be able to lift the PC board from the heat sinks so that both front and rear of the PC board can be seen.
The circuit design of this amplifier is somewhat unusual. Some of us who have worked on audio amplifiers for years might be able to tease out its schematic from looking at a really good set of photos of this PC board.
Because you have...Leave a comment:
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Re: Samsung LN-T4665F bad PFC caps, what series to replace with?
[B][/B]Yes, I solved the exploding L5973D issue:
[B][SIZE="5"]Re: magnavox 32mf231d/37 no power backlights flicker[/SIZE][/B]
My solution:
Often the 16V to 12V regulator IC at l[B]ocation 7003 on main (scaler) PC board [/B]will be burned, cracked, or have a small hole near pin 7.
The part # for this IC is [B][SIZE="4"]L5973D[/SIZE][/B]. It is on the back side of PC board: To see it the main PC board must be removed from TV...Leave a comment:
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Re: Samsung LN-T4665F bad PFC caps, what series to replace with?
LCD tv sets contain multiple regulated power supplies: These certainly DO interact with each other. For example, your faulty 2200uf/25V caps could interact with the PFC controller in such a way that the voltage on the 450V cap might become unstable or become too high.
Don't forget the small electrolytic capacitors either: It's best to either test them all with an ESR meter, or (since small caps are cheap) replace ALL of them.
I went around the block several times with a Magnavox 32MF231D:...Leave a comment:
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Re: Samsung LN-T4665F bad PFC caps, what series to replace with?
Large high-voltage electrolytic capacitors (such as 150uF 450V) normally have a much longer life than the lower voltage capacitors located in power supply secondary circuits.
Therefore, whenever a PFC capacitor fails, it is important to search for faults in the power supply which might have caused over-voltage on the PFC capacitor. Other posts on this forum have confirmed that a failure in the PFC circuit can cause the voltage across the PFC capacitor to rise up to 600 volts or higher, which (not too...Leave a comment:
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Re: Help identifying part on samsung lcd main board
I have personally seen a faulty LM1117 LDO chip. Not burned, no physical damage, but still it had no output. There wasn't a short circuit on the output. In my case, replacing the faulty LM1117 permanently fixed the problem.
The LM1117 is such a popular chip that many different companies make them. Perhaps they are not all equal in quality?Leave a comment:
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Re: magnavox 32mf231d/37 no power backlights flicker
Update:
Replacing capacitor 2002 (100uf/25V) [SIZE="7"]IS[/SIZE] the solution to the fried voltage regulator IC 7003 (L5973D).
I've been using the repaired TV set for a full week. It is continuing to work perfectly!
-EBRe: magnavox 32mf231d/37 no power backlights flicker
Update:
Replacing capacitor 2002 (100uf/25V) [SIZE="7"]IS[/SIZE] the solution to the fried voltage regulator IC 7003 (L5973D).
I've been using the repairedLeave a comment:
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Re: magnavox 32mf231d/37 no power backlights flicker
Good news!
I have 2 main boards that are working perfectly after replacing the L5973D (PC board location 7003) [SIZE="6"][B]AND[/B][/SIZE] capacitor 2002 (100uF/25V).
This confirms L5973D fails because capacitor 2002 has excessively high ESR (or is totally open circuit).
Details will follow
-EBRe: magnavox 32mf231d/37 no power backlights flicker
Good news!
I have 2 main boards that are working perfectly after replacing the L5973DLeave a comment:
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Re: magnavox 32mf231d/37 no power backlights flicker
I'm on the trail of finding out why so many of the main PC boards on these TV sets fail.
Quite often the 24V to 12V regulator IC at location 7003 will be burned, cracked, or have a small hole near pin 7. The part # for this IC is L5973D. This IC is on the back side of PC board. To see it you must remove the PC board from the TV set.
I suspect that the L5973D fails because capacitor 2002 (100uF 25V) has high ESR. This cap is mounted on the front side of the PC board. It filters the 24V rail input to...Leave a comment:
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Re: Magnavox 32mf231d no picture or sound
Please check the thread below for additional info about replacing the L5973D voltage regulator.
[url]https://www.badcaps.net/forum/showthread.php?t=28636&highlight=l5973%2A[/url]
It is 100% necessary to replace the SMT filter capacitor marked 2002 whenever the L5973D IC is replaced. This capacitor 2002 (100uF 25V) tends to develop high ESR without bulging or leaking. The high ESR of this capacitor is (I think) what causes the L5973D IC to fail!
I have 2 main boards for the 32MF231D which...Leave a comment:
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Re: L5973D IC info for Magnavox 32MD231D
UPDATE:
The non-working main PC board (p/n 313815866881) I bought on ebay is now working.
I replaced the L5973D (item 7003 on PC board). I also replaced capacitors 2002 (100uF 25V) and 2007 (47uF 35V). Both of these capacitors had very high ESR ( > 9 ohms), even though they weren't leaking or bulging. Cap 2002 is directly at the 24V input pin of the L5973D. Although there are several other filter caps on the 24v power rail, none of them are physically close to the L5973D.
Important note: The...Leave a comment:
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Re: Old caps...
Even more important than the electrolytics, electronic gear from the 1950's and 1960's contains many "paper" capacitors. These are sometimes referred to as "wax paper," "bumblebee," or "black beauty" capacitors. In most cases these will ALL have compromised performance after 50-60 years. DC leakage, even direct shorts, are common. Also, the uF capacitance value often changes with time. For some reason it often increases: A capacitor marked 0.1uF may measure as 0.15uF.
Check "Antique radio forums" as...Leave a comment:
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Re: Saw a cap vent today
Black Beauty caps.
Guitar players will pay an arm and a leg for these things. They use them inside the guitar body as a tone control cap. I guess that's OK since there's no DC voltage and the AC voltage is only millivolts!
So should we all start selling the old Black Beauty and "bumblebee" caps on ebay that we take out when we are recapping? I've probably thrown away a couple hundred of them! Silly me!
EBLeave a comment:
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Re: L5973D IC info for Magnavox 32MD231D
I purchased a 32MF231D scaler board on eBay. It was advertised as "needs repair." Because of that it cost me only $22.
It's L5973D IC is burned, same as my TV.
In fact, the eBay scaler board has a 1 inch long black smoke trail which starts at the L5973D! This board shows no sign of previous repair, which is good.
Hopefully I will be able to remove the bad L5973D without damaging the PC board. My plan is to unsolder the 8 IC pins first. Then I will use my Dremel tool to grind down the...Leave a comment:
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