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Louie
Louie, Technician
Category: TV
Satisfied Customers: 11756
Experience:  25+ yrs. experience in the field and 5+ yrs. online, down to component level.
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Magnavox 32MF338B/27Replaced FETs on both boards and current

Customer Question

Magnavox 32MF338B/27
Replaced FETs on both boards and current sense resistor on inverter, replaced two C1815 transistors, one each board. All caps check good. Replaced all three opti-couplers on both boards. Doesn't blow fuse. Red front panel LED will stay on forever, power switch turns on green LED...then it blinks 6 to 10 times and turns back to red. Inverter board never lights bulbs. Replaced main power board with a NOS one. No signs of hot parts anywhere. Have checked and found no other bad parts. Have not replaced the micro-p board. It does provide control signal to the inverter board. Where one should see about 36 volts on the main board is only about 8 volts.
Submitted: 1 year ago.
Category: TV
Expert:  Russell H. replied 1 year ago.
Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
If the 36 volts is only 8 volts there are two possible explanations:
1. the power supply for the whole TV, is defective.
or
2. there is a short (or, current overdraw or current overload) somewhere in the TV, that 'weighs down' the 36 volt line.
In either case, or to tell one case from the other, I suggest investing in a service manual for the TV model in question. It would have troubleshooting guides etc. for you to use.
This service manual is for-free, provided you download it correctly:
http://elektrotanya.com/magnavox_32mf338b-27_chassis_fuz2.ou-la_sm_1.pdf/download.html
but! you must let the page load, wait a minute and a half or so, then look below the preview image of the first page, for a small link like this:
Get Manual
and click on that link to download the manual. Do not click on anything else, you do not need anything else, all other links and large 'DOWNLOAD' or 'VIEW PDF' logos are inessential!
...and this one is for-pay, but is from a very reliable online source that I have often recommended and have never heard a bad work about:
http://www.servicemanuals.net/ServiceManual/MAGNAVOX/32MF338B27/503749/detail.aspx
I hope this advice helps. Thanks.
(PS: the variable number of blinks on the Power LED or green LED, suggests to me that various things may be detected as gone-wrong when a wrong voltage is applied - as well as, perhaps, a shorted board or shorted component - to the TV's circuitry.)
Customer: replied 1 year ago.
I have the factory service manual, the main power supply board has been replaced with a NOS board. All transistors, resistors, caps, diodes have been checked on a Tek 676? or so curve tracer and any part replaced as needed. This one is a stinker but I have been doing this stuff for 37 years...it still could be the micro-p board. Thanks, ***** ***** AI4WZ.
Customer: replied 1 year ago.
Russell, thanks for your canned answer. I stated I have curve tracer, 4 wire ohm meter, cap meter and the service manual, 37 years working with electronics and hold a US patent so I'm armed. I was hoping for a know problem solution with this model and not a bs boiler plate answer. I won't waste your time again as well as my time and money.
Expert:  Russell H. replied 1 year ago.
I don't think I have any further thoughts to offer on this case, unfortunately, so I have Opted Out in favor of another Expert in this category.
If and when another Expert takes up the case, you ought to receive a notification email.
Expert:  Dimmu TV replied 1 year ago.
.
Hello my name is***** look forward to assisting and am very sorry to hear that you are having this problem.
After changing those parts do you get the exact same light pattern?
.
Customer: replied 1 year ago.
Yes
Customer: replied 1 year ago.
Which tells me i have not found nor corrected the problem. The uP board is about all that is left and it does control the switching on of the inverter or backlight board. No other bad parts have been found and it does not blow the fuse..which is 3 amp vs a 4 amp as printed on the main board.
Expert:  Dimmu TV replied 1 year ago.
.
That sounds like it could be the panel itself. Please double check and let me know which it shows exactly now 6 or 10 flashes. If it is a pattern like 6 and then 10 instead give me that please.
.
Customer: replied 1 year ago.
Plugging in starts with what seems like it blinks 16 times (kinda fast and continuous), turns solid green then back to red and will repeat each time the power button is pressed. The CCFT lamps never light and the fuse does not blow.
Expert:  Dimmu TV replied 1 year ago.
.
Thank you for clarifying that for me. 16 blinks is normally caused by the main board usually a bad cap but if you already tried that the entire board may need to be replaced. If you are not sure which board that is it should have the a/v ports on it here is an example:
http://www.shopjimmy.com/magnavox-a8afcmps-mps-board.htm
If you have any questions at all click on reply to expert and let me know.
If not please don’t forget to leave a positive rating for my help - it is greatly appreciated.
Please note you are rating me only not this site or your Magnavox.
.
Customer: replied 1 year ago.
Please remember the power supply board (main board) has been replace with a NOS part. The factory service manual does not have the blink codes listed or we might not be here. I have seen tons of LCDs with broken glass that still light so I think that can be ruled out. Just about everything on the left half of the inverter schematic has been checked or replaced. I have not replaced the OZ9966SN inverted controlled IC but can be done for cheep.
Expert:  Dimmu TV replied 1 year ago.
.
The power supply and the main board should be two different boards.
Can you give me the part number of what you replaced? Also since you are ruling something out without any kind of test simply because it lights up (doesn't mean it is working fully) I am going to be opting out and opening this to all experts.
.
Customer: replied 1 year ago.
Okay, on this turkey the AC comes into the inverter board first containing the line fuse and full wave bridge, common mode filters and DC filter caps - which have all been replaced or checked. The main switching FET was open (hot side) as was a .27 ohm resistor, a small signal C1815 transistor (cold side) was also open. Both opti-couplers replaced with know good ones. The inverter board is passing on DC to the power supply board the correct 168 VDC. The inverter board is BA8AF01032 A8AF2D2141.
The main power board, also a switcher is BA8AF0F01023-1 A8AF2MPS which had an open FET (hot side) and an another open C1815 transistor. New board number here is (NOS) BA8AF0F01023-1 A8AFEMPS seeming to have only outside AV onboard connector differences vs the original. Both boards provide the same results.
.
Expert:  Louie replied 1 year ago.
Hi and welcome to JustAnswer,
Thank you for the question and your patience. Should you still need assistance for information purposes only...
Initially, on the inverter power supply:
• if Q1930 failed, were zener diodes D1930 & 1931 checked out-of-circuit particularly for reverse/leak?
• which 2SC1815 please; T1970? If yes, were zener diodes D1970 & 1971 checked also?
• and is the 0.27Ω R1937? If yes, was zener diode D1934 checked as well?
Basically the same check on the zener diodes of the main board power supply.
And where was "...36 volts on the main board is only about 8 volts..." checked please?
Customer: replied 1 year ago.
Louie, glad to get more indepth questions. Diodes D1930 & 31 checked for Vz but I will look closer at Vr leakage.
Transistor Q631 on power supply board was open. D1970 & 1971 on the inverter were ok, again will look at Vr.
R1937 was open and replaced. D1930 was open and replaced. I will revisit D1934 for Vr. It did have 36 for Vz. Most all resistors have been checked.
The power supply board was replaced with an NOS unit but didn't change anything.
For the two leds, when plugging in AC the green one blinks about 14 to 16 times, turns solid for a second then the red comes on and will remain on. Pressing the power button will repeat the above cycle.
"...36 volts on the main board is only about 8 volts..." that is on the inverter board not the main board as I said.
Expert:  Louie replied 1 year ago.
Sorry but the "...36 volts on the main board is only about 8 volts..." was part of the initial posting which made me think/suspect tuner circuit.
And, "...NOS unit...", not familiar with the term.
Focusing on the inverter board first; what are the pin voltages of connector CL1000A
Customer: replied 1 year ago.
NOS means 'New Old Stock'
CL1000A
1 1.14v red or green, should be 3.4 Q1002 checked good. ICs 1500 &1550 replaced, all diodes check good.
2 3.15v then .5 at red led
3 3.0 then .1 at red led
4 GND
Expert:  Louie replied 1 year ago.
Thanks.
What is the voltage at the base of Q1002? If not 0.0, please check voltages of pins 1, 7, 8 & 14 of both IC1500 & IC1550.
Should you venture to consider/try, attempt to turn on with CL1000A disconnected. Then re-check the voltages of CL1000A pins. Do the same for CL1000B pins.
The idea is to determine if the 3.4V for Q1002 is brought down by IC1500 & IC1550 or originally low/missing from the main board's PSU.
Customer: replied 1 year ago.
Will be tomorrow to get IC 1500 & 1550 readings as I will have to run sniffer wires or find top side points....
pin1 of IC1550 stays at 0v
base of Q1002 stays at 0v
Quick check finds most power supply voltages correct until red LED turns on.
Tried unplugging CCFT tubes one at a time ...no change. Not surprised as they act like zeners @ 700 volts or so when on.
Unplugging CL1000a from PSB still blinked as above but it then stayed green (no jump to red) but the backlights did not light. Think its time time to find a new inverter board on e-Bay...just trying to fix this turkey for my wife as she is totally disabled and home stuck all day. Me, ole school..can't trash anything.
Expert:  Louie replied 1 year ago.
Appreciate the post back now and an update later or when able.
Incidentally, if base of Q1002 is 0.0V, then collector should be +3.4V not unless:
• the main board PSU is supplying 1.4V instead of 3.4;
• or an issue with Q1972 whose base should be -0.2;
• if not -0.2, then a suspect D1980.
Customer: replied 1 year ago.
Turns out IC 1001 is out of service. I hoked up 32 volts to the secondary side of T1951 and everything behaved except IC1001. Thats a $3.00 part but takes 2 weeks to get one from Japan.
Thanks for your help and I will advise how it come out.
Expert:  Louie replied 1 year ago.
Appreciate the update now and again later when able. Cheers.