Vinylcafe
Linvin' the Dream
This is Part 2 of a thread started in April this year.
I felt it it better to start fresh and not have to wade through a long/ old dead thread - sorry for the length of this one.
This project may also end up being the poster child for the old axiom, "if it aint broke, don't fix it".
The monsoon season has set in here, so time to get back to work.
I am rebuilding the power supply board of my 8550, with new electolytics, transistors,resistors, and diodes.
Happy to supply complete list of components replaced when needed.
My challenge at the moment is that voltages are both low and high (depending on the winding in question), and I cannot determine why.
I've floated the entire board for easier access and testing. Also allows for reading directly off the transformer.
I fear the issue is as follows ... during the rebuild one of the shielded leads from the filter caps somehow nicked a post on the power supply board - odd given the metal sleeves of the connector are slightly recessed inside an insulated shield.
The contact was with the post with the green wire, supplying a nominal ~85 volts direct from the transformer.
The unit was unplugged, but the caps were still charged.
Big bang/spark ensued and a slightly melted lower post and metal sleeve inside the shielded connector resulted.
Also a resistor and diode were knocked out on the amp board (these two components are just voltage regulation for the lamp).
Who knows what else - there must have been a bit of wire or solder protruding from the bottom of that post.
My fear is I have damaged the transformer, which is right up there with frying a V-FET.
Is it possible that there is a component on the board that is somehow dropping the voltage right off the full wave rectifiers?
Am really hoping someone is willing and able to do a step-by-step walk-through of trouble shooting.
I have already taken many measurements and tested virtually all of the components. Happy to share all the data if there are any takers.
I feel like I've driven a vintage Porsche into a pole.
Hoping someone can please help out?
Here are the voltages off the transformer's various windings:
Ground>red 45.1 Vac Board marked ~60
Ground>orange 45.2 Vac Board marked ~60
Ground>green 66.7Vac Board marked ~85
Ground>yellow 66.6 Vac Board marked ~85
Measuring straight across red/orange and green/yellow without reference to ground/center tap - doubles those reading.
This was with all of these leads floated off the power supply board.
While both halves of the two secondary windings match, they appear low.
There is continuity between them, and no continuity between them and the 11 volt secondary winding for the lamp.
As for resistance in the secondary windings, they too are symmetrical:
Here are the resistance measurements for each winding:
yellow/green (80v) 4.5 ohms
yellow>center tap 2.5
green>center tap 2.5
red/orange (60v) 0.8 ohms
red>center tap 0.5
orange>center tap 0.5
Here are the voltages at C 335/336 off the 2 full wave recitfiers:
C-335 positive lead (+88v nominal) actual +62.5v
negative lead (+60v nominal) actual +72.5v
C-336 positive lead (-60v nominal) actual -70v
negative lead(-88v nominal) actual -58v
This is with no connection of the multi pin connector to the power amp board, except for a ground connection to the center tap of the transformer.
Any and all insight appreciated.
Service manual here for anyone interested:
https://drive.google.com/file/d/0B2_JfFthlwCqd0FCUFF3VUdESzg/view?usp=sharing
You can view the entire service manual here.
I felt it it better to start fresh and not have to wade through a long/ old dead thread - sorry for the length of this one.
This project may also end up being the poster child for the old axiom, "if it aint broke, don't fix it".
The monsoon season has set in here, so time to get back to work.
I am rebuilding the power supply board of my 8550, with new electolytics, transistors,resistors, and diodes.
Happy to supply complete list of components replaced when needed.
My challenge at the moment is that voltages are both low and high (depending on the winding in question), and I cannot determine why.
I've floated the entire board for easier access and testing. Also allows for reading directly off the transformer.
I fear the issue is as follows ... during the rebuild one of the shielded leads from the filter caps somehow nicked a post on the power supply board - odd given the metal sleeves of the connector are slightly recessed inside an insulated shield.
The contact was with the post with the green wire, supplying a nominal ~85 volts direct from the transformer.
The unit was unplugged, but the caps were still charged.
Big bang/spark ensued and a slightly melted lower post and metal sleeve inside the shielded connector resulted.
Also a resistor and diode were knocked out on the amp board (these two components are just voltage regulation for the lamp).
Who knows what else - there must have been a bit of wire or solder protruding from the bottom of that post.
My fear is I have damaged the transformer, which is right up there with frying a V-FET.
Is it possible that there is a component on the board that is somehow dropping the voltage right off the full wave rectifiers?
Am really hoping someone is willing and able to do a step-by-step walk-through of trouble shooting.
I have already taken many measurements and tested virtually all of the components. Happy to share all the data if there are any takers.
I feel like I've driven a vintage Porsche into a pole.
Hoping someone can please help out?
Here are the voltages off the transformer's various windings:
Ground>red 45.1 Vac Board marked ~60
Ground>orange 45.2 Vac Board marked ~60
Ground>green 66.7Vac Board marked ~85
Ground>yellow 66.6 Vac Board marked ~85
Measuring straight across red/orange and green/yellow without reference to ground/center tap - doubles those reading.
This was with all of these leads floated off the power supply board.
While both halves of the two secondary windings match, they appear low.
There is continuity between them, and no continuity between them and the 11 volt secondary winding for the lamp.
As for resistance in the secondary windings, they too are symmetrical:
Here are the resistance measurements for each winding:
yellow/green (80v) 4.5 ohms
yellow>center tap 2.5
green>center tap 2.5
red/orange (60v) 0.8 ohms
red>center tap 0.5
orange>center tap 0.5
Here are the voltages at C 335/336 off the 2 full wave recitfiers:
C-335 positive lead (+88v nominal) actual +62.5v
negative lead (+60v nominal) actual +72.5v
C-336 positive lead (-60v nominal) actual -70v
negative lead(-88v nominal) actual -58v
This is with no connection of the multi pin connector to the power amp board, except for a ground connection to the center tap of the transformer.
Any and all insight appreciated.
Service manual here for anyone interested:
https://drive.google.com/file/d/0B2_JfFthlwCqd0FCUFF3VUdESzg/view?usp=sharing
You can view the entire service manual here.
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