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Ask Louie Your Own Question
Louie, Technician
Category: Home Theater-Stereo
Satisfied Customers: 12459
Experience:  Have been in the business 25+ yrs., down to component level.
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I have been given a sherwood RX4503 Tuner/amplifier to look

Customer Question

I have been given a sherwood RX4503 Tuner/amplifier to look at and, repair if possible. As a radio tech (retired) I can see that the unit has the start up circuitry for the power supply, and connecting relays for the speakers etc, as is normal for large amplifiers. I have no schematic or service information. On setting up on the test bench, and turning on, there is no response, nothing. the main transformer is not being energised. there is another small transformer on the main board. I do not know what it`s status is. do you know where I might go from here ? Ray
Submitted: 2 years ago.
Category: Home Theater-Stereo
Expert:  Louie replied 2 years ago.
Hi and welcome back to JustAnswer,Thank you for the question and your patience. Should you still need assistance for information purposes only...Initially, bear with us; post back with a clear closeup picture(s) of the inside circuitry.
Customer: replied 2 years ago.

Hi Louie, we have worked before, on aVCR combo I think.

with the Sherwood RX4503 I have checked the 230v mains through to the main board, checked the primary and secondary windings on the power transformer, the on off switch on the front panel. there are no signs

of heat stress any where. I have never worked on a unit that has a small instrument type transformer to supply a low control voltage for relay operation and start up etc. I am just at the stage where I am about to remove the main board and have look underneath. Are there any voltage checks I should do before that ? RayCircuit Board close up 1Circuit Board close up 2

Expert:  Louie replied 2 years ago.
Hi again Ray and appreciate the pictures.Yes, the small green transformer is responsible for the standby voltage; i.e. in most Sherwood designs; a) +5.6V and b) POWER CHECK.Proposed first check would be for the integrity of this transformer's primary winding. With the unit unplugged, please measure the resistance prong-to-prong of the AC plug. Since the standby transformer is directly connected to the AC plug, then it should show a certain reading, not a short and not open or very very high.
Customer: replied 2 years ago.

Hi Louie, I`me back, had a few dramas around the ranch ! Well, the small control transformer is OK, and producing a nominal 5 v DC out. this comes and goes with the operation of the front panel on off switch. I have done a diode test to all the semiconductors and diodes in the area, and all the coils of the relays are OK, including the back EMF diodes across the coils.every thing seems good. can't help thinking I am missing something here. With out a circuit of the relay drivers etc. I am really working in the dark here. RAY.

Expert:  Louie replied 2 years ago.
Sorry Ray as I too do not have the schematics/Service Manual for this particular model.Might you perhaps want to solicit a second opinion from another member of the site? Hopefully another member has access to the schematics and/or offer an alternate approach to your concern.