While testing a head unit at home, like a genius, I accidentally hooked up an RCA cable to the rear pre-out, with power on. Thought I had flipped the switch on the power supply to off, but it was on. The other end of the RCA was also connected to an amp. So now there is no output from the pre-out at lower volume, and when the volume is turned up, mostly static with a tiny bit of musical signal, as volume gets turned even higher. It was working before, and the front pre-out is just fine.
My question is what gets shorted out, some internal fuse, the RCA jack itself, or something else. Could there possibly be an easy repair, or if repair may be difficult, does anyone have a recommendation for a repair technician? Or did I just ruin my head unit for good?
Thanks,
Darin
My question is what gets shorted out, some internal fuse, the RCA jack itself, or something else. Could there possibly be an easy repair, or if repair may be difficult, does anyone have a recommendation for a repair technician? Or did I just ruin my head unit for good?
Thanks,
Darin
Connecting or disconnecting the RCA cables with the amp on will not damage the head unit.
Check the shield ground on the RCA output cables.
http://www.bcae1.com/images/rca/temporaryrcashieldrepair.html
Check the shield ground on the RCA output cables.
http://www.bcae1.com/images/rca/temporaryrcashieldrepair.html
Hi Perry. Thank you for responding. The head unit was hooked up to the power supply too, it just wasn't powered on when I connected the RCAs.
I checked the RCA shields as in the guide, and got 0.2 ohms resistance on all of the shields. I did this with no power hooked up to the head unit. Does there need to be power connected at all while checking? What else could it be?
I checked the RCA shields as in the guide, and got 0.2 ohms resistance on all of the shields. I did this with no power hooked up to the head unit. Does there need to be power connected at all while checking? What else could it be?
I would suspect the muting transistors in the head unit. These are generally damaged when 12v contacts the center conductor of the RCA jacks.
What make/model head unit?
Are the amp and the head unit connected to the same 12v power supply?
What make/model head unit?
Are the amp and the head unit connected to the same 12v power supply?
Head unit: Panasonic CQ-TX5500W (Panny Bottlehead)
Yes, hu and amp were connected to the same 12v power supply.
Yes, hu and amp were connected to the same 12v power supply.
The head unit uses muting transistors. For the read channels, they are Q241 and Q341. Check to see if they're shorted (no power applied).
If you're not sure, compare the readings to the front muting transistors (Q255 and Q355).
If you're not sure, compare the readings to the front muting transistors (Q255 and Q355).
Looked up part numbers and they are NPN resistors.
In resistance mode on Fluke 87 (+ probe on base pin):
Q241 (+ on base) 0 k-ohms and 9.97 k-ohms
Q341 (+ on base) 0 k-ohms and 10.0 k-ohms
Q255 (+ on base) 0 k-ohms and 10.0 k-ohms
Q355 (+ on base) 0 k-ohms and 9.98 k-ohms
In diode mode:
Q241 (+ on base) 0 V and 2.51 V
Q341 (+ on base) 0 V and 2.52 V
Q255 (+ on base) 0 V and 2.52 V
Q355 (+ on base) 0 V and 2.54 V
When in reverse bias in diode mode (negative probe on base), there is a voltage reading from emitter and collector (0.7/2.5 V on all transistors), and one way from collector to emitter (0.7 V) but 0 V from emitter to collector. Maybe those are backwards, but one way there is a voltage. Shouldn't all the reverse bias reading be 0? Just going by what I saw on uTuber.
In resistance mode on Fluke 87 (+ probe on base pin):
Q241 (+ on base) 0 k-ohms and 9.97 k-ohms
Q341 (+ on base) 0 k-ohms and 10.0 k-ohms
Q255 (+ on base) 0 k-ohms and 10.0 k-ohms
Q355 (+ on base) 0 k-ohms and 9.98 k-ohms
In diode mode:
Q241 (+ on base) 0 V and 2.51 V
Q341 (+ on base) 0 V and 2.52 V
Q255 (+ on base) 0 V and 2.52 V
Q355 (+ on base) 0 V and 2.54 V
When in reverse bias in diode mode (negative probe on base), there is a voltage reading from emitter and collector (0.7/2.5 V on all transistors), and one way from collector to emitter (0.7 V) but 0 V from emitter to collector. Maybe those are backwards, but one way there is a voltage. Shouldn't all the reverse bias reading be 0? Just going by what I saw on uTuber.
Nothing that has to be shipped to me.
The readings on the transistors were inconclusive but since all read the same, they're likely OK. I think you'll need to check the signals with a scope to see where the distortion starts in the circuit.
The readings on the transistors were inconclusive but since all read the same, they're likely OK. I think you'll need to check the signals with a scope to see where the distortion starts in the circuit.
That might be a little beyond my skillset, unfortunately. At least not without some serious handholding/guidance. Any recommendations for a trustworthy repair technician that may be able to help diagnose/repair?
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I don't have anyone that I'd recommend. You can get the local or closest authorized repair station from Panasonic.
Find An Authorized Service & Repair Center For Panasonic Commercial Food Products
Panasonic Repairs U.S.A., Panasonic Service Centers
The service manual is available on the following site:
ElektroTanya | Service manuals and repair tips for electronics experts
Find An Authorized Service & Repair Center For Panasonic Commercial Food Products
Panasonic Repairs U.S.A., Panasonic Service Centers
The service manual is available on the following site:
ElektroTanya | Service manuals and repair tips for electronics experts
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