QRX-5500 120hz hum caused by my mistake

[crkeith65]

While troubleshooting a dial lamp short that was blowing F1 fuse, I had a wire disconnected from the light box that has 6.3vac. (Lamp Power). When I was moving the light box around to get a better look, the loose wire accidentally contacted a wire on the 330ohm resistor that had 58vdc. See my picture with the blue circles drawn in. Those are the contact points. The protection relay kicked it off but when it immediately came back on I now have a very annoying 120hz hum. The hum goes up and down with volume and changes with tone control. All other functions work as they are supposed to.

QRX-5500

• crkeith65
• Apr 20, 2016

 

[Tom B]

That point that the AC wire touched the 330 ohm resistor should be less voltage. One side of the resistor should be 13V, the other side should be 42V. The temporary short you did could have done damage to the regulator supply. Check the voltages from this supply and make sure they are all correct. If the voltage on the one side of the 330 ohm 10W resistor up at 52v to 58v check to see if the 2SD313 is shorted E to C. Also any of the other transistors or diodes could have been damaged. I would check out all of the components, and check the underside of the board to make sure than none of the circuit runs on the board have been burnt open. Make sure then, that all voltages are correct. Check all of the caps on the regulator supply and replace if necessary.
If you check the power supply and everything looks fine, you may have damage to the preamp board, which is where the 42v goes to and has N channel JFETS which are very sensitive to overvoltage.

 

[crkeith65]

Thanks, I will check that tomorrow. It appears I have some kind of hybrid receiver. My receiver matches more of the schematic of the QRX-5500A than the QRX5500. Is that possible? The Power Supply/Regulator board and one of the vario-matrix boards match the QRX-5500A

 

[Tom B]

i think there are misprints in the manuals.
I have never heard of hybrids. It is usally one or the other. The "A" version puts out a bit more power so I would expect either the same driver output board with heavier outputs, or a different driver board. Regulator supply board and 4 channel boards should be the same

 

[crkeith65]

i think there are misprints in the manuals.
I have never heard of hybrids. It is usally one or the other. The "A" version puts out a bit more power so I would expect either the same driver output board with heavier outputs, or a different driver board. Regulator supply board and 4 channel boards should be the same

I'll get the exact differences tomorrow and report back

 

[crkeith65]

check to see if the 2SD313 is shorted E to C

Good job Tom B that is the second time you have come through for me. 2SD313 was shorted E to C. On the differences I was speaking of : the four channel boards in my unit are F-2048 and F-2084 as is listed in the QRX-5500A manual. Not the F-2048 and F-2047 as listed in the QRX-5500 Manual. The driver board and the protector boards appear to be the same but the Tuner board is also different, F1517 instead of F-1479A. My serial number is 234100287 if that helps anything.

 

[Tom B]

Make sure that you don't have other issues related to the shorted 2SD313. But if your voltages are 42v, 25v, 25v, and 13V coming off the regulator board, you can assume that the remaining transistors and diodes are OK. If voltages are sill off, you may have more work to do.
As preventative maintenance you should look to replace the electrolytics on that regulator board as well. And if you still have a hum after getting the voltages back to where they need to be, I would change those regulator board caps immediately.

FYI, I received my basic electronics training while in the USAF in 1969. We spent hours and hours troubleshooting "inserted problems" onto massive schematic diagrahms we called "table Cloths" that were the entire schematic for some sort of black box used on an aircraft. Inserted issues were things such as a certain transistor shorted, "what happens to the box and signals", or a resistor opens and "what happens". So looking at your issue is was just reversing the USAF troubleshooting. I knew the problem with the output (overvoltage 58V instead of 42V). So now it was kind of easy to find the primary suspects.

 

[crkeith65]

Make sure that you don't have other issues related to the shorted 2SD313. But if your voltages are 42v, 25v, 25v, and 13V coming off the regulator board, you can assume that the remaining transistors and diodes are OK. If voltages are sill off, you may have more work to do.
As preventative maintenance you should look to replace the electrolytics on that regulator board as well. And if you still have a hum after getting the voltages back to where they need to be, I would change those regulator board caps immediately.

FYI, I received my basic electronics training while in the USAF in 1969. We spent hours and hours troubleshooting "inserted problems" onto massive schematic diagrahms we called "table Cloths" that were the entire schematic for some sort of black box used on an aircraft. Inserted issues were things such as a certain transistor shorted, "what happens to the box and signals", or a resistor opens and "what happens". So looking at your issue is was just reversing the USAF troubleshooting. I knew the problem with the output (overvoltage 58V instead of 42V). So now it was kind of easy to find the primary suspects.

COOL, similar situation here. Electronics technology at junior college in 73,74. Then Army electronics training in 77. Studied radio repair. We were troubleshooting modules by then, very little component level stuff. Was Commo Chief for several years. You are right, I do have other issues. Meter lamps is where I wanted to start, then one of the four channel boards is causing crackles and pops. With it removed 2 channel sounds perfect with all other controls functioning. After that comes the tuner. I get a good strong signal but have to crank the volume all the way up to hear it. Am works fine. This is the same problem I had with my 8080DB that you helped me with. Thankful for a good driver board. I do plan to recap where necessary. I am putting together a parts list now. Thanks.