Randall rg75d humm

[cmanningjr]

It did when I gave it the ”smack” test for about half a second...

[Enzo]

Meaning something is loose, and most likely a main filter cap. Resolder the main caps.


The reason a company like Randal suggests trying a new tube to someone writing them for advice: it is the old user-servicable thing in the amp. If it works, great. If it doesn't, then they will recommend going to a qualified service shop. The factories rarely make suggestions that lead a consumer to tinker in the circuitry.

[Enzo]

Meaning something is loose, and most likely a main filter cap. Resolder the main caps.


The reason a company like Randal suggests trying a new tube to someone writing them for advice: it is the old user-servicable thing in the amp. If it works, great. If it doesn't, then they will recommend going to a qualified service shop. The factories rarely make suggestions that lead a consumer to tinker in the circuitry.

[audiopete]

Try cleaning and re-tensioning the 12AT7 socket. Check for bad solders on the tube socket legs and re-solder all 9 joints anyway.

[cmanningjr]

Ok, I have narrowed it down to a loose filter cap.....I FINALLY took time to pull the chassis out and have the speaker connected. Visually I didn't see anything at all (I know that still doesn't mean anything) Turned the power on touched one of the filter caps and vola....NO HUMM!!!

Now, to get the board out and get it resoldered...I may post pictures and ask for advice before I attempt this task. I think I have decent soldering skills....But I want to check it out before I go in...

Thanks for everyones help this far....Great board, I have enjoyed reading about other projects as well.

Thanks again....

[cmanningjr]

The one with the arrow is the one I touched to make it quiet....

[cmanningjr]

Ok, advice on bleeding the caps before I do this?? What do you recommend??

[52 Bill]

Ok, advice on bleeding the caps before I do this?? What do you recommend??

If you just turn off and unplug the amp the the caps should drain down to a safe level in a few moments. Removing that board shouldn't pose much of a problem. You will probably need to clip off the cable ties to free up enough of the wire to get the board turned over to solder it up. If you take cables off of the board, either take a photo of the board or draw up a little diagram so that you will remember which plug went where.

When you solder the cap, you might as well resolder all of them while you're in there. Use a low wattage iron and rosin core solder.

[cmanningjr]

One of the pins is broke off the cap..

Where can I get a replacement??

It's a su'scon 4700uf 63v lx 85' c

Thanks again fir the help

[52 Bill]

One of the pins is broke off the cap..

Where can I get a replacement??

It's a su'scon 4700uf 63v lx 85' c

That's a very common value. Try Mouser or Jameco or DigiKey or Allied or even eBay. Any electronics supply house should have one. Measure the diameter of the cap and the height in millimeters. Check online and be sure you order one that will fit into the same space and has the same lead spacing.

Radio Shack may have a 4700uF but probably not one rated for 63 volts dc.

[cmanningjr]

Got the caps replaced, thought everything was going well. Plugged everything in. No sound. Double checked my work and found QC10 was not plugged in all the way..(mistake) R13 cracked..4k7/1w

Those were the only visibe signs...

Where to go from here??

[52 Bill]

Got the caps replaced, thought everything was going well. Plugged everything in. No sound. Double checked my work and found QC10 was not plugged in all the way..(mistake) R13 cracked..4k7/1w

Those were the only visibe signs...

Where to go from here??

I hate when that happens! It may be that leaving the one connector unhooked caused something to draw too much current. What did it connect to?

R13 is in the high voltage power supply for the tube. If it cracked, did it burn? If it broke from overheating, then something is drawing a lot of current through it. Or did it break from some sort of physical damage?

I'd start by replacing the resistor. If you pull the plug at QC14 that will isolate the power supply. If it powers up and doesn't overheat R13 then the problem is in tube part of the circuit. If it does overheat then C13 is suspect.

I'll suggest that if you don't already have one, you may want to build a light bulb limiter to help in the troubleshooting process.

[cmanningjr]

There is a tiny burn spot on R13 with a crack all the way around it. It was very hard to see. QC 10 was just barely sitting on top of the spade. I hope this silly mistake is What caused it...

I'll look into a bulb limiter...

[Jazz P Bass]

May I suggest, as you appear to know which schematic you found QC10 & R13 on, that you advise all in the post.

[52 Bill]

May I suggest, as you appear to know which schematic you found QC10 & R13 on, that you advise all in the post.

Both are shown on the power supply schematic. QC10 is the main power supply ground point. I don't know what it was connected to. R13 is in the high voltage tube power supply.