Musicman RD100 - Low B+ readings

[Enzo]

OK
260VAC ought to rectify and filter to about 360vDC across each cap. SO do you have about the same DC across each cap, but half what it ought be, or are they way apart from each other?

[Enzo]

OK
260VAC ought to rectify and filter to about 360vDC across each cap. SO do you have about the same DC across each cap, but half what it ought be, or are they way apart from each other?

[Gaz]

OK
260VAC ought to rectify and filter to about 360vDC across each cap. SO do you have about the same DC across each cap, but half what it ought be, or are they way apart from each other?

I get 357vdc across C32, and .070vdc across C33, so no screen voltage

[Enzo]

You have two caps in series. The power supply charges them alternately with each cycle of the AC. They are in series - they add. Add nothing to 357 and you get 357. So either C3 is shorted, or shorted across by something else, or charging current is not getting to it.

Screen schmeen, you are missing half your power supply, regardless of where it might be used.

[Gaz]

You have two caps in series. The power supply charges them alternately with each cycle of the AC. They are in series - they add. Add nothing to 357 and you get 357. So either C3 is shorted, or shorted across by something else, or charging current is not getting to it.

Screen schmeen, you are missing half your power supply, regardless of where it might be used.

Enzo, I swear I'm not as absent minded as you think! I'm trying to be a good student

I have the new caps jumpered in, so I know it's not the cap itself. I'm not measuring continuity with ground from the junction of C32 and C33, so it seems like C33 is not being shorted. That would surely cause the fuse to blow or something to burn up, wouldn't it?

So right now I'm measuring voltage at the red/yellow tap, but it 'disappears at the cap. I just can't figure out where this dang voltage is going!

[Enzo]

Clipping in the new cap is not enough, disconnect one end of the old cap.

I don't know that the cap is bad, but I will say that meters don;t test caps. They will tell you if a cap is a dead short or otherwise grievously wounded, but a meter uses a volt or two to test anything. Your 400v cap might check just fine at volts, but be an open door at anything over 20v. So your cap could still be dragging this whole deal down.

Isolate the problem.

Look at the schematic, D10, D11. I am going to call them upper and lower, since the "D10" is not next to one or the other. When current flows one way, it flows through the lower diode, the winding, and charges C33 to some V+. At that moment, the lower D is forward biased and conducting, and the upper diode is reverse biased and blocks anything through itself. Then when the AC flows the other way, the upper D conducts, and the lower blocks current. SO the current then is through the upper D and the widning to charge C32. SO it alternates, charging C32 or C33. They are in series so their voltages add for use in the circuit.

Now what if we disconnected one end of the upper diode and left it open? You would have just the lower D and the winding, and that rectified AC would charge just the C33. There is nothing simpler than that circuit - one diode in series with an AC winding, to charge a cap.

SO do that, lift the upper diode fom the circuit, and see if what remains can charge C33.

And before you do that, I don't recall that we checked the lower diode for ground connection, did we? From the anode lead of the lower diode, is there continuity to ground/chassis? The diode might be OK, and the cap may be grounded, but if the diode is not also grounded, there is no complete circuit - and no cap charging.

[Enzo]

Clipping in the new cap is not enough, disconnect one end of the old cap.

I don't know that the cap is bad, but I will say that meters don;t test caps. They will tell you if a cap is a dead short or otherwise grievously wounded, but a meter uses a volt or two to test anything. Your 400v cap might check just fine at volts, but be an open door at anything over 20v. So your cap could still be dragging this whole deal down.

Isolate the problem.

Look at the schematic, D10, D11. I am going to call them upper and lower, since the "D10" is not next to one or the other. When current flows one way, it flows through the lower diode, the winding, and charges C33 to some V+. At that moment, the lower D is forward biased and conducting, and the upper diode is reverse biased and blocks anything through itself. Then when the AC flows the other way, the upper D conducts, and the lower blocks current. SO the current then is through the upper D and the widning to charge C32. SO it alternates, charging C32 or C33. They are in series so their voltages add for use in the circuit.

Now what if we disconnected one end of the upper diode and left it open? You would have just the lower D and the winding, and that rectified AC would charge just the C33. There is nothing simpler than that circuit - one diode in series with an AC winding, to charge a cap.

SO do that, lift the upper diode fom the circuit, and see if what remains can charge C33.

And before you do that, I don't recall that we checked the lower diode for ground connection, did we? From the anode lead of the lower diode, is there continuity to ground/chassis? The diode might be OK, and the cap may be grounded, but if the diode is not also grounded, there is no complete circuit - and no cap charging.

[Gaz]

Clipping in the new cap is not enough, disconnect one end of the old cap.

I don't know that the cap is bad, but I will say that meters don;t test caps. They will tell you if a cap is a dead short or otherwise grievously wounded, but a meter uses a volt or two to test anything. Your 400v cap might check just fine at volts, but be an open door at anything over 20v. So your cap could still be dragging this whole deal down.

Isolate the problem.

Look at the schematic, D10, D11. I am going to call them upper and lower, since the "D10" is not next to one or the other. When current flows one way, it flows through the lower diode, the winding, and charges C33 to some V+. At that moment, the lower D is forward biased and conducting, and the upper diode is reverse biased and blocks anything through itself. Then when the AC flows the other way, the upper D conducts, and the lower blocks current. SO the current then is through the upper D and the widning to charge C32. SO it alternates, charging C32 or C33. They are in series so their voltages add for use in the circuit.

Now what if we disconnected one end of the upper diode and left it open? You would have just the lower D and the winding, and that rectified AC would charge just the C33. There is nothing simpler than that circuit - one diode in series with an AC winding, to charge a cap.

SO do that, lift the upper diode fom the circuit, and see if what remains can charge C33.

And before you do that, I don't recall that we checked the lower diode for ground connection, did we? From the anode lead of the lower diode, is there continuity to ground/chassis? The diode might be OK, and the cap may be grounded, but if the diode is not also grounded, there is no complete circuit - and no cap charging.

Enzo, you are wise and persistent man!

It turned out that the bottom diode was not connected to ground. Naturally, I had checked the ground eariler, however at the joint rather than the short lead of the diode itself. It was hard to tell without close examination, but large solder joint/blob had cracked, and the diode was not making contact. I resoldered it and voltages are back to normal! Thanks so much for helping me though the trouble-shooting process, and for explaining the functioning of the rectifier as well.

And I think I should mention that the original problem with the amp was that heaters were intermittent, and my friend could see the tubes glowing on and off. He said that when he would hit the top of the amp they would sometimes come back on. I thought: "what an easy fix! Just a bad bad connection in the heater circuit."

Sure enough, someone had replaced one of the octal sockets and done a really sloppy job. One of the wires had broken, and so I repaired it. That's when I noticed the low voltage and very little strained sound coming from the amp. My friend then informed me that the amp had behaved this way as well in addition to the intermittent heaters. He also noted that when he could get the heaters to light up long enough by hitting the amp it would sound fine, but that this other problem only happen more recently.

I wanted to share the whole story because what I think happened is that all that hitting and banging of the amp to get the heaters to come back on broke the weak joint of the bottom diode, creating the other problem.

And sad but true: another friend just brought over a V4 that was acting up. Sure enough he said that when he hit it sometimes it would be fine. I plugged it in and played it. It sounded fine. He was kind of frustrated and hit the top to "show me what it was doing." Sure enough, after about 5 smacks the sound became faint and a loud buzz appeared. He said, "Well, it didn't do THAT before." He proceeded to turn red, and I told him I'd take a look at it after I looked at Evan's Musicman.

Enzo, if you're still reading, thanks again.

[Enzo]

The essence of troubleshooting is to isolate the problem. Your B+ was half what it should have been, we then found one of the series caps was not being charged, we then had to find out where that simple diode/transformer/cap circuit was open. It is the systematic search that finds the problem.

I am glad you found it.