I picked up a BAT VK-5i preamp that has a hum problem in the left channel. I found a schematic of the amplification circuitry, but I don't have any info on the input selection, volume control, power supply, etc. I haven't started digging into the potential problem(s) yet, but I'd appreciate any information anyone might have on documentation, similar problems, etc.
The preamp was sent in for service to BAT a few months ago and received new tubes and some cap replacements. But it started humming again within a month or so of getting it back. The previous owner decided it wasn't worth continuing to try to fix it, so he sold it to me as is.
The preamp was sent in for service to BAT a few months ago and received new tubes and some cap replacements. But it started humming again within a month or so of getting it back. The previous owner decided it wasn't worth continuing to try to fix it, so he sold it to me as is.
Well, I was a bit premature in my previous post. The previous owner had reported a hum problem, but when I reseated all the tubes and then powered it up in my workshop system, it seems to sound pretty good with no hum problems.
The previous owner said that he had the caps replaced, but in looking over the preamp, it looks like the only caps that were likely replaced were the output coupling caps. The preamp has a number of Nichicon electrolytics in the power supply that appear to have datecodes from the 90's (9609, 9642). Am I interpreting these datecodes correctly? If so, these clearly need to be replaced.
EDIT: The codes I am referring to are stamped on the top of the caps. I have read elsewhere that Nichicon uses a code with a letter (indicating the factory) followed by a four digit code. I'm not seeing a code like this, but I can't see all the surfaces of the caps without desoldering them and pulling them out. The code stamped on the top is just four digits.
The previous owner said that he had the caps replaced, but in looking over the preamp, it looks like the only caps that were likely replaced were the output coupling caps. The preamp has a number of Nichicon electrolytics in the power supply that appear to have datecodes from the 90's (9609, 9642). Am I interpreting these datecodes correctly? If so, these clearly need to be replaced.
EDIT: The codes I am referring to are stamped on the top of the caps. I have read elsewhere that Nichicon uses a code with a letter (indicating the factory) followed by a four digit code. I'm not seeing a code like this, but I can't see all the surfaces of the caps without desoldering them and pulling them out. The code stamped on the top is just four digits.
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I've attached a schematic I found online. There are a couple features of this design that I haven't seen before and would appreciate any thoughts.
The positive phase input signal is connected through a cap to the negative phase anodes and vice versa. What is the purpose of this?
Each channel has a dual triode (V105, V205) which have the anodes connected to B+ and the gate connected through a cap to B+. Is this some form of B+ noise reduction mechanism?
Thanks.
The positive phase input signal is connected through a cap to the negative phase anodes and vice versa. What is the purpose of this?
Each channel has a dual triode (V105, V205) which have the anodes connected to B+ and the gate connected through a cap to B+. Is this some form of B+ noise reduction mechanism?
Thanks.
The small plate capacitors control the feedback response.
The large grid capacitor sends supply ripple to the grids, which try to cancel it.
The large grid capacitor sends supply ripple to the grids, which try to cancel it.
V105 and V205 are not connected to anything in this schematic , so the purpose is not clear . Maybe you can look in the preamp where are supposed to be connected .
The small caps 4,7pF are normally used for stability , supress some tendency to oscillate on very high frequency .
The small caps 4,7pF are normally used for stability , supress some tendency to oscillate on very high frequency .
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The tubes V105/V205 draw current from the supply. There's hum cancellation, using the grid connection to the B+
to draw current in the reverse polarity of the hum. Some of the old Marantz tube amps used a passive hum cancellation loop.
to draw current in the reverse polarity of the hum. Some of the old Marantz tube amps used a passive hum cancellation loop.
Thanks @rayma and @Depanatoru.
@rayma - that was my assumption regarding V105/V205. It looks like I can pull V105 with no significant ill effect to see if that is contributing to the low-frequency noise and swishing sound in the left channel. It only seems to occur once the preamp warms up.
The previous owner had reported it as hum, but it is much lower frequency than mains hum and is not a constant tone. I can't really hear the low frequency, but it does make the woofer cone move in and out. But the swishing sound is audible although fairly well masked when music is playing.
@rayma - that was my assumption regarding V105/V205. It looks like I can pull V105 with no significant ill effect to see if that is contributing to the low-frequency noise and swishing sound in the left channel. It only seems to occur once the preamp warms up.
The previous owner had reported it as hum, but it is much lower frequency than mains hum and is not a constant tone. I can't really hear the low frequency, but it does make the woofer cone move in and out. But the swishing sound is audible although fairly well masked when music is playing.
Seems like low frequency instability, but be careful as it might get worse instead of better, if you pull the tube.
Check on a scope first, before connecting to a system.
First check the power supply to make sure the filament missing won't cause a problem.
You could just remove the capacitor. That would be a more scientific approach, but be careful.
Good point about the filaments. Unfortunately I don't have a schematic of the power supply, but I should be able to figure out if the filaments are in series by pulling the tubes and checking the connections with an ohm meter. Removing the cap will require pulling out the PCB (which I'll probably have to do eventually anyway).
If the filament checks out, and you have a scope, you can just pull the tube and check for the noise.
I suspect the supply is not regulated. Apparently a previous model had shunt regulation.
I suspect the supply is not regulated. Apparently a previous model had shunt regulation.
The problem is only on the positive phase of the left channel. The negative phase seems ok (based on just a very short warmup). I verified that the filaments for V101, 102 and 103 are connected in parallel, so I tried removing V105 but this had no effect on the low frequency noise (which I wouldn't expect since it is only on one phase).
The schematic I have (and posted above) doesn't match exactly. I think this is the schematic for the VK-5, not VK-5i. One difference is that the preamp I have does not appear to have a balance pot (R168).
Any suggestions on how to proceed with finding the problem would be appreciated.
The schematic I have (and posted above) doesn't match exactly. I think this is the schematic for the VK-5, not VK-5i. One difference is that the preamp I have does not appear to have a balance pot (R168).
Any suggestions on how to proceed with finding the problem would be appreciated.
The positive and negative circuits are identical. Have you tried swapping the set of 3 tubes between phases?
Yes, I have tried moving the tubes around, but haven't tried swapping between channels. I did check all the tubes on the left channel on my Amplitrex and they all tested pretty good. I'll try swapping the tubes between channels to see if that makes a difference, but I am thinking that it might be as simple as a bad output coupling cap.
The previous owner said that some of the caps had been replaced when he sent it in for service a few months ago. I had assumed that the output caps were what was swapped since the date codes on the electrolytics appear to be from the 90s (although they all visually look fine). I did check the B+ rail and it looked pretty clean.
The output caps are Mundorf MCAP Supreme Silver/Gold/Oil caps, which I thought were only introduced in the last ten years, but it's always possible to have a cap that goes bad fairly quickly. I will swap this one cap with something in my parts bin to see if the problem goes away.
The previous owner said that some of the caps had been replaced when he sent it in for service a few months ago. I had assumed that the output caps were what was swapped since the date codes on the electrolytics appear to be from the 90s (although they all visually look fine). I did check the B+ rail and it looked pretty clean.
The output caps are Mundorf MCAP Supreme Silver/Gold/Oil caps, which I thought were only introduced in the last ten years, but it's always possible to have a cap that goes bad fairly quickly. I will swap this one cap with something in my parts bin to see if the problem goes away.
Swap the tubes first. A tube tester probably won't detect noise unless severe.
Only replace parts as a last resort.
Did you say the hum is now gone, or intermittent, or is it still there?
Only replace parts as a last resort.
Did you say the hum is now gone, or intermittent, or is it still there?
Have you cleaned the tube sockets?
If not, you apply cleaner to the tube pins, and insert and withdraw the tube several times, to wipe the socket contacts.
Don't spray the cleaner directly into the socket, though.
If not, you apply cleaner to the tube pins, and insert and withdraw the tube several times, to wipe the socket contacts.
Don't spray the cleaner directly into the socket, though.
Yeah, I used some Deoxit on the sockets yesterday.
Well, I replaced the one cap and have been playing the preamp through my speakers for the past 15 minutes. So far, no noise. Fingers crossed that this did the trick.
Now I have to figure out whether I want to get another Mundorf cap or replace them all with something else.
Well, I replaced the one cap and have been playing the preamp through my speakers for the past 15 minutes. So far, no noise. Fingers crossed that this did the trick.
Now I have to figure out whether I want to get another Mundorf cap or replace them all with something else.