Foreplay II buzz-need help with snubbers

The diodes conduct from the unbanded end to the banded end.  That is how they rectify.  Banded diodes like the UF4007s that Bottlehead now uses go in the same way that the current diodes are put in.  Be certain that you remove only one diode at a time replacing it with the new diode.

You will notice in your picture that two of the diodes have a banded end toward the blue (DC) wires and two have the banded end away from the blue wires.

There should be an electronics supply house in Portland that sells UF4007 diodes.
 
Hey there Grainger.

I think what he wants to do is replace the UF's with Crees. That way he can dispense with the whole snubber nonsense (well, not nonsense, but a pain in the butt if you can just replace some rectifiers with 5 dollars worth of schottkys).
 
Len said:
Hey there Grainger.

I think what he wants to do is replace the UF's with Crees. That way he can dispense with the whole snubber nonsense (well, not nonsense, but a pain in the butt if you can just replace some rectifiers with 5 dollars worth of schottkys).
Click to expand...

Yup.  As I remember a poster once mentioned that after the UF4007s you could put a Cree or other soft recovery diode and eliminate the reverse recovery spike.  Since a Cree won't pass it and has a minimal forward drop.  I don't remember if it takes 2 or 1 in series.
 
Before the bridge you have AC, and four legs, after the bridge you have a positive buss and negative buss.  The noise is supposed to be riding on the high voltage so you just put a soft recovery, Cree diode in the direction of the current (cathode to the capacitor) on the positive leg.  I still can't remember if there is any need on the negative leg.  It is already grounded so it may not need it.

I think VoltSecond is who suggested this.
 
OK, so my memory is shot!

First, there were two different power transformers used with the early Foreplays; one had a center-tapped high voltage section (which needs two diodes) and the other had a single winding and needs four diodes. You have the latter.

Second, the earliest Cree silicon-carbide high voltage Schottky diodes had a fat section of the cathode pin, but the current versions don't - you now have to identify pin 1.

It looks like the pins on the Cree diodes are not long enough to reach the full length of the terminal strip, which as you can see in the picture is the current setup.

So, whether you use the Cree diodes in place of the UF4007, or add the RRSF (reverse recovery spike filter), you will need to come up with a replacement mounting and wiring scheme.

I don't recall in this thread any mention of grounding the chassis plate by connecting terminal 14 to terminal 13 - that has helped many an old Foreplay. In the pictures the chassis plate does not look like an aluminum color - I hope that's just the computer's color rendition!
 
Chassis plate is aluminum, but painted a gold color. I ordered the diodes from PartsConnexion. I'll post again after the install. I'll also get a manual from Bottlehead. Plus try and see if I can ground the terminal 14 to 13. or maybe it already is.

Thanks again,
Tom
Portland, Or.
 
Tom, you should make sure that the circuit common terminal, terminal 12 is wired to the safety ground (where the incoming power wire grounds).  Try it first with a jumper.  But I think it is a good idea regardless for safety sake.

Just about everyone who put in the jumper lowered the noise.  A very few rare instances the noise increased.  That is the reason for trying the jumper first.
 
So I tried connecting terminal 12 to the ground leg of the power, very loud hum. Terminal 14 is connected to terminal 13, as suggested in one of the previous posts. I tried one of the Cree diodes( I have 4), connected after one of the existing diodes(being careful to connect the cathode end correctly). The buzz is a little quieter, but not much. I tried connecting a second Cree to the other diode that is oriented in the same direction, a lot more buzz. Removed it, of course. So I have only one Cree diode installed.

One issue that may be related is a grounding noise when ever I touch the volume. It does have a stepped attenuator. I appreciate all the help so far. I feel that I am getting somewhere, slowly but surely. Just that last bit of buzz....
 
Tom,

I'm going to look and see which terminal I should have suggested be connected to T13. (I'll post here to verify whether T12 was right or wrong.)

Looking at your quote in Red, this makes some sense, depending on which diode you connected the Cree after.  That is, it should be after the diode that is connected to the positive on the first capacitor in the power supply filter.

Looking at your quote in Green, I'm not sure what was done.  If it is connected between the power supply common and the anodes of the two diodes that are connected to the power supply common then that is where it should be inserted.

Cree diodes negate the need to install the RRSF since they have no reverse recovery spike.  I'm pretty sure one between the + of the bridge to the first filter capacitor should block all the reverse recovery spikes that the stock filter could pass.  So if the red text is correct the bridge is not your source of buzz.

manfred99 said:
So I tried connecting terminal 12 to the ground leg of the power, very loud hum. Terminal 14 is connected to terminal 13, as suggested in one of the previous posts.

I tried one of the Cree diodes (I have 4), connected after one of the existing diodes (being careful to connect the cathode end correctly). The buzz is a little quieter, but not much. I tried connecting a second Cree to the other diode that is oriented in the same direction, a lot more buzz. Removed it, of course. So I have only one Cree diode installed.

One issue that may be related is a grounding noise when ever I touch the volume. It does have a stepped attenuator. I appreciate all the help so far. I feel that I am getting somewhere, slowly but surely. Just that last bit of buzz....
Click to expand...
 
I put the first diode after the diode that is pictured here on the right, the second diode I installed was to the diode next to it. Both diodes have the banded(cathode) end going toward the cap. I will take another pic this afternoon of what I did.

foreplay3.jpg


Tom
 
Tom,

This sounds like you put a Cree diode from a diode cathode to the terminal strip.  Is that right?

Looking at your picture it seems the terminals are numbered from top (1) to bottom (5).  What should help is putting a Cree diode from (anode) terminal 2 to (cathode) the blue wire.  I.E. in series with the blue wire.  This should guard against reverse recovery spikes.

This all assumes that your problem is caused by reverse recovery spikes.
 
Well, this is what I did. So it should have gone from the blue wire, cathode end,to the second diode, anode end? I have a pic with a line drawn as to wear the diode should connect.
cree1.jpg

cree2.jpg


Tom
 
Tom,

The simplest way to do it is in series with the blue wire.  I would try to swap the UF4007/Cree combination on the left with the diode that is second from the right.  Be careful to keep the directions right, that is, both have to be swapped to give you a full wave bridge.

I wouldn't try to mess with the other diodes, just swap 1 and 3 (numbering from left to right).
 
I put the Cree diode as per your previous message, Cree diode from (anode) terminal 2 to (cathode) the blue wire.  I.E. in series with the blue wire, the buzz didn't change.

So I have enclosed another pic with the wires numbered 1-4, please explain again, I didn't quite understand which left and right.

cree3.jpg


Tom
 
Tom,

The Cree diode is not really where I suggested.  For the Cree diode to be as I suggested it would have the anode on terminal 2 and cathode soldered to the blue wire.

However, I saw something completely different in your previous picture.  I don't know why.

What you did is put the Cree diode in series with one of the two diodes that feeds terminal 2.  That would eliminate 1/2 of the RRS noise.  Another Cree diode with its cathode on Terminal 2 and in series with diode 3 (last picture) would eliminate the last half of the RRS noise.

I'm still wondering why I didn't see the Cree diode correctly in the picture.  Age maybe?
 
Sorry, that was a previous pic, just to get the numbers on. Here's how I have it. Cathode on the blue wire, anode to terminal 2. Still have the buzz. I'm pretty sure at this point that the buzz is coming from somewhere else.

cree4-1.jpg
 
That is just as it should be.  If there is buzz the diodes are not at fault.  No RRSF will help.

What is your situation now?
 
Buzz remains. I have a spare preamp to use. I may take it to a local tube audio repairman. Not sure what else to try. Filament snubbers? I'll make a copy of Voltseconds web page when I take it in.  I never learned to read schematics.

Thanks for your time and effort.
 
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