Foreplay II buzz-need help with snubbers

[Paul Joppa]

Sounds like you got a kit preamp without the manual. Never a good idea. Ping Eileen next week when they are back from CES and see about getting a manual so you know the terminal designations and have some pictures of the parts - it will help a lot. Make sure you ask for the C4S upgrade manual as well.

The 3-legged TO-126 black flat-pack transistors are on the four C4S boards. Somewhere further back there is a power supply, with some large round electrolytic capacitors. Scrunched down among the terminal strips are two diodes - black cylinders about 1/2 inch long and 3/32 in diameter, with a wire out each end. Those are the rectifier diodes; they have a white band around one end to indicate the cathode. The Cree diodes are TO-220 flat packs, quite a bit larger; and one of the leads has a wide chunk near the plastic body, indicating the cathode end.

Do be aware that buzz can have many sources, so nothing is a guarantee that it will kill the buzz. Just thought I ought to say that. Very many hum and buzz problems have been siolved on this forum over the years, sometimes on the first try and sometimes only after many experiments and exchanges. It can be frustrating while it happens, but it's a real thrill when you succeed!

[manfred99]

The only components that look remotely like the Cree diodes are on these 4 small boards, pic enclosed. I also have enclosed another pic of 4 small components that are connected to a cap. While I was in the preamp, I checked for any cold solder joints, redid any questionable  joints and it still has the buzz.

Where in the preamp are these 4 diodes located?

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi146.photobucket.com%2Falbums%2Fr271%2Fmanfred8%2Fforeplay3.jpg&hash=10c32a0fe08f0fca5cf236de0f16d4f0229f0049)

(https://forum.bottlehead.com/proxy.php?request=http%3A%2F%2Fi146.photobucket.com%2Falbums%2Fr271%2Fmanfred8%2Fforeplay2.jpg&hash=49ccc2d57d69d8256feec724701e21a0c5e838ec)

Thanks, Tom
 Portland, Or.

[Grainger49]

Tom,

The four diodes are the black cylindrical black components with silver bands around them in the first picture.  (red wires coming in and blue wires going out)  The ones you think are Cree diodes are on the constant current source (C4S) circuit boards.  Those are transistors.  The C4S boards have two transistors, one with black plastic on the outside one with a metal can for the outside.  Both can be seen in your second picture.

[manfred99]

Got it. When I put in the new diodes, do they need to go in a specific orientation, i.e.( +)(-), and is the silver band on the current diodes positive or negative?

Thanks for all this help, I do have a lot to learn. Next week I'm replacing the driver boards in my Dynaco Mark III's.

Tom
Portland, Or.

[JC]

The silver band on the UF4007 indicates that that end is the Cathode of the diode.  The un-marked end is the Anode.

I'm not sure how this is designated on the Cree package, but you do indeed need to pay careful attention to which lead goes where.

[JC]

In looking at a likely Cree suspect, I would say you will find that pin 1 on the Cree package is the cathode, same as the banded end of the UF4007.



http://www.cree.com/products/pdf/CSD01060.pdf

[Grainger49]

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.

[Len]

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).

[Grainger49]

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).

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.

[Len]

I'm missing something. Why after? Why not instead of?

[Grainger49]

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.

[Paul Joppa]

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!

[manfred99]

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.

[Grainger49]

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.

[manfred99]

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....