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Old 16th August 2009, 11:12 PM   #1
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Question Pass F5 Help Needed

Just finished th F5 today. Initial setup looked good, both sides were about .59v for R11 & 12. Both channels are hot to the touch and can hold my hand to them for 5 seconds or more without any problem. The temp ramped up for over an hour.

However, I have -8.5v DC on the output of 1 channel and .5v on the output of the other channel.

I shut it off and checked the wiring and it looked ok.
There is 23v+- from the power supply. Before adjusting the voltage is was showing 25.1v.

I might have a short somewhere. Not sure where though.
The + and - on one channel output are continuous, the other is not. I took the board off the heat sink and the shorted output went away. Could it be a FET shorted to the heat sink? I didn't see any shards poking the FETs.

Sounds like I have bad FETs. Can anyone help me verify? Any other ideas where to check?

LEDs light up so at least I have an expensive, heavy and hot night light!

Thanks in advance!

Vince
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Old 16th August 2009, 11:38 PM   #2
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Vince,
Before I hook up the Power supply and signal grounds, I do a continuity test between the metal tabs on the FETs and the heatsink. A shard going through the pad will BEEP every time...
So I guess I would try to lift grounds first.
If it doesn't short out and blow a fuse or smoke, with those kind of voltages, I would think a resistor might be out of place somewhere, or the pot's are way off, but they would not both read that voltage if that were so. At that voltage, they should be pulling plenty of current.

Another stunt I have done is get the postive or OUT binding post touching the chassis. That will cause some problems.

I think your close. These things happen to ensure that we stare troubleshooting in the eye.
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Old 17th August 2009, 12:15 AM   #3
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Thanks Tea. I will check the tabs...these are accessible from the front of the Fet correct? They look like this |) o (| The divit shows some metal. This is what you are referring to?

Funny thing is, the gold anodizing on my heat sinks doesn't conduct well. I am using a pad between the sink and the FET.

I'll keep looking...it's always the last thing you find!

Thx,

V~
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Old 17th August 2009, 12:42 AM   #4
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I would check to see if the Source resistors are really the value you
think they are.

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Old 17th August 2009, 12:52 AM   #5
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I checked the case to sink and there is a short at least in one channel. Both N and P channel FETs short to the heat sink.

Nelson, the Source resistors are 0.47 Ohm 5%. They measure .9 Ohm in circuit.

Thx,

Vince
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Old 17th August 2009, 01:26 AM   #6
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Are you using Cviller's boards? -- make sure to read the entire design pamphlet
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Old 17th August 2009, 10:51 AM   #7
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Yes, used the Cviller F5 update manual. Followed it carefully. I did not install FETs while on the heat sink. Don't know how critical this is. Is it?

Thx,

Vince
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Old 17th August 2009, 12:50 PM   #8
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with grounds lifted from ground bus, and boards attached to sink, does it still short between tab and heatsink.

I've had to file and sand (with 1200 grit) the heatsinks spots and tap hole to get a perfect fit, bit I never attach to heatsink first, it would keep bending to a minimum on a FET I suppose.
Often movign the heatsink pad around the whole is needed to keep it from shorting, if it's not a metal shard.
I assume you shorted inputs.
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Old 17th August 2009, 01:05 PM   #9
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Quote:
Originally posted by vdi_nenna
Yes, used the Cviller F5 update manual. Followed it carefully. I did not install FETs while on the heat sink. Don't know how critical this is. Is it?

Thx,

Vince
If you are using the Zetex transistors, make sure that you have the P and N devices correctly oriented (not as shown on the silkscreen).

Examine the board with a magnifying glass -- I fixed one board for someone and there was a tiny ball of solder shorting out TO-92 transistor leads. In another the boards were finicky about soldering (I like tinned boards, not plated boards.)

A burr on the back of a TO-247 transistor, or on a mounting hole with a burr, or even some drilling detritus will cause a "bad-hair day".
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Old 17th August 2009, 01:30 PM   #10
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Default Re: Pass F5 Help Needed

Quote:
Originally posted by vdi_nenna

However, I have -8.5v DC on the output of 1 channel ...

With my latest F5, I had a similar problem, which was due to a collapse of balancing between the upper (positive) side and lower (negative) side. I did check all passive components and found they were all right. In addition, I did check whether the drains of the mosfets were shorted to the heat sinks, but they were all found as well insulated. My problem was finally found as having come from the input jfets damaged due to over-temperature of soldering iron.

Hope you will find a solution soon . . .

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