Topics

Unexpected Blowing of Fuses on Transmit

harryoliver196410@...
 

Hi and regards from the UK.

 

I have a uBitx V5 which has been working great for the last few months and I have been loving it. I have incorporated a few mods – Allison’s power mods, AGC & Dr Ian Lee’s firmware – latest mod was about a month ago.

 

Yesterday it blew the 2A fuse on transmit. I have checked everything I can think of from the mic to the cables to the power supply, checked earths, checked for shorts all to no avail. In the meantime, whilst continuously testing it, I have blown a further 6 fuses. I even turned RV1 down to practically nothing and still the problem continued. I also replaced the FET’s just in case, but unsurprisingly, that made no difference. It receives perfectly, but as soon as I transmit, bang goes a fuse.

 

I am a relative newbie – currently studying for my Full license, so, far from experienced. My test equipment consists of a DMM.

 

Any help to point me in the right direction would be enormously appreciated as frankly, I am at my wits end.

 

Regards

Harry

wnpauls
 

I think it would be safe to try a 3 amp fuse.

Paul K0ZYV

harryoliver196410@...
 

Just tried it Paul - Same result ...

Evan Hand
 

Are you using a dummy load or antenna?

 

I think same problem with RV3 if skewed then lead is cut and connection intermittent.
This would lead to sudden jump in current thus fuse blowing.

The ladies at HFSignals are twisting the RV3 to accommodate the heatsink and causing
failure.

Raj


At 01/12/2019, you wrote:

Hi and regards from the UK.

 

I have a uBitx V5 which has been working great for the last few months and I have been loving it. I have incorporated a few mods – Allison’s power mods, AGC & Dr Ian Lee’s firmware – latest mod was about a month ago.

 

Yesterday it blew the 2A fuse on transmit. I have checked everything I can think of from the mic to the cables to the power supply, checked earths, checked for shorts all to no avail. In the meantime, whilst continuously testing it, I have blown a further 6 fuses. I even turned RV1 down to practically nothing and still the problem continued. I also replaced the FET’s just in case, but unsurprisingly, that made no difference. It receives perfectly, but as soon as I transmit, bang goes a fuse.

 

I am a relative newbie – currently studying for my Full license, so, far from experienced. My test equipment consists of a DMM.

 

Any help to point me in the right direction would be enormously appreciated as frankly, I am at my wits end.

 

Regards

Harry

harryoliver196410@...
 

Hi Evan and thank you,
I was using my 'droopy' dipole, but all testing has since been done into a dummy load - which I have also checked as I have two.
Regards
H

harryoliver196410@...
 

Hi Raj,
That's interesting and I've seen your post regarding this from a day or two ago.
I can see the logic to what you're saying and I'm definitely going to have a look at them this afternoon - I just think it's strange that this problem would 'suddenly' occur?  Or am I being stupid and this is something that would randomly occur?
Thanks again.
Regards
H

 

Harry,

I think a pin is broken and still connected. If the ground pin momentarily disconnects then the IRF
will draw high current and blow the fuse. I've seen stranger things!

Raj

At 01-12-19, you wrote:
Hi Raj,
That's interesting and I've seen your post regarding this from a day or two ago.
I can see the logic to what you're saying and I'm definitely going to have a look at them this afternoon - I just think it's strange that this problem would 'suddenly' occur? Or am I being stupid and this is something that would randomly occur?
Thanks again.
Regards
H

harryoliver196410@...
 

Thanks Raj, 
Yes, that makes sense. 
I'll have a crack at it this afternoon. 
Regards
H

harryoliver196410@...
 

Still no joy - I had high hope from Raj's suggestion - which makes a lot of sense - unfortunately. the pins and the pots were all in excellent condition (never thought I'd say that ...).
So I'm still blowing fuses.
Any suggestions gratefully received.
Regards
Harry

Gordon Gibby
 

Well, you were going to have to divide up the paths where all the current can go, and interrupt some or all of them until you work out where it is going.    Now is the time to take a look at the schematic


On Dec 1, 2019, at 11:19, harryoliver196410 via Groups.Io <harryoliver196410@...> wrote:

Still no joy - I had high hope from Raj's suggestion - which makes a lot of sense - unfortunately. the pins and the pots were all in excellent condition (never thought I'd say that ...).
So I'm still blowing fuses.
Any suggestions gratefully received.
Regards
Harry

harryoliver196410@...
 

Agreed Gordon - Just started doing something along those line now.
Regards
H

Jerry Gaffke
 

Here's an outline for a step by step procedure to find the fault.
Whether you choose to follow it or not, it should give you some idea of what is going on.

We've all been assuming it's around the IRF510's, since those are the only things that typically pull a few amps.
So disconnect the PA-PWR connector pin in from the power supply, does the fuse still blow when you press PTT?
I assume not.

Now reconnect PA-PWR, with a DVM inline, set to 10 Amps max.  A $2 Harbor Freight DVM will work fine.
Pull up one end of L8,  and also of L9.  So PA-PWR cannot get to the IRF510 drains.
Rotate RV2 and RV3 fully clockwise for 0v at the IRF510 gates (minimum is clockwise, backwards from most pots!)
With 0v on the gates, the IRF510's will not conduct.  They start conducting when there is 3 or 4 volts on the gates.
Apply power and press PTT, does it still blow? 
I assume not.

Should be just a few milliamps going into PA-PWR when you press PTT, I'd guess down around 10ma or so.
A very rough guess, I have not measured it.
But with the DVM at 10 Amps, you should see pretty much zero current on the meter.

Measure the voltage at the IRF 510 gates with PTT pressed, should be zero volts.
(Ideally this would be done with a second DVM, as we will soon need to measure PA-PWR amps again.)
Adjust pots RV2 and RV3, verify that the gate voltage on Q95 and Q94 respectively 
goes from 0v to 5v as you rotate the pot counter clockwise.
Return both pots to 0v, fully clockwise.

Reconnect first L8, press PTT, does it blow?
If not,reconnect L9, press PTT, does it blow?
Since the gate voltages are both still at zero with RV2 and RV3 fully clockwise, the IRF510's are not conducting.
Even though both L8 and L9 are restored, that roughly 10ma of current into PA-PWR should not have risen hardly at all.

Now with PTT pressed, first rotate RV2 counterclockwise very slowly until the PA-PWR current rises to 100ma.
This will happen very suddenly when the gate voltage reaches 3 or 4 volts, so be careful.
If rotating RV2 can bring PA-PWR up to 100ma, then the IRF510 at Q95 is probably ok.

Then rotate RV3 until PA-PWR current increases another 100ma to 200ma total.
If that works, then the IRF510 at Q94 is ok.
Rig is ready to use.

I assume somewhere in that procedure, something went terribly wrong for you.
At exactly what step things when haywire will be a significant clue.

Jerry, KE7ER

MVS Sarma
 

You cant have all of 
1. Preset gate more than 100mA drain current,
2. Hi rf drive with q90 collector preset and
3 boosted mic gain with presetbfor modulation level control. 

You may have to reduce at two of the locations. 

All the best 

On Sun, 1 Dec 2019, 9:49 pm harryoliver196410 via Groups.Io, <harryoliver196410=yahoo.com@groups.io> wrote:
Still no joy - I had high hope from Raj's suggestion - which makes a lot of sense - unfortunately. the pins and the pots were all in excellent condition (never thought I'd say that ...).
So I'm still blowing fuses.
Any suggestions gratefully received.
Regards
Harry

harryoliver196410@...
 

Thanks Jerry & MVS Sarma,
That is comprehensive help - really appreciated.  It will have to wait 3 days as I need to prep' for working away from tomorrow but should be back on it Wednesday/Thursday.
Thanks again - I will report back!
Regards
Harry

Jerry Gaffke
 

If the fuse blows when you push PTT but before speaking into the mike,
it probably is not mike gain or the RV1 drive setting.
Most likely trouble around the IRF510's somehow.

##########################
You cant have all of 
1. Preset gate more than 100mA drain current,
2. Hi rf drive with q90 collector preset and
3 boosted mic gain with presetbfor modulation level control. 
 
You may have to reduce at two of the locations. 
 
All the best 
 

MVS Sarma
 

Correct Jerry. 


On Mon, 2 Dec 2019, 10:45 am Jerry Gaffke via Groups.Io, <jgaffke=yahoo.com@groups.io> wrote:
If the fuse blows when you push PTT but before speaking into the mike,
it probably is not mike gain or the RV1 drive setting.
Most likely trouble around the IRF510's somehow.

##########################
You cant have all of 
1. Preset gate more than 100mA drain current,
2. Hi rf drive with q90 collector preset and
3 boosted mic gain with presetbfor modulation level control. 
 
You may have to reduce at two of the locations. 
 
All the best 
 

Gordon Gibby
 

Great work , guys— thanks. 


On Dec 2, 2019, at 00:38, MVS Sarma <mvssarma@...> wrote:

Correct Jerry. 

On Mon, 2 Dec 2019, 10:45 am Jerry Gaffke via Groups.Io, <jgaffke=yahoo.com@groups.io> wrote:
If the fuse blows when you push PTT but before speaking into the mike,
it probably is not mike gain or the RV1 drive setting.
Most likely trouble around the IRF510's somehow.

##########################
You cant have all of 
1. Preset gate more than 100mA drain current,
2. Hi rf drive with q90 collector preset and
3 boosted mic gain with presetbfor modulation level control. 
 
You may have to reduce at two of the locations. 
 
All the best 
 

 

The only thing I can think of that would suddenly cause high current
is a floating IRF gate. maybe a dry solder on R97/98 or IRF gate leads.

A 2A fuse will need double the current to blow instantly. If I remember
my EE a normal fuse rating is the current that the fuse will survive for
10hours without blowing, at double the rating it will blow in seconds
depending whether it is slo blow or fast!

Read more
https://en.wikipedia.org/wiki/Fuse_(electrical)

Raj

At 01-12-19, you wrote:
Thanks Raj,
Yes, that makes sense.
I'll have a crack at it this afternoon.
Rgards
H