Short in F5T

[dewdrop]

Hi there,
I have built one channel of the F5T and tested with the main light bulb. Unfortunately, the bulb did not dim after start up and became brighter. I did test the PSU without load and there was no issue.

There does not seem to have any physical shorts on the boards. I will check again tomorrow.

I measured the resistance of the speaker output and it is 47.5K Ohms whereas my Arcam integrated amp was OL. Is this normal?

What else can I check?

TIA

 

[6L6]

Make sure the bias pots are turned all down all the way. Measure ohms across R5 and turn P1 so those ohms read the minimum you can get it to

Do the same with R6/P2

Reconnect power and see if lightbulb does same thing.

That you tested PSU and fond no issue (an hopefully good voltage rails) is a good sign.

 

[Zen Mod]

for start - are trimpots in min position (shorted across pins ?)

if amp is biased even just 50% , bulb will not go dim m simply because amp is drawing current

so , condition for first start is , set trimpots to short , thus output mosfets not conducting

edit: toobhead was faster

 

[dewdrop]

Thanks guys. I did check P1 and P2 via TP1/TP2 and TP3/TP4 respectively. They are already at 1.2 ohms. Hmmm. This is frustrating.

What about the typical resistance at V+/V- relative to GND at the PSU? I assume it won't be at OL.

 

[Zen Mod]

dunno for that resistance ...... not overly informative , so I usually don't care for it , except in case of dead short

care to post some pictures ?

checked trice orientation of power rails , parts position , resistor values, mosfet/heatsink isolation ?

 

[dewdrop]

dunno for that resistance ...... not overly informative , so I usually don't care for it , except in case of dead short

care to post some pictures ?

checked trice orientation of power rails , parts position , resistor values, mosfet/heatsink isolation ?

Here you go. How do I know if the PSU has a dead short?

 

[Zen Mod]

sorry , didn't read properly your last post , thinking wrong that you asked ( again ) for impedance reading from output node to GND , as you already asked

anyway - no proper reading between rails and gnd ....... it goes from dead short , rising as your ohmmeter is filling PSU caps

not seeing much on your pictures ..... can you post proper ones , 1600xsomething pixels ?

checked what I wrote ?

thermistor legs not making shorts with washers ?

 

[dewdrop]

sorry , didn't read properly your last post , thinking wrong that you asked ( again ) for impedance reading from output node to GND , as you already asked

anyway - no proper reading between rails and gnd ....... it goes from dead short , rising as your ohmmeter is filling PSU caps

not seeing much on your pictures ..... can you post proper ones , 1600xsomething pixels ?

checked what I wrote ?

thermistor legs not making shorts with washers ?

Sorry, I realized my photos did not come up nicely. Let me take again during the day.

Will check the thermistor legs too.

 

[Gyuri]

Are there enough space between PS wires connected to top of TB-218908 board and heatsink?
If not, then can be a short circuit to heatsink, as I can see:

 

[dewdrop]

Are there enough space between PS wires connected to top of TB-218908 board and heatsink?
If not, then can be a short circuit to heatsink, as I can see:

Hi Gyuri,
Thanks for the heads-up, I checked and the wires do not touch the heatsink.

 

[dewdrop]

Did some more checks, here's better quality photos.

 

[dewdrop]

LATEST UPDATE - after fiddling around to look for shorts, I powered up the F5T to try again. The light bulb dimmed after 3 seconds, HAPPINESS!

I will try to bias the channel now. Stay tuned.

 

[dewdrop]

ok, i got both channels built up and did the light bulb test. The

I started biasing the right channel first with one turn on P1 and P2 and the bulb began to glow. Suddenly, the bulb became brighter and i thought I saw smoke coming out from the other channel!! Even though there wasn't any bias applied to it.

Yikes, quickly power off the amp, what could be wrong?

 

[Zen Mod]

no bulb tester after initial test !

you can't bias bloody thing , with rails choked!

back up pots , remove bulb tester , try again

two DVMs , as per building guide

 

[dewdrop]

no bulb tester after initial test !

you can't bias bloody thing , with rails choked!

back up pots , remove bulb tester , try again

two DVMs , as per building guide

Yes sir, I thought I read that I could do an initial bias with it. I take away the bulb gig.

I have started it again, this time with one channel at a time - seems that the smoke hasn't come up again. LOL.

 

[dewdrop]

I notice that after a initial bias - I place the cover over the unit and notice the bias voltage starts to climb slowly. Is this because my NTC thermistor isn't working?

if it is normal, what is the range?

TIA

 

[Zen Mod]

you really didn't read any (of myriad) posted setting procedures?

final Iq and output DC offset setting is in thermal equilibrium , which means - with top lid on

so , pull lid off , re-set , put lid on , wait for temp stabilizing , observe values

repeat if needed

 

[vvs07]

Did some more checks, here's better quality photos.

Please upload your pictures to Diyaudio. Photobucket links don't work.

 

[dewdrop]

you really didn't read any (of myriad) posted setting procedures?

final Iq and output DC offset setting is in thermal equilibrium , which means - with top lid on

so , pull lid off , re-set , put lid on , wait for temp stabilizing , observe values

repeat if needed

Sorry if I sound that I didn't but I did. What I am asking is typically how much does the drift normally occur? I will make sure I cater for that and not set it too high a bias in case there is a thermal runaway and cook the amp.

 

[dewdrop]

Please upload your pictures to Diyaudio. Photobucket links don't work.

Yeah. It worked for a while, let me use Google Drive tomorrow.