F5 Turbo Builders Thread

[omishra]

Originally Posted by anilva

Thanks. Will compare every line. The PCB is from a India forum group buy. It has been designed by fellow member Om Mishra. While the PCBs look well made, I am probably the first guy to use these boards. There could be manufacturing issues or may be not. With time at premium, this debugging hogs quite a bit of that.

Anil, it seems I need to finish my boards early. I am waiting diodes as all earlier parts are distributed. Since your one board worked nicely, only problem with particular board. Chances of manufacturing defect are far as they are tested for continuty at vendor by automated process and marked ET OK. I still feel problem around JFets, source resistors and P3. Please test them off the board. The jefts are sensitive to static damage by hand too.


Output Fets are from buzzforb and well packed in anti static bag. So they seems okay as part are matched quads.

[buzzforb]

I dont think p3 is the problem. Even though it can effect equilibrium of FE, it cannot do so to that level. If jfets are showing proper currents across their source resistors. Thfe next thing is gate voltage for output fets. If that is in proper range, then outputs should be biased properly. We know they were before, but we had muted sound on output. I have had that happen only when something was wired wrong and signal was shunting to ground.
Alniva, check all the voltages again, starting at the FE.
Let us know what they are.
Who knows what happened after the change. Perhaps he burnt up a pot desoldering it. Had that happen before. I have never lost an output fet to static damage, but it definitrly sounds like one polariity is operable, and the other is off or shorted.

[anilva]

Originally Posted by buzzforb

I dont think p3 is the problem. Even though it can effect equilibrium of FE, it cannot do so to that level. If jfets are showing proper currents across their source resistors. Thfe next thing is gate voltage for output fets. If that is in proper range, then outputs should be biased properly. We know they were before, but we had muted sound on output. I have had that happen only when something was wired wrong and signal was shunting to ground.
Alniva, check all the voltages again, starting at the FE.
Let us know what they are.
Who knows what happened after the change. Perhaps he burnt up a pot desoldering it. Had that happen before. I have never lost an output fet to static damage, but it definitrly sounds like one polariity is operable, and the other is off or shorted.

Thanks. I have disconnected both the boards and started comparing. Good news is that the MOSFETs seem OK and measure similarly between both the channels in terms of DC resistances.

I have found one loose solder on one of the MOSFET source resistors and bad solder on centre leg of P3. I have now re-soldered every thing that is there on the board. Will passively measure all the lines and components and compare both for any anomalies and report back, before I connect them to Power.

Cheers.

[Zen Mod]

DC resistance of mosfets will tell you nada

test them , a per Pa's article

[anilva]

Originally Posted by Zen Mod

DC resistance of mosfets will tell you nada

test them , a per Pa's article

This will require setting up a jig if I understand the article correctly. First I will try to see if I can solve the problem without desoldering the mosfets and will do that if there is no option but to suspect the Mosfets.

Thanks.

[sangram]

Might I suggest checking the circuit instead of individual components, so you can eliminate the board or soldering as an issue.

Also do check the input cables. Insulation tears can cause shorts. Try connecting the inputs together at the board, from the working board all the way to the bad one.

It's probably a simple oversight of some kind, probably before the signal even gets to the JFETs.

[anilva]

Originally Posted by sangram

Might I suggest checking the circuit instead of individual components, so you can eliminate the board or soldering as an issue.

Also do check the input cables. Insulation tears can cause shorts. Try connecting the inputs together at the board, from the working board all the way to the bad one.

It's probably a simple oversight of some kind, probably before the signal even gets to the JFETs.

I too think so. It is not the components but basically something has gone wrong during assembly. Either a dry solder or a PTH problem or a bad input cable. I am trying to eliminate one at a time. New work week has begun and hence need to squeeze time along with way and continue. I am confident of identifying the problem soon.

Cheers.

[anilva]

OK, got the board that was giving problems (left channel), wired up again after fixing few dry solders and a loose connection at middle leg of P3.


Board biased nicely. Took it up to 200mV bias and let it settle. Then connected the speaker and played music. Seems to play well and without any gain issues, previously encountered, but I could hear distortion a little bit, compared to the other channel (right channel) which is playing music brilliantly like with any Pass amp, no distortion and full dynamics.

Tried taking the problem channel (left) to bias around 350 mV. That's when I realized there is a problem. The trimmer P1 maxed out and could not push the bias anymore than 320 mA. Then twiddled around with P3 and there seems to be a big impact on output null and the bias levels. Altered a little bit of P3 and managed to set the bias to 350mA. Connected music and played - same problem as earlier case. No gain. Very feeble. Then turned the P3 the other way and connected, while monitoring the bias. Music played back normally (with little bit of distortion) and in a short while, smoke came out from 2SK170. Quickly disconnected the power and removed the board to examine. 2SK170 split open near the base of it's legs. I suspect the P channel MOSFETs are also gone. Luckily the speaker is a test one and nothing happened to it.

Any insights what could have happened? May be it's a bad P3 component. Rest of the components are checked. Seem OK.

I will park this board for a while to debug in leisure. I cannot afford blown MOSFETs. I have a spare PCB. Will stuff it up, double checking this time with presets and everything else.

Cheers.

[buzzforb]

Sounds like you had bad jfet. Not being able to bias output stage is result of idss and can be dealt with using bigger pot. P3 really should not have that big of an effect on output. It is sized to provide trimming of the amount of degeneration on the FE fets, not major swings. Bad jfet or short is my quess.

[AndrewT]

I said a long time ago that the adjustment VR needs fixed resistors either side to limit the extent of adjustment. Seems folk either don't read or have a memory as bad as mine.