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Old 14th May 2019, 12:25 AM   #5621
chemmech is offline chemmech  United States
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just curious what voltage is present from gates to ground, is it full potential? when you begin bias settings.....

Last edited by chemmech; 14th May 2019 at 12:27 AM.
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Old 14th May 2019, 12:48 AM   #5622
BigE is offline BigE  Canada
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It's pulled apart now.

I Think either the cascode or the jfets are gone. I stuck a positive probe on pin one of the jfet (the sqaure one) and measured an ohm reading to ground of 46.3 ohms. The pchannel one measured 49.3.

Should this be open loop?
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Old 14th May 2019, 01:44 AM   #5623
chemmech is offline chemmech  United States
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ok I gotta go but I just happen to have a spare set of these jfets, readings are from pin 1 to pin 3 lsj74 40 ohms and pin 1 to pin 3 2sk170 38.8 ohms (source to drain readings), I think yours are o.k. in the ballpark I have put My jfets thru hell and back on My 2 F5's I built with no problems there pretty tough..... I'd recheck the bias proceedures.....

Last edited by chemmech; 14th May 2019 at 01:46 AM.
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Old 14th May 2019, 02:47 AM   #5624
BigE is offline BigE  Canada
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Thanks! I really appreciate that!

I am going to check the cascodes tomorrow. Will pull and test hfe.

Possibly run output boards with no fe board driving
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Old 14th May 2019, 03:54 AM   #5625
chemmech is offline chemmech  United States
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Quote:
Originally Posted by BigE View Post
Thanks! I really appreciate that!

I am going to check the cascodes tomorrow. Will pull and test hfe.

Possibly run output boards with no fe board driving

I wouldn't run or power up the output boards with no front end / input board hooked up to it, full voltage on the source and no voltage on the gate might blow the output mosfets, max Vgs difference between the source and gate is +/- 4 volts respectively, not sure but it might blow the mosfets maybe somebody else knows for sure..... however I have run the front end powered up with the output board not hooked up no problem.....

Last edited by chemmech; 14th May 2019 at 04:03 AM.
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Old 14th May 2019, 02:32 PM   #5626
BigE is offline BigE  Canada
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Max Vgs is not 4V, it's 20V, but I see your point. It still might not work at all, but there is part of the biasing circuit on board. Needs further inspection.

Checking for short between rail and output would check if any OP devices are shorted.

Thanks for the reality check.

Last edited by BigE; 14th May 2019 at 02:34 PM.
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Old 14th May 2019, 04:13 PM   #5627
chemmech is offline chemmech  United States
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Vgs +/- 20 volts respectively is right not 4 volts I had the wrong datasheet..... too much coffee last night.....

Last edited by chemmech; 14th May 2019 at 04:28 PM.
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Old 15th May 2019, 12:44 AM   #5628
BigE is offline BigE  Canada
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FE board Cascode transistors are fine. NPN tests 170 hfe and PNP tests 163. I'd accept any non-zero quantity within their range.

Time to test a P-channel output devices.

OMG.

After removing the first board, I found a 4-40 screw used to hold the boards on their standoffs shorting the Drain and Source on a P-channel output mosfet!!!! Sloppy....very sloppy.

So I unsoldered that mosfet and did the PNP diode test I found here just to make sure ( it must work, the short is a bypass...

How to Check a MOSFET Using a Digital Multimeter | Homemade Circuit Projects

It tests fine. Initial OL, then 0.3, just like a new one.

However, this does not address the issue with the funny biasing -- that I hope goes away with matched outputs.

Also, since the other channel threw the speaker protection relay, I suspect that something is horribly wrong there.... last time that happened *every* output mosfet was dead....

And the speaker protection needs to be fixed too, clearly with a more robust relay.

It is better than nothing for the F5T, but it won't stand the rail voltage. The Relay I used was part of the original BOM....it's good for 24 VDC. I had no idea.
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Old 17th May 2019, 03:46 AM   #5629
BigE is offline BigE  Canada
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Horror show:

I've not touched it for days. It's been unpowered.

I removed the FE board from the broken channel, and check as the previous post. Removed, tested and resoldered the low powered resistors. All seemed well.

I verified that the other channel was not broken. It works fine. During this time, the power supply for the other channel was disconnected.

When I tried to re-install the FE board, I shorted the gate wiring to the "empty" power supply with the FE board, and two N channel outputs blew up.

I'd been working on the P-channel. The N-channel PS caps still had power after days of inactivity. Imagine my surprise.

Clearly, the drain resistors in the power supply are not doing their job - 2.2K at 3 Watts. Should be more than enough for a max 40 volt rail -- P = V^2/R, 1600/2200. Under a watt dissipation.

So, I replaced the outputs, checked that the output wiring to case was OL, and reinstalled the FE board and powered up.

No measureable flow across either of the N or P source resistors on the board I measured. DC offset was 35mV.

It would not budge when P1 and P2 were turned. Turned off the amp, the PS leds stayed on. Drained the PS manually with a 10ohm power resistor. NOTE: if there was a shorted output, the led in that channel would be off, and the DC offset would be rail voltage.

Checked the FE board Jfets. Seemed ok, but replaced anyway. Measured P1 P2. They were up around 400ohms and 700 ohms -- enough to drive the outputs.

Reinstalled the FE board to same effect, except DC offset now at 2 mV.

Checked Vgate voltage to gnd on the FE boards: 38 volts.

Pulled out the cascodes. Their HFE measures fine, at about 170 each.

The DC offset does not budge, no flow across the two source resistors.

The Speaker protection relay closes, showing continuity from output to binding post. If any flow anywhere, there would be a significant DC offset.

Horror show. I have no idea.

Last edited by BigE; 17th May 2019 at 03:48 AM.
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Old 17th May 2019, 12:03 PM   #5630
BigE is offline BigE  Canada
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pulled a P channel mosfet. It tests fine with the online test I found. So, that one should drain Vpos of the Power supply. It does not.

going to remove and retest all mosfets.

Perhaps try driving that channel with the known working FE board ?
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