Toshiba outputs need alot less resistance to the trimpots, change R3-R4 to 1K to make the bias easier!
600ohm is very high for Toshiba !!!!!!!!
I use this schematic for toshiba output and with a few turns of the trimpots from zero I reach very easily the bias! for bias across R11-R12 (0.22ohm) 0.30v is ok for 1.3A (0.425 = 1.9A you need big heatsinks!)
(
Last edited:
Toshiba outputs need alot less resistance to the trimpots, change R3-R4 to 1K to make the bias easier!
600ohm is very high for Toshiba !!!!!!!!
I use this schematic for toshiba output and with a few turns of the trimpots from zero I reach very easily the bias! for bias across R11-R12 (0.22ohm) 0.30v is ok for 1.3A (0.425 = 1.9A you need big heatsinks!)
(
Would this account for the high voltages he's experiencing?
russellc
He already is having problem. Light bulb arrangement will not allow voltages to measure properly during fireup, making setting anything correctly impossible.
Russellc
I'm using a variac. When I reach the 15% mark (.15x140 = 20vac) I'm already over driving my bias.
This is on both positive and negative voltages, not just negative.
Both boards I built behave the same way.
The idea for starting the F5 is to preset it so that the output devices do not conduct.
The only current passing through from the PSU goes through the jFETs.
If this has been done properly and there are no mistakes in the wiring, the bulb will flash as the transformer starts and dim gradually as the smoothing charges up. When fully charged the bulb should not even glow.
Check voltages. Check wiring. Check initial pot adjustment, but do not bias the outputs into conduction. Convince your self which direction each of the pots should be turned to increase the bias. Remove the bulb tester and start up direct to mains. Now you are ready to start the biasing procedure.
No damage, no smoke, no anxious moments.
When I set the pots to their minimum - as directed by NP in the SM/OM - AND when they are set so R3/R4 are at 600 ohms, I get the same result.
I'm not using a bulb tester, I have a variac for slowly applying power.
I am looking for the answer for why I have such high current through R11/R12, even at only about 17% of mains power.
I understand that when it is built/wired/set/no mistakes it will work right. I've built one before and it came up the first time.
So, there is a mistake.
What would cause such high current through R11/R12?
Don't think bulb testers, setting the bias, etc. I must have full V+ and V- across the resistors somehow.
I'm not using a bulb tester, I have a variac for slowly applying power.
I am looking for the answer for why I have such high current through R11/R12, even at only about 17% of mains power.
I understand that when it is built/wired/set/no mistakes it will work right. I've built one before and it came up the first time.
So, there is a mistake.
What would cause such high current through R11/R12?
Don't think bulb testers, setting the bias, etc. I must have full V+ and V- across the resistors somehow.
The beginning of what? I did that last night by starting over with the other board.
I set the trim pots to the minimum, same result. I set the trim pots to 600 ohms, same.
I checked the wiring, V+ goes to V+, etc.
Just now I powered up again. Turned up the variac so I had .6 vdc on both r11/r12. I measured the output of the power supply at that point - 1.3vdc!
Such low power in results in high current through (high vdc across) R11/R12.
So just where do I start over? Order new boards?
I'm no EE but I get a little of this. Something is causing this high current through both sides. The active components are in the right position and orientation.
Hi,
disconnect the amplifier.
Check the PSU, check voltages, check polarity. Is the bulb off?
If the PSU does not measure correctly then disconnect that and check the transformer.
Once you know that the PSU starts up and measures correctly then connect the amplifier.
BTW,
1.3V from the PSU and 0.6V across each source resistor sounds like a dead short from source resistor to source resistor.
disconnect the amplifier.
Check the PSU, check voltages, check polarity. Is the bulb off?
If the PSU does not measure correctly then disconnect that and check the transformer.
Once you know that the PSU starts up and measures correctly then connect the amplifier.
BTW,
1.3V from the PSU and 0.6V across each source resistor sounds like a dead short from source resistor to source resistor.
Power supply is fine. With the variac sending 117vac, I get +\- 24.7vdc with no load. When I connect the ps to the board at full ac input, the dc volts across r11/r12 are way too high. I have to turn the variac down to around 20vac to get ,6 across r11/r12.
At 20vac in and .6vdc at r11/r12, there is only 1.3 vdc from the ps.
Please read and understand that part. The ps is fine.
At 20vac in and .6vdc at r11/r12, there is only 1.3 vdc from the ps.
Please read and understand that part. The ps is fine.
yes, you told us that.
But look at the two resistor voltage drops. Each @ ~600mV adds up to almost exactly the 1.3V you have measured as supplied by the PSU.
Where is the dead short?
Are the output FETs damaged, Are they the right orientation, are they in the correct locations?
Note this is a FET equivalent of a CFP and the Nchannel device is on the negative side, with the Pchannel on the positive side. Exactly the opposite to a source follower arrangement.
But its 2.6 vdc rail to rail, not 1.3. I measure .6 across each resistor when the potential between the + and - rail difference is 2.6vdc
Excellent question, thank you.
Yes, the input is shorted.
You have obviously duplicated the same error with the second board. The three scenarios I can see are, all your output fets are shorted, the output fets are turned full on, or you have made an error hooking up the PS to the board. Remove the output fets from the board and see if you can get the correct gate bias voltage on the pcb. And triple check your PS hookup.
Thank you Bill
Agreed!
I measure 47 ohms in circuit, open out of circuit (pin 1 to 3)
If the above isn't an issue, I think this is the case
Nope... I'm 1000% positive of that.. see below where I pulled the outputs as advised. I measured right to the V+ and V- pads and got +24.6 and -25.1
Done, I get 0 volts across R11 and R12, no matter where the trimpots are adjusted.
Issue?
Deca-checked.....
Could the output FETs be bad? I have a set of Fairchilds I can try.
Agreed!
I measure 47 ohms in circuit, open out of circuit (pin 1 to 3)
If the above isn't an issue, I think this is the case
Nope... I'm 1000% positive of that.. see below where I pulled the outputs as advised. I measured right to the V+ and V- pads and got +24.6 and -25.1
Done, I get 0 volts across R11 and R12, no matter where the trimpots are adjusted.
Issue?
Deca-checked.....
Could the output FETs be bad? I have a set of Fairchilds I can try.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.