F5 power amplifier

Just began testing my newly assembled F5 kit with Universal Power Supply and Deluxe 4U chassis. Blows fuses on power-up. Power supply tested fine before connecting to amp boards. Removed and inspected amp boards and found R1 (4.75K) and R2 (47.5K) flipped. Repaired and began test with only one board wired. Quick power-up with meters connected and I see 2.8V across both R7 and R8 so I quickly shut down. Also about 15V at output! Tried the other board alone and same result.

Just bought parts for a light bulb tester. So, I will have that as test tool shortly.
Have built electronics since 13 years old when I became a ham (now 68). Good soldering skills.

Things I've checked and double checked:
all resistors in the right place
orientation of Q1 & Q2, 2SK170 & 2SJ74
orientation of Q5 & Q6, ZTX550 & ZTX450
orientation of Q3 & Q4, IRFP9240 & IRFP240
Q3 & Q4 are not shorted to ground through the heatsink
power supply +24V and -24V right on

Any ideas for the next steps to troubleshoot would be most appreciated.


Thanks in advance for any guidance and advice.

Wil
 
Some people advise after 300VA a thermistor I would definitely use something at 500VA especially if the amp biased into Class A. Also depends on how much capacitance use at your PS. Usually, we use a lot of capacitance at Class A building so a soft start would eas the power surge stress.

Using two CL60 in series with 115V primary per build plan. Problem is that there should be no voltage across R7 & R8 until bias is set. Both pots, P1 & P2 are set at maximum resistance (5K ohm), fully CCW and I see ~2.8V instead of 0V. Also ~15V at speaker out which should also be 0V. Something is very wrong.
 

Attachments

  • IMG_0403a.jpg
    IMG_0403a.jpg
    855.1 KB · Views: 354
for starters , both drain pots need to be at bloody minimum ..... zero...... nada........ zilch


I've never built one of these and the thread is really long but looking at the schematic on page 9 shouldn't R1 and R2 be 10ohm resistors as well? The 4.7k resistor w the thermistor and the 47k resistors for the gates of the output mosfets? I just like reading other peoples issues to try and learn how to go about trouble shooting and fixing them.
 
Last edited:
I've never built one of these and the thread is really long but looking at the schematic on page 9 shouldn't R1 and R2 be 10ohm resistors as well? The 4.7k resistor w the thermistor and the 47k resistors for the gates of the output mosfets? I just like reading other peoples issues to try and learn how to go about trouble shooting and fixing them.

Definitely no 47K resistors anywhere on schematics....

EDIT: I see you edited your post, what is fitted ?
 
Last edited:
Official Court Jester
Joined 2003
Paid Member
from post #89 (schm attached)

Ra and R2 are 10R ( each)

mosfet gate resistors are 47R each (R13 and R14)

in fact , observe that numbers without suffix are for Ohms , all other having approporiate suffix K for KiloOhms

NB that there is later iteration , with P3 added in sources of input JFets , for altering THD spectra and lesser demands for matching of active parts

106270d1250563083t-f5-power-amplifier-fig06-jpg
 
for starters , both drain pots need to be at bloody minimum ..... zero...... nada........ zilch

Just for clarification, zero ohm or 5K ohm to begin bias settings?

The build guide says to turn pots fully CCW to start bias setting which should give you zero volts across R7 and R8. So, I have pots positioned as in the pictures and turned pots fully CCW but I get 2.8V across R7 and R8 instead of zero.
 
from post #89 (schm attached)

Ra and R2 are 10R ( each)

mosfet gate resistors are 47R each (R13 and R14)

in fact , observe that numbers without suffix are for Ohms , all other having approporiate suffix K for KiloOhms

NB that there is later iteration , with P3 added in sources of input JFets , for altering THD spectra and lesser demands for matching of active parts

106270d1250563083t-f5-power-amplifier-fig06-jpg


ok thank you. When the previous poster had commented


" Removed and inspected amp boards and found R1 (4.75K) and R2 (47.5K) flipped."


I was confused and thought that may be were theyre having some issues
 
Official Court Jester
Joined 2003
Paid Member
CW or CCW is thing I'm often remembering for second or two ; I'm used to believe in my instruments , not my memory

so , for proper setting operation ,connect as many DMM you have , prepare coffee or tea , relax and enjoy in process.

Assembly of Japanese bicycle require great peace of mind

:)

learning new trick/thing or two is for me more exciting than (having) entire new amplifier .......
 
Last edited:
Hi All - this is my first post on Diyaudio.

I just finieshed first channel of F5 and powered it up. Everything was going smoothly, bias setting and DC off set are good but when I connected speaker an played music there is no sound. Any idea what might have gone wrong? Were should I start the troubleshooting?