F1 Voltage sanity check

[rpafenberg]

I would like to know from those that have built F1 if they noticed a substantial power supply voltage drop under load. I recently built a dual mono F1 using 2 Avel Lindberg 250AV transformers powering separate banks of 4 15,000 uF filter capacitors. While I obtain 24V under a light resistive load, the voltage drops to around 18V on both channels under load. I am able to adjust Q1-Q4 drains to the required 13.8V and the audio sounds very good. The base response is considerably better on my Fostex FE127e Metronomes with the F1 than with my Aleph 30s; however, I want to be sure that I have it operating properly before I attempt any fine tuning.

I made the PCBs using the layout that acaudio provided back in 2005 http://www.diyaudio.com/forums/showthread.php?postid=758534#post758534. Checking my voltages against those provided in the F1 service manual provides the following results.

Expected Right Left
Q7 Drain 3V 2.628 2.623
Q7 Gate 3V 4.992 5.019
Q1 Gate 7V 7.96 7.96
Q2 Gate 7V 7.97 7.94
R27-R30 .6V .58 .58
Offset <50mv 11 31

Are these reasonable variations?

Roy

 

[woody]

The 24v that you get under a light resistive load is this
just a light load on the power supply without the amps
connected ? If you built the crc power supply what value
resistors did you use for r 1 2 3 4 5 6 7 8 and what voltage
are you geting across them when conected to the amps?

 

[rpafenberg]

Woody,

Thanks for responding. Yes, the light resister load was with the amps disconnected. I did build the CRC supply and I used 1 ohm for resisters 1-8 and the voltage drop across each 4 resister set is just under 1 volt (.969).

Roy

 

[woody]

That is strange. Under load what is the voltage of the
first filter cap and the 2nt filter cap?

 

[rpafenberg]

Woody,

The value from the rectifer bridge through the first two capacitors to the resister bank is 20.85V. After the resister bank the value drops to 19.90V. As expected from a straight wire connection, his is the voltage value at the board input on both channels.

Thanks,

Roy

 

[woody]

Does the transformer you have have 2 115v primaries ? if so
are you using them in parallel? If not this could account for the
power loss under load.

 

[rpafenberg]

Woody,

The transformer does have two primaries and they are wired in parallel with each side connected through CL60 thermistors. The transformer also has two secondary windings and they are also wired in parallel. I rechecked the wiring to insure that I wasn't using only one set of the secondary wiring prior to starting this post.

Thanks,

Roy

 

[woody]

Could you have a bad cl60? what about shorting them
out after the caps charge?