RMI-FC100, a single stage audio power amplifier

...measured values are all very close to what is stated here.
...de diode string on target (1,5 VDC)

If the voltages in red are your readings,then the voltage at Q18 collector looks to me to be too low by about 11 volts, and the voltage across the diode string should be around 2.6 volts, not 1.5.
I successfully built this using a normal Vbe mutiplier and non-thermal track devices.
 
Are you seeing 8.25mA passing through Q1,Q16,Q15 and Q6? This can be read from the voltage drop across R17 or R18.

I =E/R ?/150 should be 8.25mA. Have you considered the possibility your 500 Ohm pot is not 500 Ohms.

You can substitute the thermal track diodes with ordinary ones but it will not solve your problem.

Keith

Thanks for helping me out here, yes, I measured the voltage drops across R17 & R18, it's 1,272Vdc what makes 8,48mA. I also looked at the trimmer pot, it is definitely 500 ohms and it has a 500 ohm resistor in parallel.
 
If the voltages in red are your readings,then the voltage at Q18 collector looks to me to be too low by about 11 volts, and the voltage across the diode string should be around 2.6 volts, not 1.5.
I successfully built this using a normal Vbe mutiplier and non-thermal track devices.

I just checked the voltage on the collector of Q18, it is indeed higher than stated in de schematic, I measured 34.0Vdc.
Interesting to read that you have made some adjustments to the design. Maybe this is something to look at in future, but first I would like to understand what is going wrong.

When I isolate R7 the voltage over the diode string jumps to 3V and is adjustable.
 

Keith Taylor

Member
Paid Member
2007-11-04 2:06 pm
Adelaide
I think you need to look at what is happening on the right hand side of the diode string (towards the output) As you have close to 8.25mA it seems some of this current is being shunted by a problem in the following stages.

Check that you have the correct devices in Q19,Q20,Q3 and Q1. Maybe you have swapped NPN and PNP. Measure the Vbe of these devices; it should be around 0.7V

Are you seeing 77mA through R5? E=IxR .077x16=1.232V across R5

Do you have a stereo pair of amplifiers or are you concentrating on powering up one at a time?

Keith
 
I think you need to look at what is happening on the right hand side of the diode string (towards the output) As you have close to 8.25mA it seems some of this current is being shunted by a problem in the following stages.

Check that you have the correct devices in Q19,Q20,Q3 and Q1. Maybe you have swapped NPN and PNP. Measure the Vbe of these devices; it should be around 0.7V

Are you seeing 77mA through R5? E=IxR .077x16=1.232V across R5

Do you have a stereo pair of amplifiers or are you concentrating on powering up one at a time?

Keith

I guess you're right that something is wrong on right side. I have looked again very closely on the type of transistors. I also tested them again, I am sure they are allright. I have measured 0.66Vbe for Q19 and Q20. There is no voltage drop on Vbe of Q3 and Q1 because Main PS is off.
For the current through R5 I see only 33mV (2mA). Can this be right?

The values I measure is with one channel at the time and only frontend PS.
Should I turn on the main PS for this?
 

Keith Taylor

Member
Paid Member
2007-11-04 2:06 pm
Adelaide
You need the main power supply on for the OPS (OutPut Stage) biasing to work.

If applying both supplies for the first time it would be a good idea to use a "bulb test', something often mentioned in these forums. Bulb means light bulb/globe of the incandescent variety. Wire a lamp holder in series with the incoming mains to the amp. 3 pin plug with lamp holder in the active lead and a 3pin socket on the other end. Be very careful that there are no exposed bare wires.

Turn the bias pot to zero Ohms. Put a 40 watt globe in the lamp socket, plug the tester into the mains and the amp into the tester and switch on the mains.

The lamp will glow brightly for a moment due to the inrush current of the reservoir caps charging. After a second or two the brightness should diminish considerably. If the amp seems starved of mains voltage you could try a 60 watt or larger bulb/globe.

If the bulb stays on full brightness you have a problem that, were it not for the tester, you would be blowing fuses or worse. The bulb is acting as a current limiter and also exhibits a temperature coefficient where the hotter it is the higher its resistance.

If all is well you will find that adjusting the bias will cause the bulb to glow more brightly thus preventing you from reaching the full quiescent current. If this is happening you can dispense with the tester.

Good luck

Keith
 
Good news, one of my amp channels is working now. At first, when I switched on the main powersupply, the bias current was too high, even with R22 set on 0 ohms I measured almost 3V over the diode string. I needed to shorten two thermal track diodes before I could adjust the bias properly. Everything is running nice and warm now.
I have hooked-up at a sinewave signal at the input of 1khz. On the oscilloscope the amplificated signal looks good. I have to do some more testing and measuring but I am very happy to see that I am making progress now.
 

darcing

Member
2020-11-17 11:22 am
pcb for amp. RMI-FC100

ITCO:

since I do not know how to add a .pdf-file to the answer of a PM, I will add the drill-template of my FC-100 PCB into this post.

Please compare your version of my FC-100 PCBs with the image attached:
Best regards - Rudi


Hello/ Hi, my name is Darek, I live in Poland,
do you have pcb for this amplifier ??

Thank You,

Regards,

darcing
[email protected]
 
Boring Days...

I'm also planning to use these days of lockdowns in a constructive way, i.e. building an FC-100 ;)
And I am sure I have seen a zip in this thread including a mini build guide and the different parts options. Unfortunately, even after hours of searching, I couldn't find it anymore :confused:
Maybe someone can help me with a link, here?
Thank you!!