[tsteahr]

Hello,

I'm having a problem with my FryPro.  I built a half kit and have been using it nearly 24/7 for a several months.  I ran into a problem when I switched the unit off and then back on.  The output level has level has diminished greatly and I suspect there may be some unusual characteristics to the output signal.  To check the output signal I connected the rca out to a tabletop stereo and verified the signal sounds like the chuffing steam train starting up.  In the past I have set the FryPro output to med and that would provide plenty of signal strength for the table stereo.  Now I have to set the output to hi and I have to raise the volume level on the stereo to hear the steam train.

I took the cover off and none of the individual components appear to be visually damaged or burnt.  There do not appear to be any shorted wires or any forgin material creating any shorts on the boards. 

The only diagnostic tool I have available is my Fluke 73/3 DMM.  This fluke unit is speced for a AC frequency range to 1k Hz.  When I first built the FryPro, I measured a max voltage of approximately 1.2 volts to ground at all four outputs (A-D) with the unit set to Hi.  Now I measure a max voltage of approximately 0.36 at the Hi setting.  Over the duration of the signal sweep, the voltage ranges between 0.21 to 0.36 at A and B, while C and D range from 0.18 to 33 volts (Hi setting).

Measuring ac voltage between A and C gives a range of 0.39 to 0.66 volts (during the singal sweep).  Measuring between A and B (and also between C and D) yields 0 volts.

I also set the DMM to VDC and measured output to ground (at each output A through D).  This ranged from -0.2 to +0.2 volts over the course of the sweep (all outputs measuring similar).

The walwart is providing 15.1VDC, no problem.

I discovered this problem as I was using the FryPro to provide a signal to a old B&K preamp, which is feeding a old Adcom GFA-555 power amp set to mono.  I have been using this setup to burn in speaker cable with high current (2-3 amps).  I use a power resistor in place of connecting a actual speaker to the Amp.  I connect the speaker cable of interest between the amp and resistor, set FryPro to Med, adjust volume on preamp until I get desired voltage (and current) across the power resistor and let everything roast for 10-14 days. 

The only other observation I can offer occurred while I was using the FryPro in the above rig.  I switched the FryPro off, made a adjustment to the speaker cables and then switched the FroPro back on, to Med.  I then noticed the resistor was not getting hot.  I found I had to set the preamp volume very high to get a similar voltage across the resistor.  I also noticed the Adcom has distortion indicators which are now flickering (they never lit in the past).  The amp is still running cool however.  The flickering distortion indicators suggest to me that the adcom is getting a signal from the FryPro that it is unable to amplify, hence the distortion indication as the input is not the same as the output.

Any thoughts on what might be wrong with my FryPro?

Thanks,

Tom

[hagtech]

It sounds like maybe the LM386 are damaged.  But that is only a guess.  I haven't seen it before.  First check all of the supply and bias voltages within the FRYK circuit.  Maybe an electrolytic has gone south, but I doubt it.  Does the LED blink as before?

Have you tried the Adcom with a normal music signal to make sure it is ok?

jh

[tsteahr]

Thanks for the reply Jim,

The LED blinks just as before.  I checked the Adcom with a music signal and it is fine.  I checked a bunch of the supply voltages.  All of the following measured approximately 13.3 VDC:  R1, R9, R10, R11, R12, R22, R23, R24, R25, R34, C17, C18, C20, and C21.

Do you have any idea how I might be able to check the LM386?  Considering there are two LM386 in the FRYK, would both of them had to have gone bad for me to observe the reduced output at all four terminals (A-D)?  I do have one spare LM386 and one spare LM358.  Not sure how I would get one of these chips out of the board without damaging the board...

What about the modulator, U4, or something else upstream of the amps?  The odd thing is I do hear the noise being swept with the ramp when I listen to the FryPro output on a table radio.  I'm wondering if a dc bias could be leaking through to the amps, reducing the AC output voltage I measure.

Please consider that I have no clue about electronics, I'm a ME.  So any of my conjecture could be pure BS.

Thanks for your help,

Tom

[hagtech]

I can't really think of how to debug this (actually quite complicated) circuit without a bunch of test gear.  It could be a whole number of possibilities.  All I can suggest is to send it in.

jh