Sorry for the delay I've been at the Intersolar convention in SF, CA.

I replaced the differential amp transistors and the amplifier did not work. After several power up cycles of the board with a VARIAC I heard a soft poping sound and smoke appeared in the region of Q403/Q412/Q413. I turned off the power (fast wrist method) and checked these components along with the associated diodes and everything checked out okay. I couldn't see any heat markings on any other components (used an eye loop).

I decided it was time to build a light bulb current limiter and I'll use this to retest the board sans Q412/13. I'll get to it this weekend. Thanks Teemuk, Enzo and especially JM Fahey for the help. I will post outcomes soon.

Teemuk,

    I believe I found the problem. When I replaced Q401/2 - the BJT parts bin had a mixed stock 2N2484 and 2N4248. The diff pair does not work with NPN's  For some reason I didn't see this until I started modeling the circuit.

How do you model a potentiometer in LT SPICE (I'm assuming that's what you use?)

Thanks

george

not to get to sideways off topic but - how do you do an attachement on this forum/site???

I'm desperate enough now to start modeling this board on LTSpice (too bad they don't have a simple component model for a potentiometer though).

Any other ideas - comments - suggestion would be appreciated.

http://i284.photobucket.com/albums/ll33/bonosurf/img163.jpg

better image of the 170045 board

I can send you a pdf - send me your email address.

I'll try and rescan it as well.

I'm having problems with the bias on a 470 PA board. schematic link below

http://i284.photobucket.com/albums/ll33/bonosurf/470PA.jpg

This is the 170045 version of the board since it is a 470.

The issue is I can not get the 46V bias at various points on the board. The 92V rail is fine. I have replaced Q401/2 Q412/13 tested all other devices and they pass.

The signal ground and PS ground are connected.

I have tested almost every resistor on the board and replaced all the caps but C404.

Needless to say the board does not pass a signal.

Q403 has .85V on the base and .25 volts at the emitter.

I get 63V at the node of R418/R417 but only 4V on the other side of R418 - and R418 is in spec???

If I didn't know better I would say there is a ghost in this machine.

Thanks for the info - what's the largest portable amp design out there? I'm looking to scratch build a biamp design and would like to get 20W for the low end.

I'm toying with a portable (i.e. battery powered) amp design and would like folks opinions on low power ICs for this type of application. Think Micro Cube only better.

BUMP..........

Okay the Accutronics site has a table which gives the specifications on the different type # and the bobbin/coil color. They refer to a "coil color but I believe that they mean the actual plastic bobbin when they refer to color codes. Seems to make sense for the tanks I have because all the coils have yellow tap but the bobbins are black, red, etc. Already found one coil is intermittent

I have three reverb tanks from Accutronics that were in 1) Acoustic 150 and 2) Acoustic 470 amplifiers. The input and output on the "type 4" units are 160-175 ohms for both the 150 tanks and the 470 appears to have a very high impedance on on of the ports. I noticed that the Accutronics site has the type 4 tanks specified with different input and output impedances.

Are these good units? Is there a way to bench test these tanks with an signal generator/scope?

THX

I'm starting to believe this is the amp from hell. I spent way too much time trouble shooting and replacing the A/B channel lamps. During that process, a bit of lead triming accidentally shorted the 90V rail to the zener in the BJT regulator on the PA board. I had to replace the zener in the regulator and all those voltages are now passing to the other PCBs just fine.

Here is a link to another forum where you can find the PA schematic - http://music-electronics-forum.com/showthread.php?t=10360

The current problem is no output on the PA board - signal from the other two PCBs are loud and clear but right after the 47K reaistor on the PA input I get 12Vdc on top of the input signal. By the time the signal reaches the base of Q401 (differential pair) it's almost non-existent and noisey. Checked all the active devices in the circuit and they are passing.

The sad thing is that this board was working fine before the regulator ckt was damaged. Maybe someone else can see something I've missed.

BTW I've modeled this board in TINA and I get agreement on all the posted factory diagnostic nodal voltages.

QuoteIf you want to get down to it  with  electronic history son I can go there , I have my degree and time in as well.

Okay, I'm waiting for the history lesson...............

I have a B15T with several burned resistors on the output board. Anyone know if Loud Technologies has schematics for this amp? I tried Schematic Heaven and they only had the BT-15 series.

Okay I swapped the lamps and the B channel works now and the A channel doesn't. It is the bulb. I had tested the bulb with a bench supply and it worked . I have to assume that it acts differently in the circuit - though both bulbs have the same DC resistance???

These bulbs are grain of wheat style Incandescent Pilot Lamp Lights with 6" of purple wire attached to them. I think I found a source on Google - I'm assuming they are 14V bulbs?

Thanks for the help...............

Teemuk,

  Thanks for getting back to me. All resistor values are in specification. R255/256 needed to be desoldered to check - I assume they are in parallel with the BJT junctions.

  The voltage at the node of R252 and R253 is only 26V when the B channel switch is turned on and it returns to 33V when the switch is in the A channel mode.

  The voltage at the node of R254 and the B channel lamp is only 2.9V when the switch is in B mode.

  Lamps are original and correct and function normally with a bench supply.

  The collector of Q211 reads 70mV when the switch is in the B mode. If Q211 was not in saturation, I should see a greater voltage at its collector (i.e. transistor in on state).