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[Joe Russo]

I've got an old Farfisa F/AR reverb/PSU. The power supply works, but the spring is physically broken. It looks like it's an accutronics type4, but what impedance?



Cheers - Joe

[trunkline]

Is the actual spring broken or is it just a problem with connections?  A common failure point for spring reverbs is the wires leading to the transducers in the tank--they're generally fairly delicate & prone to breaking or coming loose.  If you're even moderately comfortable with a soldering iron, it can be an easy fix.  It's probably worth checking out before replacing the entire tank.

Tom

[Joe Russo]

Unfortunately, the spring itself is broken.

[Jay Kadis]

If it is an Accutronics, the impedances are coded in the part number.  Their web site has the info.

http://www.accutronicsreverb.com/

[Joe Russo]

My problem now is that i cannot find the part number on the unit itself. The chassis is identical in size to a type 4 accutronics but is painted brown and has a locking mechanism screwed to it.

Thanks,
Joe

[Jay Kadis]

That does sound like a type 4 as a locking mechanism is an option.  If there's no way of finding the part number, which is just stamped on in ink, you can deduce the input and output impedances by running a signal into the input transducer and shunting the input with a resistor decade box.  By finding the resistance that cuts the output in half, you will have determined the input impedance.  Similar should work for the output transducer, of course provided the unit passes signal.

You might be able to get a clue about the output impedance from the circuitry to which it attaches if you have a schematic.  There also might be part numbers stamped on the transducer elements themselves that Accutronix might be able to identify.

Are you sure the spring is broken?  Some of these units use double springs counter-wound that hook together in the middle: could it just have come unhooked?  

[Joe Russo]

Thanks Jay,

I'll give that a try.

Here's the schematic of the amp

Cheers,
Joe

[johnR]

Jay Kadis wrote on Thu, 22 July 2010 15:06

If there's no way of finding the part number, which is just stamped on in ink, you can deduce the input and output impedances by running a signal into the input transducer and shunting the input with a resistor decade box.  By finding the resistance that cuts the output in half, you will have determined the input impedance.

Shunting the output like that will tell you when the total load impedance equals the output impedance of the driver stage, which is not necessarily equal to the drive coil impedance. A discrete driver like the one in the schematic might suffer damage due to overcurrent too. Using a resistor in series with the drive coil instead would give the right result without over-stressing the driver.

[Jay Kadis]

The schematic helps: R208 shunts the reverb tank input and at 100 ohms, it would just decrease the input level if the tank input impedance was high, hence I would have to guess it's an 8-ohm input type4, since the next possible input impedance is 150 ohms.  The output impedances possible are 500, 2250 and 10000 ohms.  The load on the tank output seems in the range of 5kohm, so I would have to guess the output would be the 500 ohm option.

The best way of telling for sure would be to remove the tank from the circuit and try the resistor decade box technique, since the above depends to some extent on guess work.

Also, if you can see the bobbins or coils, they seem to be color coded:

http://www.accutronicsreverb.com/main/?skin=sub05_04.html

(Of course this presupposes it actually IS an Accutronics.)

[johnR]

Those Germanium transistors aren't very large. I suspect 150 ohms is more likely than 8 ohms, but it would be best to measure it using a decade box in series, assuming the drive coil is intact. If the spring is broken that may change the measured impedance, so it's still likely that some guess work is needed.

It's possible that measuring the dc resistance of the coil will provide a clue, but I don't have a spring line here to check how close the nominal impedance is to dc resistance. With a speaker drive coil, dc resistance is typically around three quarters of the nominal impedance, but I don't know if that would be the case here.

[johnR]

Might be a stupid question, but is it possible to join the broken ends of the spring together?

[Jay Kadis]

I wouldn't use DC - I'd use a 1 kHz sine wave to deduce the impedance using the resistance decade shunting the input transducer while monitoring the output with the tank disconnected from the circuit.

But this is all moot if he can see the transducer bobbins as they are color coded by impedance.

BTW, johnR is right about using the test resistance in series with the transducer input rather than as a shunt: it loads the test signal source more safely.

[Joe Russo]

OK, The input transducer was open. So I tried another approach. Still a little unclear...

It has a brown bobbin which is listed as 250 ohms/688 turns, but I unwound the broken coil, and it was only about 145 turns, which looks closer to the 8 ohm specs.

Maybe this was custom..


The output is red bobbin and the DC resistance matches the with the list.