dana exact model 170 amplifier: looking for manual/schematic

[theHWcave]

Hi, I got a Dana Exact Model 170 amplifier but its dead, high DC on the output. Power rails measure fine.

I searched all the usual places for a manual or schematics but no luck. Does anyone know where to find schematics?

Thanks in advance

[lowimpedance]

 Manuals in general for Exact equipment are rare and this unit seems even rarer ! However you should be able to fix it without a cct.
Most likely all BJT and a shorted device/s likely, use your multimeter to check.
Other suspects would be resistor drift shifting bias points as this amp is likely fully DC coupled. As well as any tantalum caps if used.

Post some internal pictures, which would be helpful in giving any further suggestions.

[TerrySt]

I have a working Exact 170 and have also never been able to locate a manual.  I did make some measurements on mine a while back that might be helpful.

Terry

[theHWcave]

Thanks for these. But it will be a while before I will have anything to compare (if ever)
Yes, the input switching is straight forward and the wiring easy to follow. In x1 mode the amp does absolutely nothing and does not even need to be powered on. The input is connected straight through to the outputs, putting 600 Ohm or that massive 50 Ohm resistor in a heat sink in the back in series.

[theHWcave]

Yes, I forgot to mention that I already did the old multi meter test of all transistors and they seem all fine. There is a N-channel JFET 2N5457 in the input that I desoldered and tested separately and that worked fine too, so its back in. Most diodes seem to be fine too. There are 2 that test 0.6V in one direction and some weird value (not OL) in the other which might be because being in circuit.  I guess I have to start drawing a schematics myself. I started with the output because it looks as if the positive part of the amp is fully on all the time causing 65V DC on the output

On the picture, the left side is cut off but there are 3 power transistors similar to the ones on the right side. I identified them showing xxxx which is meant to be "2Nxxxx" 

Some resistors seem to have gotten pretty hot...

Unfortunately I wont have time to work more on this until the weekend. Will report back on progress

[theHWcave]

Ok I spent the last weekend reverse engineering this thing. In tracing the circuit out I found a cold solder joint on a wire bridge. After fixing that, the amplifier works - sort of. It still has a large DC offset of nearly -15V but if you ignore that, it works just fine. The gain is nearly x5 and I can't see any distortions on the signal shape.

I have added two sets of measurements, one is without input producing just the neg. DC offset, the second one is using -4VDC as input. For both of these I powered it with just 115VAC instead of 230VAC because the resistors get very(!) hot. Especially the large 22K resistor hanging basically between the rails. For these measurements I have to turn the amp upside down to access the underside of the PCB. That upsets whatever thermal flows and heat is even more an issue than it would normally be. I found that the amp works (for your definition of works) with half the operating voltage, staying much cooler which gives me time to do measurements in peace.

To be honest that schematic has me baffled. I am not certain my schematic is 100% correct.  I can't really figure out how it works and what causes the offset. Any hints would be appreciated.

[TerrySt]

Something is wrong with the open input measurements.  Some of the bipolar transistors have way more than 0.6V forward bias (like several volts), which is of course impossible if the base - emitter junction is intact.  Something must be throwing off your measurements.  Either you aren't probing where you think you are, or something like an oscillation is going on.   Or maybe the voltages change when you probe.

[TerrySt]

It would be helpful if you added reference designators to the schematic so we can discuss it easier.
The connection from the 2N2484 collector through the 1k resistor should probably be going to the base of the 2N2905 instead of its collector.

[TerrySt]

I opened mine up and I'll make some measurements, but I'm working on some other things, so it might go slow.
The first thing I noticed is that the input FET is a 2N5245 (your schematic says 5457) and the input goes to the gate, not the drain.  Could it be someone replaced the 2N5245 with a 5257 and didn't realize the pinouts aren't the same?

[theHWcave]

Good points. Need to double check (again) the pin-outs of all transistors... Could you send a picture of the PCB of your model for comparison?

[TerrySt]

Sure.

Here is the bottom side.

[TerrySt]

...and the top side.

[theHWcave]

Very interesting. The FET in my model is indeed a 5457 and its pin out according to an On-semi datasheet is 1  = D, 2 = S, 3= G.  The pin out for the 5245 is 1 = G, 2 = S, 3=D, yet the transistor on your picture is oriented the same way as on my PCB... Looks like someone replaced the transistor and thought it was pin-compatible when it isn't.

I also notice you have a "big" 13.1K resistor while mine looks like 0.6W type with clear marks of overheating ... 

[TerrySt]

Hopefully just fixing that FET orientation issue gets it working.  I don't know if you can see it in the picture, but the 4.99k and 30.1k resistors next to the 2.2k power resistor are discolored a little from heat.  They weren't real conservative on the power ratings evidently.
Let me know if fixing the fet gets it working.  If not, I can try to help with some voltage readings on mine.

[theHWcave]

Success Turning the FET around fixed it.   +40mV DC offset and amp works as it should. I will put out an updated  schematics with the FET fixed. Not sure if I can muster the energy to look for other errors in the schematics. I will also look into replacing at least the 13.1K resistor
Many thanks for your help, Terry
Heinz

[TerrySt]

 
Glad it’s working. 

Terry

[theHWcave]

As promised, schematics for the Model 170 that fixes the error.  Despite best effort, no warranties that this reverse-engineered schematic is fully correct.

[theHWcave]

attachment as PNG

[TerrySt]

I know this is an old thread, but while looking for something else, I found a copy of the manual for the Exact 170 Amplifier.  Unfortunately, it is a very light copy, but is still (barely) readable.  I'll attach a pdf in case someone is looking for it.  If anyone knows of a better place to upload it, feel free of course.

Terry