Welcome to Mindburner's DAC71-CSB-I & Prophet 5 Site - FAULT FINDING



 

This 16bit Digital To Analogue Converter is used in the Prophet 5/10, T8, some early 600's as well as the Studio Electronics rack. It has a vital role in Prophet 5 tuning and operation. It can be one of the main components to fail. On 600's, check as most use DAC700 KH-1

DAC71-CSB-I & Tuning

After suffering lots of tuning problems, I found that the DAC71-CSB-I chip made by Burr Brown (now Texas Instruments) was faulty. The synth was still functionally operational but sounded dire across the entire keyboard range. If one voice is out of tune it is usually the oscillator/filter. The sample and hold circuits can be at fault if one voice across both oscillators is off. These DAC's are complex with several sub components. They can get quite hot and operate at a relatively high 15v so failure is not uncommon in an older instrument like the P5.

Other DAC Symptoms

Remember, faulty logic IC's can also throw up a lot of the same symptoms, so check these also. The power supply can be the cause of many Prophet faults so check this before suspecting the DAC.

A faulty DAC can also lock up the synth with all the LED's lit or cycle the LED's round and round a couple of times a second. It's output is very precise so even a small deviance can create problems. Sometimes you will get a very slight droning sound or a low frequency clicking sound out of the synth if playing the upper keys.

I have seen Prophet 5's where the controls all work fine but there is no sound at all. This was also due to a bad DAC.

Odd voltages on the outputs of all of the sample & hold UAF772ltc IC's can be a sign of a bad DAC, however there may be other reasons for this i.e. associated logic chip etc. Again I would replace these op amps with TL082's or TL072's (low noise version)

Here is a link to a great youtube video by Keith Murray on DAC issues.

Right is a pin out for the DAC71-CSB-I

There is a voltage version of this IC DAC71-CSB-V

It can be used with a modification of bypassing the external op amp on the Prophet 5 board. This mod is shown in the service manual schematics.

 

dac circuit

Some dead unloved DACS awww

 

The below left image shows the underside of the 2 BB DAC package types. One has an extended 'skirt' of PCB but the pinouts are the same and both can be fitted to a Prophet without modification. The right image is of a CBI version, which is a current output 12 bit device but I am unable to find any more information on this.

Availability

Analogue Devices also made a DAC chip ADDAC71-CSB-I as do Micro Networks but these also very hard to find and have not been in production since 1995. CSB stands for Complimentary Straight Binary. The BB DAC was available in Ceramic and Metal base versions.

The DAC71-CSB-I can be an expensive and difficult chip to get hold of. I managed to get a couple of BB versions. I have both package types. So if you need one, let me know by

email

They aint cheap but DAC's shipped will be tested on a Rev 3.3 and well packed. I will ship worldwide with tracking.

 

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