Marantz 3800 help needed.

[Narodow]

I am repairing a Marantz 3800 preamp that has a bad UA749 op amp. Marantz's part number is F749PC. I am not having luck finding a UA749 since it is obsolete. Does anyone know of a replacement for this op amp? Any help is greatly appreciated.

[phatterplatter]

THE OP AMP YOU'RE LOOKING FOR HASNT BEEN MADE IN OVER 20 YEARS BUT THE POSSIBLE SUBS FOR THE PART ARE:

UA749 ( metal can) Fairchild
LM1303 Nat'l semi
Raytheon RC4739

There are people who have these but they need a min buy , usually $100

I'm trying the same chip search

[Nakdoc]

NTE 725 or ECG 725 will work. Prelco has the ECG. 888-773-5268. This will be an exact ic even though ECG transistor were subs, the ICs were not.

[McMarantz]

NTE 725 or ECG 725 will work. Prelco has the ECG. 888-773-5268. This will be an exact ic even though ECG transistor were subs, the ICs were not.

Hi All
ECG 725 is not work with marantz 3800 pre amp.
I teste the ECG 725 my 3800 pre amp this chip work , 15 minute after very warm and the sound to go bad ! NOT USING THIS CHIP WITH 3800 PRE AMP !
IS NOT REPLACEMENTH PARTH TO F749PC!:thumbsdn:

[catrafter]

The F749PC and the also unobtainium RC4194 power regulator is why I don't recommend the 3800 unless someone has to have the Dolby processing for tape work.

You can use an LM1303 or NTE725 but you must limit the power supplies to a maximum of +/- 15 Volts and remove the bias source from the output pins.

To limit the power supplies:
Remove R267,268 and install 220 Ohm 1/4 watt resistors in their place. Install 14 or 15 volt zener diodes (500mW is fine) across C241 and C242. Observe polarity!

Then remove Q211, Q212 (these were used to bias the F749P's output stage into class A operation).
Power up with no chip (U201) installed and check pins 14 and 7 for +/- 14 or 15V depending on which zeners you used.
If all is ok, turn it back off and install the LM1303 or NTE725.

Another method is to use a small experimenter's board and wire up an NE5532 or other dual opamp to match the pin out of the F749PC. Just be sure whatever part you use can handle the +/- 17 Volts or so from the power supply. You may still have to pull the bias transistors.

Tom

[jblmar]

I may be able to get UA749 as well as the RC4194. original part numbers.
PM me if interested.
They are expensive!!

Ron