Tascam TSR8 pinch roller rubber turns to gunk, replacement advice.

[trancedental]

So I removed the NEC 4570C opamps at U3 B output & U4 A & replaced them with sockets.

I changed U3 B for a NE5532P & U4 A for a Burr Brown OPA 2134, these were the only compatible opamps I had spare. But still had the same old problem with signal only when DBX NR was turned off.

I was just about to solder out & socket the U6 DBX 1252 chip & order a replacement when I decided to change out the NE5532P as I knew the the OPA 2134 was working, I then tried a LM833N for U3 B instead & switched on the DBX button & I finally got playback signal on the LED display the same as all the rest of the channels.

I've not got time to connect it up to listen but the faulty channel seems to be working! I'll check back later tonight. I'll see if either of the opamps I removed still work & maybe get replacements. I don't know yet if I might have oscillation or other problems using the substitute opamps.

 

[jpmorris]

Good to hear! Might be worth putting in exact equivalents once you've established exactly which chip is bad.

 

[trancedental]

The Tascam TRS 8 uses DBX "Type I" noise reduction, cassette versions are Type II & earlier Type I external unit versions used with the Tascam 48 / 38 recorders don't combine the encode & decode circuit togther like the TRS DBX does.

The op amp used on the TSR 8 R/P channel PCB's is the NEC C4570C (UPC4570C). There are 4 of these on the channel boards & two involved with DBX. The others deal with Rep EQ & Meter Level / Bias.

U3 A INPUT LEVEL
U3 B ENCODE LEVEL
U4 A DBX OUT
U4 B REC LEVEL R/P HEAD

The NEC C4570C (UPC4570C) description "Operational Amplifier, 2 Func, 5000uV Offset-Max, BIPolar, PDIP8" is no longer available & doesn't seem to have an exact replacement, it usually costs around £3-5 each plus. There are possible replacements however, but not exactly the same, LM833N, NE5532, NJM4580D, MC33078.

Out of the 2 DBX op amps I guess U3 is more critical because it sends the DBX input level & encodes the signal during recording. Changing U3 might cause problems with DBX. I'll replace this with an original C4570C.

U4 B works after the DBX channel switching & U4 A sets the output level after the DBX.

Tascam, Roland & other machines seem to have used the NEC C4570C (UPC4570C) in many preamps & recorders, some users online upgraded their preamps to modern OPA 2134 / OPA 2604 or other op amps. Others have also replaced the C4570C in DBX compressors & similar clones which use the same U6 DBX 1252 VCA as the DBX Type 1. I can't find any recorder circuits which have been modded in this manner.

I need to do a detailed comparison of the op amp dataheets & I'll do a software simulation of U4 with a 2134 / 2604 after the DBX circuit in Multisim / or Pspice to see if there any any problems, I'll have to get my old school oscilliscope out to check for high frequency oscillation, if I can remember how to use it!

 

[jpmorris]

Ah, I thought there would be a modern equivalent in production. Keep us posted, this stuff will be handy to know if I ever have to repair a channel on mine.