uPC1024H or TA7129P pre-amp IC substitute - looking for DIY solution using a "home-made" PCB and discrete components

ElectTech

ElectTech

Active Member
This vintage uPC1024H or TA7129P integrated circuit is found in many types of vintage audio equipment. This is not an op-amp; it is claimed to be a "low-noise audio preamplifier IC". Its spec and equivalent circuit are found below. After a search, I understand that an NTE1462 can be a substitute. This NTE1462 is claimed to replace an old ECG1462 (but I found the spec sheet only for the ECG1085). I was initially going to build a circuit on a breadboard, adopting a 2N2222 for Q1, Q2, and Q3; however, I have no instrumentation to reliably measure the results against the original specifications below. Please let me know if you have a solution using discrete components. Thank you.

uPC1024 pre amp IC .jpg

ECG1085 - Copy33.jpg
 
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Not much help from me, but I'd use a low noise transistor like the KSC1845F or KSC1815 if you need a higher Ic.
 
Probably those parts go back to stereos designed in the early 70's. Someone did up a another similar version of a Rohm chip BAxxx something.
Eventually opamps replaced these functions.
Its something that could be designed but it needs some effort. My approach would be to find circuits the parts used in and then simulate the designs in ltspice. Then you could build a small PCB using surface mount devices.
Nice little project.
 
This actually is an opamp albeit a very simple one. With the circuit schematic it is used in you can use a regular opamp like NE5532 (dual) or even µA741(single). What is it used in? In a stereo application you can easily kludge in an NE5532. Done that many times with always excellent results.
 
Thank you for the comments. My uPC1024H chips are installed in a cassette from 1978 (a JVC KD-85J). I am trying to improve the audio quality of the cassette deck and wanted to replace these ICs with something better to test if it is worth it (I fully recapped that deck in 2019, and it is working fine, but the sound quality is not great compared to others that I have from the same era). I understand that this IC has been used in phono stage in some receivers of that era, and an upgrade would be even more suitable, I think. I communicated in another forum many years ago, but no one could come up with a "drop-in" solution for this 7-pin IC. Someone hinted at a solution using a true op-amp, and this is what I came up with for the specific application of my KD-85J, but I never implemented it because I was not sure if I should change the original configuration like that and then make things worse.

Image4b.jpg

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replacementa31.jpg
 
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PE9ZZ said:
This would work just fine.
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Yes, I think that a solution using an op-amp like NE5534 (single) or OPA134 (single), or NE5532 (dual as you suggested) would be easier and more reliable than trying the 3-transistor solution. I am really bad at soldering SMD components, but I can try with through-hole components. I have done a similar project before, but I bought an adapter from cimarrontechnology, and that worked like magic, very easy. It is here:

example of op-amp adapter from another project
 
I just want you to know of tape head wear, as the tape head wears the tape gap gets larger therefore the high frequencies diminishes. So if the deck was played a lot , sound quality maybe down. SA heads were very good, but they still got worn down.
 
Stereo2 said:
I just want you to know of tape head wear, as the tape head wears the tape gap gets larger therefore the high frequencies diminishes. So if the deck was played a lot , sound quality maybe down. SA heads were very good, but they still got worn down.
Click to expand...
Thanks, that is a good point. The play and rec heads are fine. I cleaned them well, demagnetized the play head using a TDK HD-01 Head Demagnetizer (there is a whole thread about that ...), regulated the azimuth, and followed the other calibration steps in the Service Manual. The uPC1024H chips are working fine. I was looking for a replacement to seek any audio improvement, if that is possible to be noticeable. When I have some time off, I may remove one of these uPC1024H from the cassette deck and test it against a 3-transistor solution on a breadboard using a frequency generator and an oscilloscope.

Added: I did some mathematical analysis of the uPC1024H related circuit in the KD-85J cassette deck, applying the voltages given in the JVC schematic, and using Ohm's and Kirchhoff's Laws ... I can't make a final sense of this configuration. I guess that the JVC technician in Japan in 1978 was not using a very precise voltimeter, or some numbers are not correct, even though I verified the resistors' values in the 'BOM' of the Service Manual, and they seem correct.

In summary, for my application, I would remove the whole section involving this uPC1024H and replace it with an op-amp solution after configuring it on a breadboard.

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@rcs16 Thanks for the suggestion. I simulated the circuit using the LTspice program. I never used it before, but it is free and not that difficult to learn the basics. If my simulation was correct, I must offer my apologies to the JVC technician in Japan in 1978. He or she measured the voltages in the pins (1-7) correctly. At least the LTspice simulation confirmed that very well. I adopted all transistors as 2N2222 because of their high gain (over 100), just for the sake of the simulation.

If I am correct, using the formula Gain in dB = 20 Log (Vout/Vin), the simulation shows a signal of -40dB in the input and 0dB in the output; therefore, the voltage gain is 100. I input a sine wave of 0.01V amplitude in the input in the simulation.

I don't see a perfect "drop-in" solution using an op-amp. I would have to do a PCB (2 sides) using SMD devices to create a compact replacement. I am not up to the challenge at the moment.

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uPC1024H simulated 1.000.jpg

circuit analysis2.jpg
 
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ElectTech said:
I don't see a perfect "drop-in" solution using an op-amp.
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Oh, but I do. I see a design challenge. The problem for a universal drop-in replacement is R137. But by tying pin#4&5 together and using a single additional resistor of 220k to pin#7 it will work in this position. With pin#4 grounded (there is a schematic at Miss Tanya that does it that way) it will work too. If you attach the LTSpice (.asc) as zip file I could have a go at it. With an all SMD solution it is even possible to use a 5-pin SOT23-5 opamp. I don't have a candidate but surely they're there (bipolar, low noise, 40 V supply voltage).

Edit: LM321 is available in this package but it's not exactly low noise with 40 nV/√Hz
 
@PE9ZZ Ok thanks! I will send you the LTspice file (I understand I can't attach it here). My cassette deck (JVC KD-85J) uses 4 of these obsolete uPC1024H chips (aka TA7129P, aka NTE1462) , 2 for the PLAY head signal amplification, and 2 for the MIC signal amplification. I was looking to upgrade only the ones for the PLAY head. However, I guess a lot of folks out there ( @GPS16 @smurfer77 @Artopsy @petehall347 ) have vintage RTR decks, or receivers that use this uPC1024H or TA7129P in the phono stage, and that would probably be a bigger improvement to substitute that with a suitable true op-amp solution (please see this old thread **). An OPA134 comes to mind. I use this OPA134 in the phono stage of my Toshiba preamp (SY-665), and it sounds great to me (it was a conversion from the obsolete TA7322P to OPA134).

MIC input of the KD-85J cassette deck is also using the uPC1024H: PIN#4 goes straight to ground.

kd-85 - MIC-in uPC1024H pre-amp.jpg


OPA134 opamp:



OPA134 .jpg

(**) Pioneer RT-707 service & rebuild
 
It will be a challenge to design a drop-in replacement which caters for the various use cases for this device. In any case a DC blocking capacitor is needed in the ground leg of the feedback network. Several components need to be removed. Only then the opamp replacement can be used.
 
Well, it was easier than I thought. Just an extra resistor (R1) to fix the DC working point, done. Pin#7 is not connected anymore and pin#4 and 5 are connected together. While I did not verify whether this solution would work with the other schematics I think it will. The capacitor C1 could cause problems with the opamp but because Rc is so low this doesn't seem to be an issue. Maybe in case of a faster opamp than the NE5532 chosen here. The tradeoff of R1 is a fraction less gain. Capacitor C7, needed to make the opamp happy, could be an issue in case of SMD parts. I dunno if there is a single NE5532 in SOT23-5 package but I believe the OPA210 would work just fine.
 

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WOW! Congratulations! This is genius! I took a look at your solution with the NE5532, it looks good, and the inclusion of C7 (10uF) was smart. Please let me know if R3 (5.6k) is still needed for the op-amp solution. You are already bringing about Vcc/2 to the non-inverting input of the op-amp. Thanks.
 
@PE9ZZ Please find attached the simulation for the 'Marantz2216 phono stage TA7129P'. I was not able to use the 2SC2240 transistor in my LPspice program. I am still learning it. I then used the 2N2222. I think that the output in PIN#6 is producing a proper RIAA equalization curve, though. I checked the DC voltages in PINS 1-7, and they closely match those of the Service Manual of the Marantz 2216. I don't have any audio equipment that uses this 3-transistor chip pre-amp in the phono stage, otherwise I would be longing to upgrade it to your op-amp solution ASAP.

Marantz2216 phono stage TA7129P.jpg
 

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