Help Troubleshooting Millenia Media HV-3B PLEASE

[sonolink]

Btw, I just realized that I lifted the zener legs ONLY on the working channel. I never got to doing it on the faulty one. Does it make a difference? Shall I lift the ones on the faulty channel now?

 

[moamps]

No. First measure U6. We'll break the diodes later.

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[sonolink]

Thanks to the measurement results, it seems to me that the faulty channel now looks good (unlike the "correct" channel where something is wrong with U3). You should now finally measure the voltages at pins 1 and 7 of the IC6 on the faulty channel for the different gain switch positions.

One question: I do this with the same sinewave and the scope right?

 

[moamps]

Yes, that's very important.

 

[sonolink]

Ok. The gain pot has 12 positions. I measured U6 pin 1 and 7 for each gain position.

Here's pin1:

 

[sonolink]

And here's pin7

 

[sonolink]

Out of curiosity I just tried the faulty channel with a mic and it sounds fine now!!!

So I put back the original opamps one by one and the faulty one is U4!!

The thing is how can we determine what opamp should I replace it with. And if there's dodgy things on the working channel maybe I should change the opamps on both channels?

Cheers
Sono

 

[moamps]

Congratulations, so you found out which IC is faulty, you learned something about microphone preamps and how to handle an oscilloscope. I would suggest OPA604 for U3 and U4 and OPA2604 for U6 (with small heatsink). And yes, I think you need to check U3 on the "correct" channel. But now you can do it alone without anyone's help.

 

[sonolink]

I am really grateful for your help Moamps!
I owe you a big cold beer to say the least!

Cheers
Sono

 

[sonolink]

I have found at Mouser OPA604AP and OPA604PG4. Same price. Any difference?

But I can't find OPA 2604 any obvious subs?

Cheers
Sono

 

[ccaudle]

OPA604AP and OPA604PG4. Same price. Any difference?

The datasheet says both orderable part numbers (missing an "A" in the second, should be OPA604APG4) have the same part marking, OPA604AP, so they should be equivalent parts.

I can't find OPA 2604

OPA2604 is end of life and no longer available. OPA2134 was the TI recommended replacement, but it is only for power supplies up to +/- 18V, you can't use it with 24V power supplies. If there is not another set of regulators dropping the voltage to 18V you can't install a 2134 directly into the 2604 spot.

There are two test points labeled +18V and -18V on the first set of pictures you posted. Where do rails those go? I won't belabor the point, but I think down in your soul, you know what you need to do.

I don't see any op-amps capable of +/- 24V anymore at TI, and only LME49860 capable of +/- 22V operation.
Analog Devices has a couple that might work at 24V, but you would need a package adapter, they are SMD only, no DIP available any more.

Of course the whole discussion about +/-24V capable op-amps is moot if that location is actually running from +/-18V, you can just drop in an OPA2134.

 

[sonolink]

The datasheet says both orderable part numbers (missing an "A" in the second, should be OPA604APG4) have the same part marking, OPA604AP, so they should be equivalent parts.



OPA2604 is end of life and no longer available. OPA2134 was the TI recommended replacement, but it is only for power supplies up to +/- 18V, you can't use it with 24V power supplies. If there is not another set of regulators dropping the voltage to 18V you can't install a 2134 directly into the 2604 spot.

There are two test points labeled +18V and -18V on the first set of pictures you posted. Where do rails those go? I won't belabor the point, but I think down in your soul, you know what you need to do.

I don't see any op-amps capable of +/- 24V anymore at TI, and only LME49860 capable of +/- 22V operation.
Analog Devices has a couple that might work at 24V, but you would need a package adapter, they are SMD only, no DIP available any more.

Of course the whole discussion about +/-24V capable op-amps is moot if that location is actually running from +/-18V, you can just drop in an OPA2134.

Mmmm I see what you mean... I'll check the voltages when I get home and report back. Thanks!!

 

[moamps]

From the first post and the picture of the power supply, it can be seen that the preamp is powered by +/- 24v (7824/7924). Locally, there are +/- 18V regulators on each PCB. In several previous posts, it can be seen that U3 and U4 are supplied with lower voltage, and the output double opamp U6 with higher (post118). This can also be seen from the series of readouts of the oscilloscope where the output voltage is limited to +/- 25V. That's why I suggested 2604 for U6. It can be obtained here, DIP8

https://www.ebay.de/itm/304157745173?hash=item46d136e015:g:MdUAAOSw5ZNgc3UF
do not buy it from unverified Chinese sources.

Replacing opamps with other (better) models could be discussed, but this would also require changes in supply voltage. I now don't know any other dual opamp in the DIP8 case that would be a direct replacement for the OPA2604.

 

[sonolink]

Oh shit now I owe you a SIX Pack at least!!!!
I was reluctant to buy on ebay or aliexpress but I'll go with that link.
Thank you very much Moamps!!!
Cheers
Sono

 

[ccaudle]

the output double opamp U6 with higher

Slightly off topic, but do you happen to know what Millenia does on newer models now that integrated devices with 24V supply capability are no longer available? Did the design drop to 18V supplies for all stages, or do new preamps have discrete op-amps for output?

 

[moamps]

Slightly off topic, but do you happen to know what Millenia does on newer models now that integrated devices with 24V supply capability are no longer available? Did the design drop to 18V supplies for all stages, or do new preamps have discrete op-amps for output?

I really do not know. All I know is that in some earlier preamps they used their branded discrete opamps at the output.
In my experience, the good old NE5534 can work at +/- 24V with good passive cooling and not too much load. I discovered this quite by accident.

 

[sonolink]

I really do not know. All I know is that in some earlier preamps they used their branded discrete opamps at the output.
In my experience, the good old NE5534 can work at +/- 24V with good passive cooling and not too much load. I discovered this quite by accident.

You're probably right since on the working channel one of the opamps was filed (probably the original OPA604) and the other one was a NE5534 with a heatsink (probably replaced at some point)

Cheers
Sono

 

[Ike Zimbel]

I really do not know. All I know is that in some earlier preamps they used their branded discrete opamps at the output.
In my experience, the good old NE5534 can work at +/- 24V with good passive cooling and not too much load. I discovered this quite by accident.

I have definitely run 5534's at +/-22 successfully and feel like I would have seen that figure in a spec sheet before I tried it.

 

[Flatpicker]

Slightly off topic, but do you happen to know what Millenia does on newer models now that integrated devices with 24V supply capability are no longer available? Did the design drop to 18V supplies for all stages, or do new preamps have discrete op-amps for output?

I'd bet that they bought a gazillion of them while they still could.

 

[ccaudle]

I have definitely run 5534's at +/-22

I don't know how many companies still produce 5534/5532 devices, but the TI datasheet for their version of 5534 does say 44V max power supply. I don't know if that is different than the first generation devices or not.
Actually I just double checked the TI datasheet, and 44V is the "absolute maximum" rating, meaning the device is not guaranteed to not be damaged at anything over that. The recommended operating maximum is +/-20V, same for the OnSemi version and the Philips version.
Since all three vendors have that I suspect that was probably the spec for the original (Signetics?) design, but I don't have any datasheets going back to the early 80's, or whenever that first came out.