Help Troubleshooting Millenia Media HV-3B PLEASE

[sonolink]

Hello everybody,

A friend brought me this 2ch Millenia HV-3B that has one channel not working properly. When using a dynamic mic the channel has noise and poor signal. I have tried finding a schematic on the net but no luck. I opened it but the chips have no mark on them...

Any help/tips on troubleshooting it would be greatly appreciated.
Thanks in advance
Sono


https://ibb.co/rHJcHPxhttps://ibb.co/PmCgy50https://ibb.co/bbQjfk0https://ibb.co/0cpLvV7https://ibb.co/7nkpbWw

 

[ccaudle]

The PCB's are nicely marked. Have you started by verifying all the labeled voltages are correct?

 

[57sputnik]

Is R16 burnt on channel 1? (May be a shadow.)
I would also look for protection diodes around the input and see if any are shorted. (They may be Zeners.)

 

[moamps]

...When using a dynamic mic the channel has noise and poor signal. I have tried finding a schematic on the net but no luck. ..

Does that mean it works well with a condenser microphone? Do both preamps have the same gain then?

 

[sonolink]

The PCB's are nicely marked. Have you started by verifying all the labeled voltages are correct?

I have checked voltages and both channels seem to have the same voltages.
I also checked the opamps and they seem to read the same on both channels too

Is R16 burnt on channel 1? (May be a shadow.)
I would also look for protection diodes around the input and see if any are shorted. (They may be Zeners.)

Both R16 read 2.2k on both channels. They don't look burnt so must be a shadow on the pic.

I checked the diodes:
D2, D3 0.75 / 1.77 No difference between channels
D4, D5 0.03 both ways No diff
D6 1.99 / 0.75 No diff
D7 0.73 / 1.97 No diff
D8, D9 0.64 / OL No diff

I can't find D1 anywhere...

Does that mean it works well with a condenser microphone? Do both preamps have the same gain then?

I haven't tried with a condenser actually...I'll fetch one at the studio this afternoon, check and report back

Thanks to everybody for the help!

Cheers
Sono

 

[sonolink]

It doesn't work properly with the condenser. Channel 1 has noise and barely gain. Channel 2 works perfect.

Any tips please?
Cheers
Sono

 

[sonolink]

I have recorded both channels with a condenser and an SM58 so you can hear the problem.
The file is CondenserCH2-SM58Ch2-CondenserCH1-SM58CH1
I would greatly appreciate your help, please
Cheers
sono

 

[ccaudle]

You will have to get the heat sinks off the covered components, unless they are really well marked on the bottom side.
As far as I know (which is not far), the Millenia pre-amp design is similar to the Cohen "double balanced" design from the 1984 preprint, which has discrete matched transistor pairs providing front stage gain, a pair of op-amp channels providing additional gain and current feedback to the transistor emitters, then an additional pair of op-amp channels configured as diff amps to remove common mode (pair so that you get two copies for symmetrical balanced output).
An older GDIY post even had a reply from John LaGrou himself:
"Thanks for taking an interest in the HV-3. I'm not sure I've ever "claimed as my own" the HV-3 topology. The HV-3 "Double Balanced" topology was conceived by Graeme Cohen at Philips in Australia in the early 1980's. I can send you a copy of the original white paper, if you wish. On the Millennia web site, I give full credit to Mr. Cohen's brilliant work."

I am going on the assumption that the HV-3b is still basically the same circuit as the original HV-3.

Without schematics you will just have to trace through the circuit and find the components, then do standard circuit troubleshooting techniques to see if one of the devices is dead (check base-emitter voltages on the transistors, make sure there is no crazy DC voltage on op-amp pins, etc.).

The owner manual for HV-3b has this note right at the beginning:
"Back-to-back Zener diodes protect the super-matched octet of input transistors against high transients common when inserting or extracting microphones. There are certain rare instances which can contribute to failed zener diodes."
That seems like a good clue to check the zener diodes.

If it becomes too much and you are just going to chuck it in the trash bin, let me know and you can send it to me instead.

 

[ccaudle]

That same post has this note:
"What I can tell you is that the HV-3 front-end is a collection of large-geometry, tightly matched discrete bipolars with monolithic comparators. Cohen used LM394 transistor arrays, but I didn't like the sound of the 394 in this application. In fact, I've never heard a 394 application that sounded musically correct. The HV-3 output stage is class-A into most loads, and based on monolithic FET topology. Except for phantom blocking, the HV-3 is entirely DC-coupled without servos, including a DC-coupled output stage."

Old GDIY post on HV3

 

[sonolink]

Thanks for your reply ccaudle, but I'm a bit overwhelmed I think. I'm just a DIY hobbyist really...if only I had a schematic I could feel a little less lost hehe.

I can't send it to you. This is an old preamp from a friend and he's asked me to help him out.

I've got a signal tracer. I'll try your suggestion of taking off the heatsinks to see if I can find out what opamps are used and I'll try tracing the signal through the circuit. I hope I can find the fault like that.

It's a shame there's no schematic available for this unit. Thanks again for your input...
Cheers
Sono

 

[ccaudle]

Did you ever say which channel worked and which did not? I just noticed that CH 2 has a big chip on the package of Q1, which is presumably the input transistor array.

 

[ccaudle]

It's a shame there's no schematic available for this unit

Is it a multi-layer board? If there are just one or two routing layers you can make your own by tracing out connections on the PCB. You will have to make some guesses, but input signals will go to bases of transistors, the transistor collectors will go to resistors that connect to V+ (assuming NPN, resistors would go to V- if they are PNP), the gain setting resistors (mounted on the front panel switch) will go to the emitters, etc.

 

[sonolink]

Did you ever say which channel worked and which did not? I just noticed that CH 2 has a big chip on the package of Q1, which is presumably the input transistor array.

Channel 2 is the one working ok.

 

[sonolink]

Is it a multi-layer board? If there are just one or two routing layers you can make your own by tracing out connections on the PCB. You will have to make some guesses, but input signals will go to bases of transistors, the transistor collectors will go to resistors that connect to V+ (assuming NPN, resistors would go to V- if they are PNP), the gain setting resistors (mounted on the front panel switch) will go to the emitters, etc.

I will check if it's a multilayer but I think at most it'll be a double layer.

I have checked the opamps. And they are all filed!! One of the heatsinks has been GLUED to the opamp....this unit has been clearly manipulated in the past...

The thing is that on the good channel opamps are also filed except one: an NE5534. I don't have any spare ones. I wonder if I could use a NE5532 or a TL72 just to check if it's the faulty one...

 

[sonolink]

I can't believe someone just glued the heatsink....

 

[musgrave]

It is my understanding that generally John used an OPA604. If there are duels probable an opa2604. running them at 22v the heat sinks were a good idea. to see if its a single or duel look for the + rail on pin 7 for a single, pin 8 for a duel. 5534 is a single. maybe someone used it to repair it at another point in time.

 

[sonolink]

It is my understanding that generally John used an OPA604. If there are duels probable an opa2604. running them at 22v the heat sinks were a good idea. to see if its a single or duel look for the + rail on pin 7 for a single, pin 8 for a duel. 5534 is a single. maybe someone used it to repair it at another point in time.

Thanks for chiming in Musgrave. Excuse my language barrier but do you mean that I have to check if pin 7 or 8 have positive voltage to determine if the opamp should be an OPA604 or an OPA2604?

The NE5534AW is on the working channel and doesn't look as if it's been repaired (although I might be wrong) because the faulty channel has all the opamps on sockets whereas the working one not.

I'll post voltages asap.

Thanks for your input
Cheers
Sono
I'm going to post the voltages of each opamp.

 

[scott2000]

Excuse my language barrier but do you mean that I have to check if pin 7 or 8 have positive voltage to determine if the opamp should be an OPA604 or an OPA2604

Yes. As you probably know, the single 604 will have + rail at pin 7 and the dual 2604 will have + rail at pin 8...Both will have - rail at pin 4...

 

[sonolink]

Thanks Scott. I'll check and report back

Thanks to all for your help!!
Cheers
Sono

 

[ccaudle]

the faulty channel has all the opamps on sockets

Have you tried just re-seating all the socketed components to see if the problem is oxide on the pins?

I have checked the opamps. And they are all filed!!

I have heard that was standard from the factory. Presumably to make reverse engineering harder. Or maybe just to make people measure and listen and not draw any conclusions just from which parts were used.
I have never heard whether the glued on heat sinks were standard as built, but possibly if there are not screw down locations for the heatsinks on the PCB.