Melcor 1731 Teardown

[TheseGoToEleven]

Let’s open up this Melcor 1731 op amp and see what makes it tick.



The plastic case has no shielding inside. Its contents are all suspended in a sort of semi-clear, rubbery gel. It reminds me of the fruit and jello molds that used to be so popular, except this was made by engineers and is probably far less tasty.



Here is what it looked like after removing most of the clear goop. It’s a little easier to see what we’re looking at now.





The circuit board only has traces on the bottom. All of the components have been installed through-hole style. Some capacitors have long leads covered with wire insulation. There is also an orange wire connecting two separate areas, jumping over the traces in between. There are a couple of unused through holes and traces. Perhaps these were used in older 1731s.

Here are two of the previous shots with component and net names added.



I have attached the resulting schematic to match, which also includes a bill of materials. I measured most of the components but since this was an old and non-working / noisy unit, their characteristics have likely drifted from spec. The carbon comp resistors, for example, all seem to have increased in ohms.

Enjoy, and many thanks to Whoops for providing the 1731! I don’t know the age of this unit, so if anyone can help with dating it, please chime in. The BOM lists some additional markings on the transistors, which I think are date codes, but I'm not 100% certain how to interpret them.

 

[Whoops]

WOW, really WOW

Amazing work you have done,
I never seen before such a complete and organized teardown and analysis like the one you have done,
Congrats.

You were for sure the right person to give this part to, so I'm really happy that I sent the opamp.

I measured most of the components but since this was an old and non-working / noisy unit, their characteristics have likely drifted from spec.

I don’t know the age of this unit, so if anyone can help with dating it, please chime in.

This is the info I remember from this particular Opamp.
It was from an OSA mic preamp unit for the 500 series, OSA was Old School Audio and the company no longer exists. It used to sell preamps for the 500 series based on the API preamp, they had different units with different Op amps and transformers, this came from an OSA model that used original Melcor 1731 Opamps.

As far as I remember the OSA unit got Noisy.
It could also be that it distorted or that it didn't pass signal at all, it's been 15 years so I'm not 100% sure, but I think it had to do with noisy. It could be noise that would come and go randomly and not constant noise.
So not sure, but as far as I remember it had Noise randomly coming and going.

I guess the fault could be a failed transistor due to overheat.
Or a bad solder or bad connection.
Or a failed passive component from overheat

 

[Whoops]

Now that you mastered it are you going to fix it and make it run again?

 

[Jarno]

That's thorough, thank you!

 

[TheseGoToEleven]

... this came from an OSA model that used original Melcor 1731 Opamps.

I'm not familiar with OSA pres but on the surface they seem similar to some of the CAPI products, which offer various transformers and allow op amps to be swapped in/out, including 1731s. In fact, I have a VP25 waiting to be assembled now, along with a few op amps kits, including a gar1731. So that'll be fun to swap that in and give it a listen.

As far as I remember the OSA unit got Noisy.
It could also be that it distorted or that it didn't pass signal at all, it's been 15 years so I'm not 100% sure, but I think it had to do with noisy. It could be noise that would come and go randomly and not constant noise.
So not sure, but as far as I remember it had Noise randomly coming and going.

While pulling the Melcor 1731 apart, I noticed that the pin connections were pretty fragile. The pins are pretty long, but only connected to one side of the board, because the through holes aren't plated. It only takes a little bit of wiggling before the solder joint cracks and the pin gets loose. The black plastic bottom of the case helps prevent the pins from moving, but on your 1731 a lot of the black plastic bottom was missing. Maybe that didn't happen until after it already was noisy? But anyway, the pins seem like a weak point, and a possible source of nasty noises.

Also, the hFE value on the TIS92 output transistor (Q7) only measured at 75, which seems out of spec. According to the data sheets I could find, the minimum value from the manufacturer is supposed to be 100. So maybe that one was degraded and producing noise.

Now that you mastered it are you going to fix it and make it run again?

Good question. I desoldered all the transistors and diodes so I could measure them and read their part numbers, but I could put them back. Maybe if I strengthened the pin connections and swapped out Q7, the noise would go away. But I would need to put this opamp in a pre that supports the BIAS pin, and I don't think there is one around here. I could modify it by replacing Q1 & Q2 with better matched parts, and match up the collector resistors, and not use the bias pin. That's how it is in the gar1731. If I ever get it working again, I'll let you know.

 

[CJ]

Great job!

Any brand markings on those caps?

 

[TheseGoToEleven]

Any brand markings on those caps?

The ceramic caps have a plain diamond on the back, which you can see in a couple of the photos above. I'm not sure whether that indicates the brand. On the red caps, the back is blank so there are no clues.

 

[Whoops]

I'm not familiar with OSA pres but on the surface they seem similar to some of the CAPI products, which offer various transformers and allow op amps to be swapped in/out, including 1731s. In fact, I have a VP25 waiting to be assembled now, along with a few op amps kits, including a gar1731. So that'll be fun to swap that in and give it a listen.

OSA pres where simple vintage API based circuits (input tx, discreet OPamp, output tx) , but depending on the model they came with different transformers and Opamps

but on your 1731 a lot of the black plastic bottom was missing. Maybe that didn't happen until after it already was noisy?

The black plastic was broken after it was noisy or failled because we tried to see what was inside to check if it was easy to repair or not

Also, the hFE value on the TIS92 output transistor (Q7) only measured at 75, which seems out of spec. According to the data sheets I could find, the minimum value from the manufacturer is supposed to be 100. So maybe that one was degraded and producing noise.

Tolerance of transistors from that era could be 20% or 30%, so I don't the transistor failed just at looking at 75 hfe only.
Maybe it failed but for other reasons

I desoldered all the transistors and diodes so I could measure them and read their part numbers, but I could put them back. Maybe if I strengthened the pin connections and swapped out Q7, the noise would go away.

Replace all Semiconductors (all transistors and Diodes, match transistors) , strengthened the pin connection.

That should fix it

 

[CJ]

Who is fabbing the boards?

 

[Whoops]

Who is fabbing the boards?

What boards are you referring to Chris?

The info here is about a teardown of an original Melcor 1731 Opamp

 

[hodad]

Also, the hFE value on the TIS92 output transistor (Q7) only measured at 75, which seems out of spec. According to the data sheets I could find, the minimum value from the manufacturer is supposed to be 100. So maybe that one was degraded and producing noise.

The other possibility is that it was selected for its low hFE. It would appear that you've got a lot of selected transistors in that circuit (no idea what they were selected for, but the little paint marks suggest they were selected for something), so I wouldn't be shocked if the TIS92 were selected for low hfe and/or some other, possibly related. quality.

 

[CJ]

Differences I see between this and the Peter C schemo:

6.8 ohm collector resistors instead of 10 ohms,

1 less diode between transistors

Trim option on first diff pair

There were some oscillation problems with some of the diy circuits, so the hfe 75 could be right, those transistors might have been chosen for low gain to squelch the osc.

 

[Walrus]

The multiple teardowns I did (#3 in the Melcor tech docs forum) also has 6.8 Ohm resistors, the single diode is actually a double diode in one package like the 2520 uses.

 

[TheseGoToEleven]

The multiple teardowns I did (#3 in the Melcor tech docs forum) also has 6.8 Ohm resistors, the single diode is actually a double diode in one package like the 2520 uses.

Is your schematic the hand-drawn one here? It matches perfectly with what I found.

 

[TheseGoToEleven]

Differences I see between this and the Peter C schemo:
...
1 less diode between transistors

That part for D2, a 1N4829, is a stabistor or double diode. Essentially equivalent to two typical diodes in series, or one diode with double the Vf.

re some oscillation problems with some of the diy circuits, so the hfe 75 could be right, those transistors might have been chosen for low gain to squelch the osc.

I get that logic but TI's datasheet shows a minimum of 100, max 300, typical 160. It seems like 75 would be a rejected part but I don't know whether that's actually causing the misbehavior in this particular 1731. Just seems suspicious based on the data sheet.

 

[TheseGoToEleven]

Here is a photo of the stabistor and zeners found in this Melcor 1731. It would be nice if the schematic showed some kind of a dual diode symbol for the stabistor but I couldn't find any symbols in KiCad that had only two terminals.

 

[Walrus]

Is your schematic the hand-drawn one here? It matches perfectly with what I found.

Yes.

 

[Whoops]

Differences I see between this and the Peter C schemo:

Trim option on first diff pair

Peter C decided to leave the "Trim" option out of his version of the 1731, it was a decision made by Peter

 

[CJ]

hfe depends on current level used, what was the current used for getting hfe = 75?

 

[z11111]

thanks so much for this clean and concise breakdown in 2024!

--

edit: also regarding OSA, there were a looot of variants out there of their preamps. I've never seen one with a Melcor 1731 though.

Some had lundahls xformers, some had Profile 4804 OTs, Jensens, etc.. some had DOAs with similar schemes to a 2520 in a gooped box, some had 5534s in a gooped box (with no driver transistors lol... these are prone to literally melting), etc etc but were (mostly) all built out on the same boards of a circuit not unlike an API 312.

We have one with a Lundahl LL7903 IT, a Profile 4804 OT, a gooped DOA *and* a 5534 doing something, and then another metal shielded box which IIRC has something to do with the D/I input but don't quote me on that. Big low end while still being pretty hifi. I'll have to trace it out sometime because it's a good one.