how to distinguish phenolic pcb from the appearance alone?

[PreSapian]

hi i'm trying to figure out if the sansui au @907i that i'm looking to buy has a fiberglass pcb material or paper/phenol. from the pictures on this repair site, Sansui AU-ƒ¿907i it's green on both sides and brown in the cross section. the amp was produced in 1984. what do you think is the most likely pcb material?

[audiomachines]

its phenolic alright, FR-4 pcb or fiber glass pcb has this semi translucent characteristics onto it plus you can see some "fibers" on the edges.

Regards,
audiomachines

[PreSapian]

on a side note, do all phenolic boards start emitting that 'sweet vintage amp' smell once they're old enough? i have an older sansui model from 1974 (AU D907) that can fill the whole living room with that lemony musk that i now know to be associated with the phenol resin- which I also learned that is not too good for my health. so now I'm on the lookout for a vintage amp, (pre 1990s preferably, for a higher price/fidelity ratio) with fiberglass pcb. sigh, i know it's probably easier to diy at this point but i'm not too confident with a solder...

[pjp]

Its probably the flux. They used rosin flux in the old days and that has an awesome lovely smell.

It could be the phenolic, but I don't think the boards smell much unless they're heated.

[audiomachines]

Yes they do have that musky smell once the heatsinks/parts started to warm up, I have a Pioneer M73 with large dimly lit VU meter on the front and a Pioneer SA-610 with an unusual tone control arrangement. The tone control is incorporated on the feedback network of the power amp. It has this haunting ice blue fluorescent level meter on the front. I love vintage amp they were built to last.

[AndrewT]

Phenolic looks like a poor quality "Formica" laminate.

I think it is paper layers in a thermosetting resin.

[Ian Finch]

Originally Posted by AndrewT

Phenolic looks like a poor quality "Formica" laminate. I think it is paper layers in a thermosetting resin.

Quite so. The Phenolic name is used generically and covers melamine and urea plastics too which are similarly processed but translucent where phenolic is dark brown or black like the old tag strips and canvas reinforced (Tufnol) laminates from "toob" days.
Laminex or Formica (Melamine) sheet is quite similar to phenolic board. You would not say these boards smelled of lemony musk unless you mean stale cat pee or something else organic but unpleasant. This odour is accompanied by formaldehyde which is sharp alright and probably accounts for the health concerns but at the ppm exposures you get in an average home, that's unlikely. The melamine surfaces in your kitchen and storage cupboards and binder of MDF and particle board will contribute 100s of times more. There, you have another concern to fret about.

The thing is, with FR4 and similar being fibreglass reinforced epoxy resin, they "ring" as hard and dense material even though quite translucent. They wear ordinary drills and saws out quickly and the residue is a white powder.

Phenolic boards are much softer and flexible. They drill, cut and break easily, just like Laminex and are often thinner, tan coloured yet more opaque because of the paper layers. They emit that urea-like odour for ever, it seems.

A quick test....Hold the soldering iron to an edge of the board until a whisp of smoke appears. Sniff it. Epoxy resin also has a nasty odour when heated but not as sharp. Check the laminate for glass fibres glistening along the edge as per audiomachines' comment. OK? The green lacquer on boards is solder masking and has been used on all laminate types for many years.

Yes, developing resins for these laminates was once my profession.

[sakis]

Originally Posted by audiomachines

Yes they do have that musky smell once the heatsinks/parts started to warm up, I have a Pioneer M73 with large dimly lit VU meter on the front and a Pioneer SA-610 with an unusual tone control arrangement. The tone control is incorporated on the feedback network of the power amp. It has this haunting ice blue fluorescent level meter on the front. I love vintage amp they were built to last.

that was a good point there audiomachines ...there was also plenty of yamaha models that worked by the same logic ...

there was wonderfull sonics produced by this type of circuits the problem was that depenting on the load ( type and impendance of speaker/ cable length and capacitance /orientation and calculation of zobel /quality of xover) the amplifier could become even unstable since a bit of gain more was required to operate with the specific tone control so most of the companies that copy that from sansui quickly quickly moved to a standard topology with transitor or ic to buffer the tone control ...

now days one could actually try to either improove such an idea or match it to a specific setup a specific source and volume control and specific amp ...i say that since designing of such a circuit will have to be moded to fit each and every amp ...

good reminder tough sonics produced from hose circuits are unbeatable

[audiomachines]

Originally Posted by sakis

that was a good point there audiomachines ...there was also plenty of yamaha models that worked by the same logic ...

there was wonderfull sonics produced by this type of circuits the problem was that depenting on the load ( type and impendance of speaker/ cable length and capacitance /orientation and calculation of zobel /quality of xover) the amplifier could become even unstable since a bit of gain more was required to operate with the specific tone control so most of the companies that copy that from sansui quickly quickly moved to a standard topology with transitor or ic to buffer the tone control ...

now days one could actually try to either improove such an idea or match it to a specific setup a specific source and volume control and specific amp ...i say that since designing of such a circuit will have to be moded to fit each and every amp ...

good reminder tough sonics produced from hose circuits are unbeatable

I agree with you 100% my friend, unbeatable sonics. On the side note I have serviced some "BIG names" proamp that utilizes "fusible resistors" on the bias servo ckt. diff. stage, current mirror etc. Are they trying to comply with some regulations? For me these is a bad idea, you'll get the picture when this "fusible" opens up when the amp is subjected to voltage surges, which is very common in the field. I think rail fuses were enough. Fusible resistor on strategic point on the amp plus a crowbar ckt. A very bad combination thenthen

[PreSapian]

so what do you think is that I'm most likely smelling? as I say, it's lemony- which means it's acidic more than anything else, and then it's joined by a 'musk' that's usually associated with antique furnitures and such- old, dusty smell. I saw from quick googling that phenol smells acidy; which's why I guessed that the smell is from the board. but the same board material is used all throughout the amp yet on other sections no smell is emitted. just near the tone control where old alps pots sit with their oils leaking. which makes me wonder if it's some kind of interaction between the oil and the amp. the oil itself does not have the smell in question and the smell is not sharp at all, so I doubt it's any kid of solvent... I hope.

p.s. melamine off-gases? this is a shocking news to me.