Suspect glue on 1960s PC boards.

[hansomcommon]

I have just had a case of the 60s conductive glue syndrome. Older engineers may remember that many old Asian PC boards used a fixing adhesive which was brownish & looked a bit like Evostik. This glue was hydroscopic & over the years absorbed enough moisture to become conductive causing all sorts of odd faults to appear on TVs, radios & all sorts of domestic equipment. The cure is to dissolve the glue, WD40 works well, & then clean and dry the board. In my latest case a 1960 clock radios display died, the clock IC being held by the glue, removing the contamination restored normal working. This is a fault to look out for when odd symptoms are exhibited in equipment.

[Nickthedentist]

Good advice.

But were ICs really around in consumer gear in the 1960s?

[dazzlevision]

Hello Nick,

Not many, but Pye fitted a TAA570 IC in the 6MHz intercarrier sound IF/detector stages of their "169" hybrid single standard mono TV chassis. The chassis first appeared in 1969, ready for the start of BBC1/BBC2/ITV three channel colour on UHF/625 in November.

Radio Rentals fitted an IC in the second version of their dual standard hybrid colour chassis (A "CA" type from RCA, IIRC).

Regards,

Dazzlevision

[Nickthedentist]

Thanks, D, that's interesting to know.

N.

[hansomcommon]

The glue certainly was used prior to common ICs in equipment, its just that the close pinout of ICs exacerbated the problem, it was common around radio output transistors where the heat quickend the moisture absorption & caused leakage currents to upset the bias of the stage...

[G6Tanuki]

The same goo was sometimes found used to 'stabilise' open-wound VHF coils in various bits of Japanese radio gear from the 1960s through to the 1980s. As well as increasing DC leakage when it deteriorates it also reduces the "Q" of any coil it's daubed on - reducing stage-gain and also sometimes adding enough loss to stop oscillators oscillating.

[ITAM805]

An interesting thread, I wasn't aware of the phenomenon. On a similar theme, I wonder if forumers are aware of, in this case the resin in the board itself becoming conductive?

Anyone who repairs guitar amps will sooner or later will have one of the faulty batch of Marshall TSL/DSL from the the late 90's on the bench, with runaway bias caused by conductive boards. I recently had one in with two of the four EL34's glowing like lamps . The recommended fix is a new board, but they aint cheap and Marshall dont give them away either...

Luckily though some bright folk have come up with several fixes and for this one, amongst other things was to remove the board around the g1 pins of the o/p valves with a hollow tile drill. This prevents the screen grid voltage leaching across to g1, driving the grid voltage too positive.

Some pics of my fix attached

Some more info on the fault here

[Restoration73]

This "glue" was also used idiotically to hold radial electrolytic caps to boards, preventing any chance of venting. The early caps did not have the indentation on top that permits the case to split, so a fault e.g. over volting, caused an explosion with the can going towards the unsuspecting repairer (fortunately I wear spectacles). This was only bettered by the technique (still seen) of replacing pcb fuseholders with a soldered wire link !

[Andrew2]

I came across this on lots of 80's video recorders. The glue starts off white or yellow and slowly 'cooks' to an orange/brown. One video (a Sharp 9700) led me a merry dance with intermittent freezing/stopping/sulking before I found that the clock oscillator for the microprocessor was stopping or not starting at particular temperatures. First I changed the pfs, then the xtal, then the chip, none of which fully cured the problem. Then I removed the glue which was liberally smeared over the general area and the video recorder returned to tip-top health.
Getting the glue off was quite a chore.

[vu2nan-nandu]

Reminds me of the goo which caused PLL lock problems in the Kenwood TS-440S/AT HF Transceiver not long ago!

Regards,

Nandu.