Quad 34 Preamp problem

[AD360 Rob]

Evening all,

Recently I dragged my Quad 34 and 405/2 out of retirement to test a pair of Tannoy Dorset T185 loudspeakers I have been given. All appeared to be fine until I pressed the mono button at which point, the sound did indeed change from stereo to mono but the LED did not light. Upon opening up the preamp I saw that one or two of the small electrolytics were leaking. After a bit of research online and here it seems this is a common fault so all of them were replaced with decent panasonic ones and the pcb cleaned off thoroughly. Fault symptoms were the same as before so further investigation revealed the track linking pin 1 of IC14 to the base of T9 (LED switching transistor) had corroded UNDERNEATH the pcb coating ! A wire link restored connectivity and the LED operation. The problem now is that although the mono button activates as soon as it is pressed, you have to wait a good 30 seconds before it will respond again (to de-select mono) all other buttons work as they should, the +and- 7.5v supplies are present, the output (pin 1) of IC14 switches every time the mono button is depressed but for it to have any effect once selected you must wait. |Does anyone have any ideas about this?

Thanks
Rob

[Ted Kendall]

Did you clean the board thorougly where the electrolytic deposits went? It's nasty stuff, and conductive. The other thing which comes to mind is how damp it got in storage. A few days in the airing cupboard may help.

[AD360 Rob]

Hi Ted,
The board was cleaned thoroughly on both sides first with distilled water then with isopropyl alcohol, dried thoroughly with a hairdryer and the preamp has been stored indoors in a centrally heated room, there's no way it could have got damp.

[Ted Kendall]

Fair enough - I'd say the smart money is on more track corrosion or more leakage under the resist. Can you look at the control voltages on a 'scope? It might give you a clue. Might be a duff pull-up resistor somewhere...

[nutteronthebus]

I had a similar fault on my 34 but caught it just in time ( a lot of 100uf 6.3v caps ) half were leaking the other half measured as low as 50uf is seems that Quads of this age do have problems with the caps

[AD360 Rob]

I've been having another look into this fault today and what is happening it that when the mono button is pressed, the voltage on pin 3 of IC14 momentarily dips to zero which is normal (confirmed with tape switch ) what doesn't happen is that the voltage on pin 3 doesn't return to its previous level, i.e -7.5v but rather hovers around -3.5v gradually increasing towards -7.5v. I have removed and checked C56 for leakage, it's ok, checked across the switch contacts (measures the same as a working one at about 1M ohm), Checked the 1M ohm resistor R90, Replaced IC 14 just in case but still the same. I'll look at C65, R89 and R88 later and double check IC 14 voltages.

[AD360 Rob]

C65, R89 and R88 checked out of circuit, they're ok, IC14 pin voltages all seem to be correct apart from pin 3 which is correct before pressing the mono button but still climbs slowly back to the required -7.5V. (I used the tape function to compare as this is identical in makeup to the mono circuit) It's driving me nuts now !

[Ted Kendall]

Surely there is a pull-up (or pull-down) resistor thereabouts? Dry joint on pin 3?

[D_S_J_R]

A quick comment after some time away -

The Quad 34 (and possibly FM4) does seem to leak its supply caps at least all over the board and sometimes it seems the track underneath may suffer too. I don't wish to appear too shill, but Dada in the Netherlands seems able to supply kits of updated parts and claims to improve the performance of these preamps in the process. I've used their kits to update and service my 33 preamp and it does seem better than ever for the effort involved. The dada forum has threads concerning 34 service so hope it helps a bit, if not the exact fault. Quad still do offer basic repairs at not silly prices.