Vox AD120VT

No, just undo the 20 or so screws needed to remove the amplifier chassis and reverb tank from the cabinet and plonk the whole lot on the bench.

Done it quite a few times now, probably takes me 3 minutes max. Certainly a lot less time than faffing around wedging screwdrivers and bits of piping and then spending half an hour trying to get the valves back in their sockets blind.

Time you bit the bullet, I reckon, and got out that powered screwdriver.

As for the melted IEC inlet, no doubt it was too close to the fire.

Cheers,

Gareth.

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I had to take the other board apart anyway to replace a pot, but wondered about more routine checking of valves. Replacement is not blind as there is an access gap and maybe finger tips, in there , to guide into place. With a radiant fire I expected some sort of discolouration of the Rexine, presumably nearer to heat source. Low heat of my heat gun says 320 degree C, whether higher or lower from numerous heater element joinings over the years , with NiCad stainless steel "crimps", anyone's guess.

As far as I can tell (present owner not from new) I'm the first to go in there from new. TOP3 are 2x C5200 but one 2SA1943 and one KTA1943

Ah, this is the Valvetronix thing, I was talking about the re-issue AC30's.

Good luck.

Gareth.

AC30's.

Those are straightforward to work on.

Whats the chance of busting off a SMD device putting all those boards back in place, like origami with spaghetti combined with that business of tapping your head with one hand while the other hand moving round stomach, getting all that stuff lined up again without breaking a valve top pip. Just as well I have a little ratchet handle to take otherwise standard crosshead hex driver bit for those 2 bright machine screws under the 7 segment LED section. Stubby screwdriver is not stubby enough. Back in place sufficient to test properly, that for tomorrow, quite enough cussing for today.

I've just downloaded the manual as although both PAs are live , only one is getting a feed , something wrong with DG211 or the rear power selector or something. Manual does not include how to dismantle or mantle. One tip is to cover the valves with some dense foam a bit longer than the valve tip , supported by the too short standoffs , to protect the valves when turning all this metalwork over for probing etc

Anyone happen to know of a designed-in function that would switch out 1 amplifier of the 2? Analogue signal of right levels is getting to the inputs of both amps, which both agree quiescent DC fashion but some control line is erroneous, I'm assuming. Anyone happen to know what switches the A/AB class function , via the 2x DG211 analogue switches , I assume the user select, amp type, 16 way encoder switch, but which for A and which for AB ? but either way seems to control both amps simultaneously, not isolating one amp. I thought it would be the 4 way power switch on the rear but that is switched attenuators only to both amps, no control function. The phones change over function via the MAX333 an. sw. should divert both amps by the single ground switch in the phones socket .

Looking at my database, one of these had a mainboard failure (i.e. digital stuff), uneconomic to repair, One I changed the MAX333, a zener and standard diode, another was dirty switching contacts (power selector, headphones, speaker sockets).

I would change the MAX if only to eliminate it. Could be an achilles heel. I remember years ago I usd to regularly find blown analogue switches - a bit like the old RAM chips in that they were obviously more prone to failure than most other chips for some reason.

Gareth.

Maybe look for the zener and diode I changed too. Don't know what that was about.

Gareth.

Oops, bit premature there,

the zener and diode was on the +15v supply.

Gareth.

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I think I now have a way to prop up a connected live PA board , to probe stuff DC and AC, without it falling over or shorting anything. A couple of plastic "ring " binder sleeves over lower front edge of the chassis so will take the half rotated back edge of the pcb and the top of the heatsink, safely over the top panel. Along with the foam protected valves

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Looks related to An Sw failures, FET failures No signal on the grids of V1 but signal on Con2 with only a couple of long-tail FETs in between. Like a stereo amp comparing channels, signals on V2 grids and con1. DC around the 2 An Sw are the same, but 0,0.4,220V dc sort of values on the bad FETs and about 60,60,140V on the good. How long do you scratch your head before you realise that the schematic is wrong. How could there be 0 V on one end of a trace at an an sw and 60V on the other at a FET gate of the working amp? Supplies to these FET are from the valve anode rail

2002 version about 1/3 down the pdf, left side Now , with hindsight , it is bleeding obvious , a shorting connection directly across both C51 and C48 is shown in the diagram but of course in reality no such short. Now to find some 240V,200mA rated FETs , dosn't seem right 240 V on a TO92 package

Much more mysterious failure mode , that yellow goo again? Removed both FETs and they cold tested ok , changing conductance state, via DVM "diode" test, so probably ok . There is that yellow goo in this area and one resistor covered with it is 120K 1/3W dropper from the anode rail, so difficult to check in situ as between 2 electros, sets the bias. Something chemically has gone wrong with it. Small patch of body coating shows perfectly normal cream and part of a red and brown band, again no thermal discolouration. Under the R the board is perfectly normal green colour. But the r is o/c , now replaced with 2x 240K 2/3W raised off the board , JIC a heat problem but I doubt it. So as well as encouraging deliquescence/hygroscopic or whatever the word is for taking moisture out of the air and encouraging corrossion it looks as though high voltage has some deliterious effect with the dreaded yellow peril.

I didn't think to mark the "amp type" knob before removing .

360 degree hex encoder , splined shaft, have I got this right

With my mark on one end of the shaft slot at 6 o'clock , (so could be 12 o/clock ) then A/AB control V at that contact is 5V. From the circuit 5V probably relates to lack of negative FB so "class A", Then going clockwise 000005555550050 Anunciators , at 6 o'clock , Boutique Cl then xlockwise black 2x12 tweed 1x12 tweed 4x10 ac 15

15TB ac30 30 TB UK blues UK 70s uk 80s uk 90s uk modern recto US higain Boutique OD

I've placed the knob so the block of 6x 5V cover Vox AC15 to UK 70s , right or wrong?

The above list roughly approximates from clean to very dirty. Probably best to try and find the point where US highgain and Boutique OD turn into the relatively clean black and tweeds, should be a step change in gain/ tone.

But you really need a guitar to hear this.

Gareth.

US higain will be some awful fizzy overcompressed/overdistorted mess, Boutique might be a somewhat toned down version, then the Black(face) and Tweeds will mercifully bring things back down to earth again, and should be pretty much clean sounding.

Gareth.