Jcm 800 boost channel not switching....

[theholyweed]

Hi all, sorry I feel a bit cheeky as this is my first post but I was looking for some advice.

I had a jcm 800 2205 in the studio with a intermittent volume drop, I found out the problem was a dodgy tube socket for one of the el34, unfortunately in diagnosing the problem I stupidly made another one, I managed to arc pins 6
& 7 on the faulty power tube socket, And thus the amp now refuses to switch to the boost channel although the clean/normal channel works fine....

could anyone please point me in the right direction of the possible fault iv created, Iv changed over the el34s to a brand new pair and swapped around the 12ax7's to no avail....

any help would be greatly appreciated!

[The Dude]

The channel switching part of the schematic is here:

http://www.davidsonamp.com/sm/images/2205preamp.gif

Does the light indicate that it is switching?

Also, here's the datasheet for the IC that does the switching. It's just a bunch of transistors in a single IC, so you can check it as you would transistors looking at the pinout.

http://www.eletrica.ufpr.br/ufpr2/pr...215/CA3046.pdf

[theholyweed]

Cheers for the prompt reply!

No the led doesnt light and the channel isint switching when using footswitch its just stuck on the normal channel, do you think the short damaged the switching ic? its strange, when i turn on the amp standby then switch off the mains you can hear the gain channel kicking in for a few seconds until the amp powers off.....

[The Dude]

I would check DC power to the circuit first (cathodes of D2 & D3). Pin 7 of your EL's is a heater pin. While pin 6 is not used in the tube, the amp probably uses it for a solder terminal. My guess is that you applied some high voltage to the heater circuit when you accidentally shorted the two together. This would have damaged the switching circuit because it uses the filament windings for its supply.

[theholyweed]

I have a reading of 3.7v on the cathode of d2 & d3.... is this normal? does it mean its the ic?

Cheers

[g1]

That sounds low. Check those diodes and C9 & C42.

[theholyweed]

Im getting around 3.3vac on pins 7 and 3, should be 6.3vac eh?

[g1]

We are concerned with DC volts for the IC. What is the DC voltage at the d2,d3 cathodes ?

[J M Fahey]

Im getting around 3.3vac on pins 7 and 3, should be 6.3vac eh?

Pins 7 and 3 **WHERE** ?
The IC, some tube, what?

3.3 VAC relative to WHAT?

[g1]

Pins 7 and 3 **WHERE** ?
The IC, some tube, what?

http://www.davidsonamp.com/sm/images/2205preamp.gif
I'm not sure what the 0V line connects to.

[theholyweed]

apologies, I thought pin7 on the el34 socket was from the heater circuit that should be supplying 6.3vac but Im only getting a reading of 3.3v...

[theholyweed]

sorry I really am being a twat, Iv just realised thats the nominal voltage split between the 2 el34s, sorry I though it was relative as the dude said earlier on that the switching circuit for the ca3046 was powered from the 6.3vac wind thats also the filament supply....

[Enzo]

I thought we were looking for DC voltages on the IC.

If the heaters in your tubes are glowing, then the heater voltage is right, regardless of your readings. Your 6vAC heater supply has a grounded center tap, so that results on 3v to ground from either side. There is still 6v across the heater.

That heater voltage is applied to D2 D3 to make a small DC supply. So I expect four and a half volts of DC, more or less, at the cathodes, which is across C9.

Looks to me like there needs to be something plugged into the input for the switching to work. The input jack sleeve shunt holds the anodes of D4 D5 to ground. So if you are working on this circuit, have a plug stuck in the input. With the ground shunt removed, then R8 is free to pull D4 D5 up to activate the circuit. I see D4 directly controlling TR1, which lights the LED.

[theholyweed]

voltages at cathode on d2&d3 are 3.6v and the same at the + end of c9, is this too low then? is its straight forward enough to test the ca3046? iv got the pinout, can i just check the bases & emitters with my dmm?

Thanks so much everyone for their patience! its much appreciated

[Enzo]

Just my opinion, but try making the LED go off and on first. Those diodes I mentioned, D4,5 control the transistor that works the LED as well as controlling things in the IC. So if the drive to the LED is stuck, it may also be the IC then has no control input.

When you probe a plug in and out of the input, does the base of TR1 show a voltage change, do the anodes of D4,5 show the voltage change? Does the TR1 collector voltage change?


I guessed at 4.5v and you got 3.6v. That ought to work. DO you find something close to 3.6v at the collector of TR1? When the base of TR1 is driven, that should drop to close to zero volts. Does it?