Jcm 800 boost channel not switching....

[theholyweed]

The anodes of d4&d5 show 0.01v without a plug in the input jack and and 0.73v witha plug in the jack, the base of tr1 shows 3.6mv without a plug in the input jack & 26.5mv with a plug in

im getting no reading at the collector of tr1 at all although the emitter is showing 2.5v with or without the plug....

[theholyweed]

the led is always off as its stuck on the normal channel....

[Enzo]

OK, nit pick: no reading at all means your meter screen went blank. Zero is still a reading.

Are you sure you have the legs right on TR1? I am having a hard time thinking of a way the emitter would have 2.5v. I would however believe the emitter was the zero volts - it is grounded - and the 2.5v was the collector.

An interesting experiment would be to ground the collector of TR1 and see if that lights the LED.

But either way, those readings make me think the cathode of D4 is stuck to ground. That makes me think either TR1 has a base to emitter short or the IC has an internal short.

[theholyweed]

Yeah im sure im reading the legs right on tr1, grounding the collector does nothing but grounding the emitter lights the led, Iv already ordered a replacement IC, will i replace tr1 while im at it too?

[The Dude]

....will i replace tr1 while im at it too?

Or, you could check it and see if it needs replacing? I see you've already ordered a replacement IC, but it should be easy enough to test also. As I said in an early post where I provided the datasheet, it's simply a "block" of transistors. You could test it in a matter of a couple of minutes with a DVM.

Also check any electrolytic caps in the circuit. An extreme voltage may have stressed them and/or even blown them up.

[Enzo]

Ok, the whole emitter collector thing makes no sense. Look at the schematic. The emitter is connected to ground, so grounding it can't make the LED light. And the collector is wired to the LED, and grounding that completes a circuit to ground and should light the LED.

Data sheet for BC184:http://www.mouser.com/ds/2/149/BC184-310135.pdf

BC184 looking at the flat side left to right is CBE
Asian types like 2SCxxxx left to right are ECB
American types like 2N4401 leeft to right are EBC


Now if you are looking at the right pins on the part, any chance the part was originally installed backwards, swapping C and E?

[theholyweed]

Im 100% sure about the legs on tr1, The replacement Ic arrived today, swapped them over and it works a treat!,

Thanks so much everyone for your help, and more importantly you're patience!!

[drewl]

I had the same problem with a 2210 I used to have.
The switching chip was bad, that's how I got it cheap.

[theholyweed]

awesome, the Ic was only £1.40

[jalexquijano]

I just bought a 1989 JCM 800 2205 both of the amp channel are working. However, sometimes i get some intermittent flashing on the red led and sound loss on the clean channel. Is this normal or should i consider also changing the CA3046 IC?

[jalexquijano]

also swapped the CA3046 IC and the intermittent problem is gone now. Lets see if it does not show up again!