Causes for 2nd chanel not working JTM60/JCM60

[torcamaniac]

Hi,

what would be the causes for amp's 2nd chanel not working? Specificaly Marshall JTM 60?
The first clean chanel works normal.
Symptoms on 2nd chanel:
Gain, Volume and Master pots flat (no sound at all)
Pots maxed (crackly buzzing noise and very low dry clean sound when playing)

I already eliminates these as causes:
1) Tubes all glowing. Tried swapping them with no difference
2) measured voltages of 2nd stage and V2 pins. Seems fine.
3) Checked for short circuits and cold solder joints on PCB and additionaly cleaned everything with contact spray.

What else could be a problem? I suspect a component such as a resistor (unlikely) or some capacitor. Just in case I ordered all the resistors and caps connecting to V2 stage and also two pots for gain and volume on second chanel.

Here's a schematic:


Any ideas?

[Jazz P Bass]

Check that the relay itself is getting power & that the power is switching at channel select.

If that is all as it should be, the relay contacts need to be checked.

[torcamaniac]

Thanks.
I didn't check the voltages on the relay. Switching in itself works , and the Led goes from clean channel(green) to gain channel (red). Anyway I ordered 2 relays from branded Finder with same specs.
http://www.tube-town.net/ttstore/Swi...24V::3816.html
I hope they work with this amp.The original ones are 47W/7 from Fujitsu and I couldn't find to buy them anywhere.
Do you know what voltages to expect on relay contacts?

[torcamaniac]

All right man thanks. So it must be the diode D6 in schematics parallel to RL2. There's also diode D1 parallel to RL1. From schematic it seems the RL2 is the DPDT coil for gain channel.
Just to make this clear if I undrstand correctly I should get aeound 0,7-1,2 Volts across the diode? I have some basic knowledge and think these diodes are for protecting the transistor against self-inductivity from the relay?
Could it be that transistor is faulty then?

I just measured D6 diode. I get around 0,6 Volts when on clean channel and around 18V after I switch to 2nd channel.
So I guess this is right and that the transistor is also functional?

I still guess that the problem is with one of the components in the V2 stage (coupling cap or resistor). Visualy everythimg seems fine though...

[Jazz P Bass]

Quote"I just measured D6 diode. I get around 0,6 Volts when on clean channel and around 18V after I switch to 2nd channel.
So I guess this is right and that the transistor is also functional?"

That 0.6 volts indicates that the relay coil is being energized.

It does not prove that the contacts work.
Use your ohm meter.
The contacts should go on & off.

[torcamaniac]

Quote"I just measured D6 diode. I get around 0,6 Volts when on clean channel and around 18V after I switch to 2nd channel.
So I guess this is right and that the transistor is also functional?"

That 0.6 volts indicates that the relay coil is being energized.

It does not prove that the contacts work.
Use your ohm meter.
The contacts should go on & off.

Thanks Jazz,
will try to check the contacts tomorrow after school. It's a real mess to get to some components in this amp.

[Jazz P Bass]

Here is the datasheet showing the contacts.

They are shown in the unenergized state.

BT47W.pdf

[torcamaniac]

Here is the datasheet showing the contacts.

They are shown in the unenergized state.

[ATTACH]38817[/ATTACH]

Thanks,

already got the sheet

[torcamaniac]

Well,
I've replaced almost all resistors and caps in V102. Also put new better tube sockets for all stages, replaced both relays and replaced all 4 22uF 450V caps.
Checked all Pots...
Boost channel still doesn't work
So it's not the relays, not the tubes and probably not something in V102 stage.
Suggestions anyone???

I've got a schematics that's a bit easier to read than most other JTM60 available on the net.

[Jazz P Bass]

Well, good luck with that.

You broke the golden rule: "fix the problem first."

Start from scratch.

Do the relays turn on & off?
Do the contacts engage & disengage?
Is the signal path secure to the relays?

[The Dude]

^^^^^^ What he said. Do some logical troubleshooting. Guessing and randomly replacing parts is time consuming, expensive, and often nonproductive.

[torcamaniac]

Well, good luck with that.

You broke the golden rule: "fix the problem first."

Start from scratch.

Do the relays turn on & off?
Do the contacts engage & disengage?
Is the signal path secure to the relays?

Yep the relays are engaging. You can hear them when switching channels also. Also checked the old ones while applying voltage. They are good.
About the rest I guess I'll have to measure piece by piece.
Should I start measuring after the V101A? The signal path to boost channel starts at the second half of V101 that is V101B?

[torcamaniac]

Hi,

what would be the causes for amp's 2nd chanel not working? Specificaly Marshall JTM 60?
The first clean chanel works normal.
Symptoms on 2nd chanel:
Gain, Volume and Master pots flat (no sound at all)
Pots maxed (crackly buzzing noise and very low dry clean sound when playing)

I already eliminates these as causes:
1) Tubes all glowing. Tried swapping them with no difference
2) measured voltages of 2nd stage and V2 pins. Seems fine.
3) Checked for short circuits and cold solder joints on PCB and additionaly cleaned everything with contact spray.

What else could be a problem? I suspect a component such as a resistor (unlikely) or some capacitor. Just in case I ordered all the resistors and caps connecting to V2 stage and also two pots for gain and volume on second chanel.

Here's a schematic:


Any ideas?

SOLVED
Can't believe it.
Was measuring the resistors and than voltages the whole afternoon and into the night. Everything seemed fine.
Guess what. Last I tried swapping the tubes again.
Can't believe I didn't get it before because that was actually the first thing I did.
I had a faulty TAD 7025 in V1. When I put it into tone stack socket V3 I got both clean and gain channel at low volume with a lot of background reverb. Amazing how the clean channel worked with this tube before
Well after I put a tung-sol from my other amp instead everything works now

[g1]

Amazing how the clean channel worked with this tube before

1/2 of V1 is used for the clean channel, that half of the bad tube must be ok. V3 requires both halves to be working and is common to both channels.

[torcamaniac]

Hi,

what would be the causes for amp's 2nd chanel not working? Specificaly Marshall JTM 60?
The first clean chanel works normal.
Symptoms on 2nd chanel:
Gain, Volume and Master pots flat (no sound at all)
Pots maxed (crackly buzzing noise and very low dry clean sound when playing)

I already eliminates these as causes:
1) Tubes all glowing. Tried swapping them with no difference
2) measured voltages of 2nd stage and V2 pins. Seems fine.
3) Checked for short circuits and cold solder joints on PCB and additionaly cleaned everything with contact spray.

What else could be a problem? I suspect a component such as a resistor (unlikely) or some capacitor. Just in case I ordered all the resistors and caps connecting to V2 stage and also two pots for gain and volume on second chanel.

Here's a schematic:


Any ideas?

1/2 of V1 is used for the clean channel, that half of the bad tube must be ok. V3 requires both halves to be working and is common to both channels.

Yeah that was crossing my mind a lot but I just didn't try to check the tubes again thinking it wasn't it 'cause I checked it first back than
And doing it the hard way blah.