Marshall JCM 2000 DSL 401 power cutting in and out

[fatcharlie]

Hi all, from sunny Pevensey Bay, Sussex, UK.

Having found this thread after a Google search:

http://music-electronics-forum.com/t12900/

I posted there for advice. I was immediately helped by OC Disroder and J M Fahey.

Thanks guys for your help.

Their first suggestion was to make a new post and refer the old one. Check.

They then suggest posting a schematic - I hope that has appeared here now (not hugely familiar with attaching in forums).

OC suggests thumping the amp on top - I'm waiting for it to warm up as I write this thread.

I've had the amp for 19 years and it's been a gem - it's had a hard life, being stored on my boat when I was a travelling musician in the Mediterranean, then stored in the garage for the last couple of years. I'm kinda thinking it's simple bad connections but i thought I'd have a look online first.

I have read the thread I mention above and tried disconnecting and reconnecting all the leads (with the amp switched off, diconnected and left for several hours in case of capacitors still holding charge), I've pulled and pushed the speaker jack in and out several times to try to dislodge any accumulation of dirt. I also wiggled (gently) all the valves in their sockets, pulled them slightly then pushed back home without actually taking them out. All of the previous is just to try to make sure the connections are all good.

In my other post I asked "Can anyone help by leading me through a diagnostic/repair method?" - J M Fahey asked me - can I be a Tech?

In answer to that:

I'm a fairly resourceful bloke. I was a professional yacht skipper before 'swallowing the anchor' and returning to the UK. I have experience with electronics, electrics, air conditioning, gas installations, marine diesels and a wide range of other stuff. I'm a flight simulator enthusiast and am building a full cockpit in the garage - this involves soldering, programming, building structures, building PCs etc. so I have a wide experience and am open to being directed.

I can handle a soldering iron - I don't have an adjustable one but I have 3 different wattages
I understand schematics but am not experienced with them.
I can identify resistors, capacitors, transistors in schematic and on the PCB etc. but may need more help on less common components.
I can use a multimeter, select the correct mode and scale.
I don't know what is meant by 'inject signal in the amplifier' though.

In summary - I cannot afford a tech, am not one, but I am willing to learn what I don't know and I'm a reasonably capable type.

Advice to carry out procedures in post 50 of previous topic:

Cable between effects loop - has no effect.
Checking solder on input jack, and check voltages on preamp tube plates and/or check plate resistors - I haven't yet dismounted the circuit board.

In any case I'll need guidance on the latter two above.

I hope that's enough info.

Amp is now warm and thumping the top is inconclusive - if I follow my hunch, it seems to be a little better after a thump - i.e. it runs a little longer before the next cut-out. But that's very subjective as the cut-outs are unpredicatble - sometimes for several seconds, sometimes flickering. The amp is flickering on and off on its own while I'm writing, so difficult to know if thumping it has any effect, really.

Thanks in advance for the help.

Roger.

Further info.

While waiting for a reply, I tried putting a jack plug in the reverb foot switch socket. This seemed to help - the amp stayed on without flickering for several minutes. I thought I might have cured it, but eventually it started to play up again.

Believing it may still be a dirty contact problem, I took apart what i could - any leads, valves out (carefully placed in order and gently handled), sprayed everything liberally with contact cleaner. Put a jack plug in each socket and sprayed with contact cleaner while contacts were open. Then moved the jack plug in and out several times.

Put everything back together and switched on in the hope that I'd cured the problem.

Now the green light doesn't come on at all!

Rats.

Hoping for some helping hands here. Gig on Saturday...

[oc disorder]

Evenin' , well it's morning .. gee wizz you have left it a bit tight ! (gig on Sat)

.. Moving along here's the schematic

dsl401schematic.pdf

and the forum help file on how to attach um... attachments !

Reading and Posting Messages

Inject a signal ( in this case) means plug in some audio (audio signal not semaphore ) into the amp .. at this stage into a input socket or jack socket!

When we become clever dicks we may make a test lead out of a disused biro... solder a wire to the tip and the other end into the tip of a 3.5 mini jack which then can be plugged into a source of audio like a smart phone with audio playback a cd player an mp3 player or if we have the luxury an adjustable oscillator which is our test signal . Of course there's more to it than that ie reducing the volume to a usable level .. having a ground lead too with a croc clip to clip on the chassis .. so if you get my drift you find a point in the circuit that works and gradually work backwards untill you find the point where the signal disappears.

Now it doesn't seem to work at all hope the contact cleaner was the evaporative type not CRC !

What you did was a logical move ... I presume you understand the concept of preamp and power amp.
Thus you will be able to grasp the logic?? of an effects loop. It sit's between the two. The level that comes out of the send jack is higher than the output of a guitar and the level that should go into the return jack is likewise. By clever switching contacts the connection is "normalised" (i prefer an s) when nothing is plugged in... ie the send and return are connected together inside the amp and the pre-amp flows through to the power amp.

Cutting to the chase inject a signal into the return jack a guitar will do for now but it will be very quiet.. by inserting a plug into the return jack socket the preamp gets disconnected ... can you hear any thing ? Trying to see if the fault is in the pre or pwr amp .. gotta make a cup of tea it's friday here already...

[g1]

Voltage checks in the power supply area are always a good start. Especially when lights are not lighting.

[The Dude]

A common problem with these amps is a faulty bridge rectifier supplying filament voltage to the preamp tubes, and they can be very intermittent. If you can get the amp to fail/cut out again, check to see that all of the preamp tubes are lit. My bet is that V1-V3 will not have filament voltage and go dark.

[fatcharlie]

Hi Guys,

Thanks for the input - you're all very kind.

Will get to these tests today.

Cheers,

Roger.

All of the valves are lit once the power switch is turned on - just no green light at the channel switcher part.


<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<< <<<<
Further update - the contact cleaner is isopropyl alcohol. Jeez I hope that's ok!
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>> >>>>

[fatcharlie]

Update for you:

1) There is 240V coming in to the main power switch and the light inside this switch illuminates when switched on.
2) Not sure how to check the Standby switch while the amp is on, but _after_ switching off there is voltage across the terminals of this switch, starting over 800 and gradually reducing to 0. I'm assuming this is one of the capacitors?
3) When the Standby switch is moved to 'on', there is a faint but noticeable buzzing coming from that area. goes away when Standby switch is moved to 'off'.
4) Plugging a guitar into the return line of the fx loop gets nothing
5) Plugging a lead from the send to return jack of the fx loop, plus a guitar in the normal input gets nothing
6) Although the green LED for the clean channel is _not_ lit, all the valves, pre-amp and power amp, are lit.
7) The characteristic 'click' normally heard when changing channels, which to my tyro ears sounds like a relay or microswitch normally, is not present.
8) When turning the Master Volume pot to 'full' (no instrument plugged in) there is a slight crackle from the speaker as if the pot is a bit dirty there.

Love the gag about the semaphore - never did learn that but I was quite good at Morse Code at one stage.

Prefer 's' myself, although ancient English used either s or z. I realised years ago that z had been demonized by the anti-z brigade ;-)

Thanks in advance for the help.

Roger.

[Ted]

A common problem with these amps is a faulty bridge rectifier supplying filament voltage to the preamp tubes, and they can be very intermittent. If you can get the amp to fail/cut out again, check to see that all of the preamp tubes are lit. My bet is that V1-V3 will not have filament voltage and go dark.

I've seen one of these amps where the bridge rectifier hadn't been soldered in, but had still managed to work for a few years.

[oc disorder]

After tracing the supply for the green led I'm wondering if your 24 volt supply is working.
Suggest measuring around the R 29 100 ohm resistor. It's a 2 watt one so slightly larger than most of the others.
It also supplies various muting functions and the relays.

You will have to remove the board if you have to solder worth taking afew photos of where all the connections go
and or label them here's a link to an old thread shows a smaller model if you do have to remove the board worth checking
the points circled in yellow mainly the left one where the bridge rect is soldered. Don't think there's a need to add a heatsink.
Anyway yours seems ok.
https://web.archive.org/web/20090105...d=22&Itemid=48

[fatcharlie]

Ho OC - thanks for the help.

Bridge Rectifier joints all nice and shiny.

Other yellow-circled joints (jump wire and one leg of a valve) all good too.

R 29 shows 100 Ohms.

tested R 7 as well - shows 4.7k Ohms.

did you get the bit about the contact cleaner? Mine is isopropyl alcohol.

Thanks,

Roger.

Thansk also Ted and Dude for your input. Everything helps!

EDIT (again) Just realised your post could mean measure the voltage across R 29 - that's 2 volts across the resistor, but I'm getting 27-28 volts from the +ve on the resistor to the chassis.

No voltage at R 7

T 2 has 27v at the collector but nothing I can discern at the emitter?
R 10, R 11 and R 12 all have 27v at the +ve side and all check out ok for their resistance values.

[DrGonz78]

did you get the bit about the contact cleaner? Mine is isopropyl alcohol.

About the alcohol, you want to use the 99% pure isopropyl. The other less pure versions have more water content and not as beneficial.

Measure all Emitter Base Collector leg voltages for T2 & T3. With amp off try using diode mode on your meter to test those transistors T2 & T3.

[g1]

Are you using the footswitch or the panel switches?
If you are using the panel switches, to work they require the switch contacts inside the footswitch jack to be closing. A dirty/oxidized footswitch jack can stop the panel switches from working.

[oc disorder]

Hmm ,Saturday here already.. G1 is on the money...
no voltage at R7 makes me think the jack socket contacts are not closing.
22v , 27v that's no concern The Marshall designer probably just stabbed at it.. mains voltage etc alters what you see
A good check is see if the filament / heater voltage is around 6.3 v AC !
Sorry if I'm a bit ambiguous but both those readings add up and that's the main thing !
Try placing a jack plug in the socket and try to get your isopropyl in between
the contacts while they are open. A thin strip of cardboard soaked in it may be pushed between the contacts.
Try to measure continuity with your meter between the the contacts when closed.

Your isopropyl is fine for that but could clean out the grease in a potentiometer making it impossible to turn !

If you want the good "stuff" check out Ted Breux who was a guru on early Plexi Marshalls and has moved on???
https://www.wired.com/2005/11/absint...the&topic_set=

Whoops don't know what came over me....meant good stuff is Caig DeOxit a deoxidizer Caig D5 (who cares about the ZED)

Electronics Cleaner Improves the Performance and Reliability of All Electronic Equipment

Can't be much more help at the moment ! Hope it works out!

Quoting my images in case they disappear...! (well took a while )

OD1 G1... ---> T9 (like a switch) puts 47K to ground across the output of V1 B which is permanently connected to the input of V2 A grid which is normally grounded by a 1 meg (ie 47K and 1 meg in parallel).
This will reduce the signal rather dramatically.
Notice the input jack for guitar with nothing plugged in, has a switch contact to the sleeve contact (ground) and does
exactly the same thing as T9. Makes the amp nice and quiet when turned on ie not roaring with Hiss !!

CLN G1 is connected to 3 Fets T12 T13 & T14 these in this instance are all used as simple on/off switches controlled by their gate G and connects their Drain D to Ground or not !
So T12 grounds the connection from VR1 (volume control output) which is normally connected to the input (grid) of V2 B
via R103 a 220k resistor effectivly stopping any sound getting to it from VR1 !
Note: the output of V2A is also fed to the grid of V2 B (pin 7) via C88 (10n) and R102 (1M).
T13 connects a 22K to ground from the output of V2B (pin 6) via C72 & R104 and also reduces the volume (signal) going to V3 A input grid via a 2M2 R93.
T14 changes the tonal response of V3 A "shorting out" R126 (22K) connecting C91 (4u7) directly to ground.
So thats how the make it "Clean" !

Just noticed errors around my Relay 1 contacts will fix later when not so fatigued

[fatcharlie]

Hi Guys.

OK:

T2
E 0.0v
B 0.6v
C26.5v

T3
E 0.0v
B 1.4v
C 0.6v

testing them for continuity with multimeter, amp switched off, leads between base and E or C, one way I get no continuity, the other way I get continuity.

tested continuity across footswitch socket - all are ok.

Inserted footswitch in any case. Still no activity from amp, but the red led for the OD1/2 switch comes on when the switch is pressed.

cleaned all sockets with small piece of card as suggested - good job I like Weetabix!

Good continuity across all sockets - I notice the reverb footswitch and actual guitar input sockets are 'normally open' at the tip - I'm assuming this is normal.

Thanks again and keep the suggestions coming :-)

Could it be a faulty relay? How are they actuated?

Cheers,

Roger.

EDIT: Have managed to borrow amp for this evening's gig!

[oc disorder]

It appears T 2 & T3 are not turned on. Try activating the switches and read again.
Your readings "leads between base and E or C, one way I get no continuity, the other way I get continuity."
indicate the transistors are ok.

"I notice the reverb footswitch and actual guitar input sockets are 'normally open' at the tip "
Um not according to the diagram we assume to be yours.


"the red led for the OD1/2 switch comes on when the switch is pressed"
Give me awhile to think about that although anyone reading feel free to answer !

"Could it be a faulty relay? How are they actuated?"
Very much doubt it..
It is actuated by the footswitch or front panel switch SW4 which in turn turns on T3 grounding the relay thus switching it's contacts !


What about Connector 1 & CONT 2 junction ?

[fatcharlie]

Hi all,

sorry for the long silence - I did the gig on Saturday night with the borrowed amp.

it went down well but the amp was a bit 'tinny' compared to my lovely Marshall!

Sunday morning I felt under the weather - obviously a hangover. Infact it was a chest infection which knocked me for 6 the last few days. Just up and about today with the help of large doses of antibiotics, Lem Sip, paracetamol and ice cream.

I'm off on hols tomorrow morning at 0300 so unlikely to be able to do anything today but I've been poring over the schematics and will return in a week with renewed vigour - especially as i have another gig on 8th October!

I can only say thanks a million for your help so far, and beg your patience during this hiatus!

Cheers for now.

Off to Greece for me - yay!

Roger.