Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: bakerlite on April 18, 2012, 08:18:21 am
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I have a jcm2000 tsl 100 here - It arrived with blown mains and ht fuses - a melted power supply and barbecued output tubes.
Luckily i have a spare PT and tubes that i can troubleshoot with -
My first step was to try to figure out what part of the PT smoked - secondaries seem ok but it looks like the primaries got the roasting.
So i swapped in my new PT - plugged in my lightbulb limiter and fired her up with no tubes and no secoindaries hooked up - all normal there.
I then proceeded to connect one set of secondaries at a time testing with the lightbulb limiter each time - all good at no point did the bulb show any increase in brightness , nice and dim (except the expected momentary brightness inrush current when i attached the ht)
so i used a regular power chord and fired up again and tested all the secondary voltages on the the new transformer just in case - GOOD all fine.
so i hook up only the heater secondaries and fire her up again - to check i have 6.3 at the tube sockets.
almost all output heaters working
BUT
V1 and V2 do not show 6.3v in fact they show nothing (to ground) on pin 9 but on pins 4/5 they show 9v to ground.
am i right in thinking C1,C2,C3,C4 and the diodes in BR1 should be replaced?
and more importantly could those parts have taken out the PT?
(http://www.drtube.com/schematics/marshall/tl10-60-02.pdf)
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How much dc voltage do you measure BETWEEN V1-9 and V1-4/5?
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none at all between v1 pin 9 and v1 pin 4/5 same goes for v2
its only when i measure to chassis ground i get 9v at pin 4/5 on both v1 and v2
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Can you measure the dc voltage directly across the bridge positive and negative terminals? Or directly across C4? These points are electronically the same as measuring directly across V1 pins 9 to 4/5. If still no voltage, pull V1 and V2. Measure resistance directly across C4. If shorted, check C1, C2, C3, AND C4. Could also be the bridge. If not shorted, replace the bridge.
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DC voltage accross C4 is -7.5v - (this is with tubes in v1 and v2 sockets)
tubes out of sockets looking for conituity - (well shorted components)
c1 600r
c2 1350r
c3 600r
c4 1350r
bridge - continuity for 4 seconds then gradually creeps up to 1350r
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DC voltage accross C4 is -7.5v - (this is with tubes in v1 and v2 sockets)
Well the bridge and the 4 caps are fine. There is an open circuit between C4 and the tube heater pins. Start looking for a broken wire or pcb trace or a bad connector. Remove V1 and V2. Check for continuity between C4 positive lead and V1 pin 4/5. Check for continuity between C4 negative lead and V1 pin 9.
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thank you for your help i do appreciate it
something is strange here,
continuity is there between negative and pin 9 and positive and pin4/5
i just double checked my heaters to be sure on all sockets by measuring ac voltage from pin 9 to pin 4/5:
with no tube installed they all showed 6.3 except v1 and v2 which both show 18.7
i'm really very sorry - i dont know why i had it different at the beginning? maybe i wasnt making good contact from the top of the socket where as now i am measuring from the bottom. i dont know
hey wait a minute -dry solder joint anyone?....
no scratch that - solder joint are fine
i may be barking up the wrong tree here but why for the first 30seconds after i turn of the amp is there continuity between pin3 and pin4/5 on V1 and v2?
is this some sort of auto discharge process built into the amp?
anyway to summarise; the problem is that i have about 19v AC on the heaters to the sockets of V1 and V2. The rest of the heaters are fine: 6.5v. (this is with tubes out)
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anyway to summarise; the problem is that i have about 19v AC on the heaters to the sockets of V1 and V2. The rest of the heaters are fine: 6.5v. (this is with tubes out)
Is it possible that you have 19mV AC on V1 filaments? It would be almost impossible to have 19VAC. All other filaments read 6.5vac. That same 6.5 is applied to a FWB and filter caps to provide V1 and V1 with DC filament voltage.
DC voltage accross C4 is -7.5v - (this is with tubes in v1 and v2 sockets)
Is this still true?
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good call steve - I had a new multimeter here ready to try out - it is better at dealing with ac ranges than my current one.
you were right: "mv AC" and fluctuating 0.1 up to 2 briefly.
my c4 reading of 7.5v still stands though its positive not negative.
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my c4 reading of 7.5v still stands though its positive not negative
That all depends on how the meter leads are connected. Anyhow, that's good. Shake off the last few posts and re-read reply #5.
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thanks steve
heater wiring is fine now
dc voltage on v1 and v2
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What did you do???
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OK I have spent a few days in the Sin Bin.....
I may have been checking for AC on V1 and V2 heaters, but to admit that would be hard so suffice to say sorry sluckey!
SO after confirming I had heater voltage and lightbulb limiter looked ok i threw in some tubes, she works and sounds nice but there is a hum - the kind of hum you get from amps with old input jack sockets that are worn.
The hum is present even with every knob turned down to zer and gone when preamp tubes removed (but with power tubes in.)
can i presume then the culprit is between the input socket and the output driver?
also if is it normal with DC powered heaters to have negative on pin1 and positive voltage on4/5??
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I have worked on a number of these JCM2000 DSL100's and the majority of the time it has been the heaters on V! and V2 the most common problem I have seen on them is the bridge recto that powers the heaters on V1 and V2.
are you using known good tubes to test with ? these units have very little hum(normally)
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cheers jack -
unfortunately I was way off the scent with this - I made a rookie mistake testing voltages and heaters were fine -
I am up to date with this amp on a new thread here:
http://www.el34world.com/Forum/index.php?topic=13692.0 (http://www.el34world.com/Forum/index.php?topic=13692.0)
this thread could be locked and with a bit of luck purged to save my embarrassment!