Shooting Trouble on. 900 SLX

JCM Range, 800s, 900s, 2000/DSL, 2000/TSL

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Shooting Trouble on. 900 SLX

Postby Sauerapple » 02 Jun 2015, 13:40

Recently acquired a 100watt JCM900 SLX head. Was missing bolts to hold chasis to the cab, screws rose cute the metal screen to the cab and mains fuse cap. With a known quad of good tubes and a borrowed fuse cap from another 900 amp blew 3 amp slow blow mains fuse immediately when powered up. Granted it was a 3 amp not the 4 amp it called for some as they say "something's wrong with it". Have a new fuse cap and correct value slow blow fuses en route. Any free advice on where my next check should be? Inside of chasis was clean. At first glance (to a novice) nothing looked amiss. No exploded caps, burned wires, etc. filter caps are not bulging or leaking, transformer looks like a transformer. Tube sockets look good. Gonna try again with no tubes or HT Fuses when reinforcements get here. Curious as to a process of elimination.

Any help or free advice would be appreciated.

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Re: Shooting Trouble on. 900 SLX

Postby surfnorthwest » 02 Jun 2015, 16:27

Welcome to the Roadhouse. Sounds like you did not get a chance to try this amp before you bought it. Also seems to me who even had the amp previously made a few entries into it, no telling what was done. Can you contact the previous owner?

Unless your an amp technician I would simply save your time and frustration and go get it serviced. You obviously have a short somewhere and those fuses are there to protect the amp from further damage. While I would love to tell you what exactly is wrong, without getting it up on a bench to run some tests I can't.
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Re: Shooting Trouble on. 900 SLX

Postby Sauerapple » 02 Jun 2015, 17:16

Thanks for the welcome! No I did not get to play it before I bought it. Looked it over when I picked it up. Filter caps don't look bloated, transformers don't look burnt. Aside from the smell of cigarette smoke the only thing visually out of order was the first owner lost the fuse cap and tried to rig it up and wire in an inline fuse holder around the PC mount fuse holder. But for the $50 investment, I figured I was safe. Soldering Iron and some heat shrink later, the rigged up fuse holder was an easy fix.

I had honestly expected that it would not be something anyone could "fix over the phone". Was just hoping there might be something stupid I could check. I hate to pay to fix/do things that I can do myself. Was just hoping someone with a little more experience might be able to offer a "check these things" list. And if it ends up being over my head/skill level to a tech it will go. Since there are potentially lethal voltages present, I fully intend to err on the side of caution and recognize the limits of my abilities.

Try to give it a visual once over once the fuse holder and fuses come in. Though, from reading about the symptoms (with a grain of salt I might add), common faults will typically lie in no particular order 1. bad/shorted output tubes. 2. filter caps 3. Power Transformer. Is that at least logical?

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Re: Shooting Trouble on. 900 SLX

Postby surfnorthwest » 02 Jun 2015, 18:38

Yes. Sometimes guys used different tube types in these amps and I would look at the power tube sockets. Also simple blown resistors.
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Re: Shooting Trouble on. 900 SLX

Postby Sauerapple » 09 Jun 2015, 17:06

No Bueno. With all tubes and fuses (HT and Internal) removed, using only appropriate mains fuse blows almost instantly.

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Re: Shooting Trouble on. 900 SLX

Postby Anitoli » 10 Jun 2015, 05:33

From what i am seeing there is only one line fuse on the 2100:

http://www.drtube.com/schematics/marsha ... -60-02.gif

The other 6 fuses are: two 6.3A heater fuses two 500Ma output fail protect fuses and 2 250Ma 15v supply fuses.

Replace all the fuses and keep all of the power tubes out and make sure the standby is ON ( no sound mode). Turn on the power only. Does this blow the fuse F3?

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Re: Shooting Trouble on. 900 SLX

Postby Sauerapple » 12 Jun 2015, 17:21

Maybe I miss spoke. It's been known to happen. I only see 5 fuses. (1) 4A mains, (2) 500mA HT, (2) 6.3A Heater. All fuses are good. Power Tubes removed, pre amp tubes installed. All fuses installed. Speaker cabinet connected (correct ohm load set), power cord connected, Standby in off position, flip main power on.............. Power Tranny starts vibrating, shaking and buzzing. Just like that bed you stick the quarters in, at the cheap motel that charges by the hour that you took your girlfriend too in high school. (Transformer make audible vibrating noise until fuse pops). My guess would be bad PT, but I am a rookie here.

I also checked the Filter Caps with a Fluke DMM (discharged of course). I understand they should both read similar values in the several thousands of ohms. One metered several thousand ohms ( can't remember exactly) the other metered like 2 ohms. But they were still connected in the circuit which I understand could cause false readings.

Thought?

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Re: Shooting Trouble on. 900 SLX

Postby Sauerapple » 12 Jun 2015, 17:22

Pardon the slow response. Daughters been sick, work has been rough. Priorities.

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Re: Shooting Trouble on. 900 SLX

Postby Sauerapple » 28 Jun 2015, 09:28

Okie dokie. After a little more shooting of trouble, here is what has transpired. Flip mains switch, mains fuse blows. Checked solders connections at plug, mains fuse holder, mains switch and primaries on PT. All good, no connection had continuity to ground. Hmmmmmmm. Un soldered the secondaries of the PT, no fuse blow (Hooray). Re attach all of the secondaries, until the purple wire (its always the purple wire) recreates the symptom. Hmmmmm x2. Follow the purple wire to the board and through the traces (with a continuity test, courtesy of my Fluke DMM) until it exits the board as a red wire bound to the rear Filter Cap. Checking (the drained) filter cap, it does not have continuity to ground. Hmmmmm x3. Solder connections appear in good order. The red wire is tied to lug "3" on the cap which is tied to lug "2". A brown wire exits lug "2". Lug "1" has green/yel (chassis ground) and an orange wire attached. The brown and orange wires run and both terminate at the OT, with the brown wire representing B+ and orange COM (ground). Hmmmmm x4. Unsoldering the B+ supply from the center tap and firing the amp up = NO BLOWN FUSE.....HOORAY!!!!

Now I am no expert on an OT. I know what it does, converts high voltage/low current signal from the output tubes to low voltage/high current signal that is safe to run into a low impedance load that a speaker can use. But I am not sure how it does it. Which I guess is acceptable for my purposes up to this point. Now logic tells me that there is a short or open coil inside the OT. But I do not know how or have a means that I know of the test it, short of throwing parts at it, which is neither economical or practical from a monetary/pride standpoint. What I can confirm is that with the COM wire removed from the OT, every lug test continuity to the ground lug. Primary from the output tubes, to the secondary lugs that go to the speaker jacks to the lugs not being used. They all test continuity to the COM ground lug. Does not seem that that is a good thing, and should be pretty much conclusive proof that I have a bad OT, most likely caused by a bad/shorted tube or who knows what freak one in a million event that might have taken place.

Any ideas? Have I missed anything? Any suggestions?

Any thoughts or criticism (constructive or otherwise) I welcome. (also forgot to add, output tubes were removed for all test performed)

Cheers!

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Re: Shooting Trouble on. 900 SLX

Postby Anitoli » 28 Jun 2015, 12:05

Here is some info about testing PT's and OT's.

http://www.ozvalveamps.org/repairs/trannytest.htm

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