Marshall TSL 100 Question


Homebrew1709
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Homebrew1709
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06/15/2007 4:43 am
So my friend and I were biasing his TSL 100 head today and the left bias test point was not giving us a proper reading...each time we tried, the multimeter would give an error message. The right bias test point worked just fine....got it to about 80 mV which is what was recommended. Any clues as to what's wrong with the left bias test point?
# 1
da_ardvark
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da_ardvark
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06/16/2007 4:24 am
Could be alot things, the first thing to determine is what kind of multi-meter? Is it auto ranging? The reason I ask is you should try and find out what sort of input into the multi-meter causes a fault.

As far as the amp, I would not use it until you figure out the problem. Check to make sure the tubes are all seated properly. It could be a faulty grid cap. Like I said try to figure out what input into the multi-meter causes a falut.
# 2
Homebrew1709
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Homebrew1709
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06/18/2007 9:25 pm
The multimeter is auto ranging and i've got it set to mV. After the first round of error messages, we took the power tubes out, cleaned the sockets with electrical contact cleaner, then put the tubes back into different sockets than we originally tried but still got the error...The multimeter doesnt seem to have a problem reading anything else other than the left bias test point on this darn amp! BTW the tubes are brand new JJ EL34's...
# 3
da_ardvark
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da_ardvark
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06/18/2007 10:36 pm
Ok, this is interesting. I really like what you did already. Great trouble shooting technique swapping out the tubes. I'm going to look for a schemo for this amp. I have never worked on a TLS. Off topic a little, head or combo (It really shouldn't matter)

Was the amp working well before the tube change and attempted biasing?

I'm beginning to suspect a bypass cap has failed. I'm sure you know there are LETHAL voltages inside the amp.

Let's do this, see if the multi meter will show you the voltages. Change the ranges to V instead of mV.

I'm off to find the schemo.
# 4
Homebrew1709
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Homebrew1709
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06/19/2007 9:46 pm
A. Amp is a head (100 watt version)

B. The amp sounded like it was lacking punch and balls which was the reason for the tube change. The last tubes in there were junky Groove Tubes in the power section and possibly the stock preamp tubes. Figured that some nice JJ's and a little hotter bias would breathe new life into it. My Carvin combo (sans speakers) into an old Peavy 4x12 has been KILLING his Marshall :p .

C. Yes, I am aware of the deadly voltage :eek:
# 5
da_ardvark
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da_ardvark
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06/20/2007 1:31 pm
Can you give me an idea as to the actual voltage from the "bad" test point? Again you may have to change the range of the meter.
# 6
ampfryer
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ampfryer
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06/24/2007 3:57 pm
I had something similar this week. Sick TSL100 with 2 tubes hotspotting -One side of bias 90mV...OK and the other reading 15V (!)..Turned out that R6 ( the 1 Ohm) was toasted.
I'm still struggling to get the bias right though - I'm getting 140mV on the side that was damaged and the adjustment is flat out ..which is way too hot..still looking for the cause... -38V pin 5 on V7&8 but only -30V on V5&6 with a pin 3 of 450V.
Anyone any ideas as to why the 1 Ohm should get toasted???.. what the other consequences may be??
any similar measurements you could make on a TSL100 would be helpful.

Cheers

Amp.
# 7
Lordathestrings
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Lordathestrings
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06/24/2007 6:30 pm
I don't know the amp you're working on, but based on general principals:

The 1 Ohm resistor connects the tube Cathode to ground, giving a 1 Volt/Amp voltage drop across it. This makes for convenient reading of the Cathode current. It also means that if it got fried, there must have been a huge amount of Cathode current flowing through.

So I suspect at least 2 problems:
- the bias circuit has a fault that causes it to deliver the wrong voltage to the grid.
- the associated tube may be shorted, or damaged by passing too much current.

Pull the tubes, and measure the voltages on the Plate, Cathode, and Control Grid.
Lordathestrings
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# 8
Homebrew1709
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Homebrew1709
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06/26/2007 12:50 am
my buddy ended up taking the amp into the shop where they determined that it was, in fact, a fried resistor...they fixed the problem and the amp is running fine again...
# 9
da_ardvark
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da_ardvark
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06/26/2007 12:15 pm
Glad it's up and running. He ain't call LordoftheStrings for nothing :D

Nice diagnosis there LOTS
# 10
z0s0_jp
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z0s0_jp
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06/27/2007 12:57 am
it's LATS!!! -----Lord "AT" the Strings--- :p
"Dammit Jim!! I'm a guitarist not a roadie...so haul my gear"
# 11
da_ardvark
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da_ardvark
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06/27/2007 12:11 pm
Opps :eek:

Bring on the Bad REP :(


HAHA
# 12
Lordathestrings
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Lordathestrings
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07/15/2007 5:56 am
No worries. :)

I used to have a D'Addario T-shirt that said "Lord Of The Strings" on it. D'Addario got sued by J.R.R. Tolkien's estate for copywright infingement.

I use Lord a the strings, cause I tend to drawl a bit when I speak.
Lordathestrings
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# 13

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