tsl problem-blows fuse-only runs on two tubes

[wyomingrocks]

Hi group, I have a Marshall TSL head. It started blowing fuses last week on back of the unit. The HT fuse. I saw the 3rd tube over red plate. I shut it down, put in new tubes in and started to bias and smelled smoke. It ends up that some of the associated circuitry to the third socket has went bad. I am currently running on two tubes on the outer sockets and it is acting fine, I need to open it up this week and take a look. The TSL looks like a night mare to trouble shoot. Any area's in particular you can think of for me to look at?? Thank you for any help.

[Robert M. Martinelli]

Hi group, I have a Marshall TSL head. It started blowing fuses last week on back of the unit. The HT fuse. I saw the 3rd tube over red plate. I shut it down, put in new tubes in and started to bias and smelled smoke. It ends up that some of the associated circuitry to the third socket has went bad. I am currently running on two tubes on the outer sockets and it is acting fine, I need to open it up this week and take a look. The TSL looks like a night mare to trouble shoot. Any area's in particular you can think of for me to look at?? Thank you for any help.

First of all, as long as you're running on only two tubes, be sure to connect the speakers to a tap on which the impedance shown is ( nominally ) half than the speakers' one. ( e.g. if your speaker system is 8 Ohms you should use the 4 Ohms tap ).
The rationale for this is :
You removed two of the tubes, so the output transformer "sees" an input impedance which is double than the one it "sees" with all the output tubes on, so also the output taps will match a load which is double than their nominal one.

As to your problem, usually a tube heavily "redplating" is a symptom of a grid bias voltage too close to zero, so i would check for problems on the biasing network ( you can easily measure the bias voltage right on the sockets even with the tubes off ). Another thing to check, should the bias network be OK, is (are) the screen grid resistor(s). If those go open the plate currents tend to rise and the tubes can "redplate".

I would like to add a word of caution, even if you have some experience, be aware that the voltages inside your amp can kill you with the same ease of a bullet in your head. Even highly experienced techs got shocked at least once in their life, so ALWAYS make yourself sure the amp is off and the caps discharged before touching anything inside.

Hope this helps

Bet regards

Bob

[wyomingrocks]

thank you vox rules

Thanks voxrules. I had already done what you said about the cab adjustment. I use a 16 ohm cab and so I plug into the 8 ohm head connector. Thanks for breaking the speaker and tube relationship down for me I will be thinking about that as I get further into my studies on amp repair. I was a electronic tech for about 10 years mostly working with lasers, before becoming a stay at home dad. I have been out of the field of elecgtronics for a few years. I have been reading alot on amp repair lately trying to keep my skills up to date.

This tsl is a major maze, I guess I will jump into it this week and see what I can find. I may have more questions as I get a little further along. As far as I can tell I should have a good signal to compare to on the tubes that are working vs: the socket that keeps blowing.

ON a side note, it may be ovious when I open up the amp but Marshall uses a designation of W1, W2, W3 etc. on there schematics, (it is like a dark shaded block with a thin line drawn around three sides) I am not used to that symbol, what is it standing for, like I say it may be really ovious when I open the amp up.

Anyway, thanks alot for taking the time for your feedback. I am sure it will be easier once I get inside. I guess being a tech is like riding a bike, no matter how many years it has been you can get back on and ride even after a few years.

[Enzo]

W. Like where the power transformer leads connect to the board? Like W6 and W7 where the HT (high tension or high voltage) winding of the power tranformer connects to the B+ supply. (D3-D6) In my mind it means "wire." Look at the push-on connectors where the wires enter the board, are there not little printed W numbers next to each? The symbol represents that push-on connector.


Turn the amp on and measure voltages in that bad socket. Since the tube red plates, we can assume there will be B+ on pin 3. Is it also on pin 4? (the screen) SHould be. And most important, is there bias voltage on pin 5? like -50v give or take.

Power off, unplug, pull all four power tubes. Measure resistance from pin 4 of each socket to one of the neighbor's pin 4. SHould be about 2000 ohms - you are measuring through one screen resistor and back through another. If open readin, then a screen resistor is open,.

The two tubes on the left are a pair, and the two tubes on the right are a pair. Measure between the pins 5 on hte two left sockets. Then do between the two right sockets. My TSL100 schemo shows 220k grid stoper resistors, so there ought to be about 440k between pin on the two sockets. Now read the other pair. SHoud read the samer. If not, then a grid stopper might be burnt open.

SInce the one tube in the pair still works, I tend to doubt the bias feed itself is faulty, otherwise both tubes on that side would redplate.

[wyomingrocks]

thanks guys

I am sorry I wasw so slow getting back, your help is really appreciated. I hope you all had a great Christmas. I spent the past few days playing with my children and visiting with company.

I finally got around to pulling the chassis out, and the w designation was self explanatory after seeing it.

I did as you suggested enzo and took some measurements. Here are my findings.

on on all power tubes pin 3 I measured 490 volts.
Pin 4 on all power tubes measured 490 volts.
pin 5 on v5 and v6 = 41.5 volts --- pin 5 on v7 and v8 = 43 volts

Pulled all 4 power tubes
pin 4 on v5 to v6 measured 2k ----- in p4 on v7 to v8 = 278k (see my visual findings below)

pin 5 on v5 to v6 measured 11k ohms and pin 5 on v7 to v8 = 11k ohms

On my visual inspection I found one smoked capacitor C46 right off of v8 and right next to it I see a red wire coming off of the power transformer, connected to W9 on the board it has a black smoke look around it, the actual transformer appears to look alright.

I guess the only way to get the bottom board out is to remove the rear board with all the jacks on it first and then unscrew the bottom board and replace the cap that smoked. I guess I will measure R76 while I am there replacing the capacitor, is there anything else you can think of that I can measure or look into while I have the board out?

Thanks again for all your help

[Albert Kreuzer]

Yes, R76 is most likely blown (if not, then R74). Could also be a smoked circuit trace or a bad solder joint. The tube associated with the blown resistor is probably shorted (in this case V8).

That's all that comes to my mind right now. Oh, and a new fuse

Cheers,
Albert

[wyomingrocks]

C46 and R76

C46 and R76 are what I know for sure are bad. My pads under the cap ended up getting burned off, I ask a question about repair of that on another post. I went to mouser online to order a replacement for R76 but stopped just short of ordering. Does any one know where to get a close replacement for these resistors. 5 watt 1k ohms. I wasn't sure which variety to order from mouser. Does something like this seem like what I need.

http://mouser.com/Search/ProductDeta...8%2f4ncA%3d%3d

Thanks every one for your help.

[Robert M. Martinelli]

Any 5W 1K resistor will do just fine, but, obviously, if you manage to find something with the same physical dimensions the original had, things will be easier....