TSL100 bias again

[killertone]

Thanks to everyone for the help. Now I need to locae some 22pf 2000v caps. Found the ones at Mouser but don't need anything else currently. Hopefully I can find them locally.

[Enzo]

Unless you live in a large city with stocking electronics part distributors, good luck finding a 2000v cap locally.

[killertone]

Any chance any of you guys would send me one? I would gladly pay you for your time.

[Mars Amp Repair]

sure thing,
I'll drop it in the mail. send your address to me @ glen@marsamprepair.com. no charge for that little thing! glen

[killertone]

Thanks a lot, Glen! Email sent!

[Mars Amp Repair]

well,
I guess it makes sense that an open speaker jack when you're using the 8 or 4ohm jacks could certainly cause flyback conditions & blow that cap. I think most are just jumping that jack switch & informing the cust not to try & put speakers in all the jacks at once. At least that's what I'm doing.

I have had the cap blown & there be nothing wrong with the jack switch, so who knows. I think just the tube blowing can cause the flyback effect & blow the cap, too. either way, I replace it with the 2KV variety.

glen

[Landshark]

I noticed R9 is pretty much completely obliterated on my TSL's main board too as well as C46. I saw what C46 was but does anyone know what R9 is? Looks like a 1w 1 ohm, but I just want to be sure before installing the wrong part. If this head drops out again I'll get a new main board... Anyone know the distributor for US?

[Alex R]

R9 is a one-ohm resistor between one pair of cathodes and ground, used to produce the bias measurement in millivolts that you read off on the three pins at the back. They often burn up when a tube overheats. I'd fit a +-1% tolerance type to replace it, so the bias readings are accurate. Yes it's a 1watt resistor.

Don't know the US distributor but Marshall in the UK are very helpful on the phone. But I don't think they will send parts to non-approved repairers.

I now fit new boards to these amps when they come in if they have the old paler green board that you can't see the fibre weave on. I haven't seen one of those boards in an amp that has overheated, that doesn't start to drop bias voltage when I blow hot air at the bias components.

However, do have a look at the mute/power drop components too - the two little 0.1uF 50v caps in there can get leaky and sap away your bias voltage. C34 and C35 - they are on the right hand side of the preamp board looking from the front of the amp. There's a connector next to them - if the bias voltage comes back when you disconnect it then you will need to replace them too.

[Landshark]

R9 is a one-ohm resistor between one pair of cathodes and ground, used to produce the bias measurement in millivolts that you read off on the three pins at the back. They often burn up when a tube overheats. I'd fit a +-1% tolerance type to replace it, so the bias readings are accurate. Yes it's a 1watt resistor.

Don't know the US distributor but Marshall in the UK are very helpful on the phone. But I don't think they will send parts to non-approved repairers.

I now fit new boards to these amps when they come in if they have the old paler green board that you can't see the fibre weave on. I haven't seen one of those boards in an amp that has overheated, that doesn't start to drop bias voltage when I blow hot air at the bias components.

However, do have a look at the mute/power drop components too - the two little 0.1uF 50v caps in there can get leaky and sap away your bias voltage. C34 and C35 - they are on the right hand side of the preamp board looking from the front of the amp. There's a connector next to them - if the bias voltage comes back when you disconnect it then you will need to replace them too.

Thanks for the info Alex. I'll be checking those too.

[DivAud]

I have fixed two of the bias drift problems recently, looks to be R4 is too close to a B+ line. I moved R4 and drilled out the pads, removed part of the trace from R4 and tacked R4 on to the back side of the board. problem gone. I took a pic with my Iphone, but it is kinda fuzzy. Will take better pics on the next one I see......

[Alex R]

Interesting. When you say problem gone... it sometimes takes a few hours to build up the heat and the loss of bias voltage, especially if you make a circuit change. A good test is to clip a vm to a power tube pin 5 and blow hot air at the board in the area in the middle of the power tubes. After a while as the board heats up the bias voltage starts to drift down, and comes back up when you take the heat away. There's no trace of this effect if it's a good fix.

[DivAud]

I understand what you are saying, but you may want to look at the area around R4, and the very close location of the B+ traces to the C- at that location. The two units I looked at had slightly discolored traces in that area due to the heat. We ran both units and monitored the output tube current for several hours after the work was done. Our customer picked up the first one several months ago, The second unit is for a different customer. We have a third one that just came in a couple days ago. We are a repair shop, I do not expect either of these units to come back, but will post anything if they do.

[Alex R]

Hey if it works it works. The hot air thing does indicate very quickly and clearly whether or not it's cured. I've swapped about 20 new boards into these over the last few years, next one that comes in I might give R4 a try!