Marshall VS100R transistors

[markorock37]

So...I have a Marshall VS100R combo on the bench. Blown transistors, and also a bad J111 (T16) and a bad TIP29CH (T4). I already have the output transistors TIP 142 and 147s. Do these equivalent transistors need the mica spacer? I couldn't find the info from the datasheet.

[Tassieviking]

If you mean the insulator between the transistor and heatsink then I would say yes.

[Jazz P Bass]

Short answer: It depends.
 Some transistors have the collector tab totally encapsulated in plastic.
These do not need a mica insulator.
Just heat grease.
 If the collector tab is metal then, yes, it must be isolated from the heatsink IF the circuit calls for it.
 Some amplifier topologies use a grounded collector.

[markorock37]

Well the original transistors were BDV65 and BDV64 and there was no spacers. However the amp was worked on by a previous tech and he deemed it non-repairable, which of course is not the case. TIP 142 and TIP 147 are suitable replacements, I'm just confused as to if that spacer was there previously or if now with the transistor sub it may be required. I'll check the schematic to see if I can tell if the collector was originally grounded.

[markorock37]

According to the schematic not grounded. I'll order the spacers.

[Jazz P Bass]

Not to beat a dead horse but if the TIP 142/ 147 transistors are plastic enclosed at the collector then you do not need the mica. Just grease.
As a matter of fact, it may hinder the transfer of the heat.
It would be nice if you would indicate what you have as both the BDV & the TIP transistors come both ways.

[g1]

The metal tab type has not been made for over 10 years, if you found some they are old stock or possibly fake.
Like Jazz said, you don't need the spacers for the plastic backed type.
Attached datasheet shows both types.
*EDIT TO-247 package does not have the back insulated and requires insulator.


If you do happen to have found some old style metal tab types, you not only need the mica spacers, but also special plastic washers for the screws.

[markorock37]

It originally had transistors with the metal tab. Only held to the heat sink with a surface clamp. The replacements I have are the newer type.

[Tassieviking]

If they are fully encapsulated in plastic then you just need some thermal grease.
A thin layer of grease and try to move the transistor around  a bit against the heatsink to spread the grease evenly, clamp it and solder in.
Just poke the legs through the PCB enough to solder them in, it keeps the transistors more towards the center of the heatsink.

[markorock37]

There is some metal exposed on the back of these newer transistors. I think it will be best to use the insulators.

[g1]

There is some metal exposed on the back of these newer transistors. I think it will be best to use the insulators.

Thanks for the update.  Yes, you need the insulators.  I was mistaken as far as the datasheet goes.  I found pictures of other transistors with the same package (TO-247) and they are not insulated on the back either.

[markorock37]

I replaced the output transistors (TIP 142, TIP 147) and installed with the micas. Also replaced TIP29C and a J111 that were bad. Im getting a low brightness (not really dim) on my bulb limiter and a buzz/hum through the speaker. No guitar signal coming through. Tested for DC at the output jack and it was at 33V! Shut it down and started testing components. All diodes test good, and all capacitors test good. Checked some resistors in areas i felt were critical and haven't found anything off. Do you think one of the TO92 transistors is at fault? Or did I blow an output again?

[markorock37]

Looks like I'll have to wait until tomorrow to start testing this entire board.

[g1]

What is the wattage of your bulb?
I doubt that the 33VDC at the output was when speaker connected as that would be over 130 watts and your speaker would surely be toast.

Never try to power anything up on a limiter bulb while a load is connected to the amp.
Power up with no load.  Then, if no DC is present on the output, connect load while unit turned on.
Many amps will not power up properly on a bulb with a load connected.

[markorock37]

100W bulb. DC was present with no load.