Re: TM5xx/TMxxx Pass Transistors. SUBJECT CHANGE, WAS: DC-508 repair advice

Dave Seiter

Thanks, Dennis!
I hadn't seen this list before, and I'm sure it will come in handy.

On Sunday, December 15, 2019, 12:55:17 PM PST, Dennis Tillman W7PF <@Dennis_Tillman_W7PF> wrote:

Hi Michael and all TekScopes members,

As my late friend, Jerry Scheltgen (AKA Kilowatt on eBay), and I once documented, knowing which pass transistors are used in which TM500 / TM5000 mainframe, is not easy to figure out because Tek changed them often and some mainframes use different ones for no apparent reason. To make matters worse on some occasions Tek changed to a different transistor with ENTIRELY DIFFERENT lead order so they require contortions to reorient the leads to make them reach the holes the old leads went into.

To solve this mess in 8/2014, Jerry and I documented which pass transistors were used where and when you had to be careful because the lead orientation mattered.

I uploaded the list we created just now. It can be found at

If you search our archives of messages regarding any topic with "pass transistors" you will find several recommendations our members made for "better" or "more powerful" or "more sensitive" or "less expensive" or "more available" substitutes for what Tek used. The only consideration I would caution that you observe it is desirable (to prevent oscillation) that the pass transistor have a low cutoff frequency (Ft). Less than 1 MHz would be ideal.

Dennis Tillman W7PF
-----Original Message-----
From: [] On Behalf Of Mlynch001
Sent: Sunday, December 15, 2019 11:51 AM
Subject: Re: [TekScopes] DC-508 repair advice

The hard part is finding replacement transistors for the pass transistors.  I blew one up by plugging one of my plugins onto my extension cable upside down.  The plugin extension did not have an indexing pin, so it would fit either way. 
Michael Lynch
Dardanelle, AR

Dennis Tillman W7PF
TekScopes Moderator

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