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

 

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
https://groups.io/g/TekScopes/files/TM5xxx%20Pass%20Transistor%20List.pdf

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: TekScopes@groups.io [mailto:TekScopes@groups.io] 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

Mlynch001
 

Dennis,

I was able to find the original specs for my particular unit and then found some replacements that I thought might work. It has 5 bays with the same NPN and PNP parts and the high power compartment used something different. I found some replacements and then used the curve tracer to match the old parts to the new parts as close as possible. I’m going to look at what you have written, to see how close I got by dead reckoning and using my Curve Tracer. I appreciate your guidance, I always learn something from the members of this group.

--
Michael Lynch
Dardanelle, AR

 

Hi Michael,
There a lots of possible replacements provided the voltage, current, and power ratings are equivalent. The other thing to watch for is be sure the replacement pass transistor leads are in the same order as the originals. Tek changed the parts they used over the years and you must make sure your replacements have the same order for the E, B, and C pins. Beyond these 4 things the package outline is the only other detail that you need to match.

The rightmost slot of the TM504, TM506, and TM5006 are high power compartments so they probably have beefier pass transistors.

Dennis Tillman W7PF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Mlynch001
Sent: Sunday, December 15, 2019 1:49 PM
Subject: Re: [TekScopes] TM5xx/TMxxx Pass Transistors. SUBJECT CHANGE, WAS: DC-508 repair advice

Dennis,

I was able to find the original specs for my particular unit and then found some replacements that I thought might work. It has 5 bays with the same NPN and PNP parts and the high power compartment used something different. I found some replacements and then used the curve tracer to match the old parts to the new parts as close as possible. I’m going to look at what you have written, to see how close I got by dead reckoning and using my Curve Tracer. I appreciate your guidance, I always learn something from the members of this group.

--
Michael Lynch
Dardanelle, AR




--
Dennis Tillman W7PF
TekScopes Moderator

Dave Seiter
 

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

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
https://groups.io/g/TekScopes/files/TM5xxx%20Pass%20Transistor%20List.pdf

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: TekScopes@groups.io [mailto:TekScopes@groups.io] 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