Tektronix 2235

[bsco]

Hi there. I recently got my hands on a Tek 2235 scope and downloaded the manual from the net...In the schematic the component designations are not very clear...R909 is burned in my unit..Does anybody know what value this is supposed to be?? The FET on the heat sink is completely blown apart and I also traced the connections back to a small signal transistor which also shows a short...Is this scope worth repairing?? Somebody had cut off the wiring from the AC plug and fuse to the board..I know where all the wires go except for one..It has a yellow trace in the insulation. The wiring was cut at it's midpoint so it was easy to tell where they went except for this one. I can't find the other end of it...maybe the person has removed the other end completely..any help or insight greatly appreciated. I would really like to get this up and running....if it is worth fixing....
Cheers,
Bernie

 

[davenn]

hi there Bernie

most of them are worth repairing even the much older Tek scopes like the 465B I have at home
have you actually tried to physically measure that resistor with out meter ?
It may be browned but still close in value
The manual should have a parts listing to go with the numbers on the assoc schematic.

Dave

 

[shrtrnd]

The manual parts list P#8-17, says (This is the A1 board):
A1R909 Tekt Part #315-0390-00 Fixed Film Resistor 39 Ohm, 5% Tolerance, .25W
I've got the manual, but don't know if I'll have time today to look up your other info.
If I don't get back to you today, look for a post from me tomorrow.
(Make sure your resistor is off the A1 Board)

 

[shrtrnd]

I don't have the unit in front of me, just the manual.
I see two FETs on a heat sink, are one of them the blown FET your're talking about (On A1, the Main Board)?
Q946 Tekt 151-0476-02 SJ389 (Selected)
Q947 Tekt 151-0476-02 SJ389 (Selected)
To save space on the package, the Tektronix numbers might be abbreviated:
"1-476", or something similar.
If I'm sure what area of the boards you're looking at, I can trace down your shorted transistor from the schematic.
(But that'll have to be tomorrow, I'm tied-up right now)

 

[bsco]

Thank you shrtrnd. The resistor in question is on the A1 board...under the power supply shield...Actually it connects to the Fet 9070 which is blown apart....The Fet is on it's own heat sink....It is cracked into 4 pieces and when you start o remove the 3 wire terminal that connects it to the board you can actually see the pins moving...I could not find the resistor on the schematic that I have or in the parts list either...It is not listed but it is on the board..must be a different revision...maybe?? I could see the first two colors of the resistor but after that you can't make out what they are. The first two colors are Orange , then White..so 39 is correct.I have a couple of guitar amps to look at over the next few days, then I am going to see exactly what parts are gone in this unit...somebody chopped off all the wiring going to the power filter board...I'll have to reconnect all that as well.....by the way, the fet is an IR type....not sure of the number and I don't even know if is the right one..I'll have to check the number and get back to you....
Cheers,
Bernie

 

[shrtrnd]

I agree with davenn, the 1980's Tektronix scopes were designed to be repaired, I'd fix it if I could.
Manual revisions are tied to the instrument serial number, and this particular scope also has a military version variant.
A1Q9070 Tekt 151-1151-00 N-Channel MOSFET, SI, TO-220 pkg = MPT3N40
The shorted transistor feeding your destroyed MOSFET (above) is:
A1Q908 Tekt 151-0164-00 PNP Xsistor (TO-92 pkg) = 2N2907A
But you've got a diode, (emitter-base) across Q908 that will also be blown & you'll need to replace:
A1CR908 Tekt 152-0141-02 Switching Diode, 30V, 150mA, DO-35 pkg = DA2527
You'll have a hard time finding the DA2527 Diode, but it's an exact equivalent of a 1N4152.
Good luck with your guitar amp repairs and work on the Tektronix 2235.

 

[bsco]

Thanks very much for the reply....I did notice the Q908 transistor was shorted as well so I figured there were other components gone as well.. I have military grade 2N2907's here.I think the Mosfet that is installed at present is an IRF741?? I would have to double check on that...but I do know that it is an International Rectifier number..I will let you know how I make out over the weekend...Once again, thank you for your time...
Cheers,
Bernie

 

[shrtrnd]

The MTP3N40 crosses to and IRF720
If you're pretty sure you've got an IRF741 in your circuit, either:
1) you've got a revision I don't have a manual for, or
2) somebody intalled the wrong part, and that's what blew-up your circuit.
good luck

 

[bsco]

Thanks for the update..More than likely I would guess that the wrong part was installed...I got this from a guy who did some electronics a few years back...I don't think he actually new what he was doing???then again I could be wrong...I try not to judge people.. I was in the same position when I first started.......Thanks for the info..I'll let you know how I make out....
Cheers,
Bernie

 

[bsco]

Actually got a chance to check out what you told me...The 39 ohm resistor is open, the diode is open, Q908 is shorted between all three pins and the Fet is an IRF710....and that is blown apart. I'll have to see if i can get the parts locally early next week...if not I will have to order them....once I get these installed I'll have to hook up the wiring that has been cut...they were obiviously thinking there was a problem in the power supply but they never had a schematic and were probably just shooting in the dark...I'll let you know how I make out.......
Cheers,
Bernie

 

[shrtrnd]

Looking-up the data sheets, you may already know:
IRF710 is a 2A 400V N-Channel Power MOSFET and
IRF720 is a 3.3A 400V N-Channel Power MOSFET
The line cord connects to the A6 Filter Board, and it looks like a pretty simple connection to me. I don't see a color-code for the wiring in the manual, so I don't
know what your yellow-insulation wire is.
The manual's 'Power Supply, Z-Axis & CRT' diagram shows you all the connections
to the Filter Board. If you have it, it should be easy to trace where your yellow wire goes.

 

[shrtrnd]

I just had one additional thought. I don't know where you're at.
Make sure you check the identification plate on the back of the scope before you plug it in. With ebay and everything else out there world-wide. I don't know if the scope
was made for a country other than the one you're in.
That might be why they cut the power cord to begin with.
If you have a scope wired to run American 120VAC, and you're in another country that
uses 220V or 240V, maybe the last owner just thought he needed to put a power plug
on that fit the outlets in your country, without considering the fact that the scope might
be wired to run at a lower input voltage. (And you may have other damage to the A6
Filter Board).

 

[bsco]

Ok. Thanks for the info...I am in Canada....So the installed Fet is the wrong one to begin with...I have a copy of the schematic of the net......Thanks for the extra info....I'll let you know how I make out...
Cheers,
Bernie

 

[Jaco Goeman]

For Tektronix component data download the Tektronix semiconductor catalog of October 1982 from datasheetarchive: http://www.datasheetarchive.com/MPT3N40-datasheet.html
It is just over 30 Mb and is searchable.
I am busy repairing a 2235 with a dead power supply.
Hope the info helps,
Jaco.

 

[bsco]

Thanks Jaco....I still haven't been able to really spend time at this....I do all the repairs for the local music stores and musicians and it keeps me very busy...when I do get a break for a day or two I have so much to do around the house that I can't get at anything belonged to me....maybe over the summer I'll get this scope working...Thanks for the advice........
Cheers,
Bernie