Channel A failure on Tek 2235

Terry Ilardi wrote in news:c1votg$gao$ snipped-for-privacy@news.btv.ibm.com:

First,take off the cabinet,and see if there's a broken BNC-PCB connection. (wire,solder joint) Or even a bad Ch.1 BNC(bad center receptacle) Then maybe you could have a bad AC-GND-DC switch. Jumper around it,see if you get a signal then. If you blew up the input FETs,you would have a DC offset on Ch.1.A DMM would allow you to check the input FET pair for that offset.

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Jim Yanik
jyanik-at-kua.net

Thanks. Will try. The bad connector sounds like a possibility since, now that I think about it, Channel A has been a little flakey in the past. I always thought it was my fault due to poor setup.

By the way, I've read that the 2235 was not one of Tektronix' better products. Do you know anything about that?

Thanks again for the help.

Terry Ilardi

Terry Ilardi wrote in news:c2295s$7mk$ snipped-for-privacy@news.btv.ibm.com:

2235,2236 IMO were very good scopes,although not comparable to a 465 or 465B.The 2200 series was designed to be low-cost,high-volume manufacturing.

The 2245 series was not very good,IMO.

300 series were garbage(Sony-Tek)

I have a 2213,myself(built from scratch! I repaired PCBs other techs had scrapped,bought the mech parts I couldn't salvage(less than $50),and got Vern Isaacs of CRT manufacturingto give me a new CRT that was graded OK but only for internal use.

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Jim Yanik
jyanik-at-kua.net

Jim is the resident expert on Tek scopes. I agree with him; check the mechanical connection on the BNC connector center pin. There is a carbon comp resistor that connects the center pin of the BNC to the attenuator PCB. Check that resistor & it's connections to the connector and PCB.

I actually think the 2235 is a good scope. More cheaply made than the

400 series, yes, but they were like half the price when new. I use a 2235A as my main troubleshooting scope & have fixed a couple dozen 2235's.

BTW, you can use your scope to check itself! You don'e need a spare to troubleshoot this type of problem. What I mean is that you can feed a signal into channel A, and use channel B to troubleshoot the signal thru the attenuator to see where the problem lies. Be careful when you have the cover off, of course!

Jeff

Very cool! I once built an Eico 460 (not quite the same-but hey, I was sixteen at the time).

Terry Ilardi

Jef,

Thanks. I had thought of trying to use the good channel to troubleshoot the bad but then decided I was deluding myself that it could be done. It's nice to hear the experts say that it is possible. If its more then a bad connection (I can use my DVM for that) I'll try it.

Thanks.

Terry Ilardi

Terry - When you say "Channel A" or "Channel B", I presume you mean Channel

1 or 2. "A" and "B" refer to the timebases, so check that you have selected "Horizontal Mode A" for the timebase, and that the outer timebase knob is engaged with the lower. For the "Vertical Mode" select BOTH and CHOP. Do both VERTICAL POSITION controls work? If they do, the pre-amplifier is ok; if not, compare the voltages around the Ch.1 pre-amp U130 (M84F), Q114+115 with CH.2 pre-amp U180, Q164+165. Your 'test signal' here is the position controls! Also check the switching voltages at Pin 14 of U130+180; according to the service manual, should be +0.7V to switch that channel ON, -0.7V for OFF.

If position controls are ok, switch vert mode to ADD and see if you get a signal through each channel. If not, look first at the attenuators, then at the buffer amps and phase splitters on the attenuator PCB. The buffer amps are U10 (CA3180), Q13+15+18, the phase splitter is U30 for Ch.1, U60, Q63+65+68, U80 for Ch.2. Compare DC voltages, or trace an AF signal from your sig gen, listening with any small audio amp but with a 100nF capacitor to block DC.

HTH - Julian (Manchester, UK)