Tektronix 191 Burnt Resistor ID in the voltage divider switch.

[BlownUpCapacitor]

I am servicing a Tek 191 which is outputting incorrect amplitude. I belive I have successfully traced down the problem to this burnt resistor (see photos).
The problem is, that I do not know the specifications of the resistor.

It measures 37 ohms in circuit.

The resistor is located on SW40, the switch that controls the amplitude adjustment range.

I think those details are maybe enough for someone who also has a 191 to check and see the resistor resistance. Looking at the schematic, the resistor is either 40.9 ohms, or 20.2 ohms. So it's 50/50.

I also do now know the reason why the resistor burnt up.

Thanks.

[med6753]

20.2 Ohms.

[BlownUpCapacitor]

Thank you! I was looking at the schematic more and looking at the physical construction more, and had "concluded it was 40.9 ohms, but I guess I was wrong!

Almost possibly made the problem worse...

[floobydust]

It must have seen DC at the output, enough to roast the resistor. I would check others near it.
Tektronix Type 191 Constant-Amplitude Signal service manual

[BlownUpCapacitor]

It must have seen DC at the output...

My question is, where would the DC be coming from? Isn't the attenuation network of SW40 attenuating the signal directly from the oscillator? Or at least through a 50 ohm resistor no?

Perhaps there is something I am not seeing...

[floobydust]

The 191 is DC-coupled but I can't see a high current DC fault path, something that could short and put say the -11V there which should make the 20.9R hot.
There is T10, R40. Even with offset set to max. R60 or R67 are there.

Usually smoke is from connecting the sig gen to a circuit with DC at the node. Oops.

[BlownUpCapacitor]

Usually smoke is from connecting the sig gen to a circuit with DC at the node. Oops.

That's probably the answer. I got two of these from a university a while back. Both are in the salvage bins. One worked... just fine, no issues. I sold that one. This one was outputting incorrect voltage. I planned to keep this one for myself.

Perhaps a student hooked it up wrong. Seems very much so possible.

[BlownUpCapacitor]

After doing some testing, I concluded the resistor is actually 40.9 ohms.

I was also able to figure this out by examining the burnt resistor very closely and seeing a number 4 digit.

Now the problem of sourcing such a resistor arises... Digikey has some but with a 70-week lead time.

[donlisms]

68 || 100 = 40.9 within 1%...

[BlownUpCapacitor]

Turns out, if I clean off the carbonized paint, the resistance increases again from 37 ohms back to 40.5 ohms.

Would it be a good idea to put the burnt resistor back in?

[SeanB]

Not really, just order the replacement parts. The carbon is now both exposed, and cooked badly, so you never know when it will flake off the former, or the body will finish cracking through, plus a cooked resistor will be rather unstable with time.

[floobydust]

Did you note in the 191 manual corrections:
"Due to a temporary shortage of 40.9Ω 1/8W 1/2% resistors, a 41.2Ω 1/8W 1% selected to be within the tolerance range of the original resistor, may be substituted for R41, R43, R44, R46, R47, R49 in this instrument."

[BlownUpCapacitor]

I happen to have some 100 ohms and 68 ohm resistors. If using the old burnt one is not recommended, I'll use two parallel resistors.

Also would using two resistors instead of one singular one possibly cause problems in the 191 at 100MHz? Or will the differences be negligible?

[SeanB]

Negligible, provided you use short leads, and 1/8W resistors, or a 1206 SMD part and short leads to get to the tags.

[jdragoset]

Any one Tank Winding (V10 plate voltage)-to-Output Winding short, T10 thru T38, or shorted C63 or C64 could burn Amplitude Range attenuator resistors.

[BlownUpCapacitor]

Any one Tank Winding (V10 plate voltage)-to-Output Winding short, T10 thru T38, or shorted C63 or C64 could burn Amplitude Range attenuator resistors.

I find this unlikely as the Tek 191 works just fine, outputting a sexy sine wave, but at the wrong amplitude. But it wouldn't hurt to check for these I guess. I'll update soon.