开云体育

In my latest 7704A, I noticed the chain of nine 300K resistors (R4261 - R4277 and, possibly, R4279) in the CRT circuit have drifted significantly high (~20%) making what should be a total of 2.7M about 3.2M. It's rather obvious they've been running quite warm (discoloring on the PCB) and I'm going to replace them with some breathing room between the new resistors and the board but it got me to wondering what function those provide. I looked over the circuit description and didn't see those mentioned so it made me curious and, perhaps, someone could enlighten me. I presume they're probably a more-or-less standard part of a CRT circuit and that's why they're not called out in the circuit description but not sure.

Thanks,
Barry - N4BUQ

Without looking, that sounds like the typical "focus string" divider that probably includes the focus pot somewhere, to get the right operating voltage for the function, and it possibly does other things too. Carbon composition resistors are often used for this, and they tend to drift upwards over time. Your idea to mount the replacements with more board clearance is sound.

It's also possible that strings of resistors are used for feedback in high voltage regulator circuits, but if so, they would be better grade metal film resistors. Tek hardly ever did this for CRT HV regulation - they used instead a carbon or metal film resistor network on a ceramic substrate. You can usually spot this part easily in the circuit.

Ed

Barry,

These are a bleeder for the cathode circuit and partial divider for high voltage and focus circuit. I replaced mine with 1W 1% types and spaced them off the board. The 1W types I use are about 1cm in length, not 3,68 x 8,74mm types.

You will find the two 22meg resistors to be well out of tolerance. Use VR37 Vishay types.

Mark

Mark,

I checked those and one of them is well within tolerance. The other one is just at the high end of its tolerance. I may replace one or both of them but they should be okay for now.

Thanks!
Barry - N4BUQ