- 2024
- Nov
- 27
The B&W Model 200 Signal Generator Part 2 - Diagnosis
As seen in the last part of this series, there’s definitely a problem in this unit preventing it from oscillating properly. It’s time to start digging in and seeing what’s what.
I picked up a new / tested good used set of tubes at the recent Fort Wayne Hamfest. I started with a baseline measurement to make sure everything was acting as before, and then started with the rectifier. The signal output was a big stronger, so the 6X5 is probably weak. Not a big deal, and probably expected.
Going around the rest, including the PTC resistor lightbulb, nothing changed save the signal was again a little stronger with a new 6SJ7 oscillator tube. Not enough to worry about, so all tubes were swapped back to the originals. The problem is in the components.
The device is mostly disc capacitors, and while they do go bad, it’s not often. Ignoring those for now, I decided to do a quick check to spot any obvious failures before powering and measuring voltages. First thing I checked was the big 10k 2W that I assume is a power supply divider.
160k and dropping. Odd…no capacitors here, and it finally settled down to 9.91k. That’s probably the smoking gun right there - the resistor is changing value as it gets hot and driving B+ down to a level where the unit can’t oscillate. Once the oscillator is off and the current is lower, the B+ increases just enough to oscillate - rinse and repeat. I need to check B+ to make sure, but this sure is suspicious.
With a reasonable guess secured, I did some spot checks around the oscillator. There’s a 33k across the 6SJ7 that has increased to 35.4k - a little out of tolerance but eh…there’s also a 200k flying off the tube, and it was quite high at 223k. Ok, that’s possibly a problem. I noticed two more of this type and checked those - way way out of tolerance. Everything else seemed to be close enough that I wasn’t concerned,
I wonder what made just those particular ones that bad?
There’s a couple of other things here that probably should go. That big white CDE El-Menco ripoff capacitor is a type that can be problematic. There’s also a dry electrolytic that’s checking fine but probably can go, and a black plastic capacitor that connects to the output control that needs to be replaced because they crack and gain moisture.
There are multiple supply electrolytics, and they all seem to be good. I’ll need to let the unit run a bit and see if they get warm, but if not then I’m not going to worry about them until something happens. I’m going to replace that nasty power cord as well, but that’s just a “always do it” thing for me.
With possible fixes identified, it’s time to set this unit aside until parts arrive. Stay tuned!
Next part of this series: Coming soon.
Previous part of this series: https://wereboar.com … 00-signal-generator/