Boonton 72BD Repair

[Kjo]

I have had two of these Boonton meters for as long as I can remember. They are basically the same with one having a digital meter and the other an analog meter. (I sold the analog 72B some time ago to clear up some bench space.) I hadn’t used it for quite some time because the auto range was flakey and the meter would not remain stable at zero. But I always had something els to do instead of digging into the 72BD.

But a recent bench cleaning and clearing brought the 72BD front and center to where I couldn’t ignore it. Now my unit is a rather early version with a serial of 1937. Based on the date code on some parts, I would say the manufacturing date was 1979-1980. I have a manual from much later production (1987), but it appeared to be nearly identical. I was hopeful that the design hadn’t been changed too much in the intervening time.

As usual with electronics, one needs to check the power supplies first, as without them operating little hope is there that other parts will function properly. The meter has 3 primary supplies based on traditional transformers and linear IC regulators. (+-15VDC & 5VDC.) A quick check indicated 4.4V on the 5V bus, 15V on the +15 bus and -21V on the -15V bus. Clearly 2 of the 3 are a problem.

I next looked at the raw DC input busses to evaluate the condition of the main filter capacitors. The 5V supply was around 7V, really too low for the headroom required by a LM7805. So this input cap was likely very leaky and overloading the transformer winding.
The -15V input bus should be in the -25 to -28VDC range, but had more than 8VAC and unstable DC voltage. This cap likely had a high internal impedance, and the associated LM7805/LM723 were likely faulty. I removed all the capacitors, and all were faulty; the large caps had degassed and eroded dome of the PCB trace tin & copper. ESRs of a couple hundred ohms and leakage of 400K says it all.
Interestingly, the large cap values were marked with 10% of the values specified in the newer manual. (250uF/2200uF & 1000uF/4700uF). A look at the photo of the replacement caps indicated that there is not a lot of headroom under the transformer. Maybe Boonton couldn’t get caps to fit in higher capacity in 1979. My 21st century caps are small enough to use the newer spec size!

You can see the cap repair in the back side PCB photo where the eroded traces are repaired. On applying power, all three supplies were well within spec after the required adjustments were made. The range switches all work properly and the 2pf range will zero out and remain stable.

Next I will be checking some of the performance specifications, but I do not have capacitor standards to do a good calibration at this time…..