BC-610-E exciter problem

[n5ama]

I have a BC-610-E that I have been working for a few weeks. I've modified a BC-614-I speech amplifier to open up the audio a bit and using it to drive the 610-E. This was recently discussed on another topic about the BC-614 and the modifications seemed to help with the audio quality of the 610-E.

I recently had a problem with the BC-610-E and thought it might be good to take the discussion off the BC-614 mod topic since the issues with the transmitter no longer relate to the BC-614.

I had the BC-610 on for several hours on day and returned to it finding it dead and found a blown 5A fuse. I replaced the fuse and it didn't blow. I had no exciter plate supply and found the 5Z3 rectifier dead. I didn't have a replacement so I made an adapter to replace the 5Z3 with a 5U4G until the replacement arrives. The 5U4G appears to work good and the fuse still doesn't blow, but there are issues that remain.

When I attempt to tune it up either with a crystal, the M.O. or the VFO, the first operation after letting it warm up and turning the exciter plate power on, is to set the excitation meter switch to "int. amp grid" and adjust the doubler knob on the tuning unit to maximum meter reading. As soon as I switch the exciter plate on, the meter goes to half scale (about 8ma). Within a few seconds the meter climbs to full scale (15ma). The doubler knob will reduce the meter a small amount but not much. The int. amp. grid current is way higher than normal.

I checked the voltages on the two terminal strips under the exciter chassis and all seems normal except Terminal Strip #1 position 7 is supposed to have 100vac and it has 9vac. Position 9 is suppose to have -180vdc and it has no voltage present.

Terminal Strip #2 positions 5,6&7 are suppose to have -180vdc and it has -9vdc.

I haven't traced these test points on the schematic yet but hope to have a chance to do that tonight.

Either the 5Z3 failed and caused some collateral damage or something in the exciter failed and took out the 5Z3 and the fuse.

Does this sound familiar to anyone familiar with the BC-610?

Any suggestions greatly appreciated!

Tom N5AMA

[pa3hco]

clean the exicter switch that is a normal problem with the bc610-e
when you go a little to the left or rith! you have some other mesurments
73
pa3hco

[n5ama]

Thanks so much much for the suggestions in looking for this problem!!
That’s exactly what I was looking for here and trying to find someone with experience working on this old rig. It worked great prior to this problem, so I know it’s capable.

I checked a number of voltages on the two terminal strips under the exciter chassis.
I measured TS-1 voltages and found 9vac on positions 7 and 8 (suppose to be 100vac that traces to supply on T-4). I also found 0 volts on position 9 (suppose to be -180vdc).
I measured TS-2 voltages and found -9vdc on position 6 (suppose to be -180vdc).

*****I was hoping I didn’t have a bad T4.*****

I plan to remove the top on the transmitter so I can check tube pin voltages from the top versus having to crawl upside down under the exciter chassis to measure tube voltages. It’s uncomfortable at 76 getting into these positions much less staying there for very long. LOL

I had planned to remove the top to sand and paint it, so that will work well with the trouble shooting task.

Thanks again for your suggestions!! 73

Tom N5AMA

[pa3hco]

hello tom
before you put of the top deck !! chek the exiter swith if this one is working ?i had some trouble with a t55(bc610)and this one had a bad switch so there was nothing to switch hi

[n5ama]

Not making much progress. I checked the switches and they appear ok.

It appears that some component is failing after about 15-20 seconds and causing the exciter current to go from 1/2 scale to full scale. Negative bias voltage measured to pin 3 on the 807s goes from around -12vdc to over -100vdc when the meter goes full scale.

I’m also getting 600vdc versus 450vdc on the 807s in places like the plate cap and grid. I’m measuring almost 600vdc at L1 with a little drop as might be expected and that voltage is on terminal 8 of TS-2 (the voltage/resistance test says it should be -180vdc).

I replaced the 807s just to see if I’m developing a short after 15 seconds. I’ll do the same with the regulators if I can find 3 in my spare tube box. The regulator tubes appear to be firing as expected but my tube tester doesn’t do a good job of testing them.

I don’t know if the suspected component breakdown is related to both the bias change and the 600vdc overage problem or if the are two unrelated issues.

Resistance values appear to be within limits. L1, CH7 appears to be good. Someone has replaced the two 50 ohm resistors with a few turns of air wound coil like a parasitic  suppressor (maybe a field modification??).

I’ve pulled the top off the rig just to make it easier to get to the test points and tube socket pins. Of course I haven’t turned on plate power but the 600vdc still makes me a bit nervous while poking around. :<{

Any ideas greatly appreciated.
Thanks
Tom N5AMA

[n5ama]

I’m still unable to identify the reason I’m loosing voltage for both grid bias voltage and screen voltage to the 807s. This is occurring only after the exciter supply has been on for a few minutes.

I’ll check the primary to T-4 being fed from the VR-150s next. I checked earlier and it measured 100vac but perhaps I didn’t leave the meter on long enough.

Tom N5AMA

The primary on T4 is getting the correct voltage from the regulators (102vac) and it’s steady ( the only voltage found that doesn’t change with time). At this point, I’m grateful for these small things! LOL

There is a voltage drop at L1 and the two 8mfd caps but only 460 to 424vdc over a few minutes.

It makes it difficult to localize the problem or problems with voltage drops across almost all components and functions of the exciter. Going back to the original issues of a blown fuse and rectifier would make me think something failed that in turn caused the rectifier and then the fuse to fail, but I can’t find any problems with components connected to the rectifier or the fuse and if something caused them to fail, it’s not doing it now even with a undersize fuse in place.

Very interesting indeed.