Problem Seduction pre-amp_radio station in background

[choff]

Also, I just measured the ground on Seduction to Foreplay chassis and it came up as 0.00 mV. 

Note : This was measured without the Dynaco connected to the system (not sure if this would have any effect on the reading?).  I can hook the Dynaco back up for a similar reading between the Foreplay, but was holding off to see if there was anything more i should do with the Dynaco while I have it taken apart?

 

[Paul Birkeland]

That green wire absolutely has to go to the chassis. That is the big reason to add the three wire power cord in the first place.

The reading between chassis plates is a resistance measurement, though if the problem is really bad you may be able to measure voltage.

I would bet that if you provided a proper ground at the outlet and grounded the St70 chassis that you'd be in good shape in terms of noise.

 

[choff]

That green wire absolutely has to go to the chassis. That is the big reason to add the three wire power cord in the first place.

The reading between chassis plates is a resistance measurement, though if the problem is really bad you may be able to measure voltage.

I would bet that if you provided a proper ground at the outlet and grounded the St70 chassis that you'd be in good shape in terms of noise.

Sorry, messed that up. Checked again using ohm setting for resistance on the meter and it does the same thing when I measured terminal 13 and 14......beeps showing 0.0 and then oscillates between 0.3 and 0.6. Does that check out?

I did think it was really odd at the time that I wouldn't be using the ground wire.  Any suggestions on how best to properly attach this green wire to the chassis? 

 

[Doc B]

This is a classic example of "they don't make them like they used to".  You have discovered that there is a very good reason they don't make them like they used to! There is a two terminal strip  to the left of the power transformer and more or less behind the power switch. One terminal has a yellow-looking high voltage wire from an output transformer connected to it. The other terminal has nothing connected to it and is bolted to the chassis. That would be a place you could try attaching the green power safety ground wire, as it looks like it should reach. UL approved connection requires a dedicated screw, solder tab and lockwashers, which is what we do in our kits. But attaching the ground wire to that empty terminal will yield the same result.

Another way we don't make them like they used to is we isolate all the iron from the chassis with insulated washers and run a drain wire from each transformer or choke body (metal) to the same safety ground point. This reduces eddy currents in the chassis that can create hum and buzz and still keeps the amp safe in case of a short from a transformer winding to the lams and end bells. That's a  lot more work to retrofit than properly attaching the safety ground wire to the chassis.

I can't tell for sure, but it also looks like old filter cap is completely disconnected? If so, that is probably fine.  Otherwise there might be some concerns about ground loops there, too.

 

[choff]

This is a classic example of "they don't make them like they used to".  You have discovered that there is a very good reason they don't make them like they used to! There is a two terminal strip  to the left of the power transformer and more or less behind the power switch. One terminal has a yellow-looking high voltage wire from an output transformer connected to it. The other terminal has nothing connected to it and is bolted to the chassis. That would be a place you could try attaching the green power safety ground wire, as it looks like it should reach. UL approved connection requires a dedicated screw, solder tab and lockwashers, which is what we do in our kits. But attaching the ground wire to that empty terminal will yield the same result.

Another way we don't make them like they used to is we isolate all the iron from the chassis with insulated washers and run a drain wire from each transformer or choke body (metal) to the same safety ground point. This reduces eddy currents in the chassis that can create hum and buzz and still keeps the amp safe in case of a short from a transformer winding to the lams and end bells. That's a  lot more work to retrofit than properly attaching the safety ground wire to the chassis.

I can't tell for sure, but it also looks like old filter cap is completely disconnected? If so, that is probably fine.  Otherwise there might be some concerns about ground loops there, too.

Very interesting info. I think I have some isolation feet I purchased a while ago for those transformers and never got around to doing that install.  Sounds like I might want to find the time one of these days. 

I was eying up that empty terminal as well as a possible good spot.  Assume this is the one you are referring as shown in this photo?  I have plenty of slack in the green wire to reach that point. If so, I'll get to work on that and report back once I have everything back up and running.  I can't thank you all enough for the guidance.  Fingers crossed that does the trick......

 

[Doc B]

Yes, that is the terminal. Make sure the nut and screw are good and tight. Your amp will be much safer with that connection even if it doesn't completely solve your noise problems. But it may very well help.

 

[choff]

Awesome....will do and let you know.  Thanks Doc!!

 

[choff]

Well.....no luck.  Made the green wire ground connection and with high hopes fired everything back up.  Radio signal remains unchanged.

I also measured the resistance between the Foreplay and the Dynaco.  This was exactly the same as between Seduction and Foreplay as well as terminals 13 and 14 on the Foreplay.  Meter beeps with 0.00 reading and then climbs to 0.5 and settles on that number.

Lastly, I hooked the Dynaco up to another outlet on a different circuit using a heavy duty extension cord thinking this may determine if the dedicated outlet had a grounding problem as some had thought possible. Fired it all back up and everything remained unchanged, including the voltimeter readings. 

I'm obviously disappointed the fix was not obtained, but still determined to do what I can if there are any next steps you all can suggest?  Appreciate the continued support and guidance with this problem.

 

[choff]

Update : on a whim I took the rca cables running from Seduction to Foreplay out and put them back in, reversing the orientation of both lead ends.  This nearly eliminated the radio signal!!  No idea why?  The more I mess around with those cables the more it seems like they might be the source of the problem.  Perhaps picking up the radio signal like an antenna?  Going to try and borrow some more cables from a friend to continue experimenting.

Note :  looked up the radio station I am receiving and it is broadcast from a tower on a huge skyscraper about a 1/4 mile from my house.  I can see the building from my back window.  I'm clearly getting blasted by it's signal and not surprising that this is causing issues. 

 

[Doc B]

That is why I was asking if the interconnects had the shield connected at only one end. You have moved that end of the shield to the most optimal ground point.

Sometimes it may feel like we are barraging a person who asks for tech support with questions. It can helpful to go back in a thread and check to see if there are questions about the configuration that we asked which may have been missed. They might lead to more things to try to solve the problem. Tech support by the forum can be a cumbersome process, but it puts more than one brain on the problem.

 

[choff]

Thanks Doc.  I have to admit I didn't really understand what you were referring to when you suggested that earlier in the thread.  I should have just asked for clarification, but probably didn't want to show my ignorance. 

Anyhow, I would be nowhere had I not had this forum and you all to provide guidance to resolve the issue.  Plus I was able to make my Dynaco safer in the process!!  Listening to a record now and all is well.  This hobby sure can be frustrating, but is whole heck of a lot of fun when you get to the positive end result.  Can't wait to have time to eventually upgrade and build another Bottlehead product in the future. 

Thank you again...cheers!!