FM-3 tuner volume drop and hum

Hi everyone...I am working on getting my recently acquired Dynaco FM-3 tuner up and running and have encountered an issue which I am not sure is common with these or not...
I have replaced the electrolytics in the power supply and the tubes test good. When I turn it on it works fine for a minute or two then the AC power transformer hum level increases, the B+ drops a bit and the output audio has a low frequency 60 or 120 Hz hum present.

I have not replaced any other caps except for C31, C82 and C83.

The tuning eye tube is not installed ( it has this problem with or without it installed ).

I am suspecting a possible short in one of the tubes and will pull them one by one as a first step. Any other suggestions?

PH

VE6ZRT wrote:Hi everyone...I am working on getting my recently acquired Dynaco FM-3 tuner up and running and have encountered an issue which I am not sure is common  with these or not...
I have replaced the electrolytics in the power supply and the tubes test good.  When I turn it on it works fine for a minute or two then the AC power transformer hum level increases, the B+ drops a bit and the output audio has a low frequency 60 or 120 Hz  hum present.

I have not replaced any other caps except for C31, C82 and C83.

The tuning eye tube is not installed ( it has this problem with or without it installed ).

I am suspecting a possible short in one of the tubes and will pull them one by one as a first step.  Any other suggestions?

PH

The symptoms do seem to suggest that your problem is heat-related, or at least related to the ramp-up of B+.

a) Do you have an ammeter - and can you install it to see what happens with B+ ramp-up? The key is whether you are drawing more than you should and/or there is a B+ ramp-up, then after a few moments another increase in current draw concurrent to the onset of hum and B+ reduction.

b) You state that the tubes test good. On what, and does it test for shorts and gas? If so, you may need to leave the tube on the tester with the filament hot to see if there is a temperature-related short that does not manifest immediately. One of the two 12AX7s come immediately to mind as the primary suspects.

c) How and with what did you replace the filter caps? These are very fussy in placement - location matters. It can also be helpful for hum to bridge each cap with a 0.01uF film cap. The caps should be *above* the chassis, for best shielding.

Let us know.

I have a unit that can measure AC input voltage and power and current so I will use that to see what happens when the hum starts. I will monitor the B+ at the same time. I have a Heathkit tube tester which does test for shorts and I will leave the tubes in the tester for a longer period of time to see if anything shows up. I will bridge each filter capacitor with a .01uF as you suggested going forward but I am thinking I have a bad tube somewhere since this is a time related issue...it works great for the first few minutes.
PH

http://www.curcioaudio.com/Dyna%20FM-3%20Mnl.pdf

Here is a link to the manual. Page 20 gives pin voltages for all the tubes. This *might* help if you do not have it already.

But I tend to agree that you have a heat-related issue, likely a bad tube.

VE6ZRT wrote:Hi everyone...I am working on getting my recently acquired Dynaco FM-3 tuner up and running and have encountered an issue which I am not sure is common  with these or not...
I have replaced the electrolytics in the power supply and the tubes test good.  When I turn it on it works fine for a minute or two then the AC power transformer hum level increases, the B+ drops a bit and the output audio has a low frequency 60 or 120 Hz  hum present.

I have not replaced any other caps except for C31, C82 and C83.

The tuning eye tube is not installed ( it has this problem with or without it installed ).

I am suspecting a possible short in one of the tubes and will pull them one by one as a first step.  Any other suggestions?

PH

was it working well before you did any changes ?

Update: ( Some background info on this tuner...I recently purchased this unit and do not know the history of it or if it ever had any issues...so this is the first time for me trying to use it)
Seems I had a problem with the original 6V4 that was causing the B+ to drop after a while so that was a red herring with respect to this issue...all other tubes test fine and have no shorts. I measured the output at the RCA output jacks and found that there was almost 200mVpp of 60Hz...I have read elsewhere that these units have a grounding system that is not that good...so I connected individual ground wires from the RCA output jack to the center pin of V72 12AX7 tube and that helped a bit...then I started to reflow various connections on the various ground eyelets and that helped as well...I finally added a solder braid ground strap from eyelet 73 directly to the chassis...now the 60Hz level on the RCA output jack is zero...so I am happy now...

Thank you peter W and peterh for your suggestions and comments...

Peter H