Bottlehead Forum

Bottlehead Kits => Legacy Kit Products => Foreplay III => Topic started by: R.Mackey on October 24, 2011, 08:28:56 AM

Title: Repairs -- any comments
Post by: R.Mackey on October 24, 2011, 08:28:56 AM
Hi all,

I'm diagnosing a problem with a Foreplay III.  Think I have it figured out, but before I order parts and start cutting wires I could use some independent assessment.

It's a 2006 build with no real modifications and worked well for years.  I recently moved, and it may have taken a hit somewhere in transit.  (My other pre-amp failed at the same time!  It's solid-state, and looks to be a total loss.  But this is a kit.  We can rebuild it.  We have the technology.)

The only symptom is that when I hooked it back together, I got a very loud 60 Hz buzz.  Definitely not a ground loop either.  The buzz increases slowly as the preamp warms up, reaching maximum in about ten seconds.  It's identical on both channels and does not change with input or volume settings.  There's no obvious damage and no release of magic smoke.  Took it back to the bench, all resistance and voltage measurements check, all tubes light, all LED's light.  It also still passes an input signal, just nearly drowned by the buzz.  In every way it looks 100% except for the buzz.

Since it was on both channels, my first thought was cold solder joints or wiring being knocked loose in the high-voltage supply.  I found three -- one at station 21, which could affect B+ to both 12au7's.  The others were on stations 4 and 9, where the bleed resistor spanning them had come completely loose.  However, fixing these had no effect.

Next I looked at the heater supply, reasoning it was also common to both channels.  Couldn't find anything.  I also swapped and re-seated all the tubes, no change, tube bases and pins look good.

Finally I dug out the oscilloscope.  The buzz on both channels looks suspiciously like an RC charge-discharge curve, amplitude about 0.3V, faster time-constant on the charge leg.  There's no obvious 120 Hz component.  There is a nearly identical waveform across B+.  Just to be sure I pulled both 12au7s and checked again, and it's unchanged.

What I think is going on is that one of the two 220uf voltage doubling capacitors has given up, and it's only charging on half the AC cycle.  Whatever jarred loose the bleed resistor could have broken the cap, I guess.  It looks a little bulged.  I also tested all diodes and they're fine.

Does this make sense?  Or have I overlooked something?
Is there anything downstream that could have been permanently affected?
How closely do the doubling caps need to match?  (I'm guessing not very close, just better than 100% different)
How much ripple should I tolerate on B+ when it's working correctly?

Thanks in advance...  if it will help I can take more data.
Title: Re: Repairs -- any comments
Post by: Doc B. on October 24, 2011, 08:41:00 AM
I think you're right about the vicinity of the problem. Try putting an ohmmeter across each cap and see if the resistance increases to some level and stops, or if it reads shorted. This is not an absolute test because sometimes a cap can read good at the low voltage the meter injects, but short at B+ voltage. We can supply caps that will be exact replacements in terms of capacitance. I'd be inclined to test the rectifiers for shorts with a meter too.
Title: Re: Repairs -- any comments
Post by: R.Mackey on October 24, 2011, 02:51:09 PM
I got the filter caps off and they do look like the issue...  My DMM has a rudimentary "capacitance" feature.  The other 220uF doubling cap and the 47uF filter cap both read as "large uF," but the suspect doubling cap comes back with an almost null reading of 5pF.  Not a dead short.  Looks like it broke/shorted open, or maybe melted together internally somehow.

The rectifiers both test fine, cut-in voltages of 0.438 and 0.431V respectively, reverse bias showing open.

I think I'll replace all three caps while I'm at it.  The other two are probably OK but I'm not sure what happened to the other one.
Title: Re: Repairs -- any comments
Post by: R.Mackey on October 28, 2011, 06:49:52 PM
So, update.  Problem looks solved.

I got replacement caps today and wired 'em back up (both 220 uF and the 47 uF, even though I think only one 220 uF failed).  There's now no discernible 60 Hz component on either output or on B+.  Whatever ripple there is while it's quiescent is below 4 mV AC, which is about where my oscilloscope starts picking up FM radio.

Too late to play some tunes tonight, but it won't be long now.
Title: Re: Repairs -- any comments
Post by: R.Mackey on October 29, 2011, 10:39:46 AM
Music is playing!  That was it.

There's not even a trace of hum remaining.  Right now I'm forced to use an extremely sensitive +28 dB SS amp and 89 dB/W speakers, and even with ears up against the cones there's no hum or buzz, just a tiny bit of pink noise, audible to about a foot when everything else is silent.

My other broken SS gear, still dead...  probably for good. 

So, anybody looking for an excuse to jump into building one of these kits, here's another one:  It can be fixed.  You can "fix" it to suit your musical tastes, or you can really fix it to keep it running.  No reason I can't keep this preamp running for the next fifty years, easy.  Simple is good.  Simple is clean.  Simple works.
Title: Re: Repairs -- any comments
Post by: John Roman on October 29, 2011, 11:42:30 AM
Your troubleshooting process made this an good learning tool. Glad you got it fixed. You should post more often.
Title: Re: Repairs -- any comments
Post by: R.Mackey on October 29, 2011, 03:07:34 PM
Thanks, I think I will...

I have some unfair advantanges -- while I didn't have any classes in tubes, I did study laboratory physics, and I work on diagnostic systems for aerospace.  Nonetheless, if you've got friendly experts around like we do at this place, ASK THEM FIRST! 

While I was in there I put in the -18dB pads for my hair-trigger amp.  Playing "Pet Sounds" now -- boy those drums and horns are nice.  Despite the rest of my cobbled-together setup being decidedly mid-fi, it sounds suprisingly good.  The "fog of sound" you get with SETs and lots of headroom is terrific.

One of these days I'll build some high-efficiency speakers and SET amps, and do it right, I swear!