Foreplay with C4S - having circuit issue

TheReelMuhcoy · 4051

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Offline TheReelMuhcoy

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on: August 27, 2017, 07:31:34 PM
Hello everyone,
Its been a while for me... I bought this Foreplay probably around 2004, I was fresh out of high school and passionate about music and in love with hi-fi audio; very inexperienced and quite ignorant. I've grown more patient and have a high regard for quality and craftsmanship.

That being said, the Foreplay was the first DIY electronic I had built. After some time, work getting in the way, life, growing family, I was forced to get away from things for some years. Now to today, I am still just as ignorant and inexperienced with electronics and circuitry, but still love hi-fi audio. It was always a dream of mine to buy one of Doc's Paramour or ParaBee(?) kits. I have come to terms that between financial means, and family demands, and the fact that Doc is not producing those kits any longer, I will not be able to spend that kind of money on my dream to have a pair of 2A3, or 300B tubes pumping my Fostex FE167's (also built two sets of enclosure with these, neither will ever leave my possession).

I am now starting to gather parts to build myself a 300B two channel amplifier. In preparation I pulled the Foreplay out of storage, blew the dust off and fired it up. Needless to day some things were not right, and decided to use my ever-growing patience and craftsmanship to clean things up under the hood. Well lets just say I am not quite sure how this thing was running before. How ever it was, it did have a faint hiss and was unbalanced L to R. I fixed the obvious issues and scrutinized every connection, transistor, & resistor. I do not know (or have to looked into yet) how to test a capacitor. Here is what is going on...

Left channel is perfect, My right channel is dead, completely silent, no hum, no hiss, nothing. The tube is very dimly lit, and depending on the tube I have in it the voltage readings change. I had the original pair of GE tubes that came with it and a pair of Amperex 12AU7A's that I can thank my kids for bending the pins on and me breaking when trying to straighten them back out. Here is the hard Data I can provide:
Terminal    | Expected V | Amperex V  | GE V
2              |  164           |    193         |  188.7
7              |  153           |    174.5      |  177.8
10/11/12  |  143           |    162.3      |  167.3
A1            |    47           |    62.2        |  61.7
A3            |   1.7           |    1.9          |  1.9
A4/A5       |    44           |    48.2        |  49.8
A6            |  143           |    161.6      |  166.7
A7            |    47           |    62.1        |  61.9
A8            |    51           |    67.4        |  67.5
A9            |    44           |    48           |  49.6
B1            |    47           |    122         |  59.9
B3            |   1.7           |    1.8          |  1.8
B4/B5       |    44           |    47.9        |  49.8
B6            |  143           |    161         |  166.9
B7            |    47           |    122.9      |  59.9
B8            |    51           |    131.4      |  2.7
B9            |    44           |    47.9        |  49.9


I did not check resistances with the tubes in place but can provide similar info as above if needed. Anything else that would be helpful I'l happily get any thing to get this figured out.

I used the following procedures to check the FET's on the C4S boards:
"
1) Hold the MosFet by the case or the tab but don't touch the metal parts of the test probes with any of the other MosFet's terminals until needed.

2) First, touch the meter positive lead onto the MosFet's 'Gate'.

3) Now move the positive probe to the 'Drain'. You should get a 'low' reading. The MosFet's internal capacitance on the gate has now been charged up by the meter and the device is 'turned-on'.

4) With the meter positive still connected to the drain, touch a finger between source and gate (and drain if you like, it does not matter at this stage). The gate will be discharged through your finger and the meter reading should go high, indicating a non-conductive device.

Such a simple test is not 100% -- but is useful and usually adequate.

When MOSFETS fail they often go short-circuit drain-to-gate. This can put the drain voltage back onto the gate where of course it feeds (via the gate resistors) into the drive circuitry, possibly blowing that section. It will also get to any other paralleled MosFet gates, blowing them also.
So, if the MosFets are deceased, check the drivers as well! This fact is probably the best reason for adding a source-gate zener diode; zeners fail short circuit and a properly connected zener can limit the damage in a failure! You can also add subminiature gate resistors -- which tend to fail open-circuit (like a fuse) under this overload, disconnecting the dud MosFet's gate."

I used the following procedure to check the PNP & NPN transistors on the C4S boards:
"
Step 1: (Base to Emitter) Hook the positive lead from the multimeter to the to the BASE (B) of the transistor. Hook the negative meter lead to the EMITTER (E) of the transistor. For an good NPN transistor, the meter should show a voltage drop between 0.45V and 0.9V. If you are testing PNP transistor, you should see "OL" (Over Limit).

Step 2: (Base to Collector) Keep  the postitive lead on the BASE (B) and place the negative lead to the COLLECTOR (C). For an good NPN transistor, the meter should show a voltage drop between 0.45V and 0.9V. If you are testing PNP transistor, you should see "OL" (Over Limit).

Step 3: (Emitter to Base) Hook the positive lead from the multimeter to the to the EMITTER (E) of the transistor. Hook the negative meter lead to the BASE (B) of the transistor. For an good NPN transistor, you should see "OL" (Over Limit).If you are testing PNP transistor, the meter should show a voltage drop between 0.45V and 0.9V.

Step 4: (Collector to Base) Hook the positive lead from the multimeter to the to the COLLECTOR (C) of the transistor. Hook the negative meter lead to the BASE (B) of the transistor. For an good NPN transistor, you should see "OL" (Over Limit).If you are testing PNP transistor, the meter should show a voltage drop between 0.45V and 0.9V.

Step 5: (Collector to Emitter) Hook the postitive meter lead to the COLLECTOR (C) and the negative meter lead to the EMITTER (E) - A good NPN or PNP transistor will read"OL"/Over Limit on the meter. Swap the leads (Positive to Emitter and Negative to Collector) - Once again, a good NPN or PNP transistor should read "OL". If your bipolar transistor measures contrary to these steps, consider it to be bad.   You may also be able to use the voltage drop to determine which lead is the emitter on an unmarked transistor, as the emitter-base junction typically has a slightly higher voltage drop than the collector-base junction. Remember: This test only verifies that the transistor is not shorted or open, it does not guarantee that the transistor is operating within its designed parameters. It should only be used to help decide if you need "replace" or "move on to the next component". This test works on bipolar transistors only - you need to use a different method for testing FETs.
"

Considering I probably have not posted since the antiquated forum 12 years ago, I hope I have posted this in the correct section of the forum.

My unofficial signature:
Jon Yochum (muhcoY)
Construction Super & carpenter by trade
Diesel Performance junkie by adrenal necessity - YouTube video: quCD90SB5gA
Bottlehead by the need to relax and meditate

Jon Yochum (muhcoY)
Construction Super & carpenter by trade
Diesel performance junkie by adrenal necessity - YouTube video: quCD90SB5gA
Bottlehead by the need to relax and meditate after a long day.


Offline Paul Birkeland

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Reply #1 on: August 28, 2017, 05:28:54 AM
A1            |    47           |    62.2        |  61.7
A3            |   1.7           |    1.9          |  1.9
A4/A5       |    44           |    48.2        |  49.8
A6            |  143           |    161.6      |  166.7
A7            |    47           |    62.1        |  61.9
A8            |    51           |    67.4        |  67.5
A9            |    44           |    48           |  49.6
This is a decent set of working voltages.


B1            |    47           |    122         |  59.9
B3            |   1.7           |    1.8          |  1.8
B4/B5       |    44           |    47.9        |  49.8
The differences in B1 here tell some of the story.  I would be looking at the connections on board labeled "VA" that feeds pin 1 on the B side.  Despite the fact that the second tube shows perfect voltages, there's still likely an issue with the build that is coming up here.

If the glow differs severely between sockets, the green twisted pair of wires that connects between A4/5/9 and B4/5/9 is responsible for heating up the tubes.  If one of these connections is loose, you would get low emission.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline TheReelMuhcoy

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Reply #2 on: August 28, 2017, 06:10:47 AM
Why would there be such a difference in voltage from one tube to another?

I was thinking about removing the C4S boards and putting the original resistors back in to see if it would fix the issue, but I would prefer keep the C4S in and repair the issue. If one of the LED's was originally wired in backwards would that have damaged anything on the C4S circuits?

I also wonder with the test I did on the transistor's and FET's, either I did them incorrectly or there is something wrong with them that is not detectable through those tests.


Jon Yochum (muhcoY)
Construction Super & carpenter by trade
Diesel performance junkie by adrenal necessity - YouTube video: quCD90SB5gA
Bottlehead by the need to relax and meditate after a long day.


Offline Doc B.

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Reply #3 on: August 28, 2017, 06:44:35 AM
Where have you installed FETs in a Foreplay? The C4S board only uses PNP and NPN bipolar transistors.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.


Offline TheReelMuhcoy

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Reply #4 on: August 28, 2017, 07:13:28 AM
Where have you installed FETs in a Foreplay? The C4S board only uses PNP and NPN bipolar transistors.

I am calling it the wrong thing. It looks like a FET, but it is just a transistor I did not realize there was a difference (excuse my ignorance).  See attached photo.


Post thought: this makes me realize I tested those transistors wrong using. I should retest using the other procedure and see what I come up with.
« Last Edit: August 28, 2017, 07:29:25 AM by TheReelMuhcoy »

Jon Yochum (muhcoY)
Construction Super & carpenter by trade
Diesel performance junkie by adrenal necessity - YouTube video: quCD90SB5gA
Bottlehead by the need to relax and meditate after a long day.


Offline Paul Birkeland

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Reply #5 on: August 28, 2017, 07:44:05 AM
Why would there be such a difference in voltage from one tube to another?
How many times have you repeated the test?  It could be a loose connection that is bend just the wrong way with one tube, but not the other.
I was thinking about removing the C4S boards and putting the original resistors back in to see if it would fix the issue, but I would prefer keep the C4S in and repair the issue. If one of the LED's was originally wired in backwards would that have damaged anything on the C4S circuits?
You have correct voltages on both sides with one tube in particular, at least that's my conclusion, so I'm not so sure why you believe that there is an issue with the C4S boards.

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man


Offline Doc B.

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Reply #6 on: August 28, 2017, 08:10:48 AM
You say the Amperex is broken. So for now I'm going to bypass that data. Let's look at the GE readings. The only number that is way out is B8. Could you double check that reading? It should be in the low 60s.

Dan "Doc B." Schmalle
President For Life
Bottlehead Corp.


Offline TheReelMuhcoy

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Reply #7 on: August 29, 2017, 03:49:12 AM
I apologize I was not very clear.
To simplify reference of channel to data I will call them channel A (left) and channel B (right).
Channel A is functioning perfectly.
Channel B is dead silent (with both GE and Amperex tubes)

You say the Amperex is broken.
I had a pair of the Amperex tubes, only of of them broke. The other I was able to straighten the pins out and use to obtain the readings in the "Amperex V" column. That was the only thing that changed between the two different readings of B8.

Could you double check that reading? It should be in the low 60s.
I have rechecked all numbers probably 10+ times. and also checked it from different points to try to rule out a bad solder joint. Each time I came out with pretty much the exact same numbers.

I'm not so sure why you believe that there is an issue with the C4S boards.
I am not certain of it being an issue with the C4s boards. I suspected it because I believe I had a LED in backwards, I was not sure if that could have damaged something else on the board. The one of the CF board's LED's were not lighting up at all. Now the one side takes a long time for he LED's to illuminate, and if memory serves me right they are dimly lit now.

All of my P2P wiring was with magnet wire. I had read something back then that it was better than standard solid wire. I have been mostly concerned about making sure that I had the ends cleaned well enough and have a solid connection. I have been testing the voltages at the solder joint, and at a non-soldered point of the terminal. If nothing else makes sense to be wrong then I will likely rewire it so I can neaten up my 12 year old wiring slop job.

Something else to note, when I built the unit, I added a second set of outputs using a 3.3 uF 630V ClarityCap. I dont think this would have anything to do with my issue but may be worth noting.

Jon Yochum (muhcoY)
Construction Super & carpenter by trade
Diesel performance junkie by adrenal necessity - YouTube video: quCD90SB5gA
Bottlehead by the need to relax and meditate after a long day.


Offline TheReelMuhcoy

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Reply #8 on: August 30, 2017, 08:48:10 AM
It's the tubes!! I was too focused on a circuit issue I didn't think to swap the tubes from channel to channel. For some reason the amperex and the original GE tubes were both affected when I first fired it up and had an issue I automatically just swapped out the tubes to see if that was the issue it blew out the both of them.

There is a local shop here that has two amperex 12au7 tubes in stock for $9.99 each!! I am excited to have this Foreplay up and running again!

Jon Yochum (muhcoY)
Construction Super & carpenter by trade
Diesel performance junkie by adrenal necessity - YouTube video: quCD90SB5gA
Bottlehead by the need to relax and meditate after a long day.


Offline TheReelMuhcoy

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Reply #9 on: August 31, 2017, 07:58:17 AM
Well I take that back... It just ate another tube. I had it on for a few hours yesterday some TV for the kids, some music for dinner, etc. At the end of the night i noticed the output on the left channel was way down, and the right channel is thumping away just fine. I swapped the tubes momentarily and sure enough it followed.

what on earth could be wrong that it is killing these tubes so quickly???

I suppose that I should pull out and freshen up my Eico tube tester also.

Jon Yochum (muhcoY)
Construction Super & carpenter by trade
Diesel performance junkie by adrenal necessity - YouTube video: quCD90SB5gA
Bottlehead by the need to relax and meditate after a long day.


Offline Paul Birkeland

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Reply #10 on: August 31, 2017, 04:51:01 PM
Be sure you're actually eating tubes and that it's not something like a loose socket connection or flaky solder joint that's making contact when you roll tubes.

-PB

Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man