Continuing problems with C4S

[keithp]

I damaged the 2907 transistor on the "B" side of one C4S board when I was performing the voltage check after assembly (see previous post).

 I received my replacement parts and rebuilt the C4S board. I replaced both 2907 transistors, one of the MJE5731A transistors, all four diodes and a resistor that was out of spec. Resistance readings were normal. When I did the voltage check the tubes glowed as expected but only one of the four new diodes came on. Voltage readings on the B side were:
terminal   specified value     actual value
11                146                        146
15                186                        149 
16                   5                              5.1   
17                 90 - 110                143   
19                   9.85                        9.75

I resoldered the C4S and most of the wiring on the left channel power path. When I powered on the preamp, three of the four diodes were glowing. When I touched terminal 11with my DMM probe to take a voltage reading, there was a slight spark and the two diodes on the "B" side of the C4S went out and the 2907 transistor shorted. Is there an easy explanation why probing terminal 11 shorted out my transistor for the second time? I triple checked that the transistor orientation was correct. What else can I test to help determine which part of the power path wiring on that channel may be causing the low voltage on terminal 15? Is an unlit diode either a symptom or a cause of the voltage irregularities?

[Paul Birkeland]

Terminals 11 and 6 are essentially electrically the same within the circuit.  If probing 11 blows out your C4S board, but probing 6 does not, this would suggest a wiring error in the amplifier.

Your 300B on the offending side still isn't drawing any high voltage current, but is drawing filament current.  I would be looking at the jumpers on the C4S board first thing!

[keithp]

Can I reasonably rule out problems with the power supply wiring ahead of the C4S? I did the voltage check after installing the transformers and everything was perfect. Do I also need to check the output voltage or anything else on the regulator circuit board or on the D tube?

[Paul Birkeland]

Eh, yes and no.  Your voltage at 15 is really low, but I suspect this may be part of the wiring error as well. 

[Paul Joppa]

It could be that the C4S 300B plate load is trying to feed too much current, thus pulling down the power supply and still having the plate essentially at the regulator voltage. Possibly bad solder joint on teh PC board, or perhaps a reversed LED?

I'm just speculating here, haven't reviewed the wiring and circuit in detail yet.

[keithp]

I have checked the C4S board as thoroughly as I know how. All the components are tightly attached and the solder joints all pass a current at least to the next adjacent component, but I still only have one LED lit when I plug it in. I retested the voltages on the C and D tubes. Voltage at C7/D7 is approximately 147 as specified. One other possible clue is that the right channel EL84 glows dimly while the left channel glows brightly. Is it possible that the C4S is damaged in some way that I can't see, i.e. should I rebuild it using all new parts? Should I replace the LEDs that do not light up?

[Paul Birkeland]

Can you check the jumpers on the C4S boards?  Can you verify that the R1 values are in the correct positions?

[keithp]

The jumpers and R1 resistors are in the correct positions. I verified the values with my DMM and visually verified that the board matches the pictures in the assembly directions. I am attaching a picture if that helps, but it is slightly blurry.

[Chris65]

Are you certain those are PN2907a transistors on the board? They look more like the 2N2222a transistors on the SR boards.
Of course they could look different but maybe the same part. Here's a picture of mine for comparison.

[keithp]

The transistors that came with the kit looked like yours (black plastic with one flat side). The replacements they sent from Bottlehead are shiny metal and are marked 2N2907A - they look just like the 2N2222a. Since Bottlehead provided them to me, I figured they were interchangeable. When one of them shorted out during testing I borrowed one from my unassembled Eros kit that is marked 2N2907 (no "A" as a suffix). It is slightly larger than the part with the "A" suffix. Could this be my problem?

I also borrowed one LED from the Eros kit to replace an LED that got mangled during construction. This looks exactly the same to me as the originals. Can it not be interchangeable?

[Chris65]

Ok, that's fine then, just thought to check. Transistors probably just vary a little as BH gets new stock.
As far as I'm aware all the kit LED's are the same.

[fullheadofnothing]

PN2907s and 2N2907s are interchangable, as long as mounted correctly. You should have gotten PN2907s, however. The board shows the part outline, so is the tab on the transistor where the tab on the outline is?

[keithp]

Yes the transistors are mounted so that the tabs line up with the outline printed on the board. Are the 2N2907s interchangeable with the 2N2907As?

[fullheadofnothing]

Yes.

[Paul Birkeland]

You can also try swapping the large C4S boards between channels.  In the swapping process, you may notice an inconsistency, or the other board will work in that channel.

The only voltages you need to measure are IA, OA, and OB.  That will give a pretty complete picture of the operation of the preamp.