In this preamp circuit, Q24 is after the preamp Q23 to act as an inverter with 1:1 gain, per the service manual. Q24 Base DC voltage at idle current is noted to be about +20vDC, the collector is noted to be +15vDC. If base voltage is being picked from the Collector how is higher than collector?
How is the base current larger than the collector current in this setup?
DC Voltage Debelopment Base to Collector Resistor
Correction to language, Base “voltage” higher than Collector voltage, not current. If the feedback resistor is the source of the voltage, and the 2N3391 BJT is setup as an inverter not a gain config?
@onedamnote The way I see it is the following: Considering the “given diagram” Q24 is clearly set to work as an inverting amplifier with 1:1 gain (meaning no gain at all). in such a configuration the amplification factor of Q24 (HFe) becomes irrelevant, simply because the gain is set by the “mathematical division” of the resistors network formed by R93 and R94. In this case R94/R93 = 4700 Ohms/4700 Ohms = 1. The roll of R91 (1M Ohm) here is to bias the base of Q24 via the “induced negative feedback” and yes indeed the base is getting its voltage/current via the R93 (1M) resistor.
Now regarding the Voltage @ the base in relation to Ground ? Well… in theory! (if you treat the circuit as a stand alone circuit) it should be lower than the (15V) collector voltage. Which is why it could well be a “typo” in the service manual
Now, saying the above, a question strikes my mind: Are you asking just as a “theoretical curiosity” or do you actually have the unit at hand and try to repair it ? In other words, have you actually measured those voltages (in situ) to see is they really are within the same “ballpark” specs/voltages as suggested by the manual ? I would also look into thoroughly testing the values of the (3) resistors to see if they really “match” the diagram and are still withing the acceptable tolerance. And…it goes without saying…would definitely verify/test Q24.
PS1: I would definitely also test C55(1uF) just in case is “somehow” shorted or “extremely leaky” and therefore very High ESR in which case could conduct some amount of DC voltage (backwards) from the next stage (base of Q26 that has 31.4V of DC voltage there) to the collector of Q24. Just a…thought.
PS2: It would be nice to hear what other people have to say regarding the raised issue. I’m all “eyes and ears”. 😎
@and yes indeed the base is getting its voltage/current via the R93 (1M) resistor.
PS: Just noticed that I’ve made a little writing mistake saying the (above quoted) on my very first post but unfortunately couldn’t edit my own post since the 3-4 minutes edit time “threshold” have long past. I should have said R91(1M) resistor instead of R93.
@ovi4 This is from an amp I am restoring. While the question was mostly for theory, I will be getting back into and probing all of these test points out. Limited time getting to the bench at the moment, so it’s schematic reading and working on understanding the sections as best I can.
Having found several type-os in the service manual, that was a guess I had too, as I’ve not yet probed out those DC voltages.
This is one of the Power Amp Boards recently updated with new parts. I can hunt and peck out bad parts but my theory understanding is limted at the moment.
In the picture I see some resistors that look to be of “Carbon Style” (EI: Allen Bradly style) I would definitely check those around the discussed circuit. If the amp. was kept in the loft or a “humid” environment for a long time chances are that they shift in value (sometime by a huge margin!)
Also have noticed that there is no silk screen on the board (as guidance) for the polarity of those small electrolytic capacitors. I would double check them just in case they have been mistakenly fitted in reverse. Good luck.
PS: around the final Power Transistors I see that the PCB has been badly burned (actually looks to be carbonized “literally”) Now, I can say that this is already a RED flag. In such case the PCB itself can become conductive !!!. Is always best to verify the burned area between the tracks with a High impedance ohm-meter (usually digital meters have high impedance) Use the highest sensitivity scale (in the order of Megohms) If you get some reding it means that the board is conductive and in some extreme cases the only way to fix this is either to go to the extreme and make a brand new PCB or make small sliced cuts so to clear the burned PCB away (using a “Dremel” tool or similar)
@ovi4 Yes!
Here is what I did with the back side of the really bad spot. Might not be the best fix, It did surivie a 12 hour none stop test thus far.
Before cleaning the flux.
Oh, that is one really badly affected PCB. Huh….. 🤔
@ovi4 C55 isn’t the one I’d be looking at, it’s C53, as according to the specs it has a higher positive voltage on the negative side of the cap (which is apparently an electrolytic) – can’t believe that’s right as written.
Another odd thing, it looks like the ‘+’ notation on all the electrolytic caps is put on the curved side of the symbol, which in my experience is always the negative side… hopefully it’s just the image/original not being clear enough and the ‘curve’ is an illusion.
Nope, after zooming in it looks like they’re using the curve to denote the positive side of the electrolytic cap, which is the opposite of pretty much everywhere else I’ve seen it used. Maybe they’re trying to use the curve like the triangle on a diode symbol, to denote the positive-to-negative direction?
And yes, it would be nice if the board software gave us a little more time to edit a post.
Yes, the schematic has the cap symbol backwards, perhaps why they added the “+” symbol. It’s like that throughout the entire schematic, at least it is consistent.
Following some lessons on calculating Collector Feedback biasing. I’ve found Vce=9.8 to match the provided table, assuming the gain to the transistor is 145=beta.
Ic=2.0955mA, Ib=.01445mA, Ie=2.11mA
The board did get smoked at some point (way before I aquired it). After updated the out of tolerance and shorted parts, the unit is holding up nicely.
Q24 Base Reads 10 volts DC, service manual error (there is another error where the 1 and 2 are swapped, (sticky typewriter I am guessing)
- Two more errors I found are (R119 is 15k not 50k and R120 is a 10k VR not 100k).
So, basically multiple typos not just one… Hmm, it looks as if they have literally rushed to put the manual out without thoroughly examining the final printed manual version or, even worse the person who verified it wasn’t a “Teck”
Well, this printed errors are quite often found on manuals (unfortunately) therefore is always wise to first do some thinking and measurements before treating/assuming that the actual circuit is faulty. It has happened to me quite a number of times in the past when I blindly trusted the manual and looked for “imaginary faults”… 🤭…until I finally came to the above conclusion.