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So this is a loaded question I know, but I'm stuck and I don't know where to go from here...

My mom has a super old record player. It's build into a bright orange suitcase which is pretty cool. We pulled it out and cleaned it up, and put a new record on it. Faint music can be heard, but the volume doesn't do anything. I assumed the amplifier wasn't working, and told her it would be easy to fix.

How hard could it be figuring out a circuit made in the 70s???

enter image description here

Well, here is the schematic I've pulled (painfully) from the pcb. Some of the values don't make sense, but it's the best I got. The two left transistors are covered by metal, so I don't know if their PNP or NPN.

I have no idea how this works. It's not like any amplifier I've learned about. I've tried modeling it in EveryCircuit, but it's obviously wrong somehow.

Does this look familiar to anyone? Any help would be greatly appreciated.

EDIT:

Here's a picture of the bottom side of the PCB. enter image description here

The top side is a little bit trickier as you can see. The POT/switch is huge, and it's riveted to cardboard stuff. Here's an attempt. Let me know if you want any more angles.

enter image description here enter image description here

enter image description here

Update:

Man you guys are fast! Looks like I got the transistor pinouts wrong. Figures.

I just checked out the POT, and as far as I can tell it is working... which I assume means one of the transistors is compromised? Thanks for the tips on the flux and solder joints, I'll try cleaning those up. We'll see how well 40 year old solder reflows...

Now that I have the proper schematic, I'll try simulating it later today. For those that already have it in LTspice, do these voltages all look consistent? They were measured with a DVM in DC mode...

enter image description here

Plot twist... If I put my finger on the needle input, I get feedback. Turning the POT does adjust the volume of the feedback!

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  • \$\begingroup\$ I think that we have to assume that your schematic, painful as it may have been, is not an accurate representation of the actual circuit. Can you post good closeups of both sides of the PCB? \$\endgroup\$ – Dave Tweed Mar 23 at 23:28
  • \$\begingroup\$ That's what I was afraid of. Sure thing! \$\endgroup\$ – Tarrinr Mar 23 at 23:46
  • \$\begingroup\$ maybe the needle cartridge is bad \$\endgroup\$ – jsotola Mar 24 at 0:15
  • \$\begingroup\$ Can you post a photo of the top of the PCB that's been mirrored left to right please? \$\endgroup\$ – ThreePhaseEel Mar 24 at 0:38
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    \$\begingroup\$ Wow, TO-92s with heatsinks, that's an unusual sight. Well, even without any analysis of the circuit, here are some basic troubleshooting steps: Are the capacitors still good? Does anything get inordinately hot when it's turned on? \$\endgroup\$ – Hearth Mar 24 at 2:13
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I redrew your circuit with corrections to match the board photos, and it turns out to be a fairly conventional class AB audio power amp.

The pickup is probably a ceramic cartridge which has high impedance, so Q1 and Q2 are arranged in Darlington configuration to give a high input impedance. R1 raises the input impedance even higher, and keeps it high at lower volume settings.

R2 baises Q1 and Q2, with negative feedback to stabilize the DC operating point. R7 applies AC negative feedback to reduce distortion. D1, R4 and R6 bias the complementary output transistors slightly on to reduce crossover distortion.

The volume potentiometer is wired 'backwards' to provide constant DC bias at all volume settings. If the pot track or connections break or short out it could upset the bias, causing the voltage on R6 to go close to the supply voltage or ground rather than about half the supply voltage.

I can see some flux between the pot center terminal and ground, which might be hiding a short circuit. I would clean between any pads that don't have a clear open area between them. Some solder joints look partially cracked, so I would re-solder all of them.

If the DC bias point is correct and the output capacitor is good then you might just have a bad cartridge. To prove this, disconnect the input and inject a test signal (eg. mains hum from your finger) to see if the amp is working.

enter image description here

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  • \$\begingroup\$ Great job Bruce! I just disagree with your input impedance analysis. True there's a Darlington stage but shunt voltage feedback by what you call R7 turns that summing node pretty much into a virtual ground. Input impedance is then ruled by R1 and volume pot. \$\endgroup\$ – carloc Mar 24 at 10:45
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    \$\begingroup\$ @carloc you are partially right. I simulated the circuit in LTSpice and input impedance at Q1 Base was 9k. With Q1 removed and R2 reduced to 220k to maintain bias the input impedance at Q2 Base was 450 Ohms. With the Darlington configuration the overall voltage gain (including loss through R1) was 2.2, With a single transistor it was only 0.18, 12 times less! To get the same gain R1 had to be reduced from 330k to 33k, making the amp's input impedance 10 times lower. \$\endgroup\$ – Bruce Abbott Mar 24 at 12:57
  • \$\begingroup\$ Oh, again good in depth analysis. Of course the Darlington stage is indeed needed to have enough loop gain. \$\endgroup\$ – carloc Mar 24 at 13:05
  • \$\begingroup\$ Thanks for your awesome help. Looks like I'll be buying another cartridge. Is there any way I could get your LTspice file? \$\endgroup\$ – Tarrinr Mar 24 at 16:00
  • \$\begingroup\$ LTSpice file \$\endgroup\$ – Bruce Abbott Mar 25 at 1:22

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