This is a photo of P701 underside. The two heat sinks are straight (walls) on either side, transistor legs bent 90 deg to sit flat against sinks. The three small trans' in the middle use thermal grease, while the power trans' use thermal pads
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Pcb sits about in the middle of heat sinks. Transistors bolted directly to heat sinks.
I want to finish up re soldering but it will take a few days
Thanks. I tried the Canadian place I linked. Fingers crossed. Ordered new ceramic emitter resistors as well
A small update; Finished soldering board P701. Got new power transistors in the mail, still waiting for emitter resistors. Built and tested bulb limiter.
I was wondering about the test mentioned with only one pair of power transistors, any other measurements I should do before and during?
I was wondering about the test mentioned with only one pair of power transistors, any other measurements I should do before and during?
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Also I would very much appreciate a elaboration about this trick, what to measure and how to conclude when to remove it
Thanks in advance!
The amp has a double pair of output transistors in each channel which are in parallel so you only need one pair in each channel for power up testing without speakers .
Before power up turn the bias setting pots back to minimum then If you have reinstalled RN05 and 06 and the amp comes out of protection on power up that would be a good indication that it is mostly ok . I would then check the DC output offset voltage and various voltages over the amp just for reassurance . Then tap over the various boards with an old plastic tooth brush handle to check for any further bad joints ( vibration testing ) . All of this with the bulb limiter in use . You could then install all the output transistors and try again . The choice is obviously yours .
I personally would not use the "yellow" straps as they collapse the biasing of all the output devices and this is the area you are trying to prove is now ok . I think the bulb limiting should be sufficient .
PS you have done a neat job on the bulb limiter , I like it .
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Thanks! Fun little project, witch I should have done earlier.... Oh well, live and learn.
Ok, I will desolder the power transistor pairs that is now sitting on P701 and do as you suggest. RN05 and 06 is back on. While waiting for emitter resistors I guess I can have a closer look at solder joints on all the other boards properly
Thanks for taking your time explaining, I really want to learn in the process and not just make haste to get the amp done. Was wondering if my understanding of Output DC Voltage is correct, as marked in red in picture
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Update; The DC offset is off the charts - in the twenty volt region. I've measured as far back as R755 and get around 18V. Same value both channnels both +VE and -VE
So obviously the amp does not come out of protection. 60W bulb starts bright but dims to a quite faint glow
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Some more observations:
- Rail voltages are low (33V + and -)
- DC offset on outputs are negative, same on both ch
- When using voltmeter between 0V and C701 or C702, voltages drop suddenly - bulb glows bright
Was this not mentioned early in the thread from someone, about some of those caps might leak DC? Edit; John Snell mentioned C711 and C712
- Rail voltages are low (33V + and -)
- DC offset on outputs are negative, same on both ch
- When using voltmeter between 0V and C701 or C702, voltages drop suddenly - bulb glows bright
Was this not mentioned early in the thread from someone, about some of those caps might leak DC? Edit; John Snell mentioned C711 and C712
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R706 and R705 has stable and good ground connection, both resistors have correct value as well.
J704 though, has no ground connection. Or it had some intermittent around 60kohm and slowly rising, like a charging cap
Correct me if I'm wrong, but is there supposed to be a ground connection on pin 2 J704? This comes from main transformer CT (Center Tap I presume). Connection is there to J705 pin 2, witch is called GND, and comes also from CT
BTW; I see there is differences in schematic for PM80 MKII and MKII SE in where GND and CT are on board P851
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