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Messages - jpcar

Amplifier Discussion / Re: Krell amplifier repair
October 29, 2020, 03:35:06 PM
Ah! Thanks for the help! I was looking in the wrong place.

On the bad channel, the signal is at the base of Q80 but then it's very small at collector of Q80. This is also happening at Q82 (the other polarity). I thought it's unlikely to be both transistors (Q80, 82) failing but I swapped them with the good channel anyway. No difference. The same channel is still bad.

Q79,80,81,82 are all getting + & - 60vdc on their collectors. And Q75,76,77,78 getting + & - 60vdc on their emitters.

Is there something a miss around Q83, 84?
All the voltages look the same on both channels.

Any thoughts..

Amplifier Discussion / Krell amplifier repair
October 27, 2020, 08:35:18 PM

I'm trying to trace through the power amp section of a Krell hifi amp. The right channel is working fine but the left is outputting a very low signal - about 10% of the good channel.
Schematic is attached.
I fed a 500Hz sine 80mVpp into the input with a signal generator. All voltages I've marked are peak to peak.
On the good channel, before R171 reads 80mV and then after is 2V. On the bad channel this point only gains to 120mV.
Can anyone offer advice as to where I should be looking next?


Ah yes, thanks for spotting that.
That's a good point. It doesn't make much sense to see anything there if it's supposed to be ground. I tested some of the ground locations and there's about 1 ohm between the ground side of speaker output terminals and that ground from U8, U9.

I tried retesting with the scope ground reference on the C46 neg side. Now there's about half the amount of noise. It's around 50-70mV pp on the scope. Maybe that's not worth worrying about.
I checked the Digital 9V supply.
There is noise there. It's not really ripple but a very high frequency noise (scope says around 170 KHz). It's around 150mV pp. I've marked on the schematic where this noise is present. It's both sides on R74, one side of L1. Pin 4 of U9. Pin 1 of U7.
Then there's also a spiky looking wave shape around 60KHz and around 600mV pp.
It's present at U7 pin 7,8; Q6 B, E and the other side of C35. I think this is the signal path so it's probably shouldn't be there?

With U8 removed:
Noise is gone and the Spiky wave is gone

With U7 removed:
Noise still there's a different shape at 30KHz.

Should I look to replace U8?
I've replaced U7 (with a new Xvive reissue MN3007) and the hiss remains.
Hmm. What to do next.

Should I tackle Q6, Q7?
Thanks for the advice!
I found a Xvive reissue on Ebay for $12 + shipping so I went with that. Should arrive in a few days.
I'll post how it turns out.

Removing U2 the noise remains, but removing U7 and the noise is gone.

Now I need to find a MN3007 online.

I have a problem with the chorus on this amp. When the chorus is switched on it comes with a white noise type of hiss. The volume controls don't change the noise. Thankfully most of the 8 pin ICs are socketed, so I was able to easily change U3 (TL072). Same problem. I'm wondering if I should change the capacitors next: C52, C47, C48, C49. Might bad capacitors cause noise like this?
Or any other parts I should look out for?

I'm replacing the big filter capacitors (17000uF) on my PS Audio Hifi amp. These are right after the bridge rectifier, one on the + side and one on the - side.

I'm seeing pretty serious ripple coming out the speaker terminals on my scope and when I shake one of the capacitors I can hear it rattle so it's dried out.

There are also two smaller film caps and a resistor in parallel with each of the two larger caps.

Can anyone explain if I am reading these correctly and also what their purpose is?

Pics below:
Yellow cap Westlake 150
= .68uF, 5% tolerance, 100V

Polystyrene Capacitor
= 10000pF = .01uF, J for 5% tolerance, 160V?

And there's a 6.2 KOhm resistor
Yes, this is on a separate board. I have made sure it's getting a ground path from the main board.

I recently built an amp from this design by Dvnator. It's called GA-40 Lite. He says it's inspired by Gibson GA-40. He designs lots of amps on his website. Check him out.

The amp works great! I'm very happy with it.

Now, I'm trying to add a spring reverb into the circuit. I got information from Rob Robinette's website. It uses a TL072 opamp for reverb driver and recovery. The same circuitry as a Fender Hot Rod Deluxe.

The problem is when I turn the volume up I get really loud squealing feedback. With the volume down I can hear the springs in the spring tank, so I guess the recovery part is working. But I still hear it with the reverb pot up or down. The pot doesn't change it.

My feeling was that adjustments need to be made for R40 (220K) which brings to dry signal to the grid of the phase inverter, and R38 (470K) which brings the reverb signal to the grid of the phase inverter.
Since the amp originally had a 100K in this place, I changed R40 to 100K. And I changed R38 to 220K to reflect the same ratio. No change. Problem remains the same.
Both schematics for amp and reverb circuit are attached.

Any ideas anyone?

Amplifier Discussion / Re: 15A5K pot question
November 06, 2019, 11:12:23 PM
That makes sense!
Thanks for the help.
Amplifier Discussion / 15A5K pot question
November 06, 2019, 02:02:29 PM
Can anyone tell me what the '15' refers to on a 15A5K pot?

This is a snap in pot for a Fender Frontman 212R amp.
I understand the 'A' is Logarithmic and '5K' is the resistance.
Amplifier Discussion / Re: Identifying a resistor
December 15, 2018, 11:06:45 PM
Right! That makes sense. Thanks Enzo.
Enjoy that beer.
Amplifier Discussion / Identifying a resistor
December 15, 2018, 09:54:20 PM
Can anyone help identify this resistor?

Pic attached
It appears to be Brown, Black, Silver, Gold, Black.

Maybe the Silver is Gray, in which case a resistor calculator I found online would say that it's a 10.8 Ohm at 1% tolerance. I'm skeptical.
It's from a power supply. And is blown open, as it's reading infinite ohms on the multimeter.

Any other ideas how to read it?
I'm assuming it starts with Brown, since they usually don't start with Black - it being zero.