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Topics - jpcar


Can anyone suggest what I should do next?

There is -42 vdc on the output of this amp (GK 200MB bass amp). See schematic below

I have tested the transistors with a diode test (Q21, 20, 19, 18, 13, 12, D20) and I can't find a short anywhere.
The +/-15v and +/-12v rails are all working.
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?


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

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 / 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 / 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.

I'm having problem with a Farfisa Mini Compact that is not producing sound. It lights up!

Doing a few tests I'm finding that the base of the transistor (input signal) has 7.3vdc on it. That's Y363 (First transistor from the left on the attached schematic). And then the base of the next transistor in line (SFT337) has 7.7vdc and the next along (Ac128) has 8.3vdc. The point '3' continues on to the right to a headphone output (with 8.3vdc). And the point '5' goes through a 50K pot and then to a line output.
I have tried disconnected point '2' and the 7.3vdc remains.
I have tested all of the resistors and capacitors on this small board and they all seem fine. I also tested the three transistors and they measure approx 1.50V in the correct direction and nothing in the opposite direction.

Any ideas what I should do next?
Amplifier Discussion / Identifying a transistor
July 25, 2017, 05:11:35 PM
Can someone help me identify this transistor?

It's from a Wurlitzer Orbit 3 organ.

It reads 7179 - which in the schematic is 657179, which I have found on a Wurlitzer page, so I'm thinking that's a Wurlitzer part no. It confirms that it's a PNP power transistor.

Next it reads SP8406A. I can't anything to do with transistors when googling this number.

And next it reads 7346. Is this an age code?

I've attached a pic of the transistor and a schematic of where it's from.

Please help!

I would love to find the specs of this transistor so if I can't find one I can get an equivalent.

Amplifier Discussion / Polytone Mini Brute II fix up
October 25, 2016, 07:46:53 PM
I'm looking to fix up a Polytone Mini Brute II amplifier.

There are a few issues I'm dealing with..

Firstly, the preamp is quite noisy. This is hiss that is present regardless of the volume being up or down and regardless of anything plugged into the input. I'm wondering if this is an inherent problem with this amp or is this something I can do to improve it. I have another Polytone Mini Brute (later model) which is quiet as a mouse.
I tested the preamp from the noisy Brute into the power amp of the other Brute and it's noisy. Then I tested the preamp from the other Brute into the power amp of the noisy Brute and it's quiet. Hence my conclusion is that the preamp is to blame.

Second problem is the reverb is very weak. With the reverb control up full the reverb is just audible but not particularly strong. At half way you can barely hear the reverb. Again this might be an inherent problem with the amp. I recently replaced the old reverb tank (which had a snapped spring) with an Accutronics 8DB2C1C. This was a recommended replacement. Input impedance of 310 ohms and output impedance of 2575 ohms.

There's a trick I've seen with other amps to get more reverb, you can take the input for the reverb driver from before the Gain pot so that it gets maximum gain to the reverb tank all the time, making the reverb louder. In this case that would mean disconnecting R23 from the wiper of R72 (Gain pot) and reconnecting it to the side that's not grounded. (Schematic attached)

Has anyone tried this? Should I give it a go?

Another question is... can anyone tell me about OP1, OP2, OP3 etc.. in the schematic? What are they and what's their purpose. They are covered in black rubber on the circuit board. Are they little current limiters perhaps?

Thanks for any help!
Hi, I'm looking for some help repairing an amp!
I'm having problems with the power amp of an Acoustic model 120. The pre amp is working fine.
The amp powers up with quite a loud thud from the speaker. Also a thud on power down.
But then there is no signal from the speaker. I measured 65mV DC from the speaker terminals. I have checked all of the transistors and they seem fine. I also checked the electrolytic caps and they seem fine. I measured +44 VDC and -44 VDC coming from the bridge rectifier. And the same +/- 44 V on the collectors of the output transistors which are MJ 15024s.

When switching the power off, I hear the signal for a split second and a thud.

Any advice on what to do next??


Here's a link to a schematic of the amp.