Classic sixty power amplifiers Maintenance

[roberto]

Hola,
If you are doing resistor measurements in the circuit, at least one side must be disconnected from the circuit. Measurements usually are wrong being into the PC board.

No bass sounds to me is a burned woofer, did you swap the speaker cable at the amp?

[stanleyyu1973]

Hi Roberto,

I have already used spared power amp to test my speakers. Everything work in normal.
I know I should remove the resistor to measure, so when I buy the new one and remove the old one to double check again. Thank you

[AnotherJohnson]

I think these would be the substitute: https://www.ebay.com/itm/20X-LOT-VISHAY ... 04f5e53f59

PS- I think posting in jpg instead of pdf for pictures is better.

So much for the idea of high cost. 5% tolerance and $.15/each. I guess that’s good enough for bias resistors.

[AnotherJohnson]

For those curious about why an in situ measurement will be wrong, it is because the meter is looking at the component AND the attached circuit in parallel. The Ohmmeter itself is also in parallel with the component.

The ohmmeter’s Z is VERY LARGE, so it’s path looks like it’s open compared to the other two paths. But between the other two, their combined Z is different from their individual Zs. If you clip the resistor, and put the meter on, the rest of the circuit is no longer in parallel and you get the correct reading for the resistor.

The OP, Roberto, and probably 90% know this, so this note is for those who don’t.

[roberto]

Hi Roberto,

I have already used spared power amp to test my speakers. Everything work in normal.
I know I should remove the resistor to measure, so when I buy the new one and remove the old one to double check again. Thank you

Understood, then you have a bad cap, not a resistor. Usually a coupling cap is the culprit for no bass. We are just guessing about the problem. I hope you will find the culprit soon...

[stanleyyu1973]

Thanks AnotherJohnson & Roberto ! May I know how can I check the caps ? I also have a cap meter, can I using same method to check ? (Removed form board first)

[admin]

You need to remove it from the board and check the capacitance. Replacement caps are so cheap. If you remove it in the first place I would just replace it.

[AnotherJohnson]

You initially implied that one whole side was out.
Maybe I missed the post about just losing the bass.

I won’t speculate further but will enjoy your report when it works again.

Would observe that

“ Polypropylene and polystyrene capacitors, chosen to achieve a high degree of temporal accuracy, are specified for both the audio circuits and the regulated plate voltage supplies for the input and inverter stages.”

The Classic 60 wouldn’t seem to me to be a good candidate for recapping. The polypropylene and polystyrene caps usually go quite a bit longer. But, YMMV.

[roberto]

Thanks AnotherJohnson & Roberto ! May I know how can I check the caps ? I also have a cap meter, can I using same method to check ? (Removed form board first)

I don't know your meter, the best way is just disconnect one side of the cap and measure it, taking in care the polarity.

Lacking bass is usually a bad cap, other possibilities might be. You should ask the service dept. (service@conradjohnson.com) They will tell you more what to do.

[roberto]

Stanley wrote:..The left channel also have a little bit sound but only have Treble no bass...this implies too that a bad signal tube might be bad too. Change the tube position. The first tube is for left and right channel, then just swap the position. Put it first at the right channel and have a listening. When the right channel goes bad, you can tell is a small tube.

[stanleyyu1973]

Thanks all kindly advise! My amp left channel have a little bit sound have treble no bass and the right channel work a normal. Previously, I only change all power tubes only. But I didn’t change the small. I think try to replace all small tubes first and seeing what happen ! Thanks Roberto & all again

[AnotherJohnson]

Previously, I only change all power tubes only. But I didn’t change the small. I think try to replace all small tubes first and seeing what happen.

Good idea.