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How to connect probes to cap with Blue ESR

wsscott
Tue, 25 Aug 2020 19:09:35 GMT

I purchased a Blue ESR Tester several months ago, and I now have a situation where I need to test some electrolytic caps in circuit in an old tube guitar amp. Some of them are individual caps, and a couple are in cans. My question is how to connect the probes. Do I need to connect each probe to each lead on the cap, or connect 1 lead to chassis ground, and the other lead to the + lead of the cap? And how do I test the can caps? I did a test and the results seem really good for caps in a 1964 tube amp—ie. too good to be true. For example, one of the can caps is a 20-10-10 at 450V and the ESR readings are 0.80-0.98-2.4. All well within range if I'm reading the chart correctly and have the probes attached correctly. I’ve also got a single 20uf at 500V that reads 0.95 and a 47uf at 160V that reads 0.57. I just want to be sure I’m testing this properly and that these readings show that the caps are good. Thanks for your help.

Bob Parker
Wed, 26 Aug 2020 04:55:13 GMT

Hi from Down Under. It's not usually old age which kills electrolytic caps, but high temperatures and high ripple currents. Back in the 1960s guitar amp capacitors were very well made and not under much stress. I wouldn't be surprised by the readings you're seeing. For the can caps, you'd connect one meter lead to chassis and check each cap, as you said (just make sure they aren't still charged). If you're not hearing excessive hum in the speaker and if the amp sounds OK, there's no reason to worry that the caps are on the way out. 👍

wsscott
Wed, 26 Aug 2020 16:51:47 GMT

The symptoms I'm starting to have with the Gretsch amp is that when I have the guitar plugged into Input 1 it has the hum, which goes away when I unplug the guitar or turn the volume down/off. The volume control on the guitar has no effect on the hum from the amp. With the guitar unplugged, I hear a higher pitched tone or hum coming from the amp. When I plug the guitar in, I still here the higher tone, but then I also hear a low hum so I'm getting both types of noise when the guitar is plugged in, and the level of the noise varies with the Volume control on the amp and will disappear when the volume is totally down to 0 level. I hear it from both of the Channel 1 inputs, but a bit lower on the Channel 1 "dual" jack (I'm still using the mono plug). Same effect on the Channel 2 Bass channel, but a bit lower in volume. Ideas on where to look?

Bob Parker
Thu, 27 Aug 2020 02:57:10 GMT

Hi again. Thanks for the detailed description. Can you tell me which model amp it is so I can hopefully find its schematic on the web? Also, is there any way you could record the sounds you're hearing? If you could do that, we can get into e-mail contact and you could send the sound file(s) to me. It's years since I did any repairs on guitar amps and you've got me interested. 😉