Hoffman Amplifiers Tube Amplifier Forum
Other Stuff => Solid State => Topic started by: punkykatt on February 08, 2013, 03:22:48 pm
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Hey Guys, Have a Crate KBA-60 amp on the bench. Everything on the amp works good except the reverb. Using my listenig device probe I get reverb singal all the way to the reverb return pot wiper to R37 (68k). on the down stream side of R37 no signal. I lifted the down stream leg to check the resistor, resistor is good, and now with the resistor lifted I get reverb signal at that end of the resistor. When I reconnect the resistor to the board the reverb signal is gone???? Please have a look at the schematic and if you have any suggestions why the reverb is not working it would be very much appreciated. Thanks in advance. Punky
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IC3a is a virtual ground. The signal at its - pin will be essentially zero.
Since all the dry signals also enter this virtual ground, and apparenly work, it is not clear what's wrong. Shorted footswitch jack and cracked PCB/ bad joint/ cracked resistor are stabs in the dark.
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PRR, thanks for the reply, time to get out the magnifying glass.
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Reheated all solder joints, all pcb traces look in tack and continunity tests good, foot sw jack test good, all resistors a caps test good in the reverb circuit. I found that I can get the reverb to work if I short out R30 (22 ohm)(resister is good) to ground. Its connected to the RCA jack cable shield on the reverb tank input, also pin 6 of IC4B, and R31. Kind of ratty sounding reverb, but hey its S/S (lol). The tank looks to be original 1BB2C1A 150 ohms input with a DC resistance of 26 ohms. In place of R30 22 ohm I tried a 10 ohm (reverb very weak) and a 1 ohm (reverb was much stronger) The owner bought the amp used and the reverb never worked. Why does the reverb work when I reduce R30? Thanks for your time and help. Punky
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> reverb never worked
Check the actual value of R31.
The gain of IC4B is R31/R30 (and some other stuff). A way-low value of R30 ought to be huge output. You seem to need R30 like 1/10th of proper value. I wonder if R31 somehow got stuffed Blu-Gry-BROWN instead of Blu-Gry-RED.
Or some solder-splash in that area or at the jack, or cable mashed against metal, or....
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R31, schematic calls for 680 ohm. What is in there is a 560 ohm (green, blue, brown)? Ohm meter test confirmed that. Maybe Crate screwed up the schmatic too? I have found some other resistors in another part of the amp to be off from what is on the schematic but not by much. 20%. I will tack in a 6.8k in R31 and the 22 ohm in R30 tomorrow and see what happens?
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> calls for 680 ohm. What is in there is a 560 ohm
Same thing, near enuff.
What about snot or short on the tank input wire, connector, whatever?
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The tank input wires test good, I have a couple of BB and DB input/output tanks I tried also. All work the same. The RCA cables are about 12" long test good and have no shorts.
When I first started checking out the amp with my listening probe, I got a good strong dry signal to the tank input and a much weaker signal with reverb at the reverb return pot wiper. I did replace IC1A with no change. All resistors and caps connected to IC1A tested good. Is the difference between the dry signal input and the wet output signal normal? or should they be about the same?
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> Is the difference between the dry signal input and the wet output signal normal?
Tank is VERY lossy. It amounts to a speaker and a microphone, both very lossy gizmos. On a 8r:5K Fender tank I'd expect the output to be 1/500 to 1/1000 of the input. A bit less with 150r input, but still 100:1 loss. VERY variable with frequency.
There's numbers on that plan. "Test Point Data". It's real smeared, but it looks like 125mV at TP5 (reverb drive) and ~~10mV at TP6 (after recovery amp). Recovery amp gain is 7.8, so more like 1.2mV at tank output. 100:1 loss.
You could try setting up just like the test conditions, feeding a turned-down guitar at the input, and comparing relative levels. 10mV at guitar should be 15 times bigger at tank input, 8 times smaller at tank output, and similar at recovery amp output/ reverb pot top.
This might really be better with a controllable signal generator and an audio milli-Volt meter. You want a few mid-range frequencies (the tank response is very peaky) and controllable down to 10mV. Most modern DMMs will read AC mV in the 10mV area, at least for 100Hz-1KHz.
> replace IC1A with no change
IC4 is more likely. However a chip failure is VERY unlikely. Bad joints and contacts out-number bad chips many-to-one.
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IC4 is more likely. Dang, this S/S stuff is confusing????!!!! :BangHead: Looking at the schematic it looks like IC4 is supplying gain to the tank input? Plus I got a strong probe signal there. How does the signal flow work on these amps?
I don`t have a scope or signal generator. I do have an antique Korg WT-10A guitar tuner that will produce sound through its output jack in three octives and I can vary the pitch between 430 to 460Hz. Can I use that as a signal generator?
PRR: thank you again for your time and help with this project.