Carvin VTR2800 combo

[Prairie Dawg]

Hi folks.

I am working on a Carvin VTR2800 combo amp circa 1976, a guy I know picked it up in a trade. It's really clean, still has the original fat bottle Sylvania 6CA7s and in all respects is a presentable amp.

The reason it came my way is that it has no bass tone on either channel. That's right. It doesn't matter where you place the eq sliders or which channel or both, you aren't going to get any low end thump at all.

I measured the bias on one tube and it was about 13 ma, so that may have something to do with it. I'm planning on getting it back to Carvin specifications or thereabouts.

It's also been suggested to me that the 4016 IC is suspect. It sits between the two channels and I'm told it is prone to failure. You couldn't prove it by me, but that's what I've been told and it's slated to get a new IC and a socket to mount it into.

Looking at the block diagram that's in the owner's manual, there is also a 4136 that is common to both channels, so I'll be looking at that too.

the rest of the ICs other than the 4016 are "war of the worlds" tripods.

Is there anything else that I should be looking at?

Thanks all.

[52 Bill]

Check all electrolytic coupling caps for loss of value. When they dry out they will lose value and allow less and less lower frequencies to pass.

The digital IC's are for channel switching. I doubt that they would cause that symptom, but who knows

[Gtr_tech]

Check the speaker to make sure the cone moves freely. If the spkr coil has been overheated and warped, it'll seize in the magnet gap and you won't get any low end at all.....'cause the cone can't move to reproduce the low freqs.

You could have a bad op amp in the graphic EQ as well. Swap IC1 and IC2 and see if the problem is at the highs instead of the lows. Those 4136 op amps are out of production, but I have a good stock of them if you need one.

[Gtr_tech]

I just noticed in the schemo that the switching IC (4016) is marked "IC1" as well, so be sure not to swap anything with that. Just swap the 2 4136 chips.

http://www.carvinmuseum.com/pdf/amps/VTR-2800.pdf

Also, if this is the 100w version with the half power switch, it takes heater voltage off the cathodes of a pair of output tubes. Not the greatest idea IMO. I'd change that so it removes the cathode connection to gnd for that pair.

[Prairie Dawg]

[QUOTE=52 Bill;162774]Check all electrolytic coupling caps for loss of value. When they dry out they will lose value and allow less and less lower frequencies to pass.

Just as an update, I removed the preamp board and changed all the small electrolytics on it and it brought this old road warrior back to life in a big way.

Working on this amp was relatively simple to do as there is a lot of room to work inside and three postcard sized boards run things for the preamp, equalizer, and power supply boards.