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HP 204C. They show up frequently on EBAY. Cheap, low distortion, reliable (had a 1975 electrolyitic cap fail a couple years ago). I own a couple. love them.

It's no secret that I'm a big fan of O'scopes. There are USB scopes. I bought the USB OWON 1021 off Amazon a few months ago to use in a class room. You have to be especially careful with ground connections and ground loops with USB scopes. I use a 40 year old Hitachi analog scope on a...

What's the purpose of all of the paralleled film caps? It is preferable that the common connection of the film caps be isolated from the chassis ground and connect directly to the ground bus bar. Likely won't make a difference, but that is best practice.

What was the test criteria for checking the TO3’s to ground? Resistance readings will only find gross errors. A hipot would require all 3 pins of all of the transistors tied together with a minimum test voltage of about 500V. That’s not very practical.

The 400 manual has a good troubleshooting section. It's attached here. The section on testing outputs is very helpful. A broken lead on the bias transistor will give the same symptoms of blown outputs.

I have the switch and also a solidstate switch upgrade so that the switch never fails again. Need to confirm which switch you have. Please post a photo.

I see you're using scope probes for the readings. Are the probes calibrated? There will be a test point on the scope to calibrate the probe for a proper square wave.

The 400 relay board fits in the Carver amp, sort of. It has to be built with the parts on the backside. It’s important to understand that the C 500 is built with one channel inverted. The black binding post on the inverted channels is actually the output of that channel. So the signal is...

You disable the V/I limiters by lifting the dides on the protection transistors. That said, I don’t recommend doing it.

I recently fixed a B&K function generator with shorted tantalum caps. If tantalums are more than 15 years old, seems as though there’s a good chance they shorted.

Photo # 3 above is taken with a Tektronix TEKSCOPE field oscilloscope. I have 2 that were replaced with Fluke Scopemeters. It's a shame the Teks are discontinued. Isolated inputs, 100Mhz, battery powered, small, and about half the weight of the Flukes.

There were lots of local bands that racks of P/L amps. 3 for each side of the stage was very common. Most were 400's though.

It's funny you mentioned switching noise. When I looked at the dwg. the first thought I had would be to hang a .1uf cap on the anode of the zener or the regulator output. No justification, but I'm always concerned about oscillations etc.

Consider Firefox. I have no affiliation, other than I've used it as long as I can remember.

The relay boards detect DC on the output and disconnect the speakers. Fast detection is critical. The 700 and 400 uses different circuits to accomplish the same goal. The higher voltages in the 700 are the reason for two different circuits. To incorporate the thermal switches in the loop...

I'm not sure how Joe implemented the thermal switches. I think there is a low probability that they are used on the main DC. Very few contacts are rated for more than 32 volts DC due to arcing with the result of welded contacts. 250 VAC, no problem. The attached PDF is a typical device we...

Looking at the photos, it appears the amp was mounted in a rack, and the matching rack handles were removed. Hopefully you still have them. The good thing is whoever installed the amp in the rack didn’t butcher the front panel. The driver board is in pretty good shape, given its age there’s...

The original input jacks used 3/8" x 1/2" phenolic shoulder washers to isolate the inputs from the chassis.

No, LF351, Not sure the difference. The TO5 can be used, you would have to bend the pins to fit the socket. The failed opamp's output went right to the rail. The board was in pristine condition. Most of this vintage are scorched pretty bad. On Jim's 2 fin 400, I plugged in the AGC fuse...