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I've got an 87 (with no version number like this one). It's never been in for cal but still reads consistent with calibrated instruments.

Crowbar the output with a miniature relay powered off of the rectifier.

Variacs do not limit current. A 100 watt incandescent bulb in a dim bnulb test set will limit current to less than 1 amp. Always use the dim bulb test set when powering up any project the first time. I use 25 to 40 watt for small receivers, preamps and signal processors. Loud hum suggests...

You got lucky with the open 2.7k resistors. When they fail the amp usually dumps DC into the speakers. Also the feedback cap and several of the display caps are failure prone. I've got a drop in relay board to protect speakers against amplifier failure. It bolts on to the binding post so no...

All that dust inside says it was in a rack with fans.

1978 bought the Dynaco 120/PAT4 from Sound Values. Later bought the Hafler 101 when Stereo Review raved about it. Then bought DH200. Mid 80's did contract work for Sound Values testing Dynaco 400 boards, warranty repairs, production testing. Could have had one of everything of the tube gear...

George, It's not to late to buy the blank 400 board so you don't have to fool with perf board. The first and third photos show two small electrolytics on the outputs. Their date code is 7251 as in the week before Christmas in 1972. They're over 50 years old now. Who says electrolytics...

Further clarification: All DC protection circuits, every damn one of them, is a compromise. To reproduce sound a speaker pushes air back and forth. The amplifier drives the speaker. The rate that the speaker moves, how many times per second, is the frequency. Bass frequencies are relatively...

George P bought a 400 and 700 board back in February. He also asked for a couple of the chips used in the 400 board and I threw them in N.C. I think I sent him the 400 schematic. I figured he was doing some research. When I developed the 400 board about 15 years ago I didn't intend to do...

Pull the bottom cover and check for cracked solder joints on the mother board. It's anything like a 4000 preamp there will be plenty of them.

Is this a bridge output? The heatsinks look like the MA10,000

If I ever get the site updated, all of the P/L manuals will be there as well as the parts database.

Replace the fuse once. If it blows again stop. Every time after that replacing the fuse does at least $100 more damge each time.

Maybe Harbor Freight for $6.99. Actually Harbor Freight has a true RMS meter. 1000v cat. III input? NFW on that. I bought some $25 meters at Harbor Freight a few years ago specifically for testing SCR's and diodes out of circuit. Someone grabbed one and put it on 480 VAC and melted the case.

I could believe 85 millivolts. Blown rectifier blows AC input fuses. Blown outputs blow fuses and/or has 80VDC on the output. First of all strongly consider wearing safety glasses. There is 160 VDC across the main caps. Get across that and you're dead. Stop and build the dim bulb test...

The service manual lists acceptable devices for the RCA 410 drivers all of which are likely hard to find. They don't fail very often. When knuckleheads wrap foil around the fuses all bets are off. When the amp is updated with Joe's full comp backplane and driver boards, the driver...

Keep the RCA drivers until you convert it to full comp. The driver board hasn't been on fire which is good. the TO92's with corroded black leads are a failure waiting to happen.

....and if you're not going to upgrade the driver board, replace the 4 x TO92 transistors (Q1-4) As they have become failure prone in their old age. This assumes you have the PL14 board. Driver transistors at the bottom of each stack need to be retained to avoid oscillations. Attached is...

The attached document provides a wealth of information on electrolytics that does not require a leap of faith or any magic. No they don't discuss audio grade capacitors, whatever that is, but a lot can be learned. Equally important to construction is reliability, shelf life and temperature...

Up on EBAY. Loftech TS1. I've been using one for 40 years. Awesome all in one, low distortion generator. Somebody jump on it.