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Were the failed transistors outputs, drivers or both? It's normal (?) to have NPN and PNP outputs fail when any one output fails. Repeated fuse replacement takes out more parts on the control board. Drivers sometimes fail. Note these observations are based on the experience of stock P/L...

If RCA jacks are needed, I bought a pile of the OEM jacks from one of the surplus vendors that was going out of business last year.

That's the mod we did for pro use. We sent out the panel to a local machine shop to accommodate the fuses and power switch. The lower left transformer bolt was replaced with a longer bolt that fit into a bracket mounted in the rack. This supported the amp and took a lot of stress off of the...

I count 4 oscilloscopes. You can never have too many I don’t seem to be able to resist the urge to acquire oscilloscopes. Last account was nine or 10. Everything from a tube base Tek, 503 to a four channel Tek digital scope. In the mix are few USB scopes. my trusty Hitachi scope that has lived...

Every amp I’ve seen in the last few years has had DC offset issues. Some of which were full rail on the output. Q3 and Q4 is the problem. Specifically, if I recall right Q4. It seems as though the leads have a black oxide on them and makes me wonder if there is internal corrosion.

is anybody proficient with Word Press?

Some day I'm going to update the watts abundant web site. It's woefully outdated. Retirement is just around the corner. I've got all of the manuals scanned that I got back in 1983. I hope to get them all posted. Until then...

I’m very familiar with quench arcs. We use them by the hundreds at my day job. Pack Tron has an excellent data sheet with scope photos of before and after voltage spikes across contacts. Will post it here later

On post number eight, Joe says the 200 series one should be stepped on. I disagree. It should be drop Kicked as far as possible.

I don't think an isolation transformer is necessary for bench testing. Any consumer electronic gadget has either an outboard power supply that is isolated or an internal transformer. The worst thing that can be done is to start lifting power line grounds. Any decent 10 amp Variac will...

OK, if I have it right, the amp was started up with a speaker connected and the series light bulb on the AC input. The bulb went full brightness. The reason is operator error. When the amp comes up on the bulb there is DC offset until everything stabilizes. The speaker is a low resistance...

.228 looks to be a #1 drill.

What drill size and tap? Must be a metric size as none of the dies I have work.

Hm. Threaded into the panel?

Anybody have a photo? Front panel is mint and I'm not liking the idea of drilling it.

Who has used an analog meter sensitivity selector switch in conjunction with the W/O LED board on a 400? Toggle or rotary?

Tektronix 503 series has multiple plug in modules that includes function /sinewave generators, power supplies etc. I found one at a local auction for under $50. I have an import split power supply at my Houston office that is a high frequency switcher. It works. Tons of stuff on EBAY...

I'm not sure that doubling the voltage serves any purpose especially if the physical size or lead spacing changes. Unlike aluminum electrolytics who tend to fail open, the failure mode of tantalums is they short. I've seen this probably a dozen times. life expectancy seems to be 15 -20 years.

Usually the smaller electrolytic caps open up when they fail. Of course if one was installed backwards it will likely short if allowed to run long enough.

There are very few, if any contactors/relays rated for anything close to 100VDC. 480 VAC no problem. DC big problem. Current never goes to zero, contacts weld. With AC, current stops (arcing stops) every half cycle. I welded dozens of ice cube relays while developing relay boards. IGBT's...