Reliability of electronics

On 30/05/2022 5:24 pm, Trevor Wilson wrote: Designed in 1963,

**Oops. Released to market in 1972, so likely designed in 1971-2.

Trevor Wilson wrote: =================

** The model 500 dates from 1973. Each channel used 8 x TO3 transistors with SJ prefixes ( SJ2404 and 2405) So specially selected types made by Motorola.

Rated power was 250W at 8ohms, so 8 devices should be plenty. Weight was 83 pounds - which is absurd.

..... Phil

**If you've downloaded the service manual, you can see the complexity of the beast. SOA adjustments are very comprehensive. The fault with the amp lay with the VCEO of the output devices. There was very little margin for mains fluctuations. Marantz finally solved the problem by specifying 200 Volt, Japanese output devices, rather than the Motorola ones originally fitted.

BTW: The Model 500 would typically deliver well over 300 Watts @ 8 Ohms. As I recall, around 320 Watts, both channels driven.

Yeah, it's a heavy sucker. Fan cooled too, so heat sink mass is not huge. Big power transformer and 4 huge main filter caps.

It was replaced by the Model 510/M, which was far more reliable, more compact, lower mass and sounded like shit. Series/parallel output devices. Yuk.

** That assertion makes no sense. Facts: 1. Output transistors do not operate in Vceo mode. 2. They are only exposed to * half* the DC supply at idle or moderate volumes. ** MJ numbered power BJTs all exceed their Vceo ratings on test. They way exceeded the ( higher) Vcer rating too - as it almost matches the Vcb rating.

SJ numbers have no published ratings.

..... Phil

**Correct. All the US built Marantz models used Motorola devices with an SJ prefix. Even some of the early Japanese ones used Motorolas as well. Very unusual. I measured a few back in the day. As I recall none of the Model 500 output devices exceeded 160 Volts. I may still have a few originals lying around. If I locate them, I'll measure the breakdown Voltages. Don't forget: It was 1973. High voltage, high power PNP devices were very scarce.

Trevor Wilson wrote: =================

** Measured with open base ? Not how they are used. ** If you do, put 100ohms between B and E. Makes at least a 10% increase . ** SJ types were factory selected from stock, mostly for Vbe matching. Crown, SAE and Peavey used them all the time.

Total PITA for repairers, who had zero clue what the types really were.

BTW

if the Marantz SJs lacked adequate Vce, examples would have commonly failed bench testing. So I don't buy the idea.

...... Phil

**Will do.

**Yep. **Point taken. When the Japanese devices were installed, the Model 500 became reasonably reliable. If I recall correctly, most amps failed at switch on, rather than under use. I could be wrong.

Trevor Wilson wrote: =================

** I once used to see a lot of Phase Linear 400 mk2 amps. A revised version of the famous PL400 hi-fi model. Complementary MJ output devices and input op-amps on the pcb. These were all used in PA systems - not a good idea.

Saw a lot of blown output stages and many that simply went DC taking speakers with them. Nothing to do with Vce or even SAO limits.

The first problem was due to overheating of output devices - since owners had to devise fan cool themselves. The installed temp cut out was useless, since all TO3s were mounted using thick, pink silicone thermal pads. I called the "thermal insulators" as they allowed devices to get 50 C hotter than when mica and grease was used.

Going DC was due to 5W ww resistors that fed +/- 16V zener regulated rails for the op-amps. The resistors were all faulty, going bright green inside and hence open cct.

Did a nice trade in fitting relay speaker protectors in many power amps. ( wired the RIGHT way so DC arcs were eliminated)

...... Phil

**Good plan. FWIW: The Model 500 never overheated. Thermostat controlled, two speed fan cooled. Output devices were all mica/thermal paste mounted. Even after repair (using original output devices) the amps failed. Only after the output devices were substituted with Japanese types did reliability significantly improve. The amp could run all day at 40% max power. Easy. Even into 4 Ohm loads.

Trevor Wilson wrote: =================

** Were the SJs in steel or Aluminium paks?

Early 70s Motorola TO3s had a big problem with thermal expansion of chip headers. The silicon and the header had differing tempcos of expansion. The end result was the chip developed micro cracks.

Such devices were speced at a mere 5000 thermal cycles. OK for some apps but not class AB audio. Sudden, unaccountable failure was the norm.

Aluminium paks got dumped and were never seen again.

..... Phil

**Like all Motorola TO3 devices of the early 1970s, they were in those horrible aluminium packs. **Yep. I recall reading the RCA white paper on the issue. **Yep. Steel or copper is the way to go for TO3 devices. RCA won the argument.

**BTW: The Marantz Models: 240, 250, 250M, 1120, 1200/B, 140, 2270, 2275 all used Motorola aluminium cased To3 devices. All demonstrated reasonable reliability. None were fan cooled like the Model 500. The Model 500 ran much cooler than all those models under normal operation.

**Oh and with all those models, a visual inspection of the output devices would frequently and quickly reveal the problem - A tiny pinhole in the case, where, presumably, a piece of white hot silicon ejected itself from the header.

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** The chip in a TO3 can heat in milliseconds - or at least a small part of it can. This is why excursions beyond SOA limits are often fatal. High Vces are the killer - aka "second breakdown".

Motorola 2N3055s and MJ802/4502 in Al paks were OK - I used heaps of them. Big chip devices like the MJ15003/4 and MJ15024/5 were not so lucky.

...... Phil

Trevor Wilson wrote: =================

** Ha ha, no way is that true.

What really happens is the Al feed wire ( aka fuse) from chip to TO3 emitter pin vaporises. This gives a path for a DC arc to jump from the pin to the inside top of the pak. A neat hole then gets drilled until the arc self quenches.

Hitachi TO3 lateral mosfets were famous for doing that !!!

..... Phil