Instrument Resources

A few years ago, (on my recommendation) a customer who became a good friend of mine, bought a Marantz 15 on ebay. How could you go wrong by buying a great sounding classic amp that’s attractive and relatively inexpensive? And in time, will only appreciate in value. He won the bid with shipping for around $245. The ad read, "Vintage Marantz Amp Mint !! - Recently Serviced". After receiving the amp, he brought it to me with a copy of the ad. The faceplate didn’t look anything like it did in the picture. The scratches and gouges couldn’t be covered up with just trick photography. I've seen a few comical repairs over the years working on electronic equipment. But this one took the cake, and have never seen anything more pathetic than this particular amp.

First of all, there was a 1 watt where there should have been a 5 watt, wirewound. There were Radio Shack replacement transistors. And there were actually a few old germanium transistors, where there should have been silicon. One totally butchered pc board with some circuitry unrecognizable. Now, it gets better. A TO-3 package was hanging off the other pc board where there should have been a smaller signal TO-5 part. And it was held in place with a tie wrap from a loaf of bread through one of it’s screw holes!

Two overload lamps were jumped over with a 1W, 5 ohm resistor. The transformers were severely rusted. There was an unmarked transistor in the bias circuit with about 4" of extension wire on its leads. It was completely wrapped in electrical tape and tucked in there somewhere, so that it couldn’t move around. That channel still didn’t have bias. The wires from one pc board to the barrier strip were cut in the middle and then just twisted back together, not even soldered or taped. Lucky they were solid wires, huh? Surprisingly enough, the amp actually worked and a had a fair to poor sound up to about 5 watts! It even smelled a little funny. He complained to the seller and wanted to take care of this matter ... No deal !! And even got a negative feedback in return!

I felt so bad for this guy (and somewhat guilty because of my recommendation for this purchase), that I just gave him one of my Model 15s. But on the bright side of things, in addition to my parts stock, now are a few extra frame parts, heat sinks, some rusty screws, and about 4 or 5 transistors.

Just about any well meaning technician with a little electronic background can replace a transistor and get an amp working. But for how long? And, how well will the amp work? Since transistors from various amplifier manufacturers are no longer available, what choice do you have? But now you have two different sounding channels. If a transistor has to be replaced in the power supply protection circuit or the like, that’s ok and it doesn’t present a problem or change the amp’s sound or specs. To change a transistor in the mainstream is a different story. Now it’s a whole new ball game. Power and voltage ratings are easy to match. Curves, speed, hfe, VBE, etc. are not. Amps with parallel outputs must share the load throughout the frequency range. Parameters such as hfe, VBE, and fT, etc. are important. The following are a few typical scenarios that I see when I buy or repair an amp.

Scenario 1 - Take a typical amp with four outputs per channel. One transistor on one rail is blown. Replace the transistor with an equivalent type and don’t match it. You find the amp will work great at lower to medium power into an 8 ohm or higher load. Push the amp or use a 4 ohm or lower load, and the amp blows. Simple!

Scenario 2 - Let’s suppose that same amp is repaired using one 250W package, where there used to be two 100W or two 150W transistors. The amp works and probably won’t blow. I see repairs like this quite often. This type of repair may have the same drawbacks as those in scenario 1 and scenario 3 (described below).

Scenario 3 - The next level. Take that same amp replace both transistors with a typical NTE matched pair with equivalent power specifications or higher. Ok, now you have a reliable amp. Run it into a low Z load, turn up the gain, it works, and doesn’t blow. But now, there seems to be a difference in sound … it didn’t sound that way originally. The left and right channels may sound different. Bias and offset may drift. The feedback loop gets pushed a little hard. THD goes up. IM goes up. But, it’s still reliable and hasn’t blown. You may get a little hum when you get into power .... someone forgot about common mode rejection ratio. 60 Hz and 120 Hz mixed with 440 middle A doesn’t really sound pleasant. IM sounds worse than THD. I didn’t even mention parasitic oscillations .... how about a little chirp or ring that wasn’t there before?

There are by far too many scenarios to list. The human ear is very discerning, and with a good speaker system (more easily found today than yesteryear), flaws in amplification can be detected easily. If you can’t hear the difference with an incorrectly repaired amp, you did not need that nice amp to begin with.

Most repair shops don’t have a good curve tracer, or one at all. Good curve tracers are not cheap, at least the ones that get into power, and there isn’t a lot of money to be made in service. I can’t blame them for not buying one, although it’s a mandatory piece to get the job done right. Curve tracers don’t measure speed, fT, or Tr the other half of the equation. Curve tracers will measure BVCEO, VBE, ICBO, beta ac, Vce and ICER, etc., but not fT or Tr. Now there’s an additional set of rules. Does slew rate ring a bell?

Tube amps, on the other hand, are so very easy to repair, modify, or restore. Unlike solid state amps, you can easily pull a tube, versus unsoldering and unscrewing a transistor with greasy heat sink compound. There are specific torque requirements for transistor mounting. Pellets can be cracked when transistors get torqued too much …. and many were. Tubes are forgiving, bipolars are not. Tubes are not static sensitive, transistors are. Of course, you don't want to drop a tube.

As a rule of thumb, there are fewer parts on a tube amp, and usually very easy to replace. Try to measure parts on a dc-coupled amp versus a tube amp, or find that small offset on a dc amp with 30 transistors in line. Many shops don’t want to invest the time and money to replace those 16 outputs per channel that are in your blown high power amp. I personally own a few low and high-end tube units, and never thought the day would come when tubes are easier to get than transistors.

The service shop buys 50 transistors for stock, and 50 each of various types. Think there's enough stock to repair quite a few amps? But, now find only 15 of 50 match, even though modern transistor production have more consistent batches. Surprise! In addition to this, now make both channels sound identical. Yes, the manufacturers and engineers have these problems to deal with, and they deal with them. Read a good service manual to see what it takes to get that shorted transistor replaced. Then ask yourself the question - Why did it blow? How about second breakdown? You may never find out why it blew. Depending on the amplifier design and a bit of luck, of course there are the occasions where that NTE or Motorola part, that you picked up a batch of, will work just fine. Any good amplifier engineer will agree with these scenarios. Did you ever try to buy a power transistor from a manufacturer that is still available? Try it!

Selecting parts is time consuming, tedious, and expensive. Many manufacturers have house numbers stamped on their semiconductors for various reasons. Some house numbers are put on the transistor and are just standard JEDEC numbers. Some house numbers are for selected and matched transistors. The number resembles a standard JEDEC number; but it's not, and you may never find any information for it. Some transistors are made specifically for the amp’s design. And, numbers can mean anything. It’s common place for a good amp manufacturer to put a color code, prefix, or suffix on the part that denotes hfe range and speed. I remember one very popular amp brand. But the manufacturer would not give out information on what the transistor really was, and their replacement parts were very expensive. Some years later, that company was bought out by another. The new company not only gave the original types in writing, but sold those original parts for about half the price.

A long time ago, I wanted to upgrade a Marantz 16B that had almost every transistor blown. I decided to start out with new transistors from top to bottom. Original types were no longer available, and had to buy batches of various transistors from different manufacturers. Of course, this in itself is difficult and pricey. So, sat down in front of a curve tracer, distortion analyzer, a scope, a network analyzer, various loads, and worked out a combination of transistors taking the original design of the amp in mind. After a few headaches, quite a few more transistors, even more hours of labor, worked out a combination, put it to the test, and it ran fine. Everything seemed ok. Put a capacitor on the output to simulate a capacitive load; turned up the gain, and filled the room with smoke from burning resistors. All the drivers and outputs shorted. You can’t use external current-limiting power supplies for test in every case.

Finally, after a few more combinations of transistors, pushed the amp to the limit, and then some. Ran it into the worst conditions possible. Shorts, capacitive loads, inductive loads, heat, 63% load lines, cold, THD, IM, clip, noise, etc. Then changed all the parts which had slightly different tolerances to make sure these conditions were repeatable. Finally the combination worked well. I did achieve a lower noise floor, and a faster, more reliable, amp with better THD and IM, but at a great cost. And, not without the help of the original engineer who designed the amp. Not that I spoke with him … it’s just that his layout (the original design) led to a good start. So in summation, if your piece does not have a standard or stock part number, or you can’t get the original part from the manufacturer that was selected for that particular position in the amp, then you have some serious work to do, unless you‘re lucky or you’re repairing a clock radio.

I have designed, redesigned, modified, and repaired audio equipment for over 50 years, and specialize in Marantz 7T, 14, 15, 16, 16M, 16B, 18, and 19. Shown below are a few of the Marantz 15s (3 versions), 16s, and 16Bs that have been restored and/or modified. Also shown is a 15 and 16, specifically used to test new types of components. These are a few from my collection.