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Other than differential transistor pairs, what else can cause high DC offset?
Other than differential transistor pairs, what else can cause high DC offset?
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Old 10th November 2017, 04:32 AM   #21
Ian Finch is offline Ian Finch  Australia
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Yes, an analog meter on an AC range would allow you to approximate the max. output voltage and hence calculate power with a clean sinewave signal. It should also give you a small visual indication of clipping by unsteadiness at some point as you crank up the volume control. A budget DMM will measure steady voltages below about 1kHz OK but you'll need to ascertain the clipping point by 'scope or by listening and an educated guess, as indianajo describes.

I sometimes need to do this test with only a partial load as it stresses old gear on it's last legs. I have dummy loads of several standard resistor values that can be wired in series for say, 16 ohms that will do the trick. This means the test is no longer accurate because the power supply voltage sags less at lower loading but it still permits a fair estimation to be made of the amplifier's capability.

The only unusual requirement is a clean sinewave signal source. The internet comes the rescue there with many test tone sites and some really great free audio software like Audacity out there to supply your every need for test tones in whatever format of constant, swept frequency, chirps and gated tones you like.

Note that some sites post tones and free software that is junk or maybe too low in resolution (low bitrate MP3 for example) for some test purposes. Also, the line level input sensitivity of these old amps is quite high and probably only requires 150mV for full output so beware of overloading the preamp with the perhaps >1V output level from your PC's sound card.
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Old 12th November 2017, 09:22 PM   #22
62vauxhall is offline 62vauxhall  Canada
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I have not gotten around to approximating output power yet. The receiver seems to work well as far as the tuner section and Aux input goes into which I had plugged a CD player. But I've just now noticed a problem with one channel of Mag Phono - it's dead and that channel has that "motorboating" sound.

FWIW, the above is when in stereo mode. Switch to Mono, and the receiver outputs to both speakers.

Prior to hooking up a turntable, I replaced the 5 electrolytic capacitors since the pre-amp board was easy to get at and some higher voltage versions of the same capacitors were at hand.

Am I wrong in thinking that a faulty transistor on the phono board can be the cause?

There are four 2SC644's on it. I ran into some of those on the Amplifier boards and replaced them with 2CS1815 which seem to function well but I have none left. What I've read seems to indicate I can use 2SC1845 which I do have.

Before I go ahead with that, I was hoping someone might know the pin arrangement of 2SC644. I was fairly sure it was ECB but not 100% positive. The notes I took regarding this "project" were on scraps of paper, now discarded, so I no longer have that info in front of me.

There is no 2SC644 datasheet online that I can find but I did come across a text only description stating the pinout was BCE.

Would I install 2SC1845's oriented the same as 2SC644's or would I reverse them?

Last edited by 62vauxhall; 12th November 2017 at 09:41 PM.
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Old 13th November 2017, 01:37 AM   #23
62vauxhall is offline 62vauxhall  Canada
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Additional:
I reviewed my "before" photo again and saw how the 2SC644 transistors on the amplifier boards were oriented. It was the same as the replacements I installed there which has the same pin arrangement as 2S1845 so I was able to install those in the phono section.

The channel is still dead but the "motorboating" is gone. Plus the "live" channel sounds a lot better than before.

The phone preamp board is fairly small so I just started poking things with my finger tip. This produced a strong hum in the "dead" channel when I touched the joined leads of a 10uF capacitor and 1K ohm resistor that are soldered in series.

I already replaced that 10uF capacitor so it is brand new and touching the same spot of the "live" channel produced no hum at all.

So, this leads me to think that 1K ohm resistor (carbon film) might be bad. I have 1/2 watt 1K ohm (metal film) resistors on hand so will replace those to see what happens.
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Old 13th November 2017, 08:23 AM   #24
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by 62vauxhall View Post
I have not gotten around to approximating output power yet.
You need to ensure your maximum power output is not clipped - an average reading voltmeter will over-read the output voltage once you start clipping the output. It averages the signal over the waveform and then applies a factor to convert an unclipped sinewave average to rms equivalent.
Quote:
...........................There are four 2SC644's on it. I ran into some of those on the Amplifier boards and replaced them with 2CS1815 which seem to function well but I have none left. What I've read seems to indicate I can use 2SC1845 which I do have.

Before I go ahead with that, I was hoping someone might know the pin arrangement of 2SC644. I was fairly sure it was ECB but not 100% positive. The notes I took regarding this "project" were on scraps of paper, now discarded, so I no longer have that info in front of me.

There is no 2SC644 datasheet online that I can find but I did come across a text only description stating the pinout was BCE.

Would I install 2SC1845's oriented the same as 2SC644's or would I reverse them?
All the 2sxxxx transistors have the collector in the middle.
The To126 has an ecb pin out.
All the others (To92 & To247/264) have bce pin out.
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Old 13th November 2017, 03:41 PM   #25
62vauxhall is offline 62vauxhall  Canada
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After some poking around with a DMM this morning, I discovered a lack of continuity on the dead side of the phono board where there was continuity on the live side.

It seems that one leg of a capacitor I replaced formed a bridge between two pads on the PC board. The live side retained enough solder to maintain this bridge but the dead side did not.

An easy fix once I knew what the problem was.

Am listening to the receiver on phono as I type and it actually has some authority to it. No bass or treble added and loudness off.

This is now the oldest piece of amplification I own at nearly 50 years so it's kind of satisfying to think it may be working as well as (or maybe better than?) when it was new. It's only 14 years younger than I am.

I really appreciate the trouble taken by those who provided advice and suggestions especially Ian. I can't return the favor as my knowledge is sadly lacking unless....

....anyone have any 8 track tape cartridges that need reconditioning? I''m a bit of a whiz at that - over the past 6 years, 2,500 under my belt and counting.
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Old 13th November 2017, 06:51 PM   #26
indianajo is offline indianajo  United States
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Congratulations on spotting your error. I'm a whiz at making cold solder joints - which a motorboat could be involving an e-cap. Lusting after a $250 Marvello? RF heated iron to replace my $45 one - but not this month.
Checking which is the base with a meter is worth it when receiving a batch of new transistors. The base is the one with two diodes coming from it.
Manufacturers change the pinout in TO92 from time to time, datasheets of different dates have different pinouts. I've seen this a couple of times from Fairchild. there are two different pinouts of BC546-556 for example, also J174. BCE and EBC are popular, then this 2SC part I'm salvaging from a Toshiba TV goes ECB from the flat side. ( I can't buy 1 Watt TO92 anymore, have to salvage some. NTE has flooded the market with "replacements" that are pieces of **** that have no heat sink access like 2n6727 of zetex, no extended case like MPSW56 of ON, a watt rating that I think is probably completely bogus) There were 70's TO92 looking things that were CBE from the flat side, look at a Wurlitzer schematic from the time.
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Last edited by indianajo; 13th November 2017 at 06:59 PM.
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Old 14th November 2017, 04:10 AM   #27
PRR is online now PRR  United States
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Quote:
Originally Posted by 62vauxhall View Post
.....does the 47 volts I measured and it's using two 2SN3055 output transistors per side say anything?
47V total DC means at-most 47/2.8= 16.6 Volts RMS. 16V rms in 8 Ohms is 2 Amps and 32 Watts Peak.

Real circuits have real losses, several Volts.

A low-price amp will have major sag from idle to full test-tone power. Estimating 20% for both sag and circuit drop, the full-load test V+ will be sagged to 38V, 13V rms, 22 Watts in 8 Ohms.

They may have rated it "32 Watts IHF", a documented way to make short-term peak numbers.

Under more demanding FTC rules (after this amp was made) it might be 22W, or it might be less if it overheats in the FTC's overly-hard test.

It may well be 13W-15W as you guessed.

We can usually figure a pair of old 2N3055 is OK to 75W, planar 2N3055 maybe 50-60W. So you are over-provisioned here. '3055 became the most common cheap output part, so they probably used it from 20W to 60W models, until plastic parts put the TO-3 out of style.

Last edited by PRR; 14th November 2017 at 04:12 AM.
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