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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 7th November 2017, 04:59 AM   #11
PRR is offline PRR  United States
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That amplifier has one main power cap (lots of little ones) and two Output caps (which appear to be new).

With NO load, there WILL be "DC" at the speaker terminals. There is a large thump at power-up. With speaker, the output cap will charge-up and the output DC voltage fall exponentially to zero. With no load at all, the output cap has no charge path and the speaker terminal will sit at half-supply "forever". This amp has 1.5K resistors from cap to ground which will tend to zero but very slowly. And if those caps leak, never quite zero. In fact a 1,000uFd cap is likely to leak near 1mA, which in 1.5K would be 1.5V, so "nearly 1V" is on the good side.

Put a 10-100 Ohm resistor on the speaker terminal. Power up, wait. If it goes down and down to below 1V, I would forget about it.

I would be more concerned that the *other* side of the cap sits near half the supply voltage, a happy amplifier ready to swing either way. If it is a 40V supply, it may sit at 16V or 22V, these old amps were not precision DC machines (also you "center" bias allowing for sag). An unhappy amp will show 2V or 38V or worse.
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Old 7th November 2017, 03:04 PM   #12
Ian Finch is offline Ian Finch  Australia
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Good pics and I can only agree with PRR's comments and suggestions.

How about the output transistors though - Those little TO39 size cans won't be capable of 12W and neither will the KSA1220/C2690 replacements be. Even with heatsinks, I think only a few watts would be possible. Assuming those are actually the driver transistors, where are the output transistors? Perhaps on the rear panel but are they all the same type?
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Old 7th November 2017, 04:01 PM   #13
62vauxhall is offline 62vauxhall  Canada
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My oversight, the output transistors are not shown. They are two pairs of 3055 (can't recall prefix). Those that were on the amplifier were from three different manufacturers. I had the notion that output transistors should all be by the same maker so replaced them with new ones. I did that after doing what replacing I did on the boards.

It may or may not be relevant but once those new 3055's were in place, a DBT glowed quite brightly. Subsequent one by one removal and DMM checking revealed one was bad. I did not test them prior to putting them in so do not know if it was already faulty or suffered damage somehow after putting it in. I'm at a loss to imagine how or why that could have happened because to the best of my knowledge, I patterned installation of components on the board the same way as the originals.

In any case, I substituted one of the original 3055's after checking that it was OK. The DBT. (60 watt) still illuminated but at much lower intensity - sort of a dull orange.

It was about that time I resumed measuring DC offset on both channels and saw the same 0.5 volts on the one I was monitoring but 11 volts on the one I ignored.

FWIW, I will be picking up a couple more new 3055's today. Might be futile, but at least they will all be alike.
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Old 7th November 2017, 04:04 PM   #14
nigelwright7557 is offline nigelwright7557  United Kingdom
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I bought in a 1980's Maplin 225wrms amplifier with 2n3055/mj2955 output transistors missing. I bought in brand new ones and the amp oscillated badly.
I tested the new transistors and they had much more gain than the spec for the original transistors. I increased the VAS capacitor a little and that stopped the oscillation.
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Old 7th November 2017, 04:15 PM   #15
Nico Ras is offline Nico Ras  South Africa
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I also agree with PPR and Ian, I do not think you had a problem to begin with.
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Last edited by Nico Ras; 7th November 2017 at 04:18 PM.
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Old 8th November 2017, 04:32 AM   #16
62vauxhall is offline 62vauxhall  Canada
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After reading the recent opinions, it seems that there was no problem to start with if 900mV offset was normal. But there is a problem now.

If bridging the +/- speaker terminals with a resistor (I used 5W 47 ohm) was to reduce that number, it did. The left side with 0.5V became about 12mV and the right side with 11V became about 8.5V and that resistor heated up.

So somewhere along the line I goofed. I had to have installed something incorrectly to create such a disparity.

I am very glad I did have a "before" photo of the boards after all that I could refer to because I installed one of those new 1000uF coupling caps backwards.

Since both boards were twins of each other regarding capacitor polarity, I assumed (why oh why do I still do that) that those larger caps would be likewise. But, on examining that photo, I saw right away they were mirror image.

I reversed the "offending" capacitor and with those resistors in place, I now get 12mV DC offset on each side. Without resistors - 0.5 volts.

I feel somewhat elated now that both channels are equal.

Thank you guys for the explanation.

I made a decision yesterday that I wonder if I can stick with and that is to stop looking for old audio gear. I have very little time at home anymore and seem to be spending all of it fiddling with the hardware I manage to find.

I also have a vinyl and tape obsession and wonder why am I not spending time listening to all the recordings I have instead of with what amounts to the uneeded gear required to play it?

Wish me luck.
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Old 8th November 2017, 01:42 PM   #17
Kay Pirinha is offline Kay Pirinha  Germany
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Guess the wrongly installed electrolytic was in the channel labelled 1? Your pics show orange and pink wires coming from the power amp PCBs. These need to be connected to the 'lytics' positive terminals, as in channel 2.


I'd replace at least the erratically reversed capacitor by a new one, it might have become severely damaged or unreliable at least. Better, replace both by axially leaded new ones with a capacity of 2,200 F, as the LF roll-off for 1,000 F @ 4 ohms is meagre 40 Hz.


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Old 8th November 2017, 07:29 PM   #18
Ian Finch is offline Ian Finch  Australia
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That's a great result and you were so close by our own efforts. There may be a few finishing details to ensure a lasting repair but at least you succeeded and learnt a few new things in the process. I know quite a few guys who've learned very little over the years they've tried repairing electronic devices. Just consider that you're a long way ahead of them and each repair gives you more experience - if you can carefully choose the jobs such that you aren't thwarted by complexity or parts supply issues.
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Old 9th November 2017, 04:07 PM   #19
62vauxhall is offline 62vauxhall  Canada
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There are no doubt "finishing details" that could be performed but at the moment I am inclined to leave well enough alone. It is not exactly what I would consider a highly desirable unit and was purchased solely because I thought it was an oddball. I guess something made for sale in an Eastern US department store winding up in Western Canada does make it a bit unique - at least around here.

There is little online info on the Rolecor brand but there did appear to be two models below this one.

Is there any measurements I can take / calculations I can make that would determine what it's power output is?

For instance, does the 47 volts I measured and it's using two 2SN3055 output transistors per side say anything?
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Old 9th November 2017, 06:42 PM   #20
indianajo is offline indianajo  United States
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You turn the volume knob up until the sound is a little strange, then back off slightly (the sound you're looking for is clipping) Compressed pop music & commercials is the best source. classical music actually varies in voltage below max for minutes at a time.
You put an 8 ohm, or on recent units, 4 ohm resistor of suitable power rating on the output. Instead of the speaker. For rough approximation you can use the speaker. Don't short the speaker terminals together while doing this, it blows up transistors. I use extra wires to run the meter feed off to the sides, then alligator clip leads on the probes. The rubber insulated ones so they don't short together.
You take an ANALOG voltmeter with a 20v or 50 v AC scale, and measure the peak voltage.
P=V^2/R
I used to get pretty good results from $6 R**** S**** meters. Their pointers were pretty fast, my $285 Simpson meter pointer is a little slow.
In the very early versions of the 2n3055 the output voltage was about 1/3 the rail voltage, but later higher gain transistors can go to 1/2. 2SN3055 sounds 1978 or later.
How long the unit can sustain this voltage is the differnece between peak, music power, average or RMS watts. Most receivers had wimpy transformers and would peak out much more watts than they could sustain. Heat sink capacity is another limit to sustained watts. Doubling up the output transistors lengthens the time that they can put out a given wattage.
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Last edited by indianajo; 9th November 2017 at 06:46 PM.
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