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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 6th November 2017, 02:28 AM   #1
62vauxhall is offline 62vauxhall  Canada
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Default Other than differential transistor pairs, what else can cause high DC offset?

I am not posting about a very desirable piece of gear, just a recently acquired, old, inexpensive, late 1960's receiver made by Rolland Electronics and branded Rolecor. Probably 12 to 15 watts per channel.

The first thing I did was test the DC offset at the speaker terminals and got nearly 1 volt one each side.

Although my ability to read schematics is minimal, I did spend a fair bit of time looking online for one but without success. No technical info at all actually.

I managed to determine what I thought were the differential pairs on the amplifier boards and substituted newer ones plus replaced all electrolytic capacitors on those boards. This managed to reduce DC offset down to about 0.5 volts.

I started removing and checking (DMM on diode setting) other transistors on the board. There was a pair of TO39's of which one was bad. I replaced those pairs with a complimentary pair of pairs of TO126 transistors which I was told would make a satisfactory replacement. Pinout pattern accounted for.

Still no reduction of DC offset below 0.5 volts.

Have now checked all other TO92 transistors present which tested OK. There are some of what I believe to be Germanium transistors (TO1?) that I have not yet removed and checked.

Can anyone sort of steer me in right direction by advising what other causes might produce DC offset of 0.5 volts and how I can get it down to a more acceptable level?

My limited experience tells me this is simple receiver based on how much "open space" there is inside. It is not crowded to the point of anything being inaccessible or even inconvenient to get at.
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Old 6th November 2017, 02:59 AM   #2
nigelwright7557 is offline nigelwright7557  United Kingdom
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As its a simple amp then maybe there is no trim pot to set the DC offset ?

I found quasi amps to be worst for DC offsets. My own designs always have a trim pot in the current mirror or in the LTP to adjust DC offset.
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Old 6th November 2017, 03:12 AM   #3
Ian Finch is online now Ian Finch  Australia
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Rolecor brand from the 1960's, predates the Rotel brand which is still produced by the Japanese Roland Electronics Corp. Without schematics for any of those early models though, how can we offer advice? There is a short thread about Rolecor products on AK collectors and repairers forum which will give you a little info but nothing useful.

It's unusual that a small amplifier of that age had a differential input but it may well have had a large capacitor (about 1,500 -2,500uF) coupling the amplifier output to the speakers. This may have become leaky and likely requires replacement. If you have the same 0.5V on both sides of such a cap, that would probably be the situation.
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Old 6th November 2017, 05:11 AM   #4
62vauxhall is offline 62vauxhall  Canada
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Quote:
Originally Posted by Ian Finch View Post
Rolecor brand from the 1960's, predates the Rotel brand which is still produced by the Japanese Roland Electronics Corp. Without schematics for any of those early models though, how can we offer advice? There is a short thread about Rolecor products on AK collectors and repairers forum which will give you a little info but nothing useful.

It's unusual that a small amplifier of that age had a differential input but it may well have had a large capacitor (about 1,500 -2,500uF) coupling the amplifier output to the speakers. This may have become leaky and likely requires replacement. If you have the same 0.5V on both sides of such a cap, that would probably be the situation.
There is such a single large capacitor. I thought it was odd for it to only have two terminals as the only other single large cap I've seen on a stereo chassis was inside a friend's large Marantz receiver but it was a dual layer.

The value of said cap is 63V 2200uF. The local shop I go to has a 100V 2200uF on their website but out of stock. Another shop also has one listed on their website but it's farther afield and more expensive.

If local is a no go, I'll get one sent from one of the usual online vendors. That's always my Plan B as it involves a 100KM round trip to avoid the extra time and expense of international shipping and the Canadian duty charged if shipped across the border.

Ian, if only I had half the smarts you do. But, if that was so, I'd be tinkering with something simpler.

EDIT:
"There is a short thread about Rolecor products on AK collectors and repairers forum which will give you a little info but nothing useful."

Yes, as mentioned I spent considerable time looking for info on this Rolecor so became aware of it's department store heritage and the Rotel connection. No doubt I read through those AK threads. I thought I had some luck once as a contributor posted photos of an old Rotel which resembled this Rolecor - including that single large capacitor. Also the manual was uploaded. But unfortunately after studying it, even with my rudimentary grasp, I could see the circuitry and component complement was considerably different.

Last edited by 62vauxhall; 6th November 2017 at 05:24 AM.
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Old 6th November 2017, 05:33 AM   #5
JMFahey is offline JMFahey  Argentina
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So old means that it might even be a transformer driven power amp.
In that case there is no DC NFB, no differential pairs of course, and output offset depends on bias string matching, which is somewhat random/iffy to put it slightly.

Are you sure it is a split supply amplifier?
If itīs a capacitor coupled single supply one, which I suspect, then DC offset is irrelevant.
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Old 6th November 2017, 08:20 AM   #6
Ian Finch is online now Ian Finch  Australia
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A single 63V, 2,200uF cap could also just be the smoothing cap for the common, single voltage power supply. I should have added that a single supply amplifier requires only one output capacitor - but that means one for each channel, or 2 in total. That won't be an output capacitor, if it's the only large cap. The 63V rating also reads like a more recent replacement but check the voltage on the cap's terminals. If negative is ground and the positive terminal is around 50V, that should be the situation instead. If looking for a replacement, the voltage rating needs to be at least 15% higher than the smoothed DC supply. The maximum capacitance allowable will be limited by the size of diodes which could pop with inrush current on startup. They are probably only rated at 1A continuous but can handle considerably more as a peak charging pulse current. A 1A diode bridge should tolerate up to 3,300uF though.

The main smoothing cap or caps are usually located near the rectifier diodes or bridge but who knows how this one is arranged. Pics, top and bottom of the innards would help a lot to identify the major parts and maybe deduce the general circuit too. I also suspect this is going to have a single rail power supply and a quasi-complementary output stage, as the later dual-rail Rotel series amplifiers and receivers did. If the power transistors (2 per channel) are all the same type (i.e. not complementary NPN/PNP types), it will assuredly be a "quasi" design.
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Old 6th November 2017, 08:59 PM   #7
indianajo is offline indianajo  United States
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digikey ships to canada without customs people report, through some sort of cross border fiddle. They are in Minnisota. Use royal mail instead of UPS, if something changed and there is a customs charge- - RM doesn't charge a customs loan fee as UPS does.
Farnell probably has a warehouse in CA - they are my first line parts house. US warehouse has a lot of 1000, 2200, 3300, 4700 caps. Look under aluminum electrolytic caps, snap in package (not axial leaded, that is now obsolete). If the short pins won't fit your board, solder a splice on or use a cinch solder terminal strip. Use glue to keep the cap from flying around in handling, like 3M weatherstrip adhesive.
And there is a supplier in Vancouver I found in a search once, Bennet's ?, Bailey's? something.
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Old 6th November 2017, 09:34 PM   #8
jwilhelm is online now jwilhelm  Canada
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Quote:
Originally Posted by indianajo View Post
digikey ships to canada without customs people report, through some sort of cross border fiddle. They are in Minnisota. Use royal mail instead of UPS, if something changed and there is a customs charge- - RM doesn't charge a customs loan fee as UPS does.
Farnell probably has a warehouse in CA - they are my first line parts house. .
Digikey is also in Manitoba and Newark is in Ontario. Digikey and Mouser both offer free overnight shipping to Canada, no need to use snail mail. I still find it hard to believe they don't offer better service to the US customer base. Electronic parts are duty free coming from the US to Canada, so unless you are using DHL there's no extra fees.
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Old 6th November 2017, 09:37 PM   #9
jwilhelm is online now jwilhelm  Canada
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A couple good pictures of the amplifier internals goes a long way. The eagle eyes here would likely be able to point out output caps and supply parts.
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Old 7th November 2017, 04:36 AM   #10
62vauxhall is offline 62vauxhall  Canada
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Thanks for all the info re parts sources.

I have obtained from Mouser & Digikey in the past - primarily Mouser and even Newark Element 14. Although Mouser / Digikey have Canadian web addresses, I assumed that both shipped from the US. They both have a basic shipping rate via USPS of 6 or 8 dollars but that is to a depot called Package Express in Sumas, WA to which is a 2 hour round trip. I am gun shy about shipping to my home address, Those times I've done so (including electronic parts) I've been nailed with either customs or brokerage fees. On weekdays I am away from home 5AM to 8PM so never home when such things arrive so must always drive to whichever outlet it winds up at. Even just a few small items is sent in a box too large to fit in my mail box of the building I live in. I had a $25 USD turntable headshell sent to my home address not long ago and by the time the smoke had cleared it was $76 CAD.

As far as this receiver goes, I appreciate the offers to provide opinions based on photos. I hope those I have taken are sufficient. It will probably be evident but they are after I have replaced certain components therefore do not depict what was on the boards originally.

Adjustment pots were mentioned previously and there are two on each board - a large one and a small one. I have encountered bias adjustment pots before but in conjunction with service manuals. As there is no manual for this unit, I have no clue what either of these two does.

Some additional info:

With the exception of that single large 63V electrolytic, I do have replacement capacitors for all the remaining original grey Elna electrolytics. I have just not gotten to then yet.

The voltage to that 63V capacitor is 47 volts.

I was somewhat less than diligent over the last couple of days. When first obtained, the receiver exhibited nearly 1V DC offset on each channel. After installing what electrolytics I did replace, DC offset went to 0.5V each channel.

But I got lazy. Since Both were more or less symmetrical, I expected them to stay that way so only bothered checking one side. After replacing the original TO39 pairs (2SA & 2SC497) with TO126 pairs (KSA1220 & KSC2690) I saw no change to DC offset in the channel i was monitoring but the other, I learned yesterday, was 11V and I'm unsure as to why this would be.

And apologies for the filth. A brush 'n' blow was going to be my final act.

EDIT: I got a NAD 710 receiver some time ago and checked it's DC offset early on and it was all over the map. I learned later this was because it was capacitor coupled. Although DC offset with the Rolecor appears constant (although grossly imbalanced) is it possible it too is DC coupled? I read recently, where it was alluded to that high-ish DC offset in capacitor coupled amplifiers was normal.
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Last edited by 62vauxhall; 7th November 2017 at 04:44 AM.
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