stereo amp cleaning

Since, by your own observations, the subsonic filter switch could be the problem, or something in the immediate vicinity of the switch. What kind of switch is it (slide switch, toggle switch, rotary switch)? If you can easily disassemble it to get at the contacts, you might find that they are very tarnished or even corroded; caused by airborne contaminants. Also sounds like the volume pots are still dirty. Use a cleaner that lubricates as well as cleans. A tip... when you spray the cleaner into the pot, turn the set so that the residual cleaner can drain back out. That allows the cleaner to float out any contaminants as it drains. I have also found transistors to cause that exact problem. I surmise that the wire bond from the external leads to the silicon inside breaks and becomes extremely intermittent, and can be very difficult to find if you aren't wary of this kind of fault. Lightly tap on each semiconductor in the area and see if the audio is affected. Actually, just about all types of components can become intermittent (microphonic) in this manner, so using an insulated tool such as a plastic or nylon tuning wand, tap components and circuit board to see if that affects the audio.

Good luck...

--
Dave M
MasonDG44 at comcast dot net  (Just substitute the appropriate characters in the 
address)

Life is like a roll of toilet paper; the closer to the end, the faster it goes.

It's a plastic push-button PCB-mounted switch, not easy to disassemble at all. I'd worry about breaking the plastic parts which are, I'm sure, quite irreplaceable. Not particularly accessible either as there is a display controller board above the tone control board where the switch is located.

I was using zero residue cleaner, I'll pick up some lubricating contact cleaner and give it another go. There appears to be a hole in the back of each pot where you can see the shaft turn, plus a small (1-1.5mm) hole on one side. Maybe a blast of compressed air after cleaning wouldn't hurt either...

This amp is totally discrete and there are no soldered wire connections (all wound) , so I think (hope) the fact that BOTH channels are messed up indicates a simple dirty pot problem... I did check voltages at maybe a dozen transistors, both left and right channel from PS through the pre-amp and amp and they were all really close to expected at idle. I don't know if

+/- 1V is significant on a 43VDC rail or not... anyway there are quite a few transistors but nothing unmanageable and as there are no proprietary, obscure, or out of production IC's in this dog, whatever I did is completely fixable. I just hope that it doesn't take me 30 or 40 hours to figure it out.

I may try just bridging the "open" pair of contacts on the subsonic filter switch with a jumper wire and see if that helps.

Thanks for your help

Dave

The still bad switch probably needs disassembling & reassembling, or else replacing the individual switch or switch block. Rows of pushbutton things on PCBs were a standard design around for many years in the 70s & 80s, so finding a replacement from some scrap item is likely.

The pot is either still dirty, probably not, or else is worn out or cracked, or possibly the rivet at the end of the track is loose.

Replacing the pot is most likely the solution, you've already cleaned it. Is its an oddity, rivets can be squashed, and worn out tracks can be painted, tho this would unbalance the volume L-R tracking,

Re the off balance, there are reasons amps have balance controls. If yours has none you can fit a potential divider in the channel with the higher output, and set it up and replace the covers. If youre lcky and the amp has external connections between pre and power sections, you can put what you want between those 2, such as a new volume & balance control.

NT

Had the exact same problem once in a mixing panel (unbalanced audio, one channel fading away and coming back, incidental cracle and pop sounds - cleaning did not help)

turned out to be a leaky electrolyte. Found it by measuring biasses on every transistor in sight. Tapping the cap did not reveal anything though.

--
 - René

I re-cleaned the volume pot with some lubricating cleaner and dried it out with a hair dryer (on low heat setting). The crackling in the right channel has now all but disappeared.

The left channel, however, will now work for about 10-30 seconds when the amp is first powered up, then distorts rapidly over about 1-3 seconds and cuts out completely after that.

The amp is set up as follows:

Input jacks --> input selector PCB --> volume knob/loudness PCB --> tone control PCB --> power amp PCB.

I'm going to take out the input caps off the tone control and route the signal directly from the volume knob to the power amp. That will at least tell me if the problem is with the preamp or power amp. I'm pretty sure it's going to be in the power amp, as the second-to-last component in the pre-amp (tone control board) is the stereo/reverse stereo/mono switch. If the problem was in the preamp prior to this point, changing this switch should move the problem from one speaker to the other. Which it does not. The only downstream component from the stereo/reverse stereo/mono switch is the balance pot.

When I replaced the caps on the power amp, I cleaned the board with spray cleaner. It was filthy, like 30 years of dust and grime. In retrospect, I probably shouldn't have bothered, maybe the cold temperature of the spray cracked off an internal lead in one of the transisotrs or something.

The last component in the amp is "speaker protector". This is a white rectangular box about 1-1/4"L x 3/16"W x 1/2"H (approx, I'm not looking at it right now.). I don't know how this works, but could it possibly be my problem? Or should I be thinking transistors? I guess if I've got the whole amp apart it wouldn't be such a big deal to measure bias currents on both channels at idle... there are 14 transistors per channel including the outputs.

My amp is integrated, but fortunately is very simple to split as the connection between pre-amp and amp is a three-conductor wire. I've been meaning to do this for awhile, I've just purchased a sub with line inputs and outputs... will have to cannibalize some piece of junk for a block of RCA jacks.

All of my signal path connections are wire-wound, not soldered. Is there some tool to do this if I wanted to stay true to design and wind the connections on my pre-out/main-in connections? Is there any benefit to doing it this way? I'm guessing there probably is as it looks like a fair bit more work than just plonking a blob of solder on a ribbon cable lead.

Thanks for the help.

Dave

In no particular order:

a) cleaning pots and switches is only the first step. You need to provide some (slight) amount of lubrication afterwards. 100% volatile cleaners can cause serious friction after the skunge is removed rebuilding additional skunge almost instantly. The idea is to exercise the pot only as long as the cleaner is wet, then stop, then re- saturate to rinse out the residue. *AFTER* that, a wee spritz of lubricating cleaner makes all go smoothly.

b) switches are even 'more so' as wear can happen in any of several ways including breaking of tabs or contact springs, and additional friction can cause failure. So, same process. Saturate, exercise for a few cycles whilst saturated, rinse, lubricate.

c) Look for broken traces on the boards, look (still) for bad caps. Do you test the new caps *before* you install them? I have found about

0.5% of new electrolytic caps are bad or sufficiently out of spec to be problematic. "Badness" typically manifests as intermittents... and your delay may be related. Or, one you did not replace for whatever reason...

d) you may believe that the power-amp section is clean, but one thing I have experienced that is quite peculiar but as I have seen it twice, it cannot be all that rare... an input transistor on the power- amp side has become sufficiently different from its other-channel mate that similar-volume signals to it are quite different in output volume. RARE, and the only way you will catch this is to remove the questionable transistors and put them either on a scope or tester that can determine their actual response.

e) lastly and I have seen this only once and it was A BEAR to find. A perfectly fine looking 1/2 watt resistor in the signal path had drifted to 3X its nominal value. How did I find it? Well, every other # $%^&*( part checked out, so I started measuring resitance "cold" across both channels and comparing the results component by component. When I found a difference, I started lifting legs about that point. And there it was.

Good luck with it.

Peter Wieck Wyncote, PA

I re-cleaned with a cleaner with lubricant last night.

Did not re-do switches. I will though.

I don't have an ESR meter. Is testing plain ol' capacitance any help? I'd like to think it's a capacitor given that that's all I changed out.

This was an overnight change, so unless the change was mechanically induced (definitely not ruling that out) I wouldn't be inclined to look there justyet.

The more I think about it the less likely a pre-amp problem seems... the signal goes through primary amplification (a few transisitors) then through tone controls, subsonic filter, high cut filter, balance control --> power amp. I've got a switch to bypass the tone control, and switches to bypass the filters, plus a switch to reverse the channels between pre-amp and amp. None of these affect the amp output. If I've got a bad cap in the amp stages and am getting a DC offset at my outputs, that MAY be engaging the speaker protection. Or, I suppose, worst case scenario I've baked one of my giant expensive output transistors. I am assuming the speaker protection (being two leaded) is a polyswitch. The distorted sound for a second or so prior to speaker cut-out might match a polyswitch tripping, I don't know as I've only ever had amps with relay protection (or none) in the past. Do you know offhand if polyswitches tend to fail? I suppose probably as much as any other component exposed to variable voltage and current.

I'm going to bypass the preamp section. If I can confirm that the problem is in the amp, I could a) check DC voltages at all transistors until I find one that's off or b) start swapping electrolytic capacitors from channel to channel. there are only four per channel in the amp section.

Yes I've had several of those; they can try your patience.

Thanks, I may just need it.

The posts I've seen in this thread do seem to suggest a problem with coupling caps. Also, I've seen bad (open circuit or intermittent) polystyrene caps on HK's in the past.

If you're not already using them, a sine/square generator and an oscilloscope would be most useful here. In addition to rapidly narrowing down to the affected stage, a square wave might show that you have a frequency response problem in the affected stage. If so, the source of the problem may well be more obvious.

Remember that with coupling caps, you can often get a clue with DC voltages. There may be DC, or a higher DC than the other channel, on the output side of a bad cap, or the following transistor stage may show a low collector voltage.

If indeed the static was not controlled by the volume or balance, then your problem is narrowed down a bit, assuming there is only the one problem involved here

I have the service manual pdf if you need it.

Mark Z.

Thanks Mark, I've got the service manual. I do not currently own a functioning oscilloscope (two fine doorstops), yes it would certainly help here. Ah, but these old amps are simple enough to diagnose without one. Nothing but resistors, caps, and transistors. And time, lots and lots of time.

The problem is definitely in the amp section. I separated the pre-amp and amp and tested both. The pre-amp drove an external amp just fine, the amp exhibited the same symptoms with another preamp.

As there are only four electrolytic caps in each channel of the amplifier, it's a fairly quick fix to swap them out one at a time until (hopefully) the amp comes back to life.

I tested the voltages everywhere in the amp on both channels, wherever voltages were shown in the service manual. The affected (left) channel had voltages a couple of volts higher ON AVERAGE (2-2.5VDC) than the corresponding right channel components which were really close to expected values. I replaced the input coupling cap on the bad channel which did not help.

Following your advice I looked at collector voltages... there is one that really stands out and that is Q405. Expected is -7.1V and I read -0.3V. This collector is tied to the emitters of Q401 and Q403 via a resistor network... the emitter voltages of Q401 and Q403 are also significantly off (low). It's a tough call when your expected voltage is less than a volt... Q401 emitter should be -0.6V and I see -0.2. Q403 should be -0.6 and I see

1. The only cap in the signal path prior to these transistors is C401, the input coupling cap which I swapped out as a first try. Otherwise there's an electrolytic from the negative rail to the base of Q405... that'll be next.

Thanks for your help.

Dave

messagenews:QYi_h.4254$H snipped-for-privacy@newssvr21.news.prodigy.net...

Do you have a good transistor checker? (one that does good/bad and is also capable of matching?)

Where are you (by region)?

Peter Wieck Wyncote, PA

I've got a transistor checker on my multimeter, it shows an arbitrary hfe which, for a given transistor, never matches what's in the data sheet. Of course the gain in the datasheet is always at a particular voltage and current and I'm not sure what voltages/currents are applied by the meter... or whether the voltage or current is controlled at all. I'm guessing 9V at whatever the multimeter battery can put out for current through an unknown series resistor. If it's a truly fried transistor, no value is returned for hfe. I've had as much luck testing transistors with an ohmmeter.

I've never really trusted it to be honest with you.

Are you thinking driver transistor? Maybe one early in the signal path which could introduce DC voltage which carries right through and would explain the elevated voltages I am seeing in the left channel?

I live in Penticton, British Columbia, Canada. It's about 250 miles inland from Vancouver and about 40 miles from the Washington border. It's a town of 30,000 and, as you may surmise, very little of my equipment and parts is purchased locally.

Dave

Exactly on the driver transistor. And "by region" I was thinking if you were relatively nearby I would send you my transistor checker to help through this process. Canada may be squirrelly what with customs and such, by the time it reached you, you would have found the problem. Then you would have to send it back.

I have had very good luck "pairing" transistors when doing repairs/ upgrades for 70s vintage SS amps (Dynaco, Scott, AR, Fisher), that is making sure the 'right-side' transistor is reasonably closely matched to the same 'left-side' transistor. This little instrument really does help.

Not as fancy as their IT-30, but it does the trick

Peter Wieck Wyncote, PA

Thanks for the link. As luck would have it, the seller lives in my home town of Camden, Maine. Weird.

This tool would be really really handy and it's a helluva' lot cheaper than a 'scope.

A couple of the driver transistors are not available, 2SC2603 and 2SA949's. NTE crosses would have to do.

Thanks

Dave

Exactly.

And why I try to get a bit closer than a cross-reference when I use even direct-replacements. Unlike tubes which can vary somewhat, transistors can vary pretty vastly, well beyond what common sense suggests. And even though transistors do not really have a "burn-in", when they begin to fail they can drift for a brief period before final failure sets in.

Peter Wieck Wyncote, PA

There used to be a spray on the market - can't remember who made it at the time, might have been GC - it was called "Blue Stuff". I worked in a stereo service department thru college, and that spray was the best we could find for cleaning stubbornly noisy pots and switches. When you sprayed it on it formed a mildly abrasive blue paste, which would actually scrub the tarnish and dirt off of switch contacts. I never saw it fail, even with the noisiest, most tarnished switches and controls. I think it has diatomaceous earth in it.

Anyway, I think TechSpray carries it - might be worth a try

Maybe not an issue, but the 2603 and the 949 are not a pair. The C2603 is a really small signal type, even smaller than a TO-92 package, The A949 is similar to a TO-92 but taller. Don't know the package designation. I think it's complement is the 2SC2229. They are readily available. I get them from Onkyo and B & D Enterprises. BTW if you register on BDent.com you can get datasheets for these really easily.

I'll try to look over your troubleshooting results with the schem and post back.

Mark Z.

I just picked up an IT-27 on eBay for $9. Plus of course the obligatory $10-15 shipping, I should have it within a week or so.

Man.

This might be really easy. Q 405 conrols the turn-on delay. HK's don't use relays, so they mute the signal til the amp stabilizes. On this model it is Q405 and Q406 respectively. The transistor could be bad, but I would be especially concerned with D401, R405, C405, C407, and D403, which is a 15 volt zener.

Could just be solder connections relating to the above, but in any case you need to see that Q405 turns off hard a few seconds after turn-on. The -12.5 or so volts at the base is critical.

Mark Z.

Mark:

First off, thanks very much for all of your insight and time.

Okay, as I had just "upgraded" C405 and C407, I put the originals back. Left channel is now functional once again, although I haven't had time to check my voltages yet nor have I given it a good listening test. I can't say why, but I've got a gut feeling my voltages are still on the high side. How common is it for zener voltages to wander over three decades?

I set about setting the idle currents and, as soon as I touched V404 (right channel), poof, right channel gone. I think the 1K pot just disintegrated when I moved it after 30 years of sitting in one position. Well, I must be doing something right, because I stumbled across a 1K 70's style (BIG) pot in one of my many doorstops which I am cannibalizing to make myself an ESR meter. I'll put it in tonight, hopefully it's in better condition than the one I'm taking out. One wonders at moments like this whether one is moving forward or backward...

I can't believe there are so few companies that make ESR meters... the more I get into electronics repair, the more critical such a piece of equipment becomes. Ditto with a transistor tester although there seem to be tons of those on the market.

I have a hard time with transistor theory... I understand what they do, but get quite confused when I see a whole bunch of them tied together, controlling each other with various feedback schemes. That's when I start testing components as I lack the deductive skills required to narrow things down, as you did, to a single transistor and associated passive components. Hopefully I'll get better at it... I've read some texts but don't find the ones I've read particularly helpful. They all seem to assume that one is in a laboratory environemnt at school with access to and training on the SPICE program, in particular. Which I do not have.

An oscilloscope and signal generator are handy, but again is somewhat hit or miss (for me) when I get into a transistor network as it's not immediately obvious where the signal path is. It's almost easier in a newer amp with IC's... what you end up doing is replacing an IC with ten transistors in it because you know the problems's gotta' be in there someplace if you've got signal going in and none coming out. If the IC is not obsolete and/or proprietary and hence unavailable. In the past I've used the scope to localize distortion which saves A LOT of time and lifting of legs to test components.

I am quickly learning that with vintage audio gear, if it ain't broke, don't even breathe on it or else...

Thanks again for your help.

Dave