Sugden A48 Mk2

[zeagan84]

New to the forum and have some hope for help from some more experienced tinkerers/repair folk.

I've got a nice little Sugden that I picked up originally to sell but I'm falling a little bit in love with it and would ideally like to get it in tip top shape. The issues are pretty minor but they're an annoyance.

The first is that at low input/no input there's a very light intermittent crackle on the right channel. It's not frequent and usually occurs when it's first switched on from being off for some time. Wondering what might be the best place to start looking, the crackle is independent of which input is selected, volume setting or tone adjustment which leads me to believe it's in the power amp section.

The second little niggle is the speaker selector switch, it doesn't really seem to do much of anything. Both l.s.1 and l.s.2 always seem to be driven and the push buttons do nothing.

As for what I'm working with, I have a multimeter, a soldering iron and patience and any parts needed I would need to order.

Some notes, the service info said to set the midpoint voltage across the first 10,000uf capacitor to 38V for each channel which I have confirmed and to set the current across the main fuses for each channel to 160mA, the VR was very very touchy and the best I could get it to was 150 on one channel and 152 on the other once they settle, much better than the 190/81 that they were at when I first tested it.

Oh and the serial is past the 4200 mark as there were some revisions at that point.

Looking forward to any help or advice any of you may have.

[dave cox]

It does rather depend on the sort of noise !

A recording of the noise might be helpful.

A first stab would be the speaker switches (especially if there is one hidden inside the headphone jack). Also a problem in the soldering between amplifier output (R34/R35 junction ?), output switches, headphones jack & output blocking cap (C17 ?). Also C17 could be a bit leaky ... Since this an ac coupled output problems here could cause noise with no input signal.

Otherwise the poweramp internals. I don't recognise any transistors that are known for noise issues. Might be worth trying to heat / cool them (small signal ones) in turn to see if the noise comes / goes. Another possiblility is the endcap of a resistor. I would avoid poking or prodding with the power on, if something breaks its quite possible it will self destruct

dc

[zeagan84]

The noise would best be described as the kind of soft static you would hear when tuning on a receiver. Reasonably quiet and totally independent of volume level.

I'll take a video when I get home later, just writing some finals and studying some laplace transforms. Good times.

I'll also take a look at the solder joints and see if any look cold or broken and check that output blocking cap for swelling or any other signs of failure. The prices for most of the parts in this amp seem pretty reasonable which is nice, it's just hunting down the problem without creating new ones.

[Mooly]

You might find freezer spray useful if the fault occurs when cold. A cheap 'Air Duster' can used inverted will give a freezing action. Practice being able to deliver the stuff a single drop at a time onto suspect components.


Although a noisy semiconductor could be a possibility, I would also look at the three small non electrolytic caps. Edit... make that four.

A tap around with a plastic pen etc is useful to determine if there is a soldering issue.

The circuit diagram isn't clear on whether the switches are push buttons or even relays. There does appear to be a relay that is intended to give a delay at switch on and to charge the speaker coupling cap first (to eliminate switch on thumps).

[zeagan84]

Originally Posted by Mooly

The circuit diagram isn't clear on whether the switches are push buttons or even relays. There does appear to be a relay that is intended to give a delay at switch on and to charge the speaker coupling cap first (to eliminate switch on thumps).

That is interesting...because it does not have a delay when I switch on the mains, and it does a little thump.

Also, will pick up some freezer spray on the way home too for trouble shooting.

[Mooly]

It may still give a small audible thump because the time constant of 10,000uF and the 47 ohm used to charge it is pretty long. Even a voltage of a few hundred millivolts will give an audible thump.

I would say you need a delay of around 6 seconds for the noise to be inaudible.


If there is no delay at all (speakers connect instantly) then I would first make sure that there are no model variations to account for that. i.e. was the relay fitted to later production versions only.

[Mooly]

This shows the relay driver and delay circuit (top right) together with the delay relay contact (arrowed) and the presumably manually operated speaker switch that selects between headphone and speaker feed.

This diagram seems to show just one set of speaker switching i.e main and headphones.

Maybe there are some variations. This diagram is from a mk III

[zeagan84]

Just found an updated diagram for the variant I have, it looks right to me. My circuit knowledge is pretty limited if I'm honest but for the speaker switching it looks like it's relays s13 and s14.

Also would RLY./2 maybe be the delay one when it's powered on?

[zeagan84]

So I did a little poking around today, tried cooling all of the transistors which didn't seem to have a noticeable effect on the static. Tapped on the boards to look for connection issues and inspected capacitors visually for any swelling and didn't find anything.

I took a video of the sound that is happening.

https://drive.google.com/file/d/1kN5...Or0SuWmwz/view

And also it seems like a few things have been...adjusted over the last few decades. I've attached pictures of the relay, the speaker switches that do nothing and also a little area that seems to have a sticky substance all over it but no source for said substance.

[Mooly]

The delay circuit in your diagram is far more conventional. The delay is set by the charging of C51 via R53 and the Zener in series with that resistor. So there should be a definite delay before the relay coil is energised. That is something that is easily checked by monitoring the voltage across the cap... as it rises the transistor should turn on and the relay energise.

When the relay pulls in it connects the speaker feed to the amp via RLYJ2

The contacts RLYJ1 are a neat trick and are used to discharge the timing cap on power off. This ensure the circuit is instantly ready to give the full delay again if the amp is turned back on immediately.

S13 and S14 look to be conventional manually operated switches.

I know from experience that working from pictures is extremely difficult but given a little time and a methodical approach you should be able to trace the wiring and see why the switches appear to do nothing.

Couldn't just listen to your file... something about needing access.

I normally use Dropbox for that kind of thing as there is nothing for those persons the file is intended for to install etc.

[zeagan84]

Thanks so much, I feel like I at least have somewhere to start looking now. Also, I hadn't set the permissions right for the video, it should be fixed now to where you can hear the noise it's making.

I've got 2 weeks with nothing to do between the end of semester and full time summer work so this is a perfect little project.

[Radio Wrangler]

Yes, that sound is definitely a fault. It could equally likely be a connection, a transistor or a resistor. The sound doesn't give any clues, they can all sound like that.

At a low level, it's difficult to trace. You wind up swapping things until eventually you get find the errant part. Slow, tedious, but worthwhile.

Your photos show carbon composition and carbon film resistors. Some audiophiles love carbon resistors of all types. An engineering approach says that a carbon film resistor in good condition is always several decibels noisier than a good metal film part. Carbon film resistors sometimes fail. Carbon composition resistors often fail. A pragmatic viewpoint says that a lot of equipment was made with them and sounded OK, but that was when the parts were much younger. Several times their life expectancy has gone past.

The low power transistors are a possible source of the noise. The types used don't have bad reputations, but they still have a low failure rate.

Careful substitution will get you there.

David

[zeagan84]

I'm thinking at this point I might do a bulk order of resistors and transistors, the only components that are above a dollar or two seem to be the screw top capacitors so it shouldn't be too costly and just start replacing and testing.

One other thing, there's a hole in the chassis for an IEC sized plug, would it be a terrible idea to upgrade the power cable to a male IEC socket with a ground while I'm in there?

[thermionic]

As David has mentioned, the carbon composition resistors are very suspect. Some lead a hard life and are prone to changing value. If you want the best out of this amp, it’s in my experience of repairing many of these over the years, that you test them all for tolerance.

Also, there is a 1 Watt, 1.5K carbon film resistor on each power amp pcb that should be changed. These get slowly cooked over time and are a candidate to cause problems.

Good luck.


SimonT.

[Mooly]

My advice (fwiw) would be to try and find and fix the fault rather than just blanket changing parts, something that can in itself cause problems unless you are careful.

I would still advise to first try freezing parts to try and get a clue, but practice dispensing the stuff a single blob at a time.

[dave cox]

How long does the noise last for and does it slowly fade out or just suddenly stop ?
How did you cool the transistors ? You need to REALLY cool them, but not quite so much that a block of ice forms!
( I know, questions, questions )

If the noise is just for few seconds then its likely not something heating up as that's probably going to take more like minutes. It might be worth checking the operating conditions L v R channel, this might give another clue.

As for shotgun replacement when all else fails ( we've all been there, even Mooly ) I would start with the small capasitors as they have quite a limited life anyway. Low levels of noise can be a real to find

dc

[michamoo]

If this was a 1970s era Pioneer i would be looking at the underside of transistors in the output stages. A black oxide used to build up between the transistor legs giving the symptoms you have there.

[zeagan84]

Originally Posted by dave cox

How long does the noise last for and does it slowly fade out or just suddenly stop ?
How did you cool the transistors ? You need to REALLY cool them, but not quite so much that a block of ice forms!
( I know, questions, questions )

If the noise is just for few seconds then its likely not something heating up as that's probably going to take more like minutes. It might be worth checking the operating conditions L v R channel, this might give another clue.

Honestly the more I test it the more I'm noticing that it persists even when warm, it's just a little less pronounced, also because it's not volume dependent it's very easily drowned out specifically while warming up the amp.

For cooling I was perfecting the trick Mooly suggested of single drops from an inverted air duster can. A tiny bit of frost was appearing on the transistors so I feel they're probably getting cool enough. Unfortunately haven't found an offending one yet that exacerbates or diminishes the noise.

[dave cox]

Sounds like you got it right with the cooling
This doesn't mean the transistors are OK, though

Another trick is to swap the small signal transistors round, one at a time, L channel to R channel. Be careful with the PCB, I don't know how durable they are in this amp, I'm sure someone here has worked on one before !

dc

[zeagan84]

That's not a bad idea, one thing that is a little annoying about the layout is that the PCB has to fully come out and around 4 connections that I can see need to be unsoldered to do so. I'm thinking take the PCB's out and solder in some extensions to get it hooked back up with them out of the chassis to do more testing?