Attempting vintage receiver repair - Please Help!

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Ok, so I received an early 70's HH Scott R 36 S solid-state receiver and all it does is hum. The hum is independent of the volume control and input selection. From what I've read on this so far, the problem is most likely bad filter can-caps.
So I opened it up and took a look at the big capacitors. There one big 70v/4700uf and two 35v 2200uf. I took them out of the circuit and started testing. Capacitance meter says they're values are good. Ohm meter doesn't indicate shorts or opens. I even hooked a 9v to them one at a time, unhooked and watched the voltage drop with a multimeter. So far the caps are testing good, but they're 40 years old and the thing just hummmmmss so I'm not convinced.

I would just replace them and see if that works but I'm on a budget and it seems like I should be able to verify good or bad caps between a capacitance and multimeter.

Would testing the caps to see if they hold a voltage over a period of time be a good indicator of anything? If I charge them up with a 9v, should I be able to let them sit for a few hours and then see it discharge with the multimeter?

I've scoured the web and I'm lost when it comes to testing caps. One person will say get a cap meter, the next an esr meter, and the next says you need both, then another says they're all worthless. So I'm a little confused!!!

If it's not the caps causing this hum, what else could it be?

Thanks in advance for your input!

 

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Also, if pics of the receiver would help just lmk and I'll post some. I'm all for helping you help me!

 

[Jagtech]

Cap testers are not 100% reliable, as some caps only fail when actually under load. Electolytic caps really aren't very costly, usually only a dollar or two. I would just replace any that are suspect in the power supply, and save yourself a lot of time and trouble.

 

[(*steve*)]

Cap testers are not 100% reliable, as some caps only fail when actually under load. Electolytic caps really aren't very costly, usually only a dollar or two. I would just replace any that are suspect in the power supply, and save yourself a lot of time and trouble.

More precisely, capacitor testers are usually capacitance testers, and capacitance is just one of the important features of a capacitor (arguably the most important).

There are other things like ESR, voltage rating, tolerance, etc that can be important.

In a power supply, ESR is often the big one, but that typically only becomes an issue for very old devices or those with switchmode supplies or those capacitors which get very hot (and typically applies to electrolytic capacitors)

 

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@Jagtech- A few dollars a pop plus shipping plus waiting time adds up. I'd like to make sure these caps are dead before wasting time and money ordering new ones, if at all possible that is.
@*steve*- Is there any way to test for ESR without an actual ESR meter? I have plenty of spare components and a few types of meters if there's any kind of circuit I can throw together to determine it.

 

[(*steve*)]

@*steve*- Is there any way to test for ESR without an actual ESR meter? I have plenty of spare components and a few types of meters if there's any kind of circuit I can throw together to determine it.

Not easily.

It's possible to do it with a signal generator and an oscilloscope, but it's not easily quantified.

With caps in parallel you often need to remove them to test the ESR anyway (as one may be faulty)

At this point I typically replace them with new ones even if the old ones test good. The only exception would be if all test good and I don't have suitable replacements on hand. If it turns out not to have been the problem you know you've eliminated it and it won't weigh on your mind.

For a single capacitor I compare the reading to a new one and replace it if it seems significantly higher. Again, I might be replacing a good capacitor, but at least I'm replacing it with a better one.

Things get more complex where you have unusual size components to allow for clearance. These may be harder to get and you might retain them if they test OK.

However, you're not in a situation where the caps will have died in the same way that switchmode power supply filter caps die. Old electrolytics can leak (electrically) or lose capacitance, but ESR is less of a problem. If left unused for a long time the capacitors can lose the ability to work at their rated voltage. There are also other types of capacitors which may have decayed over time. None of these problems will be detected by an ESR meter. (And even a high ESR might be either normal or not affect the circuit).

I realise I'm giving you somewhat contradictory advice -- sorry.

Given that your problem is hum. My first task would be to replace the power supply filter capacitors (even if just temporarily) to see if the hum is coming from there.

I'd also measure the current drawn from the various supply rails and make sure it was reasonable.

From the size of the filter capacitors I imagine this has a reasonable power output. If so, at low volume (presuming the hum isn't extremely loud) the power supply should be lightly loaded and hum would be less likely to be coming from there even if the capacitors were a little dodgy.

Given that the hum does not respond to the volume control, either the volume control is broken, or the hum is being injected after the volume control.

How loud is the hum compared to what you would imagine the normal listening volume would be from this radio?

 

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It's pretty loud. I would guess it's equivalent to about 1/3 of the total volume.
And thank you for your time and advice.
I'm digging through my broken appliance pile and I've found a 2200uf 35v electrolytic among a few other spare caps. I'm going to keep digging and maybe I'll find more suitable replacements. I've heard that with filter caps you can replace them with caps of higher capacitance than the original, within reason, and they'll work fine. If this is true, I may be able to dig up a few more caps and at least rule out whether or not that is the problem.

Edit:

It's pretty loud. I would guess it's equivalent to about 1/3 of the total volume.

Have you tried a line input signal as well as the tuner signal?

I'm digging through my broken appliance pile and I've found a 2200uf 35v electrolytic among a few other spare caps. I'm going to keep digging and maybe I'll find more suitable replacements. I've heard that with filter caps you can replace them with caps of higher capacitance than the original, within reason, and they'll work fine. If this is true, I may be able to dig up a few more caps and at least rule out whether or not that is the problem.

Yes, that's true. Make sure the new ones have an equal or higher voltage rating.

 

[KrisBlueNZ]

Pictures would be useful. One from above, showing the whole thing, then two of the circuit board - one from above and one from below.

If there's no signal coming through, just a hum, this could mean that the output stages of the amplifiers have failed and slammed against the positive supply rail. If the output stages are capacitor-coupled. It's not so likely that both channels would fail in that way, but possible I guess. But that's just one possibility.

I tried to find schematics and service information but couldn't find anything. Have you tried?

 

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I've tried it on each input and there's no change.
I've checked the output transistors also and they tested good. I also looked all over for schematics and couldn't find anything.Here are a few pics:
Top

Filter caps

Bottom (changed out a split cap for the orange drop)

 

[KrisBlueNZ]

OK, thanks for the photos. They're a good start.

You've probably figured this out already, but...

The board between the electrolytics is the power amplifier, with the output transistors mounted on the back panel and connected with wires.

The board on the underside in front of the mains transformer is the power supply. The big electrolytic, 4700 µF 70V, will be the smoother for the supply to the power amp. It will be fed from the power supply board.

Can you measure the voltage across this big electrolytic with the power ON?

Do you have an oscilloscope? If so, measure across the electrolytic and see how much ripple is there. Tell us the peak-to-peak ripple depth.

The smaller electrolytics (2200 µF 35V) will be the speaker coupling capacitors. Can you measure the voltages on the positive sides of them with the power ON? It should be about half of the voltage you measure on the 4700 µF electrolytic.

Can you trace the wires from the volume control? There's probably a twin screened cable, or two separate screened cables, connected to the middle tags of the potentiometer, and feeding to the power amp board.

A few close-up photos would be helpful - the power amp board (both sides) and the output transistors, and the power supply board.

Is there any sign of overheating? A funny smell when the unit has been on for a while?

 

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Thank you for your help.
While taking measurements on the 2200uf cap on the opposite side of the power amp of the 4700uf cap, I accidentally touched the negative probe of my multimeter to the receiver case while the positive was on the cap positive. When that happened, one of the channels went out (stopped humming), and a small puff of smoke came up from below the cap. I'm assuming that cap and the output transistor it's connected to are now blown. I'll check the OTs tomorrow when I have a chance.
With that being said, here are the measurements I got from the caps with the power on:
4700uf = 67vdc
2200uf (blown) = 54vdc
2200uf = 35vdc

I looked at the volume pot, and it has a few wires coming off of it, and is also soldered into a circuit board. I'll see if I can unmount the circuit board and take pics tomorrow when/if I get a chance.

No odd smells or sign of overheating (except when I blew that cap)

I don't have an oscilloscope right now, but I actually have plans to purchase one from a guy on craigslist this weekend. It's old school (circa 1970's I think) and needs some work done on some of the potentiometers from what I'm told, but it works and he's giving it to me for $20 so hey

 

[(*steve*)]

That puff of smoke may have been a track vaporising. But yes, you may have taken out other things as well.

Remember *exactly* what you shorted out, it may be helpful in tracking down what has been damaged

 

[KrisBlueNZ]

While taking measurements on the 2200uf cap on the opposite side of the power amp of the 4700uf cap, I accidentally touched the negative probe of my multimeter to the receiver case while the positive was on the cap positive.

You must have done more than that. Connecting the positive and negative probes to any different points in the circuit won't cause any damage (unless the multimeter is on a current range). Did you touch one of the probes to two different parts of the circuit at once?

If there's a mains voltage selector, check that it's set correctly.

With that being said, here are the measurements I got from the caps with the power on:
4700uf = 67vdc
2200uf (blown) = 54vdc
2200uf = 35vdc

Those are the voltages on the positive terminals of the caps, measured relative to the chassis?

I don't have an oscilloscope right now, but I actually have plans to purchase one from a guy on craigslist this weekend. It's old school (circa 1970's I think) and needs some work done on some of the potentiometers from what I'm told, but it works and he's giving it to me for $20 so hey

It sounds like it will be useful for this job.

 

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Hey guys,
sorry I haven't had a chance to update, but I finally have a day off, and I have detailed pics.

So I took some pics and edited them to show EXACTLY what happened when I lost a channel taking measurements the other night. In the pic below, the red circles show where my probes were when I got a puff of smoke and the channel died. The positive lead was on the caps + and the negative on the chassis. The red arrow shows where the smoke came from, and under this pic there's another of the opposite side of where the smoke came. (note: the multimeter probes aren't touching anything in these pics, it's only on to show where it was set and where the probes were plugged in at the time of the incident)

It also occurred to me that the power cord is two-pronged. Does this mean that the chassis isn't properly grounded?

I also made a pic for my attempt to take measurements last night that resulted in a blown main fuse (now replaced).

If y'all could tell me what to set my meter to and where to place the leads, I'd be happy to share the outcome.

I also got down to the volume circuit and took some pics. While I was in there, I noticed one of the two film caps attached to the volume pot is pretty bulged at the top. I know what bad electrolytics look like, but I'm not familiar with bad film caps so I don't know what's normal and what's suspect. Here are the pics:
Bottom:

Top:

Vol pot right:

Vol pot left:

Vol pot back:

Suspect film cap:

I checked the mains selector and it's set to 120v

I also took some pics of the speaker outputs and the OTs. When I had the multimeter incident the other day, it was "Speakers 2" that went out. So as it is right now, with Speakers 1 on there is a loud hum (same as before), with Speakers 2 on and Speakers 1 off, there is a very quite hum that is barely audible.
I checked the OTs and they tested fine with the diode function, and none were shorted between Emitter and Collector. (I did get some ohms between the E and C terminals. but they were several MegOhms.)
Anyway, here are the pics:
Outputs back

Outputs bottom

Outputs top

If y'all need any more pics of any specific areas to help me get Speakers 2 back on and rid of this hum, just lmk. Thanks again for your time and assistance.

 

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Oh and I almost forgot, here's a pic of the old oscilloscope I got off craigslist for $20. It powers up fine but I need to get some probes for it before I can see if it works correctly or not. It's a '64 Tektronix Type 545B. Came with the original cart too

 

[KrisBlueNZ]

OK, thanks for the pics.

I don't know what happened with the puff of smoke. Just connecting a multimeter to measure voltage between two points in a circuit won't cause any damage. You must have accidentally touched two different points with the positive probe and shorted them together. There's no obvious sign of damage though.

Looking at that amplifier board, I'm concerned about the likely condition of most of the components on it. Transistors of that vintage are common problems, and those old carbon resistors are known to drift in value over the decades. This could be a problem on all the other boards as well. I wouldn't necessarily say that there's no point fixing it, but I wouldn't invest too much of anything in it, because you may need to cut your losses :-(

Yes, if the power cord has only two prongs, the chassis is not grounded. The mains cable will only have two wires as well, so you should replace them both. The earth connection to the chassis should be done through a hole that's not used for anything else, preferably tapped (otherwise, a screw and nut is OK), using lock washers, and a tag with the earth wire crimped firmly into it. Make the earth wire longer than the others, and put the chassis connection near where the cable enters, so if the cable gets pulled out, the earth connection will break last.

That bulging capacitor was like that from manufacture.

OK, if you want to start fault-finding, this is what I suggest.

Take photos of the amplifier board from above and below, viewed directly in line with the centre of the board. Don't use a flash - you can see what happens if you do, in your second to last photo. See the tutorial at https://www.electronicspoint.com/take-photos-circuit-boards-t267883.html

I need to know how the output transistors connect to the circuit board. The pinout for the "TO-3" package is at http://pinout-circuits-images.dz863.com/131/BD121.jpg (the drawing on the left is the bottom view, looking into the pins).

Can you annotate the top side photo to show where each of the wires is connected on the rear panel circuitry. Number the output transistors Q1~4 from left to right, and for each binding post on the edge of the board, annotate it with "Q1 base", etc. There are four 0.47 ohm power resistors, and some other components, on the back panel as well, and some of the wires don't just go to transistor terminals. Can you draw this up somehow?

Also, measure the DC voltages on all of those pins, with the negative multimeter lead to the chassis. Be very careful to position the multimeter probe so that if it slips, it will not touch another post and short anything out. Also be careful not to touch the posts yourself - there's enough voltage there to give you a slight to moderate electric shock.

Also, can you annotate the top view photo with the part numbers for all of the transistors (grey devices with three leads) on the board.

There are binding posts that don't connect to the output transistors. I guess the one at the left with the red wire is the positive supply from the big electrolytic? Can you mark the binding posts with the destination of the wire that connects to them? I expect there will be two or three wires that go to posts 6, 7 and 8 on the volume board; these will be the signals from the volume control.

Any other information that you think might be helpful, please mark it on the photo as well.

 

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I'll make grounding the chassis and changing the power cord a priority before I plug it in again. I'll work on the pics/diagrams you requested. Probably have them up in the next couple days.
I'm fixing this for a good friend who has some sentimental value invested in it. Her family has had it since it was purchased new in the early 70's. I'm going to talk with her sometime this week and suggest replacing most if not all of the passive components.
I figured new caps, resistors, and transistors for it. Are there other components I should replace also? Do diodes hold up over time or should I replace those as well?
Lastly, as I was inspecting all the caps in this thing, I came across two components I'm not sure about. I think ones an inductor and the other some type of cap. Could you help with these?
The one with the two colored dots. Inductor?

And the fat blue ones here marked 103j and 563j, capacitors? (0.01uf and 0.056uf respectively?)

I looked online but I couldn't find anything that looked like either of these.
Thanks again for all your help.

 

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One last question for tonight.
What type of cap would you replace all the non-electrolytics with? I was thinking metallized polyester. That is with the exception of the ceramic caps that go from inputs to ground, I've read that those should be replaced with other ceramics.

 

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I was thinking on replacing the big filter caps with these
2200uf
4700uf

 

[(*steve*)]

Do you want them just to work, or would you also like to keep them looking like the originals?

If you just want them to work, your suggestions seem fine. Replace capacitors with those having similar or higher voltage ratings too.