Tandberg 74b noise

[midnightmarauder]

I have a 2ch Tandberg reel to reel, model 74b. One of the channels has a lot of hum, when the r2r is set to playback mode. That is, espessially from the speaker output.

I've attached a block diagram, 3d analysis picture and schematics. I would guess one of the electrolytic capacitors that I've circled out in blue on the schematic must be the cause of the hum. But they all look fine, noe visible leakage. Because only one channel is affected, so I would guess that the faulty one must be C36 or C38. Can anybody tell me if I'm on the right track here?

[midnightmarauder]

I tried interchanging C36 and C38, and the hum is still present on the same channel as before, so I guess I've ruled those out now.

I'm guessing the hum is comming from the power supply, but how can this only be present in one channel? I would say any AC current/DC ripple should be filtered out as long as the caps I've circled out in blue on the schematic are functioning.

By the way, I've replaced C204 and C254 before, because they where leaking.

Also, the hum from one channel is present in the pre-out as well, but not as much as from the speaker-out.

[midnightmarauder]

still hoping to see some response here.. should I start swapping out all the electrolytics or can anybody give me a hint? I'd be greatful for any input on this...

[midnightmarauder]

Obviously I should have said HUM, and not noise when talking about this. Anybody have any ide what parts of the circuit could cause this 50hz and somewhere around 180hz humming in the right channel?

[2N1305]

Well, congratulations on providing us data like that, not many people take the time to show shcematics of what they have, much less 3D plots of a noise analysis!
Anyway, I think it could be bad caps, or even bad grounding. Somewhere.

The thing I would do, like I've repeated SOOO many times here on this forum is get an oscilloscope and track down the noise. In your case follow it from the speaker down to the input. As soon as you see the point where the hum starts to get buried in the noise you're in the right place.

cheers and good luck!

2N

[midnightmarauder]

Thanks for the post 2N, yes I thought they'd be helpful when diagnosing this problem. I hoped a tech could tell me where the hum most probably comes from. But I guess it's time to invest in an oscilloscope. I need it for troubleshooting more gear as well...

Any tips on what to get? I need a scope that is not too expensive but still usable for higher voltages like in tube circuits.

Originally Posted by 2N1305

Well, congratulations on providing us data like that, not many people take the time to show shcematics of what they have, much less 3D plots of a noise analysis!
Anyway, I think it could be bad caps, or even bad grounding. Somewhere.

The thing I would do, like I've repeated SOOO many times here on this forum is get an oscilloscope and track down the noise. In your case follow it from the speaker down to the input. As soon as you see the point where the hum starts to get buried in the noise you're in the right place.

cheers and good luck!

2N

[2N1305]

You're quite welcome. I'd say anything on ebay, providing it WORKS. I mean, if you're looking for a bargain, some older scope from the seventies or eighties like a tek 465, 455 or an HP1201. also something that works on 115V. That's what you have in Norway, right, and 60Hz, too?

check out this page for details on Tektronix scopes. I doubt you'll need anything of this caliber, nor this frequency range, but it's vaulable reference nonetheless. Tektronix Portable Series Oscilloscopes

[midnightmarauder]

Thanks for the tips and reference link. We have 230V 50hz in Norway. But I'm using some 115V gear plugged into AC converters. Always works fine, but would that be a problem with oscilloscopes considering the different hz rating?

[2N1305]

Originally Posted by midnightmarauder

Thanks for the tips and reference link. We have 230V 50hz in Norway.

Boy, I was way off...

Originally Posted by midnightmarauder

But I'm using some 115V gear plugged into AC converters. Always works fine, but would that be a problem with oscilloscopes considering the different hz rating?

Yes, I think it would be, but I could be wrong. Actually, I think it would be quite wrong considering the RMS voltage on 50Hz is slightly lower than it is with 60Hz, so you could get lower voltages from the power transformer making the scope not work properly.
I believe their models had a transformer with two primaries, possibly more, in order to be re-wired to 230V.

If you do have money then order a new tek scope on ebay, one of thos digital phosphor, they are protable lightweight, and they have an auto-measure feature. Frankly I like to find my signal on my own...

[midnightmarauder]

Originally Posted by 2N1305

Boy, I was way off...



Yes, I think it would be, but I could be wrong. Actually, I think it would be quite wrong considering the RMS voltage on 50Hz is slightly lower than it is with 60Hz, so you could get lower voltages from the power transformer making the scope not work properly.
I believe their models had a transformer with two primaries, possibly more, in order to be re-wired to 230V.

If you do have money then order a new tek scope on ebay, one of thos digital phosphor, they are protable lightweight, and they have an auto-measure feature. Frankly I like to find my signal on my own...

So I got myself an old Kenwood scope, 230V powered. But I'm still waiting for probes to arrive. Thanks for all your input 2N1305. Hopefully I'll get to the bottom of this..the Tandberg 74b has a very punchy and nice eq thing going on when running tracks or busses (hot level) through the tape. Really looking forward to get it up to shape! I'll post back if I find out where the fault is

[LeeYoo]

Hi,
Some questions.
Is the hum in the speaker still there when you turn the main volume down.
Did you try putting the deck in record for a second or so and then on playback again. Rec/playback switches sometimes get half stuck or dirty.
Did you already swap output valves.
Voltage can be measured with a digital meter on DC.
Ripple (hum) on a cap can be measured with the meter switched to AC.
C204/254 can't introduce hum.
Yes, C36 or C38 could be faulty, but this would only affect ONE of the speaker outputs, not the line output.
Leo..

[midnightmarauder]

Originally Posted by LeeYoo

Hi,
Some questions.
Is the hum in the speaker still there when you turn the main volume down.
Did you try putting the deck in record for a second or so and then on playback again. Rec/playback switches sometimes get half stuck or dirty.
Did you already swap output valves.
Voltage can be measured with a digital meter on DC.
Ripple (hum) on a cap can be measured with the meter switched to AC.
C204/254 can't introduce hum.
Yes, C36 or C38 could be faulty, but this would only affect ONE of the speaker outputs, not the line output.
Leo..

Thanks Leo!
The hum goes away when I turn the main volume down, and it's present when the switch is in rec and playback mode. When I set the switch to Amp it is not present. So I guess the problem is in the playback preamp (channel 1): I tried swapping output valves, and also the preamp valves. The hum is still present only on channel 1.

When connecting the scope probe to C111 I see the hum is present there. when I ground the point between C111 and C112 the hum goes away.

Where can I connect the probe to check if the hum appears before or in the V1b stage?

[LeeYoo]

Hi midnightmarauder.
If the hum goes away when you turn the main volume pot down, you can rule out the stages behind it.
I think you are right. The problem must be in or around the pre-amp stage.
Any hum here must come from the supply, or from the playback head/wiring.
Hum is introduced (supply, shielding, grounding), unless the circuit itself is oscillating.
I would short the negative side of C106 to ground (head side/muting switch).
If the hum goes away, check the head/wiring.
If not, scope the +200v rail on C136. Scope on AC, 1volt/div.
It should be clean.
Then check anode V1b with the same settings.
You could also scope collector Q1.
Post the results.
Leo..

[ruffrecords]

Your hum plot seems to me to have more 60Hz than 120Hz component, which to me says it is NOT a PSU problem but plain old mains hum getting into the replay amp somehow. I concur with LeeHoo you should short the replay input and see if the hum disappears.

Cheers

Ian

[LeeYoo]

Uhhh, almost right.
Norway (and the UK) are 50 (100) herz, not 60 (120).

Did you rule out a (big) transformer nearby.

[drtechno]

try cleaning the speed select switch

[midnightmarauder]

Originally Posted by LeeYoo

Hi midnightmarauder.
If the hum goes away when you turn the main volume pot down, you can rule out the stages behind it.
I think you are right. The problem must be in or around the pre-amp stage.
Any hum here must come from the supply, or from the playback head/wiring.
Hum is introduced (supply, shielding, grounding), unless the circuit itself is oscillating.
I would short the negative side of C106 to ground (head side/muting switch).
If the hum goes away, check the head/wiring.
If not, scope the +200v rail on C136. Scope on AC, 1volt/div.
It should be clean.
Then check anode V1b with the same settings.
You could also scope collector Q1.
Post the results.
Leo..

Now I'm confused..
- When I short the negative side of C106 to ground I get insane hum on the output. Same thing happens on channel 2 when I short the equivalent C156. Don't know how to figure this out..?
- When I scope the 200V (measures 218VDC) rail on C136, with scope set to 1V/AC, it's clean. Anode V1b and Q1 collector also clean.
- I cleaned the contacts on the speed switches and pb/amp/rec switches with deoxit. No improvement.

[midnightmarauder]

By the way, I found out that one of the EL84 power amp tubes is bad. When I tap on it, it's output puts out a lot of high frequency noise. When I swap the valves it jumps to the other channel and has no effect on the forever problematic 50hz hum. So I can't see any reason this faulty valve could have anything to do with it. If I had another working EL84 I'd stick it in there but I don't. But I can rule this one out, or what do you guys think?

[midnightmarauder]

Originally Posted by LeeYoo

Uhhh, almost right.
Norway (and the UK) are 50 (100) herz, not 60 (120).

As LeeYoo says, Norway mains are 230V/50hz. Why would a 100hz hum indicate a psu problem while 50hz wouldn't?

Originally Posted by LeeYoo

Did you rule out a (big) transformer nearby.

Don't know of any, unless it's buried in the ground. Anyway, could that affect only one of the channels?

[LeeYoo]

Originally Posted by midnightmarauder

By the way, I found out that one of the EL84 power amp tubes is bad. When I tap on it, it's output puts out a lot of high frequency noise. When I swap the valves it jumps to the other channel and has no effect on the forever problematic 50hz hum. So I can't see any reason this faulty valve could have anything to do with it. If I had another working EL84 I'd stick it in there but I don't. But I can rule this one out, or what do you guys think?

Yes, valves do go bad this way.
They are as reliable as a lightbulb.
I have replaced many thousands of them.
Live with it, or replace it.
And yes, it has nothing to do with your fault.

Q: Norway mains are 230V/50hz. Why would a 100hz hum indicate a psu problem while 50hz wouldn't?

A: 50 herz is picked up by wiring, and a 100 herz ripple is present on the caps of the supply because of full wave rectification.
http://en.wikipedia.org/wiki/Full_wave_rectifier

Leo..

[LeeYoo]

Originally Posted by midnightmarauder

Now I'm confused..
- When I short the negative side of C106 to ground I get insane hum on the output. Same thing happens on channel 2 when I short the equivalent C156. Don't know how to figure this out..?
- When I scope the 200V (measures 218VDC) rail on C136, with scope set to 1V/AC, it's clean. Anode V1b and Q1 collector also clean.
- I cleaned the contacts on the speed switches and pb/amp/rec switches with deoxit. No improvement.

Shorting the negative side of those C's to ground should have stopped the hum there.
I can't explain this, unless that ground wasn't ground.
Try to scope anode V1b/V2b, and V8a/V9a. Scope to ac 1V/div or less and compare.
Maybe we're looking in the wrong place.
Leo..

[midnightmarauder]

Originally Posted by LeeYoo

Shorting the negative side of those C's to ground should have stopped the hum there.
I can't explain this, unless that ground wasn't ground.
Try to scope anode V1b/V2b, and V8a/V9a. Scope to ac 1V/div or less and compare.
Maybe we're looking in the wrong place.
Leo..

Maybe we are in the right place, I'm not sure.. because when I set the scope to 1mV I get a clearly visible (sinus)signal on V1B. It's not visible on V2B.

When I scope V8 and V9, I get the same result on both anodes on V8.

results:
V1A . clear V1B . wave
V2A . clear V2B . clear

V8A . wave (set to 20mV/div)
V9A . clear

Same thing when I turn the volume pots all the way down. when I say clear I mean with some noise but without the wave.

time to dive into the schems again. You status is mentor like in my opinion right now Leo! Thanks! very educational for me. because - I really hope to get this r2r fixed its no studer but its real nicely built and a pretty sweet coloring tool. :rainbow:

[midnightmarauder]

According to the schematic both channel uses the same filter cap C136 for V1b/2b. But for V8a/9a there is C201 for ch1, and C251 for ch2. These are all mains filter caps, right?

DC Voltage across C251: 209V
C201: 227V
(measured no AC Voltage on any of them)

[LeeYoo]

Hi midnightmarauder.
I don't think it's the supply caps, since V1b has it's own cap/resistor, and the hum is already present there.
I think it is time to look closer at the parts around Q1.
If the hum isn't pickup by the PB head/wiring, it could be the caps around Q1.
But first measure the voltages on it's collector and emitter. About 12 and 20volt, and compare with the other channel.
Start with C107. Don't take it out yet. Just put another one across.
Leo..

[midnightmarauder]

Originally Posted by LeeYoo

Hi midnightmarauder.
I don't think it's the supply caps, since V1b has it's own cap/resistor, and the hum is already present there.
I think it is time to look closer at the parts around Q1.
If the hum isn't pickup by the PB head/wiring, it could be the caps around Q1.
But first measure the voltages on it's collector and emitter. About 12 and 20volt, and compare with the other channel.
Start with C107. Don't take it out yet. Just put another one across.
Leo..

Voltages DC on transistors:
Q1 emitter: 17,8V Q1 collector: 13,4V
Q2 emitter: 18,2V Q2 collector: 14,0V

I tried putting new, unused caps across the ones around q1, here's what happened:

C107: The wave was cut and spread up and down the vertical axis.
C108: Increased the amplitude on the wave.
C106: When putting a 6,8uF cap (should have been 5uF but I don't have that) across, the wavelength and amplitude doubled.

I can't figure anything out from this, hope you can Leo..

By the way, compared to Q2, I scope hum on Q1 emitter with scope set to a couple of mV's.

[midnightmarauder]

1st picture: no caps added
2nd picture: added c107
3rd picture: added c108
4th picture: added c106

[LeeYoo]

So a bigger cap for C106 increased the hum...
To me, that indicates hum pickup from the mains transformer into the heads, or wiring.
One more trial. Completely remove C106, or lift the +side, and see if the hum disappears.
Leo..

[ruffrecords]

Sometimes hum can be due to a poor heater cathode insulation. Try swapping V1 and V2.

Cheers

Ian

[midnightmarauder]

Originally Posted by LeeYoo

So a bigger cap for C106 increased the hum...
To me, that indicates hum pickup from the mains transformer into the heads, or wiring.
One more trial. Completely remove C106, or lift the +side, and see if the hum disappears.
Leo..

I removed it completely, what happens is I get LOTS of noise. The amplitude of the noise makes it hard to see/hear but I think the hum is gone with c106 out.

[midnightmarauder]

Originally Posted by ruffrecords

Sometimes hum can be due to a poor heater cathode insulation. Try swapping V1 and V2.

Cheers

Ian

Thanks for the input and info Ian. I tried that already so here that's not the fault