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Old 4th March 2013, 04:54 PM   #21
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Quote:
Originally Posted by Ian Finch View Post
Q14 I can't locate (Is it Q401?) but it may not be the problem you suspect. Voltages at RF frequencies don't show up on a DMM or on a low frequency 'scope even, as they can be capacitively coupled so you can't measure a DC voltage. However, I'm no expert so I'll leave that to someone familiar with discrete RF design.
Q14 shows up at the top of p11, associated with the muting switch.
Quote:
Originally Posted by Ian Finch View Post
The bias adjustment is shown as "BAL ADJ" on the schematic next to the variable resistor or trimpot (R64,65). You adjust this according to the tiny voltage marked and measured across R80 or R81. That's 40-50 mV. Though probably unused since it was built, these tend to be low quality gizmos and may need replacement if you can't get a steady voltage after messing with them. If the voltage is OK to start with, don't touch.
I'll need to find a way to read mV. Maybe I can do that with my scope?

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Originally Posted by Ian Finch View Post
BTW, what was the DC voltage at R80, 81 WRT ground?
Didn't quite make it to that point, hopefully tonight, thanks!
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Old 4th March 2013, 04:59 PM   #22
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Quote:
Originally Posted by Ian Finch View Post
I had a think about the FM stereo and since it is derived passively with a diode bridge type detector, there is little to question about the signal balance there, if the stereo indicator is coming on appropriately when signal level is good and the bridge diodes are working correctly. Use your scope to look closely and compare levels at both left and right outputs from the Multiplex Decoder board. Deselect the connection to the preamp when doing this, so there is no load interfering with the measurement. If you have a dual channel or dual beam 'scope, it's a good opportunity to try that out. Without knowing the brand, 'scopes usually have a "trace rotate" or beam rotate control either on the front panel or internally. Watch yourself around high voltages, though - EHT in larger CRT scopes can punch holes in you.
My scope's dual trace so I should be able to do that, perhaps after I get the hum sorted out. Scope is a Philips, found a manual for it and the rotation adjustment is internal. Just what I need, another gizmo to fix, I have three receivers going at once right now I'll use an insulated tool when I do that, rubber boots and tie one hand behind my back
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Old 4th March 2013, 05:06 PM   #23
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Quote:
Originally Posted by gmphadte View Post
There is one more way of testing a transistor, in circuit other than measuring Vbe.
You have to short base to emitter. This will switch off the transistor and full Vcc will appear on the collector.

Gajanan Phadte
Could I also swap the suspect transistor with another in the circuit, or would I risk damaging anything in doing so?
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Old 5th March 2013, 01:08 AM   #24
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Quote:
Originally Posted by mmmalmberg View Post
Q14 shows up at the top of p11, associated with the muting switch.
Thanks, this is a DC switch and at times, all terminals will be almost shorted. As long as hiss between stations
is cut back when the panel mute switch is on, it's fine. Any NPN such as BC547,8,9 would likely work there.

Quote:
I'll need to find a way to read mV. Maybe I can do that with my scope?
Wot, no DMM? Even a $10 one should go down to mV and $25 gets a deluxe model with transistor checker too!
You can use your scope but it will need a little calibration, preferably with a square wave of known accuracy.
I'm always wary of testing DC with a scope because of the unknown ground potential, since scopes have
a grounded earth on the probe input connection. I don't think you'll have a problem though, as long as the
receiver operates OK when the chassis is grounded. Old US sets may not be.

IIRC, there is a cal. terminal on Philips scopes and this is a 0.5V square wave on my TEK and also a cheapo
Taiwanese brand I have. Obviously, you can only calibrate the 1V range with a fixed output but the range
switch should be sufficiently accurate to cover other ranges. Just don't try to measure with the variable
control on the range switch or "Mag" engaged as these are not what you want for accuracy.

Quote:
Didn't quite make it to that point, hopefully tonight, thanks!
OK, good luck!
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Old 5th March 2013, 02:58 AM   #25
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Quote:
Originally Posted by Ian Finch View Post
Wot, no DMM? Even a $10 one should go down to mV and $25 gets a deluxe model with transistor checker too!
Oh, yeah well, I probably have five analog meters and three digitals, almost all of them have something else wrong... Mostly I fry them trying to measure current somehow. But yeah I have at least one dmm that will work if I get a battery for it - guess it's time to get that together

Quote:
Originally Posted by Ian Finch View Post
IIRC, there is a cal. terminal on Philips scopes and this is a 0.5V square wave on my TEK and also a cheapo
Taiwanese brand I have. Obviously, you can only calibrate the 1V range with a fixed output but the range switch should be sufficiently accurate to cover other ranges. Just don't try to measure with the variable control on the range switch or "Mag" engaged as these are not what you want for accuracy.
I may just leave that for another day, after I've got the trace leveled and I don't have so many balls in the air... Getting some batteries sounds more attractive right now Thanks....
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Old 5th March 2013, 05:12 AM   #26
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Transistors are easy to test using a multimeter so do not swap. The hum related problems mostly do not happen due to a faulty transistor.

Quote:
Originally Posted by mmmalmberg View Post
Could I also swap the suspect transistor with another in the circuit, or would I risk damaging anything in doing so?
Gajanan Phadte
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Old 5th March 2013, 05:26 AM   #27
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OK I did a quick once over on the sockets with deoxit, carefully sliding the transistors in and out a bit. There's a lot of them and I didn't want to cause any new problems... Won't turn it back on 'til tomorrow so it has plenty of time to dry.

The schematic shows 40-60 mV bias. I found one of my analog meters has a .6V range and I used that, was able to set each side to a very steady 50 mV. This was accompanied by a reading of .5V, vbe I guess that is, rather than the .65V shown. To whatever degree my vintage radio shack vu meter is accurate.

The DC voltage at R81 wrtg was 15V, and at R82 12V. FWIW...
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Old 5th March 2013, 11:57 AM   #28
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Quote:
Originally Posted by mmmalmberg View Post
.....The DC voltage at R81 wrtg was 15V, and at R82 12V. FWIW...
This seems quite wrong. It means the output is biased unequally between the rail of 48V and ground.
12V to ground means there is 36V to the positive rail when it should be closer to half the 48V positive rail.
Because of the circuit design, there will be some DC offset from about 24V at the output (before the electrolytic).

I think I misunderstood the operation of BAL ADJ, so vary that to see if the voltage also varies. I'm sure it will
to some extent but I think we should first check that the voltage is 12V at the bases of Q6,7 as is indicated on
the schematic. I think that there is actually no separate bias adjustment on the amplifier and 40-60mA just
indicates the normal expected range with the parts fitted. Don't swap transistors here as some may have
been selected to get best operation.

You say your voltmeter is a VU meter? Only use a straight DC range measurement and when you need accuracy,
DMMs are the only way to fly. They may not be strictly necessary but allowances have to be made for mechanical
meters unless they are high quality and calibrated.

You can read DC on an analog AC (VU) meter but it wont be accurate at low voltages because of the necessary
diode rectifier losses. Then again, you may mean something else by your description of what you are using.
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Old 5th March 2013, 02:36 PM   #29
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Using analog multimeter. Sorry 'bout that.

Am not swapping any transistors, tonight I'll poke around some more. The voltage across those resistors did vary quite directly with the BAL ADJ pots and hit .5V somewhere in the middle of their ranges.

I'll also double/triple check that I'm measuring at the correct point on each side.

Thanks
-Mark

Last edited by mmmalmberg; 5th March 2013 at 02:40 PM.
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Old 6th March 2013, 05:25 AM   #30
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OK SO... deox'd all the transistor sockets last night and gave them a day to dry out.

Tonight re-checked these BAL ADJ voltages. I tried using the BAL ADJ pots to adjust the voltages to ground from these resistors. The closest I got was to get one at 21.5V and one at about 23V, each pot at one end of their range. From there, checking back to the voltage across the resistors, one was at about 20mV, and one was at at about 150mV.

I set them back to 60mV each across the resistors. The voltages to ground were still about the same. In other words the adjustments seemed to have a much greater effect across the resistors than relative to ground. I also noticed that as I brought the voltage down on the side that was temporarily high, the slight hum I've been chasing got lower. I think changing that voltage may have just been affecting the output level.
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