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 Post subject: EMUD 196 Voltage Issue
PostPosted: Feb Wed 07, 2018 2:51 am 
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Joined: May Tue 16, 2017 2:06 am
Posts: 94
All Picked up a non-working EMUD 196 and performed all the normal checks and fired it up with my Variac transformer and all tubes lite up and I had the expected hum. Replace the filter caps and now I can get audio and can inject AF to the output tube and at the test socket. Checking voltages at the tubes all were nominal except for pins 1 and 2 at the EAA91. The voltage readings were -.9 on pin 2 and +5 on pin 1 vs. -3.5v. I did not check pin 7 voltage yet, but I assumed that I don’t have a problem after EAA91 with the AF being heard. I have the same result (sound wise) on AM and FM. I swapped the EBF89 with marginal change but pin 2 was still +4.5. I swapped EAA91 and still no marked change. Checked C43 and C44 and they are good as well as R20. I was thinking changing C39 and R16 to see if it improves. The other thing is when I turn the volume up I get a high pitched squeal and it is related to volume because I can lower the volume and it gets quieter.

My question is…Why would the voltage swap from neg to pos on pin 2 on the EAA91?
Do you think IF transformer is bad?


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 Post subject: Re: EMUD 196 Voltage Issue
PostPosted: Feb Wed 07, 2018 4:24 am 
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Joined: Nov Mon 02, 2009 7:01 am
Posts: 2705
Location: Lincoln City, OR
Greetings to btcbar and the Forum:

I doubt the EAA91 is your difficulty.... you say you have no AM either and that tube is not part of the AM detection circuitry.

These European radios are notorious for problems with the key switches. I don't see how you can develop +5 on that diode cathode unless there is a bad connection in the switch. You should definitely measure pin 7 because that will also tell you what is happening with the suppressor grid of the AM detector. If this is way positive somehow, that could account for your high plus reading on the corresponding cathode.... but that would almost have to be a shorted EBF89 and you tell us that you have already tried another one.

I would look into bad connections in those key switches first (thoroughly clean contacts with contact cleaner and exercise them). Ask around for anyone who has done work on European radios for hints and suggestions about these switches.... I avoid them like the plague myself.

Regards,

_________________
Jim T.
KB6GM


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 Post subject: Re: EMUD 196 Voltage Issue
PostPosted: Feb Wed 07, 2018 4:37 am 
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Joined: May Tue 16, 2017 2:06 am
Posts: 94
Yeah I haven't messed with the switches at all other than to move from AM, PU or FM. So you think it is a high resistance coming from one of the switches is the cause? I take a look at them thanx.


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 Post subject: Re: EMUD 196 Voltage Issue
PostPosted: Feb Wed 07, 2018 5:26 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4123
Location: Near Brandon, Iowa
The AM and FM circuits in this radio overlap at some points and are independent at others.
The discriminator circuit (of which the EAA91 dual diode is a part) is one area where they are independent- it is only used for FM operation.

Since both AM and FM are inop, I think that I would concentrate on the circuitry (and tubes) that are common to both modes, since a single failure there would kill both bands.


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 Post subject: Re: EMUD 196 Voltage Issue
PostPosted: Feb Wed 07, 2018 5:40 am 
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Joined: May Tue 16, 2017 2:06 am
Posts: 94
Thats why I was looking at R16 as a possible single point of failure in that transformer and maybe why I have such weird voltages. The voltage from A4 and A3 are good if I am reading it correctly the -15/-20 coming out of 8 into C33.


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 Post subject: Re: EMUD 196 Voltage Issue
PostPosted: Feb Wed 07, 2018 4:59 pm 
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Joined: May Tue 16, 2017 2:06 am
Posts: 94
As expected the voltage at pin 7 on the EAA91 was wrong at +0.54. The C39 cap and R16 resistor checked good. The squeal was tube ECL82.


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 Post subject: Re: EMUD 196 Voltage Issue
PostPosted: Feb Wed 07, 2018 5:56 pm 
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Joined: Dec Mon 08, 2008 8:27 pm
Posts: 8586
Location: alameda,CA
I've worked on a number of EMUD sets and several have had the same problem where that the power resistors used in these will corrode and short out. Usually the ones either in the B+ power supply. I'd check all of them.


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 Post subject: Re: EMUD 196 Voltage Issue
PostPosted: Feb Thu 08, 2018 8:05 pm 
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Joined: May Tue 16, 2017 2:06 am
Posts: 94
Thanx Bob! I did find the one going to the filter cap badly corroded so I replaced that one. I did find a problem with the transformer can going to EAA91. I ohmed out the can where Pins 1 and 2 go to and they were open. I opened the can and it smelled burned plus it looked like some one tried to fix it with a soldering gun and hammer. I took the can from a spare 196 chassis I bought and installed it... I now have AM but no FM and the squeal is back coming from ecl82. Even with the speakers disconnected you can still hear it squeal. I ordered another tube so hopefully that will clear up. However, even with another can my voltages are wrong. Shooting pin1 and 2 at EAA91 I now get neg voltage but it is -.05 on both. I have some repairs left on the power connection because the pins are broken off and that is my next project because I really don't like powering it up with alligator clips. After that is done I'll move on to why my voltages are bad.


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 Post subject: Re: EMUD 196 Voltage Issue
PostPosted: Feb Sat 10, 2018 7:44 am 
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Joined: May Tue 16, 2017 2:06 am
Posts: 94
All: I fixed the power problems and some cold solder joints I noticed. Strange thing...I have AM but when I switch to FM the output tube starts squealing. I went over the connections and voltages and everything is good at the ECL82... I even swapped the .005 and .002 caps with no change. I also throughly cleaned the keyboard and polished the contacts the best I could. Any ideas as to why I get no reception and squealing on FM?


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 Post subject: Re: EMUD 196 Voltage Issue
PostPosted: Feb Tue 13, 2018 4:51 am 
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Joined: May Tue 16, 2017 2:06 am
Posts: 94
Found the problem for the FM and the squeal. The PO must have soldered a brown wire from pin 3 on the second IF can directly to the T2 terminal on the audio transformer. Removed wire and got FM stations and no squealing.


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