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P150 AM board weirdness

thxdave

thxdave

Department of Redundancy Dept. (Retired)
Subscriber
I finally stumped LeeStereo with this one. I recently got my 2245 back from my local tech who was not able to complete a rebuild for me when I was moving and couldn't do it myself. He was stricken with cancer so this receiver sat in limbo for over a year. When I got it home, it still had a few issues but I've been working through them slowly. The one thing I have not been able to get working is the AM tuner.

The P150 is receiving the required voltage when the Selector knob is set to AM but I get NO sound of any kind. I ended up replacing all the transistors out of pure frustration but it made no difference. I tested all the ECB voltages and they were all on the money except for the Base of H153. It measured 1.9v and is supposed to be 3v. Here's the mystery (and hopefully a clue): When I touched the Base leg of H153 with my insulated probe, the AM audio appeared! This was repeatable when I turned the P150 upside down and touched the Base lead with my finger. I'm obviously loading the circuit with my test lead (and/or my body) but I don't know how to proceed from here. Any suggestions?
 
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I'll assume you looked at the solder joint at that pin with good light and magnification. You might try reflowing that joint anyway. I'd like to know the dc voltage at the base when it's working and at the emitter and collector. I'm guessing these pins getting probed don't make the audio come back?
 
If the board has been recapped, make sure C183 is not in backward. The silkscreen on the board is reversed, and postive lead on cap should go to spot marked negative. I doubt if reversing this would make you have no sound, but worth ruling out.
 
I've found a few errors since getting the 2245 back so will reflow anything suspect. I have been touching and wiggling a lot of the leads trying to find the source of this problem. Will double-check those other voltages but the Base measured 1.9v at the same time as the audio reappeared. I'll have to put my second DMM to work to read the other two legs at the same time. You are correct.....the other two pins had no effect, only the Base.
 
Steven Tate said:
If the board has been recapped, make sure C183 is not in backward. The silkscreen on the board is reversed, and postive lead on cap should go to spot marked negative. I doubt if reversing this would make you have no sound, but worth ruling out.
Click to expand...
You are right about C183 and it is presently in the "correct" way. However, I did find two other caps on this board that were in backwards. Correcting them did not fix the problem.....darn it. ;-)
 
thxdave said:
I've found a few errors since getting the 2245 back so will reflow anything suspect. I have been touching and wiggling a lot of the leads trying to find the source of this problem. Will double-check those other voltages but the Base measured 1.9v at the same time as the audio reappeared. I'll have to put my second DMM to work to read the other two legs at the same time. You are correct.....the other two pins had no effect, only the Base.
Click to expand...
As well, measure E and C without a meter on the base. Hope it tells us something!
 
rBuckner said:
I'll assume you looked at the solder joint at that pin with good light and magnification. You might try reflowing that joint anyway. I'd like to know the dc voltage at the base when it's working and at the emitter and collector. I'm guessing these pins getting probed don't make the audio come back?
Click to expand...

Probing the Base lead would bring the audio back as would touching the resistor attached to the backside of the board. It attaches to the Base of H153. If I touch the body (not the wire) of that resistor, I get AM audio. As I posted previously, all the transistors measured correctly except for H153. It measured E: 1.3v, C: 7.9v and B: 2.3v which is below the 3v shown on the schema.

rBuckner said:
As well, measure E and C without a meter on the base. Hope it tells us something!
Click to expand...

See above: E = 1.3v and C = 7.9v by themselves. I attached one of my DMMs to the Base of H153 and used my second DMM to measure the two other leads while the Base was being tested and they measured C=8.9v and E=1.3v.

I went back through the board tonight with a magnifier and a strong light and touched up anything that looked suspect, but no changes. Hope something gives you a clue in this helps.
 
IF H153 were good, the base to emitter voltage would not be 1 volt! Is it open base to emitter? It should be around 0.6 volts, as shown on the schematic. If it's okay then I'd look first at the stuff on the base like R183 (12K) and the transformer secondary on the L153 part. That would be where R183 leads to from the base. Assuming here that C161 and 162 are in right.

I'd guess that the probe is acting as an antenna and inducing the AM signal which enough of which is leaking through H153 to the next stage.
 
I obviously not understanding the testing procedures so dumb it down as much as you can! The measurements I reported were all taken from Ground to each leg. This morning, I measured the Base to Emitter legs together and I am getting 0.6v as you indicated. I lifted one leg of R183 and it reads 5.6k which is below the 12k on the schema so I'll see if I have one here in parts bin. I DID check R187 (underneath the board) and it tested good at 2.2k. I double-checked all caps again and, apart from the famous backwards C183, all caps are installed correctly.
 
Is the color coding on R183 consistent with 12k? If the original was 5.6k, probably want to stay with that. Its not uncommon for these not to match the schematic, but the original is probably the way to go if they differ.
 
You must be sensing my frustration all the way out in Texas, Steve!! I pulled R183 and read the color codes and it is indeed marked at 5.6k and reads 5.6k so not sure where the 12k shown comes from. If I put a 12k resistor in there and it all started working again, would I have an official entry in the SM Error thread!!?? ;-)

(I still can't believe that I'm spending such an inordinate amount of time on a part of the radio spectrum that spews hate on one side and religion on the other....talk about the duality of man!) </end mini-rant>
 
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thxdave said:
You must be sensing my frustration all the way out in Texas, Steve!! I pulled R183 and read the color codes and it is indeed marked at 5.6k and reads 5.6k so not sure where the 12k shown comes from. If I put a 12k resistor in there and it all started working again, would I have an official entry in the SM Error thread!!?? ;-)

(I still can't believe that I'm spending such an inordinate amount of time on a part of the radio spectrum that spews hate on one side and religion on the other....talk about the duality of man!) </end mini-rant>
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Lol. I get it. I wouldn't replace it if it looks original. I've been known to replace a transistor just for a test. If it doesn't help, put the old one back in. Looks like a KSC945C would be a replacement for the 2SC372 at H153. Might even try replacing H151 and H152 with KSC1845. For some reason, it seems you aren't getting enough signal amplification through the board. It could be one of those. I suppose it could also be a bad IF transformer - L153, but not sure how to test that. Push comes to shove, I think I have an AM board from a 2270 that I think is identical to the 2245. If we need to rob it of parts or try to replace the board, it's available.:thumbsup:
 
Oh, and I think what Randy was referring to with the measurements was the B-E voltage drop. On the schematic, it shows 3V at the base and 2.4V at the emitter -- a drop of 0.6V, typical diode drop. Your measurements showed base at 2.3V and emitter at 1.3V -- a drop of 1V. Not what you would expect there.
 
Steven Tate said:
Oh, and I think what Randy was referring to with the measurements was the B-E voltage drop. On the schematic, it shows 3V at the base and 2.4V at the emitter -- a drop of 0.6V, typical diode drop. Your measurements showed base at 2.3V and emitter at 1.3V -- a drop of 1V. Not what you would expect there.
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His direct measurement was 0.6 volts so maybe a measurement error or flaky meter? Or low meter battery?

If you give up, make notes on wire colors, unsolder the board and PM me for an address - I'll fix it. But not until January!
 
I SO want to get better at troubleshooting and diagnosis. Thanks so much for both your offers of help. I may take you up on it if I become suicidal over this board! ;-)

I guess I need to know the correct way to perform these transistor tests. To briefly backtrack, I DID replace all the transistors on P150 per Steven's Substitution chart. My repair guy had put in all new caps as well so I'm just trying to get this thing across the finish line. That doesn't mean that I didn't get a defective transistor but they are all new via Mouser and DigiKey.
 
thxdave said:
I SO want to get better at troubleshooting and diagnosis. Thanks so much for both your offers of help. I may take you up on it if I become suicidal over this board! ;-)

I guess I need to know the correct way to perform these transistor tests. To briefly backtrack, I DID replace all the transistors on P150 per Steven's Substitution chart. My repair guy had put in all new caps as well so I'm just trying to get this thing across the finish line. That doesn't mean that I didn't get a defective transistor but they are all new via Mouser and DigiKey.
Click to expand...
Ah, yes. Missed that. Sorry. I really don't have much of a reasoned way of diagnosing these AM tuners. I'm sure some of these guys do. I look at the AM troubleshooting section, and hit a brick wall as soon as it says you need an RF VTVM to measure output of the oscillator. I wonder if that can be measured with a scope? It does mention that meter movement or not when you cross the dial tells something. If it does move, the problem isn't before the last IF stage. If no movement, either something in the IF stage is bad, or it isn't getting adequate signal. That could be a bad antenna or connection, or, who knows what else? The fact that you get some sound when you touch the base of H153 probably tells us something, but I'm not sure what it is. :dunno:
 
^^^ " I really don't have much of a reasoned way of diagnosing these AM tuners" - Yeah, me either. It's so rare that they are bad. Checking any and all dc voltages all over the board is the starting point. Well, after making sure the power supply voltage at the board is right, including are all grounds really grounded.
 
I'll double check all the voltages and I'm going to test all the resistors, too. I'm trying to learn how to troubleshoot these things and I'm glad this board is fairly small!!
 
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