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spamtrap1888 spamtrap1888 is offline
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Default Grundig AM/FM portable "Transistor 305" troubleshooting

On Mar 5, 7:02*am, klem kedidelhopper
wrote:
On Mar 5, 6:07*am, Franc Zabkar wrote:



On Sun, 4 Mar 2012 19:47:09 -0800 (PST), klem kedidelhopper
put finger to keyboard and composed:


http://dl.dropbox.com/u/65394789/Gru...0schematic.pdf


* * * * * * * * * * * * * * * * * * Voltage readings
* Test point * * * * * Radio working: * * *Radio not working
------------------- * * * * --------------------
* E. AF178 * * * * * * * *-1.10V * * * * * * * * * *-1.20V
* B. AF178 * * * * * * * *-1.50V * * * * * * * * * *-1.50V
* E. AF124 * * * * * * * *-1.10V * * * * * * * * * *-1.20V
* B. AF124 * * * * * * * *-1.32V * * * * * * * * * *-1.44V
* E. AF121 * * * * * * * *-0.85V * * * * * * * * * *-0.90V
* B. AF121 * * * * * * * *-1.20V * * * * * * * * * *-1.30V
* E. AF126 (1) * * * * * -1.00V * * * * * * * * * *-3.70V *
* B. AF126 (1) * * * * * -1.20V * * * * * * * * * *-4.00V *
* E. AF126 (II) * * * * * -0.80V * * * * * * * * * *-3.70V *
* B. AF126 (II) * * * * * -1.10V * * * * * * * * * *-3.60V *
* E. AC151 (I) * * * * * - 0.90V * * * * * * * * * -0.80V
* C. AC151 (I) * * * * * --3.05V * * * * * * * * * -2.95V
* E. AC151 (II) * * * * *--0.66V * * * * * * * * * -0.62V
* C. AC151 (II) * * * * *--3.20V * * * * * * * * * -3.10V


I suspect that C45 (near pin #8 of 7209-301) may be shorting.


- Franc Zabkar
--
Please remove one 'i' from my address when replying by email.


The "whisker theory was also posed to me last time I took a crack at
this repair. I disconnected all the cans at that time but the problem
still persisted. I'm trying to understand though how C45 could affect
this? It seems like a somewhat removed part of the circuit but perhaps
I'm not seeing the whole picture. Could these two affected transistors
be turning on, and if so wouldn't the collectors be very close to the
same potential as the emitters? I never looked at that. Collector
voltages are not listed on the schematic for those stages. Still if
that isn't *the case I have to try to determine how this voltage is so
drastically rising. R22 seems to set a bias point. I *subjected R22 to
some stress. It's definitely not a cold solder joint on that pot.
Unless it's breaking down internally but I would have thought that my
poking and prodding would have helped that along. In fact I had hoped
that once things became "thermal" they might also become
"mechanically" intermittent as well, but that was not the case. Now
hypothetically speaking in the case of one of the 820 ohm emitter
resistors opening, would that affect both stages? So many
possibilities here. I also thought of putting the soldering iron onto
the leads of several components upon turn on to attempt to bring on
the problem ahead of the time it usually takes but I am leaving that
as a last resort. Here is another link to the schematic just in case
the first one doesn't work or fails:http://db.tt/bbVeazAe
Lenny


I was able to open the pdf after downloading it.

While I haven't done the math, I would suspect C44. First, it is
suspect merely by virtue of being a 40 year old electrolytic capacitor
that is connected to both failing stages. Further: if, when you turned
the radio on, C44's leakage current was initially low, but with
operation it increased substantially, then a bad C44 would affect the
bias point R22 was trying to set, as current began to flow through
R21. Moreover, a short through C44 to positive should have an effect
similar to the short through C45 to ground that Franc suspects. Thus,
I would try replacing C44.