On Mar 6, 3:53*am, Sylvia Else wrote:
On 5/03/2012 2:47 PM, klem kedidelhopper wrote:
I would like to ask for help once again with the repair of my own
personal old radio. I started this project almost a year ago, and it's
been on the back burner since then. But I had a little free time and
so I put it back on the bench. I would really like to see this radio
working again.
This is a Grundig "transistor 305", circa 1970 I think. The radio is
intermittent. I have been working on this set on and off for almost
the past year. I recently took some voltage measurements and there are
some serious differences in base *and emitter voltages in the last *IF
stage and the detector stage when this radio fails. I have tried many
things, heat and cold, including substituting all the transistors in
the RF and IF sections one at a time and have studied the schematic
until I felt a migraine coming on. l think that by now I could almost
draw it from memory, and I'm still coming up empty.
I considered trying to inject a signal and/or signal tracing, however
with the circuit voltages so far out of whack when it fails I think
that approach would be futile. The base and emitter voltages in these
three stages increase substantially when the radio quits. Some
voltages are jumping from for example 1.2V up to 4.0 V. With
conditions like this you would think that the cause would be obvious,
and perhaps I've been staring at this this too long and it is. However
the remedy has stubbornly eluded me thus far. The intermittent nature
of the problem is what makes it so difficult.
This is typical of what is going on: If the radio has been off for a
long period of time it will typically work for several hours and then
quit. In the beginning of this Quitting sequence, just after it first
quits it will try to come back on intermittently for a short period of
time, crackling etc, (as though something was intermittently breaking
down). Eventually it will just remain silent. There was an
electrolytic in the audio driver section, C57 a 100uf/3.0 volt with
slightly high ESR. Replacing it improved the low frequency audio
response, however the AF amplifier is not the problem though. You can
still get a good audio signal from the volume control out when it
fails. So the AF section seems to be unaffected. The voltages to that
part of the circuit substantiate that as well. And the RF and
oscillator circuit voltages during dead time seem to be fine as well.
The problem when it occurs affects both AM and FM.
The strange thing is that when powering this radio up again after a
shut down, the time it remains on before it quits once again is
directly proportional to the time that it has been off. For instance
if you leave it off overnight it might remain on for a half hour or so
the next morning before it quits again. Recently after trying it again
after months of it sitting idle it played for about 3 hours before it
quit. Then five minutes later when trying it again it quit almost
immediately. The few other electrolytics in the affected areas check
good on ESR and bridging them during dead time yielded no improvement
either. I have the schematic and I have uploaded it to the site listed
below.
I know that there are technicians out there better at this than I am,
and I would really appreciate it if someone could please take a *look
at the schematic along with the voltage readings I obtained during Go
and No Go conditions and give me your opinions. With three stages
affected I suspect that I'm looking for a common denominator but I'm
just not sure. The initial voltage readings (on turn on with the unit
working normally) will be listed in the left hand column. The voltage
readings during a failed condition are listed in the right hand
column.
The major differences are marked with an asterisk*.
I used to strap this radio to my bicycle when I was a kid and ride
around The Bronx with it. It has a great sound. I've owned it since I
was a teenager. I guess it's just a sentimental thing. Thanks for any
assistance. Lenny
http://dl.dropbox.com/u/65394789/Gru...0schematic.pdf
* * * * * * * * * * * * * * * * * * * Voltage readingso
* * 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
Thanks once again for looking. Lenny
The bias of AF126 II is derived from the emitter of AF126 I (hereinafter
'the transistor'), so if the latter's bias arangements go awry, so will
the former's, which is thus of no interest.
If pin 10 of F IV became open circuit, the base of the transistor would
be pulled towards -9V through R5, R4, R26 and R21 in series, being a
total of 47.2K. For that to put the base at -4V requires a base current
of 0.1mA, there being no other source for the current.
For the emitter of the transistor to be at -3.7V requires a current of
4.5mA through R24 (the 820 ohm resistor). Given the calculated base
current, this would be the case if the beta of the transistor were 42,
which seems entirely plausible.
So if the transistor has a beta of around 40, the behaviour can be
explained by an intermittent open circuit at, or in the vicinity of, pin
10 of F IV.
Sylvia must mean R28 in series; R26 goes from the emitter of AF 126 I
to ground.
A reference I found says that the minimum hFE for the AF 126 is 50, so
a beta of 42 here is kind of sad.
http://alltransistors.com/transistor...ansistor=21733
Now I'm wondering if the supply voltage is really 9V.