Kenwood KR-3200 No AM, FM only in Mono

[LAylEE]

Hi AK community,

My Name is Lajos and this is my first post.
Your community helped me a lot with my other receiver KR-5150 in my living room, I appreciate it a lot.
I was unable to resolve an issue with my new receiver Kenwood KR-3200 in my working room.
I'm new to vintage audio systems, and I bought this KR-3200 knowing that it has issues with the tuner.
So, when it came I cleaned very well all the contacts, buttons, pots.
When first powered on, the main AMP worked great and the sound was pleasant, when listening from aux input.
The issue is the tuner, there is no AM tuning at all, it is just muted, and on FM I can tune in on strong local stations, but stereo lamp never came on(Checked the bulb and it's fine) and the Signal meter is not swinging.
I checked all the transistors, resistors and diodes on tuning PCB.
Everything looks fine, exept I found two of the diodes being dead Dg14 and Dg12(1N60 Germaniums). Replaced them with BAT41 Shottky diodes.
So right now, AM still nothing muted(signal meter not swings), FM signal meter swings, and stations can be tuned(only mono, stereo never locks in), but signal meter works wierd.
When I tune in to a strong local station(Using a T dipole FM antenna connected to 300Ohm) let's say 102MHz, where the sound is the clearest the meter is on 1, but when I tune to around 102MHz(up or down doesn't matter), the meter swings to almost 4.5-5, but in that case the sound is really faint, even on max volume you can barrely hear the sound.
Pressing stereo or mono button doesn't do any difference.

I'm hoping someone here can give some advice on what to check next, or if someone had this experience.

Schematics of KR-3200:
https://www.vintageshifi.com/repert...ge.php?pdf=Kenwood-KR-3200-Service-Manual.pdf

Video demonstration of FM tuning(in order to watch video, you need to download it):
https://www.dropbox.com/s/f9gotp6l9ul3eff/Kenwood KR-3200 Tuning issues.mp4?dl=0

Thanks in advance!
Lajos

 

[Ylli]

Really sounds like the unit is badly out of alignment - which may have occurred before or after some other failure (someone trying to 'fix' it by randomly turning adjustments). But there is a section that would affect both AM and FM: Qg4 and Qg5 are used as IF amplifiers for both AM and FM - a failure in this area would affect both. I would suggest you check the DC voltages on these transistors and see if anything looks suspicious.

 

[LAylEE]

Hi YIli,
Thanks you for reply.
I've checked the DC voltages, here are the results:
When selector knob S1 is set to FM:
Qg4: B=2.2V, C=12.7V, E=1.4V
Qg5: B=2.8V, C=11.9V, E=2.1V
Qg3: B=2.1V, C=13.7V, E=1.37V
Qg9: B=0.8V, C=0.17V, E=0.19V
Qg14: B=0.0V, C=0.17V, E=0.0V
When selector knob S1 is set to AM:
Qg4: B=2.95V, C=13.7V, E=0.75V
Qg5: B=2.8V, C=11.9V, E=2.1V
Qg3: B=0.0V, C=0.0V, E=0.0V
Qg9: B=1.4V, C=8.9V, E=0.8V
Qg14: B=2.25V, C=13.55V, E=1.63V
Looking at the service manual voltages seems ok, but I don't know.
What is your opinion?
Thanks!

 

[Ylli]

When selector knob S1 is set to AM:
Qg4: B=2.95V, C=13.7V, E=0.75V

Let's start here. Are you sure about these numbers? 2.95 volts on the base and 0.75 volts on the emitter would indicate an open B-E junction - i.e. bad transistor.

 

[LAylEE]

Hi Ylli,
I checked again and yes, on AM Qg4 Base 2.95V, Collector 13.7and emitter 0.69 - 0.75V
It's confusing in service manual, that in the schematic states 1.5V on Qg4 emitter but also states 0.65V in parentheses, I don't know what that means.
I will try and change Qg4.
What would be a good substitue for Qg4?
Thank you for this, appreciate it.

 

[LAylEE]

I tried to exchange Qg3 with Qg4(as they are the same transistors), but no luck, all the same.
Is it possible that both of them are bad?
While measuring PN junctions they seems to be ok.

 

[Ylli]

I don't see it documented in the manual, but it looks like without the () is in FM mode and with the () is in AM mode. So the emitter voltages you measured seem OK in both modes. That 2.95 volts on the base in AM mode is strange. Swapping Qg3 and Qg4, along with checking the junctions would seem to indicate that they are both good.

Still don't understand 2.95 volts on the base and 0.75 volts on the emitter - shouldn't be possible with a good B-E junction. If something is oscillating at RF/IF frequencies, that can sometimes confuse some meters (or be rectified by the transistor B-E junction, but that should drive the base negative). Could be the AM local oscillator leaking through. If you can get to the bottom of the board, you might try putting your finger on the B-E-C of Qg4 to de-Q things while measuring the base voltage. Also try 'fingering' Qg9 and Qg14. If you still see 2.95 volts, then there is something going on here I don't understand.

One other quick thought - does that 2.95 volts change if you tune across the AM band?

 

[LAylEE]

Hi Ylli,

Sorry for the long time to reply, I had a busy weekend.
I checked what you suggested.
I left Qg3 and Qg4 swapped, so right now Qg3 in in place of Qg4 and the other way around.
When I now measuring Qg4 voltages in AM mode, Qg4: B = 1.42V, C = 13.7V E = 0.7V and Qg3: B = 0V, C = 0V E = 0V
Fingering BCE while measuring base voltage Increases to 1.52V.
Fingering Qg9 and Qg14 does not change anything.
Base voltage does not change tuning accross AM.
In FM mode:
Qg4: B =2.2V , C = 12.8V, E = 1.4V
Qg3: B = 2.1V, C =13.8V , E = 1.4V

Qg5, Qg9 and Qg14 are as measured on the first time in both modes.
The receiver still behaves the same as previous.

 

[Ylli]

When selector knob S1 is set to AM:
Qg4: B=2.95V, C=13.7V, E=0.75V

Qg4 voltages in AM mode, Qg4: B = 1.42V, C = 13.7V E = 0.7V

That makes more sense.

So let's widen our view...
AM does not work at all. FM is weak - only works on strong stations and even then the stereo light does not come on. So what is common? Those two IF stages (Qg4, Qg5), but at least the DC voltages on those two stages look ok.

In FM - what are you using for an FM antenna? If you disconnect the FM antenna, do you completely loose FM reception, or is it still OK on strong stations? Looks like the FM local oscillator is tied into the AM loop Antenna - not sure I understand why that might be.

If accessible, try to measure the voltage on the source of Qg1 (across Rg2). Also, emitter of Qg2 and Qg8.

 

[LAylEE]

Hi Ylli,
I'm using a single wire as an antenna connected to 300ohm input, but I tried with designated dipole T fm antenna, and the results were the same.
With that wire antenna on fm I can tune to 13-15 stations(no stereo even when signal meter is at 5), when disconnecting the wire I can tune in to 2-3 stations, but the signal meter does not move at all.

As per measurements:
Qg1:
D=10.35V
S=0.27V
G=0V(it is gnd)
Qg2:
B=2.45V
C=14.3V
E=1.65V
Qg8:
B=4.3V
C=13.9V
E=3.6V

 

[Ylli]

Those all look OK The gate of Qg1 actually would go a bit negative on strong signals as there is a local AGC loop in the tuner through Cg12, Dg1, Rg1, and Rg4.

The AM and FM IF signals are combined at the base of Qg4, and go their separate ways again after the collector of Qg5. The voltages around Qg4/Qg5 seem OK. The FM 'front end' up to the base of Qg4 looks OK as far as DC voltages are concerned.

Let's check the voltages around Qg6. Would expect:
Collector +14 volts
Base +2.7 volts
Emitter +2 volts

Also please measure the DC voltage at TP7 when tuned to a strong AM station (I know you are not hearing any AM, but see if you can get any voltage at TP7 when tuning around near a strong stations frequency) and the voltage at TP2 when tuned to a strong FM station.

 

[LAylEE]

Qg6:
B=2.7V
C=12.75V
E=2V
I measured TP2 and TP7 as indicated in the schematics with orange color, hope you were refering to that.
TP7: 0V all the time, I measured while slowly going through scale in AM mode.
TP2: -0.9V when tuned in to a station and signal meter indicates almost 5 in FM mode

 

[Ylli]

TP7: 0V all the time, I measured while slowly going through scale in AM mode.

Confirms that nothing is getting through the AM circuitry.

TP2: -0.9V when tuned in to a station and signal meter indicates almost 5 in FM mode

Confirms signal is getting through on FM, but I don't know if that is enough or not - manual just says (when tuning the IF) to adjust to maximum.

Let's backtrack and see if we can find the issue with the AM front end. Physically examine the wires coming from the bar antenna, and confirm none of the are broken off. Then in AM mode, measure the voltages on Qg9 and Qg14.

 

[LAylEE]

I've checked the cables from bar antenna, all looks good, no broken ones.
Qg9:
B = 1.4V
C = decreases from 8.9 - 8.5V, as tuning to higher frequencies in AM mode
E = increasing from 0.85 - 0.95, as tuning to higher frequencies in AM mode
Qg14:
B = 2.25V
C = 13.5V
E = increasing from 1.6 - 1.7V, as tuning to higher frequencies in AM mode

 

[Ylli]

Those numbers look good. Without specialized test equipment to chase a signal down the RF/IF chain I am coming up empty. Don't know what else I can suggest you check.

 

[LAylEE]

Hi Ylli,
No worries, thank you so much for your help, I really appreciate it.
At least we confimed, that it's not just a simple fix.
I will see if I can find someone local to check with equipment, and if it worth the trouble.
Thank you again, and wish you the Best.