• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

2330b missing one fm channel and I'm stumped

phollenback

phollenback

New Member
I'm working on restoring a 2230b that's generally in good condition. The one major problem I'm trying to figure out is that it only has a left fm channel. I've tested other inputs such as AUX and they have both left and right sound. I also tried taking the audio directly from the preamp output and it behaves the same. AM radio outputs on both channels, and so do both tape inputs.

I've done the usual deoxit of the various selectors and pots.

I have the service manual and I've been poring over it for quite a while. I've checked voltages on the fm tuner chip on the p200 tuner board and it looks fine for both channels. I'm stuck at this point because I'm having a heck of a time figuring out where the left and right audio signals go from the p200 board. I've probably reached the limit of my ability to read circuit diagrams.

The STEREO light illuminates when I tune to a stereo FM station. Selecting REVERSE from the speaker output moves the output from the left channel to the right.

FM is only the one channel on the fm 25us setting as well, and I've verified operation of the microswitches in the dolby module slot.

If I turn the balance to the right and the volume knob to the max I can hear low volume and somewhat muffled audio on the right channel. Does this suggest a failed cap?

So, my guess is that something is going wrong after the HA1196 fm stereo chip, although I have yet to hook a scope up to the outputs from that chip to confirm.

Anyone have any suggestions for my next troubleshooting step?
 
Register to hide this ad
Welcome aboard..

1. Tune to a strong FM stereo station and check for audio signal at C319 and C320 <------------- and o'scope would be handy.
 
If you get stereo lock on FM, the tuner and MPX are OK. The channel is getting lost after that. Other inputs working rules out pre-amp and the amps are working so I think selector switch or wiring.
 
@avionic thanks for that advice, I will hopefully have a scope set up in a few days to check the circuit. Meanwhile, I measured the dc voltage all the transistors in that part of the circuit, since the voltages are listed on the schematic. I noticed that q303, q304, and q306 had roughly correct voltages. However, q305 is showing 38v on all legs and not the correct emitter voltage of 8.8v.
 
Cycle the FM 25 uS deemphasis switch. Last evening I "fixed" a missing FM channel on a 2275 by cycling and cleaning this switch. Oh, I see you have already checked this.
 
phollenback said:
I'm working on restoring a 2230b that's generally in good condition. The one major problem I'm trying to figure out is that it only has a left fm channel. I've tested other inputs such as AUX and they have both left and right sound. I also tried taking the audio directly from the preamp output and it behaves the same. AM radio outputs on both channels, and so do both tape inputs.

I've done the usual deoxit of the various selectors and pots.

I have the service manual and I've been poring over it for quite a while. I've checked voltages on the fm tuner chip on the p200 tuner board and it looks fine for both channels. I'm stuck at this point because I'm having a heck of a time figuring out where the left and right audio signals go from the p200 board. I've probably reached the limit of my ability to read circuit diagrams.

The STEREO light illuminates when I tune to a stereo FM station. Selecting REVERSE from the speaker output moves the output from the left channel to the right.

FM is only the one channel on the fm 25us setting as well, and I've verified operation of the microswitches in the dolby module slot.

If I turn the balance to the right and the volume knob to the max I can hear low volume and somewhat muffled audio on the right channel. Does this suggest a failed cap?

So, my guess is that something is going wrong after the HA1196 fm stereo chip, although I have yet to hook a scope up to the outputs from that chip to confirm.

Anyone have any suggestions for my next troubleshooting step?
Click to expand...
Check the LPF filter. The "big green plastic box" in the FM board. Sometimes they go kaput. Make sure signal is coming out of it.
 
Thanks for the advice everyone!

avionic said:
Have you pulled it to check for a shorted transistor?
Click to expand...
I did pull Q305 and tested it with a multimeter. It tested ok, not shorted.

I was finally able to get my scope out of storage and check signals on the board. I discovered that the transistor before Q305 (Q303) was faulty - no output signal. It's a 2SC1327T - can anyone suggest a reasonable modern replacement? 2SC1328 seems to be a similar transistor except for a higher max voltage, and http://hudsonvalleyhifi.com/technical-data/transistor-substitutions suggests that KSC1845 is a modern replacement for the 2SC1328. Thus I'm going to try a KSC1845 unless anyone has a better idea.
 
Nice work! After looking into the specs on 2SC1327 and possible subs, I have a question for the more experienced. 2SC1327 has a Vce=35V and Ic=0.1A. KSC1845FTA has a Vce=120V and Ic=0.05A - half the max collector current of the 1327. I notice that just based on those two specs, KSC1815YTA is much closer. Vce=50V and Ic=0.15A. It would appear to be a more robust sub. However looking at the Gain Bandwidth Product (Ft), the 1327 specs at 150 MHz, and the 1845 at 110 MHz. The 1815 spec is 80 MHz. This transistor being in a tuner circuit, which spec is more important - the max collector current, which is deficient in the 1845, or the Ft, which is deficient in the 1815?:dunno:
 
Steven Tate said:
Nice work! After looking into the specs on 2SC1327 and possible subs, I have a question for the more experienced. 2SC1327 has a Vce=35V and Ic=0.1A. KSC1845FTA has a Vce=120V and Ic=0.05A - half the max collector current of the 1327. I notice that just based on those two specs, KSC1815YTA is much closer. Vce=50V and Ic=0.15A. It would appear to be a more robust sub. However looking at the Gain Bandwidth Product (Ft), the 1327 specs at 150 MHz, and the 1845 at 110 MHz. The 1815 spec is 80 MHz. This transistor being in a tuner circuit, which spec is more important - the max collector current, which is deficient in the 1845, or the Ft, which is deficient in the 1815?:dunno:
Click to expand...
If the circuit demands are less than 0.05A, then the 1845 will be fine. It apparently is since it didn't fry. :D Different circuits have differing criteria. Just because an original transistor is rated at 0.1A, doesn't mean it is required by the circuit. The speed and low noise was probably the "desired" factor in this instance. Also, the specs of a device are MAX, and each independent of one another. At max voltage of 120v, you can get nowhere near the 0.05A MAX lc. But if you run that transistor at, say 30v, you can get the 0.05 lc. BUT that would be running at the maximum current and would fry anyways in time. Current, voltage, and wattage are each independent values and none are performed at the same time.
 
Skywatcher said:
If the circuit demands are less than 0.05A, then the 1845 will be fine. It apparently is since it didn't fry. :D Different circuits have differing criteria. Just because an original transistor is rated at 0.1A, doesn't mean it is required by the circuit. The speed and low noise was probably the "desired" factor in this instance. Also, the specs of a device are MAX, and each independent of one another. At max voltage of 120v, you can get nowhere near the 0.05A MAX lc. But if you run that transistor at, say 30v, you can get the 0.05 lc. BUT that would be running at the maximum current and would fry anyways in time. Current, voltage, and wattage are each independent values and none are performed at the same time.
Click to expand...
That all makes sense. I’m still wondering about that Ft spec. If it was strictly handling an audio signal, I’m guessing it is irrelevant. But in a tuner circuit, I assume it depends on its function. Not sure what this transistor does, but wondering if Ft needs to be over 100 MHz for some reason.
 
You must log in or register to reply here.
Back
Top Bottom