[jrblasde]

Just confirmed that you are correct. It’s the function generator. I connected the signal output probe to one of the inputs on my oscilloscope, and I do not have control over amplitude or DC offset. I’ll have to open it up later tonight. Fingers crossed that it’s not a total loss, but at least it was a dirt cheap oscilloscope!

Rookie mistakes were definitely made. I had started out using a capacitor (as the instructions do indicate), but it kept falling out of the alligator clip on my probe and I grew frustrated. I figured what the heck, I’ve got the radio connected to an isolation transformer. But I failed to consider the DC voltage I might introduce to my signal generator.

[462ron]

Oh that sucks. It’s something easily overlooked. Hopefully it’s nothing more than a resistor!

Ron

[jrblasde]

After a couple of days worth of troubleshooting my signal generator, I gave up. The oscilloscope still worked fine, so I used it to trace the signal on its own circuit board. I checked the signal generator output, the resistors and capacitors, the op amp, etc. but no signs of a signal. I traced everything back to the DSP IC and gave up at that. I ended up buying an identical oscilloscope with arbitrary waveform generator on Amazon. It’s a Hantek DSO2D15. For the price, it is surprisingly feature rich. The oscilloscope is rated for signals up to 150 MHz, and the signal generator can produce an arbitrary waveform (including AM and FM modulated waves) at frequencies up to 25 MHz. This enables me to produce AM and FM IF signals for alignment. For FM RF signals I can use one of those automobile Bluetooth-to-radio transmitters.

I shopped around quite a bit. I know people online tend to harshly review the Hantek equipment (bug prone, tend to lock up, etc.) but that hasn’t been my own experience. Anyway, I looked at Rigol, Siglent, and others. I couldn’t find a model with built in signal generation below the $500 price point. Even when browsing dedicated signal generators, I was easily looking at spending $300 on one of the frontline models. For me, the Hantek was a no-brainer. I couldn’t justify the higher prices for a unit I use on perhaps five projects a year. The new unit should arrive on Monday, and I can reattempt alignment with a capacitor connected to the signal generator output this time!

[jrblasde]

Got my replacement signal generator in the mail today! I was able to finish alignment of the IF section. I’ve got some trouble in the RF section though—I can’t receive AM, shortwave, or FM RF signals. The alignment procedures for the RF section don’t seem to be working. 

Either I have a bad 7F8 converter tube or I have a miswire in the RF chassis. I will first assume a miswire, so I will comb back through to make sure everything matches the schematic. If it does, I’ll replace the tube.

I can feed an AM signal at 455kHz to a loop antenna sitting just beside the loop in the cabinet, and the radio picks it up and passes it through to be demodulated. But I can’t do the same for a signal in the AM bandwidth.

[MrFixr55]

Hi Joseph,

First, verify that your band selector is in the correct position for what you want to do. I believe that the first position is the tuning buttons. If no position works, I think that the oscillator is dead. Do recheck your wiring. If this set has a separate RF chassis, it may be tight there. Of course, the tube could be bad also.

[jrblasde]

Good sanity check! You are correct—the first (as in, the most counterclockwise) position is for the push button presets. I verified my position for the band switch but no luck. I’ll verify the wiring tomorrow evening. If there are no errors, I’ll replace the tube.

[MrFixr55]

Hi Joseph,

Fortunately, Ms. Fixr does not read my computer, or she would warn you against coming to me for sanity checks. I don't know why she feels this way. I only have about 80 radios, 15 stereos and 12 TV sets. That's not too much, is it?

[jrblasde]

Not too much at all!   Sounds like you’re just dabbling into the hobby!

[morzh]

Mr. Fixr

Compared to you I am sooo green behibd the ears...I have about 40 or so sets.

[jrblasde]

I’m making progress with the alignment. It was a combination of needing to solder the connection on pin 3 of the BC oscillator coil (oops!) and needing to replace the 7F8 tube. Luckily I had a spare 7F8 in the 48-482 I have sitting in my attic.

Here’s a couple of links to videos demonstrating AM and FM. Haven’t picked up anything on shortwave just yet. I still need to finish up with alignment of the RF section, but this is so exciting after eight months of work!

https://youtube.com/shorts/wpDEIsRydR8?s...37rEL9TApC

https://youtube.com/shorts/4ATUTwN2IN0?s...Dc1KLdPuLo

[MrFixr55]

Throw in all the Audio equipment that has to be kept in a bedroom (my office)L. 8 portable self contained PA speakers, road case containing mixer and two 4 channel wireless mic receivers. Throw in a large bin containing cables, mics, other paraphernalia. 4 speaker stands, 8 mic stands under the bed of the other bedroom.

Of the 80 radios, 35 were either from childhood or street finds. The rest were from eBay. All of the stereo equipment except for the KLH and all of the TVs were either "gifts" from family, friends and neighbors, or street finds, including the 2 Fishers (400 and 800). Yes, Ms. Fixer does put up with a lot but it keeps me off the street and out of trouble.

The vast majority of the radios work and were repaired by me. The TVs will take more time.