07-05-2021, 08:00 PM
Update on transformers:
I replaced the output transformer with Hammond 125D. It works fine. Here's something to consider when using universal transformer. Initially, I missed the inverse feedback circuit implemented off the transformer secondary and missed the need to wire the secondary leads with regard to polarity. Of course, I got it wrong and had some pretty good high pitched oscillations when treble was high and volume low. I switched the leads so that the high side connects to the feedback circuit and that solved the problem. The oem transformer has one lead soldered to the transformer frame to prevent this mistake.
Regarding the power transformer, I purchased a tested replacement, but found that the DC resistance on its primary was about 2.8 ohms (just like the one in my radio). I bench tested it and all secondary voltages were pretty much on spec. So, that was encouraging. After recapping the radio, I powered up the radio with its original transformer slowly increasing voltage with variac and found no problems. The transformer output on all secondaries is pretty much per spec. So, it seems that the schematic showing the primary DC resistance at 28 ohms is incorrect.
The radio works okay on SW, AM, and FM. I will do the alignment in the next couple of days and then put it all back together. I really appreciate your help and all the other Phorum members - I've gleaned a lot of good information from a lot of previous posts.
I do have a question about alignment but will put that in a new post. Also, first chance I get I'll post pictures and some more restoration detail.
I replaced the output transformer with Hammond 125D. It works fine. Here's something to consider when using universal transformer. Initially, I missed the inverse feedback circuit implemented off the transformer secondary and missed the need to wire the secondary leads with regard to polarity. Of course, I got it wrong and had some pretty good high pitched oscillations when treble was high and volume low. I switched the leads so that the high side connects to the feedback circuit and that solved the problem. The oem transformer has one lead soldered to the transformer frame to prevent this mistake.
Regarding the power transformer, I purchased a tested replacement, but found that the DC resistance on its primary was about 2.8 ohms (just like the one in my radio). I bench tested it and all secondary voltages were pretty much on spec. So, that was encouraging. After recapping the radio, I powered up the radio with its original transformer slowly increasing voltage with variac and found no problems. The transformer output on all secondaries is pretty much per spec. So, it seems that the schematic showing the primary DC resistance at 28 ohms is incorrect.
The radio works okay on SW, AM, and FM. I will do the alignment in the next couple of days and then put it all back together. I really appreciate your help and all the other Phorum members - I've gleaned a lot of good information from a lot of previous posts.
I do have a question about alignment but will put that in a new post. Also, first chance I get I'll post pictures and some more restoration detail.
