Heathkit AR-1515: Problems after recapping

[Helios]

Some time ago I decided to recap my AR 1515 that I had built in late 70s. I relied heavily on the work of dlucy here: https://audiokarma.org/forums/index...upgrading-a-heathkit-ar-1515-receiver.891978/ (Thanks, dlucy!).
(Sorry,I did not found the trick to use a name instead of the long reference)

I changed the EC on all the boards, except the Control. On the Power Supply board, I changed too some diodes (regular and zener), as suggested. For the time being I kept the main caps, just changed the C3 6000 uF.

When reassambling I encountered, to my surprise, a couple problems.

First the readout malfunctionned. All I was seeing was 1FFF (AM) or FF.F (FM): one of the legs of the capacitor that I had changed was not making contact with the front side trace of the board. After correction though, the problem was not gone; sometimes I could see 1550 (AM) or 89.1 (FM), or another numbers, or not numbers, and the movement of the tuning shaft had no effect. What intrigued me at the utmost, was the signal strength always stayed at the null position (FM, here we have no more AM stations).

I began to investigate further and found that the oscillations on the RF board (FM and AM) were OK, I could see them changing on my scope as I turned the tuning shaft, but I could see the oscillations only when the IF board was disconnected; as soon as I connected the IF, I couldn see any oscillations. Strange!

I rechecked my work, to see if any pieces that I had replaced was right (orientation) or for the presence of any bridge (solder): found none.
I then systematically reviewed the voltages on the boards, and found the following:


I measured the voltage (pins 1 and 2) on the power supply board (P802)

(The TABs do not seem to work here)
Pin 1 Pin 2 Comment
-16,6 17,1 P802 not connected

-13,7 13,7 P802 connected; PreAmpl and Control On (Always will be)

-10,2 9,8 RF connected (FM)
-9,5 9,0 RF connected (AM)

-12,6 12,4 IF connected (FM)
-13,5 13,5 IF connected (AM)

-7,1 6,4 Multiplex (FM)
-13,7 13,7 Multiplex (AM)

-13,6 13,6 Counter REadOut (FM or AM)

-10,3 9,9 Counter ReadOut + RF (FM)
-9,6 9,1 Counter ReadOut + RF (AM)

-4,7 3,7 Counter ReadOut + RF (FM) + IF (FM)
-8,5 8,0 Counter ReadOut + RF (FM) + IF (AM)

-4,1 3,1 Counter ReadOut + RF (FM) + IF (FM) + Multiplex (FM)
-8,5 8,0 Counter ReadOut + RF (FM) + IF (FM) + Multiplex (AM)

As we can see, there is a marked drop when the IF board is added. But the board seems OK!? Or have I something overlooked ?

After days searching, I need another head the point me on the right track.



Helios

 

[txturbo]

Was it working normally before you did the re cap?

 

[Helios]

Yes

 

[txturbo]

Can you post some pictures of the IF, Multiplex and counter boards?

 

[txturbo]

Do you still have the assembly manual?

 

[Helios]

I still have the manual.

Here some pics.


IF Board



Multiplex



Counter



Power Supply

 

[Helios]

I have been searching for days, if not weeks, where the trouble came from; I have thoroughly inspected the RF and IF boards, at one time doubting if the FM tuner was still working, and found ... nothing. I was not looking at the right place: on the Power Supply board war indeed a solder bridge. Once corrected, all was falling in place: the pins 1 and 2 (P802) now gives me a steady -14,18 V and +14,21 V.





What remains is the Counter trouble; I can see on my scope the signals from AM and FM Osc. getting to the Counter board. I tested all the ICs, they seem good. On the crystal though, I see no oscillation signal on my scope.
Is it possible that a crystal get faulty ? (I have no experience with this kind of things). If so, can I replace it with any 100 KHz crystal ? or are there any precautions to take ?




Any comment would be here much welcomed.


Helios

 

[sregor]

Crystal failure is not that rare. !00kHz crystal is unusual, but there it is. Good luck.

 

[txturbo]

Have you used the build manual to do the resistance checks of the boards?

 

[txturbo]

This is a 100khz crystal available at mouser.

https://www.mouser.com/ProductDetail/Epson-Timing/C-2-1000000K-PPBFREE?qs=8DSv3jbiFFKBHBdNvsJYww==

 

[Helios]

The crystal was indeed the culprit. I used the one suggested by txturbo. At first I just soldered it in parallel with the old one: same problem.
Then I removed the old one, and it worked. The new one is rather tiny, as compared to the older.

I have lately ordered parts to reburbish a sansui 771 and some parts were hard to get, that explains the delay as I wanted not to pay expensive delivery fees for a cheap part.