Re: 8566B Repair - Page 1
Forum section: electronics / repair

author: _rushin_or_draggin_
author_rank: Regular Contributor
timestamp: June 19, 2025, 05:25:37 pm
content: Hello all,I recently acquired 2 8566b’s for very cheap. One set seems to have been in use while the other set seems to be a parts unit. I had a shorted rail on the RF section and got that repaired but I didn’t have the bus interconnect cable. After ordering one and connecting the IF and RF sections connected together everything seems to be working fine except I have no display. All regulated voltage test points in the IF section test within 0.01 volts. I just wanted some of yalls input before I started digging into the HV section as I would prefer if it didn’t have to… I do have the spare IF section but it looks like the CRT anode was disconnect from the “18KV DISCONNECT WITH CAUTION” plug…hmm, makes you wonder why.(Oh yeah, I have the intensity knob turned all the way up too!)Thank y’all!!

author: _rushin_or_draggin_
author_rank: Regular Contributor
timestamp: June 20, 2025, 12:53:11 am
content: Update: the A6A1 board was swapped and the display now works perfectly. There is the common YTO unlocked issue which I will fix. There is a bigger issue though: when connecting the calibration output to the input I get NOTHING! The output is giving me a good 100 MHz signal atleast. Probably a blown attenuator or something. I’m going to either trace the input signal…or just swap the attenuator modules

author: _rushin_or_draggin_
author_rank: Regular Contributor
timestamp: June 25, 2025, 09:27:35 pm
content: Hello again,I have let the unit run for awhile and it’s demonstrating an issue that I’m struggling to track down. On startup the unit will power up and respond to keystrokes correctly. After a few minutes all the lights turn on and it will not respond anymore. There also seems to be clicking coming from the attenuator right before it locks up. After that power cycling the unit will not fix it. Once you wait for about an hour or so it goes right back. I am no longer having the YTO error funnily enough. Still no input at all. It also sounds like the standby fan in the RF section is fluctuating in speed. These problems occur even with the IF section removed.Also, when this occurs ALL of the power supplies are pulled down about 3-4v out of spec. When there is no error they are spot on

author: klondike
author_rank: Newbie
timestamp: June 27, 2025, 09:37:05 pm
content: I had a no-input issue with one of my 8566 units. The problem is the blown power transistor on the YTX driver which kept the YIG filter inside the YTX shut.Good luck with your repair and keep us updated!

author: _rushin_or_draggin_
author_rank: Regular Contributor
timestamp: June 30, 2025, 03:33:52 am
content: Thanks for the reply, Klondike!I will have to look into that once I get the unit running. I have narrowed it down, I believe, to the filter cap for the 20V rail. It is demonstrating 1V P-P of ripple. When the capacitor is completely out of the unit the 20v is brought down to 12v and demonstrates the issue every time despite a long waiting period. Once it’s back in it does the original “run for 5 minutes then need a few hours break”. I’m going to see what I can do about replacing it. It’s a 8700 uF 40VDC cap. I’m currently looking for a similar replacement that’s new manufacture and not ridiculously expensive.

author: _rushin_or_draggin_
author_rank: Regular Contributor
timestamp: July 18, 2025, 08:12:24 pm
content: Update: 99% sure it’s the cap. I have taken a break from it to work on other projects; however, I recently just turned it on to see what it would do and it was reading the recall 8 signal (although slightly off from 100 MHz (oven was still cold, not sure if that’s enough to cause any issues since it should be locked either way I thought). Recall 9 showed nothing on screen despite adjusting the FREQ ZERO pot on the IF unit.I ran the calibration sequence and it ended on RECALL: REG 7 with “Corr’d” below that. I’m not really sure what to think at this point! I was certainly expecting it to say “Adjust AMPTD CAL” when I couldn’t get anything to show during the Recall 9 procedure. Any ideas??

author: _rushin_or_draggin_
author_rank: Regular Contributor
timestamp: July 27, 2025, 08:14:56 pm
content: Update: The issue with the fan coming on all the time has been fixed. The K1 relay on A23 Motherboard had gone bad (coil wouldn't pull in all of the way). I took the relay from the parts unit and it had also failed (just corroded contacts). Popping it back in and now the fan works as expected.As for the rest. I replaced the electrolytic cap and it had no effect. I decided to take a closer look at A17 (Positive voltage regulator board) and as I was looking Q2 completely fell off! The leads of the transistor had completely corroded through. Looking at the schematic Q2 looks to be part of the current source section for the 20V rail. I have also found a few bad caps in the feedback loop for the op amp regulator circuit. I am going to go through and test every capacitor on it and replace all of the transistors with corroded leads and then go from there.I believe my issue with recall 9 not showing anything is either a) caused by the heater not having time to heat up and causing a reference unlocked, or b) an issue inside the IF unit. I will also pull those boards and look for any corroded leads or bad caps. I feel this will take quite some time but I will continue to update.