Kenwood TS-820S, DG-1 digital VFD display troubleshooting. - Page 1
Forum section: electronics / repair

author: Y2KEDDIE
author_rank: Newbie
timestamp: March 01, 2023, 02:49:46 am
content: I have been struggling with the Kenwood TS - 820S, DG-1, digital display.I appreciate any help and or collaboration. I’ve already tried de-oxit and reflowing all solder connections to no avail.The display counter assembly consists of two stacked printed circuit boards:The counter input and ripple counter boards.On power up, each band position, the display is:JJY/WWV: 16.831.51.8: 6.8313.5: 6.831.57: 6.831.53.5: 6.831.521: 26.831.528: 26.831.528.5: 26.831.529: 26.831.529.5: 26.831.5My thoughts are the digits right of the decimal indicate the output of the first mixer (7.83Mhz). The LO input to the SN76514N balanced 2nd mixer. This being a fixed frequency.As I manually adjust the VFO, the display digits ,after the first decimal, do not change (on all bands).The receiver tunes accurately, the display just doesn’t respond.The counter input board has two signal inputs: the 7.83 LO IN, and the VCO RF IN. These two inputs are fed into the SN 75514N balanced mixer and supposed to produce an output, fed to a corresponding band filter, amplified and then passed onto the ripple counter board input.I observe signals (several hundred mV)at all test points but am not sure what I’m looking at. They are complex waveforms on my scope. The service manual does not provide waveforms)I’m considering the possibility that the HF (VCO)signal is not mixing and I’m only getting the LO displayed.I’m awaiting a replacement SN75514N , on order.I ‘m considering breaking/isolating the Cin line feeding the ripple counter, and injecting a 2.8-31 MHz signal to see if I can change the corresponding display. I’m unsure what signal level to use. Possibly this would not work due to the other circuit inputs (latch pulse, reset, etc. It would be desirable to be able to isolate the two circuit boards to determine the area of the fault.Can you give me some direction and troubleshooting tips. Possibly test points and what I should see.Thank you,Eddie

author: DimitriP
author_rank: Super Contributor
timestamp: March 01, 2023, 07:14:36 am
content: Short answer is, pull the board out and reflow everything.Clean/solder/recrimp the wires going to the display board too.What does the display do?Is it dark, missing digits or displaying random numbers?Also take a look here:https://kenwood-hybrids.groups.io/g/main/topics

author: Y2KEDDIE
author_rank: Newbie
timestamp: March 01, 2023, 01:50:47 pm
content: As I said in my original post: the display, displays the frequencies I listed for each band regardless of the VFO position. The numbers after the decimal point are always 831.5. The only time this number changes (+- 2500Hz) is when I adjust the IF shift knobI have reflow soldered both boards several times.I’m seeking advise on troubleshooting the circuitry.

author: max.wwwang
author_rank: Frequent Contributor
timestamp: March 01, 2023, 07:38:53 pm
content: A picture is worth 1k words.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 01, 2023, 11:21:30 pm
content: Since the radio actually works and receives and transmits a signal.The next step would be to Check the freq inputs to the counter with a scope or another counter. You have done this, I think.But I do not recall not seeing a clear signal on the inputs myself.And see if VCO input changes appropriately with tuning.You are probably correct, in that the VCO freq is not getting mixed in.You can inject a freq but the freq should be the VCO freq which is in the Service Manual, (I cannot find mine right now) different between models. I think this freq is a few MHz. You have to look it up.I would start with a 2 v signal.This group specializes in the Kenwood Hybrid radios 520. 530 820 and 830 radios and their repair. They have helped me a lot in the past.https://kenwood-hybrids.groups.io/g/main

author: Y2KEDDIE
author_rank: Newbie
timestamp: March 04, 2023, 03:54:04 pm
content: I isolated the ripple counter from the “signal processing “ board. By injecting a signal into the Cin point. The counter responds when I inject a 7 MHz signal. It displays 6 MHz. 1 MHz low from actual input. It responds 1Mhz. Low reguardless what frequency I inject. I believe the counter is hardwired to display 1 MHz low to compensate for the mixer action of the preceding stage. I believe this confirms this board is operating correctly and the problem is on the preceding mixer board.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 05, 2023, 01:20:03 pm
content: In case you do not have the service manual:https://www.manualslib.com/manual/999901/Kenwood-Ts-820.htmlI believe you are describing the circuit on page 72.You first need to make sure you have the two signals, VCO and Carrier. As I recall they are pretty clean signals on a scopeYou can disconnect the multipin connectors and check the inputs first. I think they are on the mini coax wires.

author: Y2KEDDIE
author_rank: Newbie
timestamp: March 05, 2023, 02:58:15 pm
content: I injected a very clean sine wave (500 mV from my signal generator) into TP4), then the counter displays correctly.I suspect possibly the gain of the broadband amplifier (Q5,Q6,Q7,Q8), might be too low,or possibly the output of Xfmr T1 too low.I replaced IC-1 (SN76514). I wonder what the iwo inputs (cleanliness), and output signals should look like and what levels.I suspect the signal at TP-4 to be somewhat complex ,(noisy), as it has the band switched filters preceding the wide band amplifier.

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: March 05, 2023, 07:30:05 pm
content: There are adjustment on the counter board.Formal adjustment is addressed on page 40Try adjusting T1 and T2 for max signal at TP2

author: Y2KEDDIE
author_rank: Newbie
timestamp: March 11, 2023, 10:12:18 pm
content: I narrowed it down to the Balanced Mixer chip, SN76514N. I ordered two and received either Bad, or counterfeit chips. I believe they are counterfeit because when I do resistance measurements pin to pin nothing agrees with the data sheet.Since this is an obsolete chip my only source has been e-bay. I wish I could find a donor board or chip from som one reputable.I’m told the FRG-7 uses the same chip in it’s display.

author: Y2KEDDIE
author_rank: Newbie
timestamp: April 20, 2023, 05:17:42 pm
content: Update 4/20/23After being side tracked with counterfeit chips, I found another e-bay seller and got a good chip. This fixed the problem. The display now works correctly.I find the TS-820S service manual quite vague and hard to follow in describing receiving signal path from antenna to the audio output. I’d like a better description of how the VFO, pd/pll, interact with the DG-1 display.I wonder if there are any other articles written to supplement the service /user manual.