Kenwood TS-870 Tech Notes

Note: I no longer service Kenwood gear.

Pulsing or intermittent ALC:
As with the TS-850, this is usually caused by failure of the DC-DC converter circuit. The TS-870 has basically the same circuit as the TS-850, except it is not on a separate board. With the TS-850, one can merely replace the entire DC-DC Unit (X59-1100-00). But with the TS-870, one must replace individual surface-mount components.

Since the TS-850 was easily repaired by replacing the inexpensive DC-DC Unit, I never saw the need to troubleshoot down to the component level. So in the TS-870, I just use the “shotgun” method, and replace all four of the active components in the DC-DC circuit:

One 2SA1162(Y) transistor (Q401 on Final/Connection Unit)
Two 2SC2712(Y) transistors (Q402 and Q403 on Final/Connection Unit)
One 1SS226 diode (D401 on Final/Connection Unit)

I’ve done this many times, and it has always solved the problem.

Raspy or garbled RX/TX signals:
Unstable trimmer capacitors in the VCO circuits can cause RX/TX signals to sound raspy. The quick fix is the give the offending trimmer(s) a few twists, then realign to factory specifications. But the best fix is to replace the offending trimmer(s).

TC1 in VCO2 affects all bands.
TC506 affects 30KHz to 7.489MHz.
TC507 affects 7.49MHz to 14.489MHz.
TC508 affects 14.49MHz to 21.489MHz.
TC509 affects 21.49MHz to 30MHz.

Front-end overload:
The TS-870’s front-end is well protected by fuse-lamps (PL1 and PL2 on the RF Unit). PL1 protects the main RX input (RAT), and PL2 protects the EXT RX ANT port.

A strong signal from a nearby transmitter can cause a fuse-lamp to open, causing a drastic loss of receiver sensitivity. Usually, the glass casing of the affected fuse-lamp is darkened.

Kenwood does not list the specifications of these lamps. Although my tests indicate they are rated at 5V 100mA. But I recommend replacing with original factory parts when available. The part number is B30-2134-05.

Note: In extreme cases, I’ve seen surface-mount capacitor C2 (120pF) on the RF Unit become shorted.

Interesting observation:
When I had a TS-870 with a 60-mil serial number in the shop, I noticed some production changes. The circuitry wasn’t changed, but Kenwood opted to use standard DIP IC’s, instead of surface-mount chips. This was done on two boards. See the pictures below.

They used short ribbon cables, soldered to the bottom of the board, where the original surface-mount chip would be. The other end of the ribbon goes to a DIP IC on top of the board. The chips are electrically identical to original, but they are DIP’s instead of SMD’s. This was done to IC1 on the Filter Unit, and IC1/IC2 on the RF Unit.

Note: A 60-mil serial number means it was one of the last TS-870’s made.

IC1 on Filter Unit:


IC1 on Filter Unit (bottom view):


IC1 on Filter Unit (top view):


IC1 and IC2 on RF Unit:


NOTE: If you don’t have the skills or equipment to work with static-sensitive surface-mount components, please leave it to an experienced technician.

Mike Nadeau – N1EQ

March 18, 2009