Archive for September 10th, 2012

Vanagon iPod Interface: Minimal Edition

My buddy Duggles, from far-off NH, restored his ’83 Vanagon to its original hippie-chick-magnet state. Late in the process, he realized that the once-fancy CD+radio widget in the dashboard lacked a line input for his iPod / iPad / iDingus. Knowing my foibles, he asked for advice.

Fortunately, he’d already discovered the service manual, without which life is always much more difficult. Search for PIONEER DEH2850MP SERVICE MANUAL and pick the site you prefer.

My first email went a little something like this, with a few updates:

BEH2850MP Audio Mux

BEH2850MP Audio Mux

The trouble with jamming a new line input into the existing circuitry is that you must match the DC levels as well as the audio amplitude. The schematic on page 19 shows the selector IC has capacitor-coupled inputs and outputs to strip off the DC level.

It would be very easy if the multiplexer (IC151, top of page 19, detail shown) had separate control inputs that we could override, but it uses a serial control stream from the CPU. No practical way to mess with that, alas.

As nearly as I can tell, the best way to do this would be to hack a DPDT switch between the FM/AM tuner and the amp, upstream of the mux. You pick the Radio input, flip the DPDT switch, and the iDingus plays through the Radio inputs.

However, an easier way is to simply inject the iDingus audio in parallel with the tuner audio, but set the tuner to an FM frequency without a radio station. The radio output should mute, leaving the field clear for the iDingus audio. This might not work, but it’ll be dead simple to try. If it’s acceptable, then you’re done.

The obvious problem is that we don’t know if the iDingus line level matches the tuner’s line level. The mux is upstream of the volume settings, so there’s hope that this will all Just Work. If it’s way too loud, that’s fixable. If it’s too soft, that’s a problem.

So, to begin…

DEH2850MP PCB Radio Jumpers

DEH2850MP PCB Radio Jumpers

The diagram on page 36/37 shows the A side of the PCB, with all the connectors & suchlike. The FM/AM Tuner Unit is over along the right side, with the audio output on pins 23/24 near the bottom and ground on pin 22. The traces proceed upward along the edge of the PCB, cross the connector near its middle, the audio passes through caps C151/152 on the B side, go through two jumpers on the A side across a mess of traces, and then dive to the B side and wriggle into the IC151 mux.

Quite conveniently, the ground trace follows along with them and is the lower of the three traces just to the right of the mux.

If I interpret the part number for C151 correctly (page 45, top right):

C 151 ... CKSRYB224K10

it’s a 220 nF cap. Anything around that value should work. This one from Radio Shack is grossly overpriced; anything with the same or larger value is OK (voltage rating doesn’t matter): NTE MLR224K100 – 0.22MF 100V Mylar Capacitor

Solder one lead of each cap to the top two jumpers, solder suitable wires to the other cap leads, solder the ground / shield wire to the bottom jumper, solder a suitable jack to the cable, plug iDingus into jack, fire that mother up, and see what happens.

The right channel is on pin 24, which goes to the top jumper of the three. Don’t bother trying to figure out which pin of the iDingus corresponds to that channel; just solder the damn wires and fix it later if it’s wrong enough to be objectionable.

I have no idea where or if you can drill hole(s) to snake the cable(s) through the housing. If the Vanagon doesn’t have a rear power amp, you could probably cut the traces under those RCA jacks (CN352, top right on page 37, above the FM/AM tuner) and repurpose them.

Give it a go…

We both attended Lehigh U, but Duggles realized early on that he lacked the personality flaws common to engineers and bailed out before damaging himself too badly. So his reply didn’t surprise me in the least…

I read your instructions carefully, examined the kindly supplied circuit diagram, and pored over the circuit boards with a magnifier. Then I blew you off (!!), threw caution to the winds, hacked off an old headphone cord, snaked the wires in, and soldered right to the very convenient L/R outputs on the RF board. Fired it up ,,, shitz, tons of background hiss, no quieting on the FM signal! A skein of obscenities was loosed in the mountain air until I thought to turn the iThang on … boom, full quieting, no hiss, and a quite substantial sound. No level issues at all, quite clean and detailed, and I didn’t even use the capacitors! (What was their purpose btw?)

After observing that a prophet is not without honor, save in his own land, I couldn’t resist going full-frontal didactic again:

The mux has a DC bias on its signal lines, with caps on both the input and output to isolate it from the surrounding circuitry. Back in the day, analog switches were fussy about their DC bias, so you had to go overboard to make them work at all.

I don’t know if the iDingus also has DC blocking caps on its output and figured that injecting raw DC from the mux into its guts could be a Bad Thing. But, eh, those engineers at Apple (‘s contractor) are smart folks and (probably) anticipated this sort of (mis)behavior.

The hiss you get with the iDingus turned off probably comes from dragging the mux bias to ground. I don’t know that’s a Truly Bad Thing, but adding those caps should eliminate any future problems.

You could even play DJ by combining radio & iDingus audio!

Rock on…

Seeing as how Duggles actually was a DJ for quite some years, I wouldn’t be surprised in the least to hear he does exactly that. We’ll be visiting him later this Autumn and I’ll inspect his work.

I love it when a plan comes together…


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