Grundig C200 Cassette Player  - Page 1
Forum section: electronics / repair

author: Michaelaudio
author_rank: Regular Contributor
timestamp: April 13, 2021, 03:03:39 pm
content: So i have this C200 which was brought to me with no sound. I Hand my hand in the PCB and then it worked, easy cold sondern Joint i thought.. So i soldered some bad looking joints, turned it in and i had a really loud nasty square wave coming Out of the speaker. (A few HZ only).The Problem hast to be at the Output Stage (all Transistors are okay) Collector of T8 i have te square wave (about 6v Peak) at the Base of ZU there is almost nothing, Just the audio Signal. So all voltages are right, except for the Voltage in the Output ofc. I dont know where to Look at now, also i dont understand what C23 is doing and why C of zu is going to the C of T10. Some Kind of feedback Loop? Id Like to understand it.

author: floobydust
author_rank: Super Contributor
timestamp: April 14, 2021, 04:49:52 am
content: It's a circuit using the minimum number of transistors.During playback, the power amplifier is driving the loudspeaker, and during record it is an oscillator? driving the erase head.I would suspect a dirty rec-play switch. If it makes a bad connection, C22 will float and it can oscillate. Try clean the switch, move it back and forth a hundred times (with no tape in).I would also suspect the electrolytic capacitors given the age, e.g. C23, C24, C26 if there is strange low freq. oscillations.What is "ZU"?C23 looks like it provides tiny +ve feedback which bootstraps the input to increase the input impedance.R42 seems to increase ripple rejection on the PSU rails, so check the rails have no excess ripple from a bad capacitor or PSU. R47 seems to be for the usual DC feedback for the power stage.T10-C you open a jumper and connect an milliammeter there and adjust R48 for 5mA bias current, and R40 might be to set DC offset 3.7V at T10-E. I think you would do both adjustments back and forth because the trimpots interact with each other.

author: perieanuo
author_rank: Frequent Contributor
timestamp: April 14, 2021, 06:34:08 am
content: check dc and ac path for loudspeaker, maybe it't just the switch that disconnects the speaker when you connect the external one or C26 that dried

author: Michaelaudio
author_rank: Regular Contributor
timestamp: April 15, 2021, 12:46:24 pm
content: Oh, ZU should be "T7"I checked some Caps and they seem okay. When i Connect Base of T7 to ground, it stops oscilating, then i Connect the left Side of C22 to gnd it still oscilates, so it must be after C22 somewhere.I cleaned the Switch, moving it really often, still nothing Changed.

author: perieanuo
author_rank: Frequent Contributor
timestamp: April 15, 2021, 01:19:15 pm
content: the switch connected to C26 directs the signal to loudspeaker? have you measured its position/continuity?

author: Michaelaudio
author_rank: Regular Contributor
timestamp: April 15, 2021, 01:42:40 pm
content: You mean If there is a Low impedance between Point 2 and 4?Edit: guess the Switch isnt the Problem, 0.1 Ohms between the Connections

author: perieanuo
author_rank: Frequent Contributor
timestamp: April 15, 2021, 03:18:04 pm
content: verify all components around T8/T9/T10don't tell us you haven't measure the loudspeaker, what do you see between 2-4 switch and GND?it should be simply to diagnose this final stage

author: Michaelaudio
author_rank: Regular Contributor
timestamp: April 15, 2021, 04:36:01 pm
content: As i can tell by the loud clicking, the speaker is ok. (As i wrote it worked for a sec) 2-4 i have some Spikes (8V ptp). The Diode is okay, and so are the Caps, and the resistors, it is pretty weird.The frequenzy of the clicking/square wave gets a bit Up, as i turn the volume Up.Edit: okay i have only 100ohms between 2-4 and 6 in Play Mode, that shouldnt be.

author: floobydust
author_rank: Super Contributor
timestamp: April 15, 2021, 06:37:38 pm
content: Did you test the electrolytic capacitors?

author: Michaelaudio
author_rank: Regular Contributor
timestamp: April 21, 2021, 01:59:50 pm
content: I did. So when i remove the inner section of the Switch, i have 2.2K Ohm between Pin 6 and 4. In Play Mode 100ohms between 6 and 4-2 but the traces in the Switch part Look okay..